 Well, I'll wake you up with some Discussion of brachytherapy this morning. I'm going to talk about in my first talk I'll talk about brachytherapy technology and dosimetry And then my second talk later on this morning I'll talk about treatment planning and some quality assurance with brachytherapy so let me start by talking about technology and dosimetry and What about categories? Well, there are categories by the route of which you administer the brachytherapy Intracavitory, you put the applicator into a natural cavity in the body, the esophagus, the rectum, the the vagina uterine canal and so on. So natural cavities in the body, intracavitory interstitial, it used to be needles and now it's catheters You put these and seeds directly into the patient. That's interstitial Surface applicators, not a lot of that work is done with brachytherapy. A lot of it would be done with electrons externally with Teletherapy, but there are quite a few people doing nowadays doing surface applicators with brachytherapy We used to do it a lot Before we had electrons. We've had electrons for 40 or 50 years now with linear accelerators But prior to that we didn't and so we did a lot of a lot of brachytherapy for treating surface lesions basal cell carcinomas and so on And then intraluminal, you put tubes into organs like the bronchus and arteries And I don't know if a lot of that is done now We went through a spell about 20 years ago when this was very popular intravascular brachytherapy, we called it and People thought this was going a long way Apparently it's kind of died out now. So not many people do that What about categories by dose rate? These are my definitions and I'll explain why mine are different from the official definitions Permanent implants typically less than about 30 centigrade per hour depending on the isotope You're using for the permanent implant Low dose rate that this is my definition from 30 centigrade up to 100 centigrade per hour You'll find in your textbooks that they go up to 200 centigrade per hour and I don't like that because I've looked at data and it seems that you get Significantly increased complications above about a hundred centigrade per hour So you start running into radiobiological problems once you get above a hundred centigrade per hour So I like to define a hundred centigrade per hour as the upper limit for so-called low-dose rate brachytherapy and Then high dose rate brachytherapy high enough to give enough radiation in a single session So above about 1200 centigrade per hour, and it's usually fractionated, but not always And then pulse brachytherapy. What is pulse brachytherapy pulse brachytherapy uses equipment just like a high-dose rate machine But in pulses maybe once every hour So the patient is usually an inpatient lying in a bed but now by by using a high-dose rate machine and Turning on the pulse for maybe a minute one minute every hour you simulate low-dose rate. You're getting repair That's what low-dose rate is all about. You're getting repair while the patient is lying in bed Which you wouldn't with a normal high-dose rate machine. You're allowing repair between pulses So it it simulates it almost simulates low-dose rate brachytherapy Problem there is the patients lying in the bed a Big advantage of that is to the staff that's looking after the patient because while the machine is turned off, which is maybe 90% of the time or 95% of the time the machine is turned off people nursing staff can go in and look after the patient Visitors can come in and so on so it has an advantage low-dose rate brachytherapy doesn't What about loading? Well, manual or hot loading is very rarely done these days, although that's not true because we do hot load Seeds into the prostate so that's that's not true. I wasn't thinking I'm thinking of the old days when we actually hot-loaded cesium sources or even radium sources if you go back far enough Right into the patient and and the physician would actually hold the source and in in a pair of tweezers and push it into the patient I've seen that done many times. In fact, I used to work with a radiation oncologist Who boasted he could push him in with his fingers and he developed lots of cancer? So it wasn't a very good example, but many manual hot loading is a problem because of the staff Manual after loading is much more common where you put an applicator into the patient And then you load it afterwards and that's pretty much what's done with with prostate implants for instance You actually put the the catheters into the patient and then you have to load it And I'll show you some examples with breast cancer too that you can have to load those into the patient so manual after loading is a lot less dose to the physician doing the job and Remote after loading we all know about remote after loading with high dose rate or pulse dose rate It's usually fractionated or pulsed Now what about different source types? We used to use a lot of tubes Tubes that would go into catheters in the patient or into Applicators that you put into the patient you put an applicator Say into the uterine canal and then you put a tube through that applicator We don't use those much anymore. Look at the dimensions of that tube. It's quite quite a large diameter We used to use cesium for instance, and they're about three millimeters diameter and that means That the tube that you put it in has to have quite a large diamond diameter and that can be a problem Patients don't like you putting needles in that have a large diameter into their breasts. I don't blame them at all So we don't use that much anymore needles again. We don't use needles all that much anymore But this is how it was done originally. We stuck needles into patients tumors and treated them that way wires much more common and just these are usually made of iridium and I'm going to show you later on iridium can be made much smaller much smaller diameter Maybe one millimeter diameter where you might need a tube or a needle would be of cesium would need maybe three millimeters Diameter so the the needles are much smaller to go into the patient Actually a lot more common are seeds in the ribbon such as iridium seeds in the ribbon Cobalt 60 Fears with spaces that I don't know if any does anyone have a high-dose rate