 Okay, ladies and gentlemen, good afternoon. I was told the most important thing of a teacher is to be able to have an imagination of your brain, what you are thinking. And probably you think, afternoon now I'm tired. I want to continue. This second part is dealing more with measurements and not so much with fundamentals. And it goes through different methods of dosimeters and mostly then going into the ionization chamber. And I told you already why I think that the understanding of ionization chamber is so important. We can understand them quite well. We can perform quite good measurements, exact measurements with uncertainties which are quite low. We had already one lecture today in accuracy and precision and uncertainties. And if I would have more time or maybe we could introduce it, it would be a wonderful exercise really to deal with the uncertainties, how to establish the uncertainties, how to determine that and how to document and what so on. So saying that the standard uncertainty of ionization chambers in the order of 1%, okay, you know it. But what does it really mean? It's not so important. Again, there's a concept behind which could be teach, which can be taught and which should be exercised. So the most, for most commonly used dosimeters are ionization chambers, radiographic films. This is still true. We have prepared this. It has, I think, a larger room. To these, still important. And diets which have a growing importance because we are getting more and more better diets. Even now we have diamonds. I have not used the diamonds because there was one famous detector offered by one German company called PDW. This diamond detector was extremely expensive. It worked with 100 volts. And if you made a comparison first farmer chamber, then you switched the detector, the diamond detector, and you have left the 400 volts, it's done. It's over. It was expensive. Now the PDW has developed a new diamond detector which seems to be much more better. I have no experience with that, but it's working without voltage applied and seems to be much more reliable. So I think this new diamond detector is a step forward, but it's not meant to cure. So these are several ionization chambers. The identical ionization chambers have now put here the chambers from PDW, but people important are chambers from IBA. We have two famous companies offering ionization chambers. We also have some companies in America, but I think it's more or less the three or four companies offering ionization chambers. I need to say some chambers. They are accurate and precise. They are recommended for calibration and no other detector is recommended for that. National corrections are well understood and we'll also later on tell a bit more on corrections and they have an instant readout. Disadvantages they have need connecting cables and sometimes this may be a problem if you have and if you have all in one from one company it's okay, but if you have, I can tell you one story which is quite nice. I will stand it later on because it was, I can tell you now, Electrometer, you need an electrometer and this is giving the high voltage to the chamber. The question is where's the high voltage? In the central electrode if you have an if you have an identical ionization chamber is it in the central electrode the high voltage or is it on the wall? It depends, it depends from the company. So the EVA, Electrometer, they have the high voltage on the central electrode. PDW normally if you have the TRIAC system, the MM connector, it is on the outer wall. So you have the high voltage on the wall. What happens? Dangerous? Ionization chambers from PDW are made from PMMA. It's not conductive. And only the inner wall has a graphic layer. So therefore, but EVA is offering ionization chambers made of conductive plastic. And if you combine such a chamber with the PDW Electrometer you have the high voltage on the outside. 400 volts here and what? On the outside. Which is not dangerous because but you can feel it. Especially you can feel it if you make measurements and water phantom and you look in the water. So all this is happening connecting cables required. And the other point is sometimes the cable are not really fitting to each other though. I will show a picture later on because PDW is offering any link between any connector which is quite interesting. High voltage supply required. Many corrections required. Many say quite a number of corrections are required. Film, Advantage, 2D spatial resolution very thin, doesn't perturbed beam is still very good. Disadvantages, darkroom processing all this is really refers to the old version which is not available anymore. I myself have made a lot of measurements with the so called Ixomat verification film and I still think it was a wonderful film. It was the symmetry with the film could be done very easily. I should go back in the 80s in 1984 it was just some years ago we developed a method to apply radio-surgery with an accelerator. It was really new because at that time only the gamma knife was existing. Gamma knife to buy a gamma knife is quite expensive and it was the idea whether we cannot use an accelerator for radio-surgery. Nowadays you can buy all this together but at that time it was people are saying it's impossible. An accelerator is so loose in its construction you cannot do precise irradiation and there was still a struggle between these two concepts. In the beginning the gamma knife people are really saying you cannot do that, it's impossible it's not precise enough. Now we know it's not a problem. Fintosimetry was the key to overcome with a dosimetry at that time. Fintosimetry was relatively easy to understand the small field of symmetry. Needs proper calibration against ionization chamber energy-dependent problems needs an approach scanner this is still true. Who is doing fintosimetry with now, with the Gavkromis films you know that and you know that you can make many mistakes with the scanner and still it's not so easy. Term TLD, small in size point those measurements possible many TLDs can be exposed available in various forms you can even have TLDs which are responsible to neutrons some are reasonable TLC equivalent but expensive disadvantageous signal erased during readout easy to lose reading no instant readout accurate results require quare who's doing TLDs what is your experience with uncertainty stable but what is the uncertainty so if you do a measurement how accurate is it is it 2%, 1%, 3% 5% yes that's quite typical normally one guy he's explaining he can do TLD symmetry with 1% but normally my own experience is all of that it is very difficult to get down to this uncertainty of 1%, 2% not recommended for beam calibration diode, small size high sensitivity instant readout no external bias voltage simple to measure relative distributions still require cables these disadvantages which are shown here may be not applicable anymore to newer developments we have from IBA I think in order for PDW shielded diode and the unscheduled diode which obviously