 Let's move on to the lymphoid cell lines from here. So when we talk about lymphoma, there are two forms of lymphoma. One is the primary lymphoma where there is no associated lymphoma anywhere in the body and we see it in the brain. That is primarily the first area to be involved with the brain that would be called as a primary lymphoma as opposed to a secondary lymphoma because there is some other entity. There's a diffuse lymphoma or the patient is immunocompromised or there's some underlying lesion that would be associated with secondary lymphoma. These are two separate imaging entities. They give different appearances. Primary lymphoma tends to be a more solid tumor with sheets of enhancement. These are again, as we talked in the case of germinoma where it is a round blue cell tumor. Lymphoma is also a round blue cell tumor. So the nuclear cytoplasmic ratio would be very high. These are densely packed cells. So they kind of restrict water movement and it causes a restriction of diffusion in that. So densely packed new plasma will be hyper dense on CT. It will be ISO to hyper intense on T1 and T2 sequences with surrounding vasogenic edema. It's a mass lesion. So it will be associated with mass effect. It almost always enhances and it tends to hub the ventricular lining. It is typically like always touching the ventricular lining either along the corpus callosum or along the superior or the peri-ventricular area. Now this is one of those entities that crosses the midline. The other entities, the frequent entities that we talk about which cross the midline would be glioblastoma multiforme or lymphoma or dimalinating disease. Although we always include metastasis and infection also has potential entities that can cross the midline. But the classic three, when we talk about lesions crossing the midline, the classic three are GBM, lymphoma and dimalinating diseases. So that would be the typical primary lymphomas. On the other side, the secondary lymphoma does not tend to hub the ventricular lining. It tends to be more peripheral. It can be anywhere in the brain parankyma but tends to be more peripheral subcortical region. This is an entity there is to be differentiated from PML. This is unlike the primary lymphoma. It doesn't follow the same characteristics. It may or may not have a restricted diffusion. It may or may not intensely enhance. There will be some enhancement but it will not enhance like how primary lymphoma enhances. Primary lymphoma almost all never bleeds. Whereas secondary lymphoma may be associated with some hemorrhage or constipation or necrosis. So lymphoma, we talked about primary lymphoma that it will always enhance intensely with sheets of enhancement. There are certain conditions when it may potentially not enhance and we should be aware of that. One is when it is a secondary lymphoma or it has been treated. So giving steroids or radiation it almost meds away the lymphoma which will probably come back later on but at that point it meds away the lymphoma and there is no associated enhancement. So we should keep in mind when we have seen a case who has come from another hospital and we see a large mass lesion with some restricted effusion but no enhancement. The first question we should ask has the patient be treated with steroids or radiation because we have to think of lymphoma if there's a lesion with restricted effusion crossing the midline, always that differential. So these are a few examples. There is a non-contrast city of an examination where we see that hyper-density hubbing the ventricular lining. You see this lesion spreading along the ventricular lining, think of lymphoma. The second case is a post-contrast image where there is a intensely enhancing mass lesion in the right paediatric region and then we are not getting all the slices but this is in direct continuity crossing over the midline along the posterior corpus callosum. Now we talked about that the three entities that we talk about when regions cause the corpus callosum, GBM that would be associated with hemorrhage, necrosis, central necrosis. The other one being demyelinating that would not be associated with this kind of mass effect and this kind of intense enhancement. So this one would almost, we can predict that this is going to turn out to be lymphoma. Team lesion on MR along the posterior corpus callosum, mass lesion with intense enhancement and restricted diffusion. Restricted diffusion is a hallmark of round blue seal tumors when we see inside the brain. So you see enhancement, you think of germinoma, you think of lymphoma, you think of primitive neuroectodermal tumors. These are the intraparenchymal mass lesions that enhance as well as are associated with restricted diffusion because of their tightly packed nature and high cytoplasmic, high nuclear cytoplasmic ratio. So this is a mass lesion cross-winkled corpus callosum intense enhancement with sheets of enhancement, restricted diffusion. This is going to be lymphoma. This is a case of secondary lymphoma. So this is where the issue arises. There's mass lesion. There is mass effect. So it is going to be a mass lesion. There's low attenuation on CT, hyperintensity on T2 and faint patchy enhancement on contrast administration. Now this patient was an immunocompromised patient. So we can predict that this is going to be a lymphoma. The other differential, if we did not see the mass, if we did not see that enhancement on the CT, it is the masses of the hyperdense on MR, it is ISO intense or slightly lower intensity along the medial parietal lobe. And that is the same area that shows enhancement. If we did not see enhancement, this would be virtually indistinguishable on imaging from immunocompromised related PML. Now we have a few factors that differentiate. Now PML tends to involve the subcortical u-fibers. Lymphoma does not involve the subcortical u-fibers. PML almost never enhances in its primary state. Secondary lymphoma almost has some enhancement associated. But what we should keep in mind is secondary lymphoma may potentially not enhance. The patient may have been treated with steroids, then also it will not enhance. And on the flip side, patient with PML potentially may enhance when he's been treated and his immunity is coming back, either entity called as immune reconstitution inflammatory syndrome or iris. So all these features further confound the imaging characteristics making it difficult. I mean, life is not simple. We have to, things are not straightforward. Whenever there is obvious findings that this is the classic appearance of PML, there is always some entity that will confound it with a secondary lymphoma. So we have to keep these two things in mind. You see peripherally located abnormal signal intensity extending almost up to the cortex. Think of these two conditions simultaneously lymphoma and PML. Both these things will be associated with impenu compromised status. Then accordingly, how the patient is presenting, what are the other features, whether the patient was neutropenic and his immunity is coming back, you can go in favor of PML with iris or if the patient did not never had any of the PML features, then this is going to be secondary lymphoma. So those are the various factors that you clinical association, serological values, CSF values, all this as a whole thing. When we sit down in tumor board, these are the things we talk about that these are the things that we'll be using to differentiate between secondary lymphoma versus PML. But those are the two differential that we have to think of.