unit with cobalt 60 spheres We've got some My first high-dose rate unit had spheres of cobalt the problem is and we'll see in a minute That those cobalt spheres are pretty big diameter compared to iridium that we using in other high-dose rate machines in most high-dose rate Machines so again, you've got big catheters that these have to fit into But there are some advantages of cobalt 60 especially in countries that can't keep replacing the source as you have to with the iridium source and In finally stepping sources in a catheter example high-dose rate brachytherapy with iridium Stepping sources in and you can create the pattern and you can create a pattern with the cobalt 60 spheres too But you've got to create the pattern and then put the spheres in whereas with the stepping source you can just step the source one source can be stepped and The dwell time of the source can be changed to match the dose distribution that you want so it's more versatile and You can do it at the time that you're treating the patient with the cobalt 60 spheres You have to organize everything beforehand because that's the pattern you're going to get in Inside the patient that you have hot spheres and cold spheres in fact You can see there are some red ones and some white ones there the red ones are the hot spheres and the white ones are like Little balls that are not radioactive same size, but not radioactive applications of brachytherapy. Let me If I can yes, here we go So low dose rate we can have continuous low dose rate or pulse low stress rate Which is equivalent roughly equivalent to low dose rate and you can see with the continuous typically iridium wires or iridium in wires iridium seeds in wires and These are just some of the things that are typically treated with this system skin and head neck Actually, all these clinical examples are just some examples. You can pretty much treat anything with anything. You can treat all So so with iridium wires, you don't just have to treat skin and head neck You can use iridium wires in other parts of the body too pulsed similar things had neck you can do skin gynecological quite often use pulsed systems and so on with permanent Implants typically prostate can use it elsewhere, but most of the time 99% of the time probably used for prostate These seeds I'm going to go over these a little bit more later on maybe in my second top High dose rate fractionated or in single doses most of the time it's fractionated Of course, you need special equipment as you do with pulse low dose rate. You need special equipment And that can be used just about treating anything here Okay What about the energy of the sources? Well, all photo emitting isotopes can be pretty much grouped into two categories High energy and I define that as greater than about 50 KV. I've seen other people use a hundred KV and These have similar attenuation characteristics in tissue Regardless of the energy they're pretty similar in tissue Whereas low energy isotopes the it varies significantly in tissue depending on which isotope you use and In shielding character characteristics because they're very low energy You don't need much shielding. In fact, you can send the patient home I wouldn't want to send the patient home with some with with an isotope in Say of 80 KV, which is why I like to define the high energy as being about 50 KV We're not 100 KV So I think of it more in terms of shielding and what you can do with the patient patient can go home Let's look at some important isotopes and cesium 137 was very important to us when we first started doing brachytherapy Well over a hundred years ago. We used radium and then eventually we started using radon seeds pretty much the same characteristics of raid radium same very high energy and radium is A big problem because it's very high energy very difficult to she shield and radium tubes can break and Radon gas can leak out and then you have to close the hospital because you've really damaged you've got radon all over We actually did that in our hospital, but fortunately it wasn't broken We thought we'd broken one, but it wasn't broken, but we had to seal off all the doors We caught in the nuclear regulatory commission to with all their instruments Thank goodness it didn't break a safe crack down on top of it and we thought it cracked right through it But it missed so we were lucky so but that's happened But hospitals have had to be closed and often you don't know it's broken He's got a crack in it and you're radiating everybody around so it's a big problem with radon So we radium so we gave up radium and introduced cesium probably about 30 or 40 years anybody using radium You don't use radium anymore Radium has a wonderful half-life 1600 years. So your source is gonna last longer than us Unfortunately, that's bad news too because if you lose one it's gonna be there for lots of years and you might be Might have a problem with with that radium source So we've got rid of our radium sources and replace them with cesium Cesium has a 30-year half-life great. You don't have to keep replacing them But they do decay you've got to take into account the decay of the source, which isn't a big problem It's easy the mean energy is about 660 kV still high But not as high as radium radium goes up above an MEV So it's a little easier to shield cesium sources The half value layer of lead is you can see it's five Just over five millimeters. So a lot of shielding is needed when you use cesium sources We used to carry shields in not carry wheel with big wheels Shears into the patient's room when we were using cesium and put it so when the nurse goes in They can stand behind the shield and stop themselves from getting too much radiation, but it's still a big problem Okay, this factor now. I'm gonna show It's the what we call the specific gamma rate constant sometimes the exposure rate constant and this tells you how much radiation comes out of a source of Unit strength say one one millicury here and and notice it's in exposure units It's rumpkins per millicury per hour of one centimeter We don't use this much anymore, and there are lots of reasons why I'm gonna spend quite a bit of time explaining Why we don't use this anymore First of all, I want you to notice it's a small number We're gonna