are working very precise so it seems good for small beam to symmetry the diode seems to be quite good nevertheless they always need to compare or compare them with ionization chambers we can rely on ionization chambers if we do everything carefully so it still needs cross calibration with ionization chambers so now I want to go to ionization chambers and what is happening by ionization chambers filled with air so some principles the measurement of a diode requires in small volume by various interaction process we know that already we can discuss the quantity of energy imparted such interactions process normally result in creation of ion pairs we know that and here is such an example we have a cylindrical ionization chamber air filled conductive inner wall electrode and the central electrode and I told you already it depends on the company where the high voltage is so it is you can do what you want so if we apply a radiation then we have the creative of ion pairs and the ion pairs are then are simply walking to the different electrodes so the question is which types of which types are this charge carrier what is this charge charge carrier air ions and electrons this is what many people think it is but it is not true it is positive ions and negative ions because the electrons very soon are sticking to the other atoms and therefore we have negative ions and positive ions and therefore both carriers are always traveling with the same velocity because they are the same so the creation of this ionization in the gas is the basic for the similar ionization chambers again we have two types the cylindrical chamber and the plane chamber I want to go in more detail in how such measurement works or some sentence those related measurement quantities charge, those related measurement those rate-related measurement is the current this is the dose in air which is the charge which can be measured we multiply this with this concept we need this and this energy on average to create one elemental charge so the total charge multiplied with this is simply the energy which has to be put into the gas divided by the mass of air so it is very simple to determine the dose in air what we are interested in is and you see this relates very well with the definition so this the charge multiplied with it is a mean energy imparted and the dm corresponds to the mass of air this is the number of this constant we call it w value and good enough this value is quite constant whether it is electrons or photons but it is not the same for protons some changes or heavy ions but for normal variations it is constant and for relative air humidity of 50% it has a value of 33.77 joule per coulomb next the measurement absorb dose in air must be converted to the absorbed dose to water because this is the quantity we want to obtain and this is the quantity which is given to the doctor and this is the quantity which is prescribed by the doctor to treat a tumor two gray per fraction and the two gray is since 50 years it was water absorbed dose again let me just say this this is a discussion between tissue dose and tissue in water one main advantage to apply or to offer to work with the water absorbed dose is we can measure it and we can verify we can do measurements in the phantom and what we do is if we do for IMRT or for now for advanced treatment methods we want to verify whether the treatment planning gives a result which is correct so it is good to calculate the dose in water and it's good to measure the dose in water we have two things which are compared it would be less good to calculate the dose in tissue and now how to measure the dose in tissue not so easy charge charge which has been collected during a certain time so if you have a session chamber it's irradiated it's creating these charges they are traveling and so what we are measuring is current and if we have it for one minute so it's current multiplied by time this is charge it will be the same if you have stable conditions it will not change because of this tochastic character because we have so many ionizations and if you have a huge number of say of a variation which is a tochastic variable a huge number the mean will not change at all okay not quite we can discuss afterwards there is only the same reading we will take right or right that is true of course if you do a good measurement you will have some variations but this will come from the accelerator or something else or say electro meter will not influence so much electro meter are very stable but what if you do measurements within one hour the temperature can change some things we have several influence quantities which may change you are correct you have to do several measurements and take the average for that so we are now on the point that we want to have to convert the dose in air to the dose in water and this conversion depends on several conditions such as the type on NSE of radiation and type on volumous ionization chamber though this translation from dose in air to dose to water depends from a series of influence factors radiation type and the construction of ionization chamber for this conversion and for most cases of the symmetry in clinical applied radiations such as hyalurgyphotons, hyalurgyllicant we use the bright grey cavity theory for that who is familiar with that it has some nice principles but it takes quite a time to really understand it so I will go into more details now so what is now we introduce here water where we do the measurements and we have our ionization chambers here and the primary interactions radiation fields of photon then are photon interactions maybe here and here and here we have photon interactions and we assume that the number of interactions in the air cavity itself is very small because it has a density lower by a factor of 1000 so we have almost no energy here so the primary interaction of the photo radiation mainly consists of those producing secondary electrons and the secondary electrons they go through the chamber and these are now the energy deposits from the elegans which are counted in the air cavity so we know the interaction of secondary elegans in any way are characterized by stopping power I have already introduced this chamber or say that if we know the fluence of the electrons and if we know the energy and we have mass stopping power then we can calculate it and here is the formula of dose in air is the integral of the fluence of the electrons differential energy multiplied by the stopping power electronic stopping power divided by rho by the density and integrate over the energy let us assume that exactly the same fluence of secondary electrons independent from whether there is an air cavity or water so now we assume that the fluence of secondary elegans is not changed at all when we introduce this cavity then we would have an air and a dose in air with this water is exactly the same with one difference now we have to apply the energy loss not in air but in water so this is the formula for the dose in water and if we introduce