show you much higher numbers later on and even more important specific activity Specific activity is how many curies you can pack into a gram This is relatively small number eighty-six curies in a gram tissue, so What that means is these sources have to be pretty big and I already explained that three millimeters diameter To be able to have a source that's reasonable for brachytherapy treatment Principal uses I mentioned it's the first Radium replacement for low-dose rate brachytherapy particularly we don't use them for I don't think anybody's ever you I might be wrong I don't think anybody's used cesium for high-dose rate brachytherapy. Sources will be too big Pack all that into one source would give you a quite a big source Let's talk about this specific gamma ray constant or exposure ray constant It relates the activity to the output the output in that case was in rumpkins not Gray or sandy gray, which is what we really want. We really want to know the Dose to the patient not the exposure of course you measure the exposure could use an ionization chamber to measure it But really we want does It's a been a big source of error in traditional systems Because it relies on an accurate knowledge of activity and an accurate knowledge of the effect of encapsulation on your measurement of activity which can be a problem with low energy sources because a lot of the Radiations that are coming out get absorbed and you don't count them and with high energy sources because you've got a very inefficient system Because you're missing a lot because they're getting out so it's been a problem with Determining the the activity of the sources, but we did it because we had no other dosimetry system It's not used much in present-day brachytherapy Source specification and most of the rest of my talk this morning is going to be about the modern brachytherapy dosimetry It's been replaced by what we call the dose rate constant So I'll be talking quite a lot about the dose rate constant So that's new and it's dose rate is not exposure anymore And it's something that's related to things that we can measure in-house very hard to measure the activity of a source in-house But this is much easier to measure in your department to check that the vendor has given you the correct Activity for the sources that they're selling you Big leap came when I iridium 192 was introduced It's got a 74 day half-life bad news. You've got to keep replacing it and you have to Very important that you keep correcting the output of this source for the K Easy to do we physicists can do that Mean energy 330 K VR a little bit better a little bit easier to shield It's half value layer in ladies now 2.5 millimeters. So it's a bit easier to shield you still need shielding though This is still pretty high energy Look at a specific activity we can pack Over 9,000 curies into a gram of iridium much smaller sources, and that's One of the reasons why it's become so important So it's used as a replacement for cesium for seed implants and as an HDR seping source and and PDR pulse dose rate to Let's look at another potential source that we have used a little bit And I mentioned it before in the early high-dose rate machines that some people still have cobalt 60 Big advantage over iridium. It's got a five-year half-life So you're not replacing the source every three months as you probably would be doing Most of the time with a an iridium high-dose rate machine with a cobalt high-dose rate machine You don't have to replace the source for a long time very useful The mean energy unfortunately is a lot higher. So a lot more shielding is needed Half value 11 millimeters lots of shielding much more than with iridium. So that's a problem It's got an intermediate specific activity It's kind of between iridium and cesium So the sources have to be somewhat bigger than iridium and that's why most of us have Now got rid of those machines and replace them with iridium machines And the principle uses high-dose rate Intracaetry and again pulsed dose rate And not just into cavity Anything but they're often used in high-dose rate. The reason it's into camera tree here not interstitial is The sources are too big and you've got to make big holes in the patient. They don't like that and so it's mainly in Natural cavities in the body. So most of the time are you using it for interstitial at all just into a cavity? Everything yeah, if you that's all you've got you got to use it for everything But you do have to put big needles into the patients and they tend not to like that too much It's got the advantage over iridium because of its long half-life But the disadvantage because of the source size and the shielding that's required Let's compare those properties that I've been mentioning easy to do With cesium In fact, I've got it here with iridium 192. It's the easiest to shield. It's got the lowest half-adulayer in lead It has the highest specific activity. So you've got the smallest sources Which is why it's possible? Why is that now it's the preferred source for modern high-dose rate machines? That source replacement can be a problem you've got to replace the source maybe once every three months because of its half-life And cobalt 60 for particularly for countries where replacing a source every three months is a problem Then cobalt 60 might be a better choice Let's look at low energy isotopes remember below 50 KV now and we've pretty much got two major Players in this field I-125 which has a half-life of 60 days Mean energy very low 28 KV Means the half-adulayer of lead is very low So it's easy to shield and very often the patient can just be sent home with the With the I-125 seeds in them as we do with prostate cancer patients I have a friend who had to have a prostate implant and he was I I sent him to a Specialist in New York who was world famous for his prostate implants and my friend was so Excited because he could take the subway home He couldn't believe they said I'm radioactive and they let me take the subway home because he wasn't not enough radiation Getting out of his body happened to be a big man. So he had a lot of natural shielding Not enough radiation was coming out that it mattered that he could go home on the subway issues for temporary implants also at high activity you can