a mean mass stopping power as this integral so you see here the mean stopping power the mean electronic stopping power as the integral over the fluence of the energy fluence times stopping power divided by the fluence we obtain the dose to water is the mean stopping power multiplied by the fluence of secondary electrons the same is true for the dose in air and therefore we came up with this wonderful nice expression the relationship the dose to water is the dose to air just by the ratio of the mean stopping power ratio in water and in air okay now it seems obviously we are finished we have the dose in air and we have the translation formula and then we get the dose in water so here but this formula is valid only the two conditions the cavity must be small when compared with the range of charged particles incident on it so that presence does not perturbs the fluids of the electrons remember we have made the assumption that the fluence is the same whether there is air or water so this is a condition the condition is that the fluence of the electrons must not be disturbed by air cavity and the other one absorbed dose in air is deposited solely by electrons crossing it I think quite important we have used the concept of stopping power that is we calculate the energy by the stopping power that means energy loss per length but if they are stopped it does not it has some remaining so the complete energy is going in and the correct characterization of the energy which is absorbed is not the energy loss it's a total energy entering so this concept it's very important to understand this this concept which is well known this is a normal I think all of you know this formula only works for crosses do we have only crosses in the ionization chamber? no so you see it is an approximation from that so these are the two break-grade conditions exactly putting these two conditions together break-grade theory provides the most important mean to determine what episodes from a tecton measurement are made of water so if the two break-frontations are fulfilled number one crosses only and no perturbation of the fluence these two things then the dose in water is given by this formula this is under break-grade conditions so but you have already seen it is not completely true so in order to do it exactly and really to understand this transition from dose to air to dose to water we have to do something else this assumption that this is true is has an uncertainty of maybe a few percent so it is not exact so to do it more exact we have to discuss the question what's going on in this air cavity with electron tracks so we have here crosser we have starters which may come and we can even have insiders if it's a gas we can say insiders and starters okay then we also have to take into account that an electron can make and knock on process so it will get the energy to another electrons and it will create really a further track of electrons which are called delta electrons and these delta electrons are symbolized here with this way in a very good approximation we can collect photon interaction within the cavity so we and we will throw them away this is if we throw this away it is a very good approximation in a very good approximation all the fluence of the pure crossers and starters is not changed this is a very famous principle in in the theory the fluence of particles say electron bond is not changed by the density so it's independent of the density the absorbed dose is different if it's a high density or low density of course the absorption is different but the fluence is not changed why? you can imagine there's two processes which are responsible for the fluence one is creation of secondaries which will be less if it's low density material but the track length is much more larger because there is almost no attenuation or no energy so these two processes are cancelling and after that the fluence is not dependent of the density so the introduction of this air does not change the fluence however we have the fluence of delta electrons and these are indeed different in air and in water so it follows that the break condition that the fluence of all electrons must not be disturbed exactly fulfilled it is exactly new to the secondary elegant to delta electrons these are different because we have different production rates of the secondaries in air and in water so it must be taken into account by a so called perturbation factor so we need a perturbation factor to correct for this effect so this is the original formula and now we have the same formula just you see the small p now the p is a perturbation factor which corrects for such influences what about the stoppers we have stoppers we can have stoppers do they create a problem yes they really do we have to take into account the stoppers so if we assume we have a crosser here so the energy and this is the entrance energy of an electron this is the track length and the energy imparted is simply the electronic stopping power multiplied by d ok this is the definition of stopping power if we have a stopper the energy imparted is simply the incoming energy you see the difference between them therefore the calculation of those using the stopping power column this formula only works for crosses as a consequence the calculation of the ratio of the mean energy collision stopping power also works only for crosses so this works only for crosses and again we need some correction for the stoppers this is done by something which you have certainly heard the Spencer Attic's stopping power ratio have you have you heard about that? that's a formula but what does it mean I think the most important thing is that it takes into account the stoppers so the ratio is given by a complicated this is a calculation of the stopping power ratio which we need to translate those from air to those to water we need such Spencer Attic's stopping power ratio if you look into the textbooks or you look in the TRS document you almost find the Spencer Attic's stopping power ratio and what you have here you have this which is very similar to the stopping power this is another expression of stopping power but you have here one term given the count the stoppers so a summary of this this is our really true translation the dose in water is a measure of charge divided by the air the W value the Spencer Attic's stopping power ratio and a perturbation factor now we are ready now if we would know this this quantity perturbation factor we would be ready and I can again refer to a dosimetry protocol like the famous TRS 398 you will find the stopping power ratios according to Spencer Attic's and you will also find the perturbation factors which are different for cobalt 60 for electrons for photons so I think this is now I am ready with my fundamental talk now I will go to the next one or we can make a small how long the time 15 we can make just a few a small break just to breathe I will go now to the next talk which is dealing with the calibration details performing a measurement with photons