I don't know anybody who's doing this anymore, but I know we had quite a service in in my town in Detroit Where one of the big centers were doing high activity? I'd seen seeds for the brain. I didn't particularly like what they were doing But an eye plaques it's been used a lot in eye plaques That's quite common still for treating oculomelanomas for instance behind the eye. You just Put the seeds in behind the eye on a special applicator And as a temporary implant then you take them out a week later on and they've had enough radiation to treat that disease And palladium 103 very similar palladium 103. It's got a shorter half-life So when might you want to use palladium instead of iodine well It's got a shorter half-life So the radiation is delivering in a relatively shorter time and that might be for a more rapidly growing cancer You want to get the radiation in a little bit quicker. I'm not sure that's the way That's the rationale that most radiation oncology shoes for using palladium It's got a lower mean energy slightly lower instead of 30 kV. It's 22 kV Means less shielding. It's got point zero zero eight millimeters Needed for half valerian lead again very easy to shield this principle use is just like ID 125 Permanent implants of prostate and a few other Few other cancers where there are some new isotopes that are being considered There's a terbium 169. It's got a mean energy of 93 kV So that's some people would still call that low energy It's got a half-life of about 32 days and more than halfway between the iridium between iodine 125 and and palladium 103 It's a potential replacement for iridium and it's got a lower energy So it requires less shielding and it's got an even higher specific activity than iridium which means smaller sources. So People are Promoting the use of a terbium. I haven't got any experience with it Anybody got any experience with the terbium? Probably not. It's a new isotope. It's got some nice properties that One of the problems is 32 day half-life iridium is 74 days half-life. So it's going to be you're going to need to replace these more often You might be buying them just for a specific patient and And that's okay. The advantage of iridium you can buy a batch of iridium and then it's useful for the next several weeks This would start decaying too much so it wouldn't be as useful. So that's a problem Another isotope this being considered as a cesium 131 got a very short half-life and it's got a low energy. So this would be replacement for For iodine implants for permanent implants of prostate for example, again, I haven't got any experience anybody Cesium 131 probably not It's a new idea that's being promoted lots of papers being published on it But I don't know too many people are using this yet but if a manufacturer if a vendor decides that this is something to to promote you might be seeing Cesium 131 and a terbium 169 sources in in the relatively near future And then something we have been seeing electronic brachytherapy anybody have electronic brachytherapy here Okay, let's talk about electronic brachytherapy that is here. I know lots of people using electronic brachytherapy So what is electronic brachytherapy electronic brachytherapy uses a miniature x-ray tube instead of a radioactive source and This is what it looks like and that's a finger behind there is a finger with that with that x-ray tube on That finger it tiny this can go into catheters in the patient into into cavities in the patient So it can be used very much like Iridium and in fact you can attach this to what basically would be a high-dose rate remote afterloading use it for high-dose rate remote afterloading and people are now doing that It's got a big advantage. You can turn it off. You just throw the switch. It's turned off now Anybody can go in and handle it and it's much easier to to protect people from the radiation So let's get some details the tubes inserted into catheters the way we do high-dose rate It replace it can be used to replace iridium. Most people haven't yet Shielding storage handling and dose distribution advantages over iridium. Let's look at what it looks like now This is a typical source. You can see it's about two millimeters diameter a little bit bigger than the iridium It's usually about one millimeter diameter, so it's a little bigger But the source 10 millimeters long not that much different from the iridium sees that we use today And and you could another thing you can do you can change the energy You can adjust the voltage if you want to so you not only can have Optimization from those distributions you can in optimization of energy Distributions as well, so it's got an added advantage in terms of potential dose distributions And the air curmer rates we're going to talk about air curmer rates later on A comparable to a 10-curie high-dose rate source, which is what's used in your high-dose rate machine So it's very similar. It doesn't decay Which is another advantage? So it's got some advantages and you're probably going to be seeing electronic brachytherapy in the near future And this is what the dose distribution looks like it looks very much like the dose distribution around the high-dose rate source Now I want to spend most of the rest of my time I think all of the rest of my time today talking about modern brachytherapy dosimetry What do we now do that will? Make the dosimetry better than it was before the current method used in treatment planning computers is based on AAPM task group 43 A lot of AAP you've heard about AAPM task group 43 and it's been updated a little bit So this has been in existence for about 20 years now And it has lots of advantages over the previous methods that we used So what was wrong with the old dosimetry that we used before 20 years? So it's just like your dosimetry with external beam radiotherapy We had these protocols come out and one protocol 20 years later Enhance the accuracy of measure of determining the dose in the patient and then another one would come out I've I've been working in the field of quite a while and I've seen three major protocols with external beam Dosimetry Same things happening with brachytherapy. What was wrong with the old dosimetry? I mentioned some before Specification of the source strength as activity This is difficult to measure accurately and reproducibly reproducibly both by the vendor and the user quite often for instance, I would get a source and they would tell me oh, this is 10 milli-curie source and then I would measure it and it would be a 11 or 12 milli so maybe a 10 or 20 percent Difference it wasn't that they were wrong. It was the way they were defining activity They may have been correcting for the fact that you're losing some of the The radiation in the encapsulation or they may not have been doing that maybe I'm doing that and this is a big problem And I'll show you some examples why it became a big problem It's a variability in the fact that to convert the activity to the dose for example prior to 1978 the specific gamma ray constants that I mentioned before Published for a rhodium mine 92 range from 3.9 to 5.0 R per hour per milli-curie at a centimeter big differences in how you converted the activity into Exposure in those cases exposure in the patient then of course you convert that with the F factor into dose so big differences and it's all to do with this business of do you correct for for absorption in the in the Material of the source or don't you and it's varied all over the between vendors It varied and it certainly varied a lot between users So that if I quoted I give my patient six thousand centigrade Somebody else is giving exactly the same dose, but they're quoting five thousand centigrade that they gave There was a difference and if you if radiation on colleges goes from one department to another and He asked for six thousand centigrade to his patient and he's only getting five thousand over here The chances are he'll never notice, but he'll be underdosing his patient. So it can be a problem and It's preferable to use only quantities that Derived from those rates in a water medium rather than things like exposure in air What we really want is what's the dose rate in our patient? So we now need to specify the strength in units that are not activity So what do we do in TG 20 TG 43 the old units Before TG 43 the old units when we had radium going way back Was milligrams of radium and then it became milligram radium equivalents when we went to cesium And I remember I always use milligram radium equivalents I didn't use the real activity in milli curies We use milligram radium equivalent because all the clinical experience Developed in the previous 50 years of brachytherapy had been developed using milligrams of radium So we wanted to just replace radium with cesium. That's long gone We lost that a long time ago Then we went to activity and then I say all apparent activity. What do I mean again? It's all to do with what gets out of the source and what you measure apparent activity is what they may be correcting the the activity that you measure with a factor to account for the attenuation in the source and So you've got the variety of different units here activity and actually apparent activity and this certainly became a problem For TG 43 we need a unit new unit that could be directly related to your in-house method of verifying the source strength And this new unit is the air karma strength So instead of activity now we express the source strength as in terms of air karma strength The air karma strength is the product of the air karma rate, which I'll define in a minute Due to photons of energy greater than delta We'll explain that in a small mass of air in vacuo. We'll explain that too and The square of the distance we put the square of the distance in there because that's the way it's defined If the distance is one meter, then it's one. So you don't modify you multiply by one and we're gonna see the Europeans actually stipulated at one meter and So you don't need to multiply by the square of the distance one meter squared Okay, what is air karma strength? This is the property that can be related to the measurement for each source The air karma strength is Usually inferred from transverse plane air karma rate measurements performed in free air geometry a distance large in relation to the Linear dimensions of the source typically one meter So you're gonna you're gonna have a source and you're gonna measure along the transverse act is at a meter away Well, there's the advantage that the source which is usually about six Millimetres long, but a time you get a meter. It's like a point source So you don't have to take into account the geometry of the source for that measurement Because of the large distance the effective source size linear dimensions is negligible Why in vacuo? Well, the qualification means that the measurements are corrected for photon attenuation and scattering in air or any other medium imposed between the source and detector and Also any nearby objects like floors walls and ceilings This isn't something we can do in our department I've tried doing this and I'll put a source in the middle of a big room like this And it's amazing how much scatter gets to that source from from the walls and the floor It's very difficult and this is something that the National Calibration Labs will will be doing they'll be specifying how this How measurements that we can do in our department relate to the source strength Why energy greater than Delta? Well, there's a cut-off Delta Intended to exclude any low energy photons that will increase the air karma strength But don't contribute to the dose to the patient for instance. They don't get further than One millimeter point one centimeters into the patient. So they're just surrounding the source We want to eliminate those because of tissue attenuation. They don't get more than a millimeter from the source Typically, this is 5 KV Units of air karma strength Okay, this is the equation for air karma strength And the unit is micro gray Meter squared or you can leave that out if it's at one meter per hour. So micro gray per hour the unit of and one micro gray per hour For many users is called one you one you unit is one micro gray per hour at one meter The alternative to this unit is this is the European Units called the reference air karma rate and this is your equate European equivalent of a karma strength It's numerically equal But it's explicitly defined at one meter. So it's exactly the same thing But explicitly they define it at one meter simplifies things a little bit It's it's usually simply micro gray per hour and it's therefore assumed that this is a one meter So that's the strength of the source Now, let's look at how can we get from that? So if we know the strength of the source How can we get from that to the dose in the patient? Okay Well, the first thing we can do and I'm going to do them in steps and show you the parameters that we have To use to do that is to determine the dose rate along the transverse axis Closer to the source. We now know what the dose is at a meter from the source Let's determine it what it is close to the source out one centimeter Which is usually the reference distance for these sources. How can we determine that? And then how do we count for the effects of absorption and scattering on this along the transverse axis? So we've got these measurements close to the source. How do we correct that for absorption and scattering in tissue? And then the next step is how can we do that? Off the transverse axis so now instead of just knowing it here How can we get it anywhere else over here? So we want the dose distribution all around the source How can we get it over here? And first of all, let's just take into account inverse square law Now this is an extended source So it's not simply one over our squared it got to take into account the dimensions of the source to get it Around the source. So that's the next step. And then finally, how do we count for absorption and scattering on these off-axis dose rates? How can we take into account the self absorption of the source itself and the encapsulation and scattering in the tissue So there are the three the four steps that we need to use to get the dose in the patient This is what TG 43 does So what about the dose rate closer to the source? We need a factor that would convert the source strength at one meter To the dose at a reference point usually one centimeter Close to the source. How do we do that? Well First of all, this is this is going to vary From type of source to type of source every type of source with different encapsulation is going to have a different factor And that's really important We need to do that for any type of source so that Our dosimetry for one type of source is exactly the same as somebody else's dose symmetry Even our dosimetry with a different source in a different in a different environment And the factor is the dose rate constant So what is the dose rate constant? This is the dose rate per unit air karma strength at one centimeter along the transverse axis of the source It includes the effect of the source geometry the spatial distribution of radioactivity within the source any Self-filtration of the source and Scattering in the water remember we define this in vacuo at a meter now We've got in water because it's now in the patient and so we have to make any corrections for absorption of scattering So how do we know that? Well, it depends on the source structure Unfortunately for us somebody's done all the calculations already and they all did it all typically using Monte Carlo studies of the disc of the dose at one centimeter away on the transverse axis and Here's a good example where a a p m an astro astro work together to to define these terms for all the different sources that we have and as I say mainly Monte Carlo and here There's the information on just High-dose rate iridium sources and you can see a whole variety of high-dose rate in different machines Sources in different machines and they have different values for this this conversion factor So the conversion factor is very highly specific to the exact Structure of the source and you can see all these different values here All this information is in your treatment planning computers It's all been put in there You don't have to do it yourself although if you want to do some hand calculations to check that you're Those symmetry systems working right on the treatment planning computer you you can use these values They're all published in In in the 8 p.m. Task Group report Now the next thing account for absorption is scattering in that dose Okay, well we accompany is this by what's called the radial dose function and it's a function. It's a small g R-minus distance R and it counts for dose falloff on The transverse axis so we're still talking about the transverse axis of the source We haven't gone over here yet Transverse actually the source and it accounts for photons scattering and attenuation and so on Consensus values of our GR. There've been a lot of studies published and the task group AAPM report number 229 Analyzes all this and comes up with a consensus value of this parameter for all the different sources that we have and Here's an example. You can't read it But it just shows you for a whole variety of different high-dose rate sources So all these things at the top there are all different high-dose rate sources This is the consensus value of that at different distances and it's defined as one at one centimeter So we start one one centimeter from the source is our normal and then anything else along the central axis only here It gives you specific values for each type of source again very highly specific to the source itself Well, what about the effect of source geometry off the transverse axis? Let's go out here somewhere and let's just use inverse square law only This function is called the geometry function It's the ratio of those rates in there at the point of interest for instance anywhere at the distance r In any direction to that reference point at r0, which would typically be one centimeter Ignoring photon absorption and scattering. It's purely inverse square law and actually as you'll see When I show you the full equation it's really relating the What's going on over here to what's going in on here in terms of inverse square law? So it's a ratio really of what's going on here to what's going on there and Because it's a ratio Lots of the Potential errors cancel out and you can use simplified form of the equation We used to call this the siever integral many years ago. We Older physicists knew we had to solve the siever that I don't think I ever solved it But it's an integral and it's very complicated to to do it I'm not going to show that because we don't need to do it anymore This is the geometry. We've got a point a distance r angle theta And we've got the source of length L and it's purely geometry inverse square law It replaces one over r squared if it's a point source. It's always one over r squared But it's not what point source anymore. It's a distributed source It counts for the distribution of activity and we can use a simplified form of the integral Mathematically simplified form of the integral for a line source. This is the equation for the the Geometry function and for a point source. It's simply one over r squared Okay So that's what goes into your treatment plan. Let's say your treatment planning computer Does it just uses a simplified form turns out the errors by simplifying it a quite negligible And then finally now we've got the dose here due to inverse square law What's the dose here really in practice? You've got to take into account absorption and scatter in the tissue And this is accomplished by what we call the 2d anisotropy function capital F Distance r angle theta And it accounts for anisotropy of dose distribution Including effects of absorption and scattering the medium And then we've got consensus values that have been published again in a APM report to 29 And that's what all your treatment planning computers have in them And this is just one example of for one type of source This is all this is for one type of source and that's all been determined by primarily by Monte Carlo Analysis and all this data from this table is in your treatment planning computer Okay, so the dose rate at the point the full equation is this you've got all these factors multiplied by each other To give you the dose rate at any point Around your source and this is what your treatment planning computer Calculates it uses that equation using the parameters that have been published Calculating the the inverse square geometry that it can calculate very easily But what if the orientation of the source isn't known or you you can't bother to put it in because all the sources are at different angles That that can be a real problem Well, what we then use is a 1d version of this. We simply have a 1d version of of this function And it's a 1d anisotropy function and what it does You've got data around a distributed source like this, but you don't know the direction of the source So it converts everything into what it would be for a point source Okay, it integrates in 3d space all around your source and comes up with a single number at a distance r That's the 1d anisotropy function the equations are a little bit simpler not much simpler, but now you've got the The values for the the 1d anisotropy function function phi and you've got this Geometry this radial dose function for a point source now instead of for a long source So it's basically based on inverse square law and all these values are published for instance for seeds Seeds are the most common source where you don't know the angle you put seeds in and they get distributed all around They're also very small and you could put that data in With all the angles of the sources, but most of the time you don't you just assume their point sources And then you use the 1d and those rate equation And let me finish by talking about some improvements on this We're already working on improving on tg-43 the next version Of of the symmetry is going to be based on models and this is just like in your External bean treatment planning system. You've got a lot of modeling that goes in there to develop more accurate Algorithms to calculate the dose This is all in in task group report 186 model-based planning so how What models are compared in tg-186 they compare their latest model-based Dosimetry with the previous versions and what do they do they look at tg-43 and compare Perit with tg-43 and then they talk about the models and these are very similar to your external bean models You've got a model that takes into a pound primary and scatter and separates them out You've got a model just like the ccc model in your external bean treatment planning system Plattscomb superposition convolution a little bit better a bit more physics and then you've got Boltzmann solvers solving the Boltzmann equation We have that in our external bean treatment planning computers now and it finally Monte Carlo, which are probably the gold standard so it looks at each of these and And I like this because it shows you gradually increasing the complexity of the rate You're increasing the complexity and you're increasing the physics In each of these go from tg-43 and and this the and separating primary and scatter These are basically factor base. There's not a lot of physics goes into that And then you've got these other methods just like in external beam treatment planning that are model-based dose Calculations and then of course Monte Carlo is the most has the most physics content in it So let's compare these methods the tg-43 is the current standard most people are using that now Has full scatter and it's a water phantom doesn't take any account things like homogeneity in homogeneity is in the body Similar thing for the separation of the primary and scatter. It's there's no transport of electrons involved in that you can get Electronic disequilibrium problems and so on not taking into account and then collapse cone does take in your account heterogeneities and it's accurate to the first scatter and It's just like external beam radiotherapy. It's pretty accurate and it gives you a pretty good idea of the dose distribution even if there are in homogeneities present and then the the the Boltzmann equation And that's in and you can see these are in different Electro has the CCC method very and has the Boltzmann equation method It's a little maybe a little bit better and it does full homogeneities In homogeneities and in Monte Carlo as I say is probably the gold standard And I suspect that that's what we're all going to be using in about 20 years time Maybe 10 years time probably not five years time because the speed of computers is getting such that we can do for Monte Carlo Calculations very quickly. That's probably where we stand Okay, let me summarize. Good. I'm about on time. Let me summarize very simple Brachytherapy can be administered by various roots dose rates loading methods source types and energies Tg43 significantly improved brachytherapy dosimetry about 20 years ago when it first started and now we don't have Significant errors. I can remember an error. I didn't mention it, but we converted when About 20 years ago when Tg43 first came out and we started looking at the dosimetry of I won 25 seeds and Palladium seeds and we discovered we were about 10% in error The doses were wrong because the old-fashioned dosimetry and now the new dosimetry and so we used to give if I remember correctly a hundred and forty five Gray for a permanent iodine implant. It came down to I think a hundred and twenty No, hundred and hundred and thirty about a hundred and thirty gray. So sudden big shift No, I I've got it wrong. It was a hundred and sixty gray and it came down to a hundred and forty five gray That a ten percent shift and we did the same thing for for a palladium So it there were big errors involved Tg43 significantly reduced those errors and it finally model-based those calculations are beginning to be incorporated into commercial treatment planning systems and Hopefully this will improve the dosimetry even more Okay, anybody got any questions or is it too early? Yes Can you speak up so everybody can hear your questions? No electronic brachytherapy can be used anywhere and is being used anywhere in the body for breast implants for prostanium plants and Anytime you can put a catheter into a patient. You can use electronic brachytherapy Let me tell you one problem with electronic brachytherapy today in America. I don't know if they'll be the same elsewhere And that is a stupid problem in my opinion. You're only allowed to use your source your little x-ray tube on An individual patient you can't use that same source on another patient So they have to throw it away and get a new one. It's what they don't throw it away They keep it for doing physics experiments on but you just you can't use it. It's silly. They're worried about Contamination and some but how can that be? I mean use the high dose rate sauce on one patient to the next Why can't you use it? So that's a problem in America, which is probably why Electronic brachytherapy hasn't taken off much more than it has but I suspect that that won't last That's people are going to come to their senses and and be able to do it. So anything that you use Regular sources. Yes. Well, there were lots of ways of doing that have been published You can't do the whole calculation. So you pick a point and you check what it is at a point I remember my wife's a medical physicist. I remember she started doing I won 25 implants for the first time 25 years ago and She she said She she got the treatment planning computer system and everything was working fine I said, well, why don't you do some tests on it? Take a point simple point take a point and do some tests It was a factor of two in error Why was it a factor of two in there because the the Treatment planning system had just been developed and they weren't calculating in the decay of the source properly Isn't it simple something like that? So it's good to do a point dose comparison now One of my ex students wrote a paper on putting a diode on the patient surface so let's say you're doing a Treatment of the vagina and you want to know is this the right dose? She was putting a December over the top of the vagina marking it on The treatment planning system where exactly it is and calculating what those that should be getting and that worked out very well I don't know if anybody's doing that probably are but I thought that was a pretty clever idea for using What everybody has in their department? and Ways of measuring point doses and that worked out rather well and you can detect two or three percent errors that way Yes, I had a question in the back. I didn't really understand the question because you're so far back Yes the residual dose anybody understand Yakov, do you understand you're closer to me now he didn't understand either Let's talk about it later because we're talking about it over the coffee break because I don't think we understand exactly what your question is Yes, another question. How about the oh to the? To the electronic brachytherapy source. Oh, it's it's just a it's not it's not a problem I mean, it's only it's it's just as if you were feeding the power to a 50 kV x-ray tube Like in diagnostic radiology, you'll be the same kind of thing and the good thing is you can turn it off Dosimetry it's all been done and calculated now. Is it in tg 43? Is it in any of these types of reports? I think it might be I think they're working on it if it's not published yet But so that could be you need accurate dosimetry in order to use it right now There are so few people using it. They've done their own dosimetry. They've studied it themselves well, I would Is there any radio? I'm sure there is a difference in radiobiology The whole body dose will be higher with cobalt-60 because it's higher energy it spreads to the whole body So that's going to be a big difference. Do the cells themselves know Probably not because the damage is being caused by the low energy electrons that have released the secondary electrons are released Probably the cells themselves don't know whether it came from cobalt-60 or iridium So the biological effects should be the same Now electronic brachytherapy may be a little different and I've done some studies on that myself I I suspect that eventually we're going to find that there is a little bit of an advantage of electronic brachytherapy I see those distributions are very similar and and if you're using it Cobalt-60? Okay, I wasn't thinking cobalt but most of the Treatments, it's mainly inverse Guelot From the source you get close to a source. It's mainly inverse Guelot There isn't much difference between the sources a little bit, but not much difference. Yes, you're good Well the cobalt-60 sources for brachytherapy are spheres Geometry is easy. So I you're right. I don't remember ever seeing that Yes, yeah, they're just spheres and it's a lot easier to do the calculations with spheres So so they probably haven't bothered. Are you speaking next? You've got some banging going on here Good luck. You got somebody banging. We have to tell him to stop