 Hello everyone, this is Amit Chaudhary, armor radiologist and lecturer at Tata Memorial Hospital and Homewabha National Institute, Mumbai. I thank the Indian radiologist team for this opportunity. I wish everyone best of health, especially with the ongoing COVID pandemic. Let's begin with the discussion on case-based approach of pediatric posterior fossa neoplasms. This is a child who wore an MRI scan of the brain. Actual T2 and coronal T2-related images had mass involving the lateral recess of fourth ventricle extending to the cerebellopontan angle, which is iso-induced to the gray matter. And this is a classic location for wingless subgroup of medialoblastoma. The medialoblastoma is so-called because of the WNT gene involved in its pathogenesis. Wingless medialoblastomas comprise about 10% of medialoblastomas. These are encountered in children under three years of age without any gender predilection. These medialoblastomas have best prognosis because they seldom metastasize. However, whenever these metastasize, the prognosis isn't as good. These are classically located in the cerebellopontan angle or along the lateral recess of the fourth ventricle. Enhancement, as in all medialoblastomas, is variable, varying from mild enhancement to avid post-contrast enhancement. These medialoblastomas can be associated with Turcot syndrome, which is a combination of intestinal polyposis. This was a child who had a mass involving the lateral aspect of right cerebellar hemisphere, which is showing restricted diffusion on diffusion weighted images. It is dark on ADC maps, iso-induced to the gray matter on T2 weighted images, hypointense on T1 weighted images, and shows heterogeneous post-contrast enhancement. The mass is seen involving the lateral aspect of the right cerebellar hemisphere extending to the surface and abutting the tentorium cerebelli. So when a mass is encountered in a child which is showing restricted diffusion involving the lateral aspect of the cerebellum extending to the surface, Diagnosis of SHH or Sonic Hedgehog type of medialoblastoma needs to be considered. Again, this is known as Sonic Hedgehog because of the genes involved in the pathogenesis of this subgroup of medialoblastoma. The gene is shown after the video game character, Sonic the Hedgehog. SHH type of medialoblastomas comprises about 30% of medialoblastomas and these have a bimodal peak, that is they can be encountered in infants and in adults. There is no gender predilection. Prognosis is intermediate. When these tumors are encountered in infants, these usually have a good prognosis because of their favourable dysmoplastic and nodular histological features. Classically, these are located in the lateral aspect of the cerebellar hemisphere, are seen as surfacing neoplasms, often abutting the tentorium cerebelli. Enhancement again is variable but most often these tumors show intense post-contrast enhancement. Metastasis are rarely seen in cases of SHH medialoblastoma. However, tumor recurrence can occur in the treated tumor bed. These kinds of medialoblastomas can be associated with Gorlin's syndrome, which is a combination of multiple odontogenic keratosis, basal cell carcinomas, craniofacial anomalies and have increased predisposition to tumors like medialoblastoma. This was a 16 years old boy who had presented with inability to kick the football, that is he would miss kicking the ball while playing football. And on imaging, this child on diffusion weighted image had a tumor mass showing restricted diffusion in the midline involving the vermus and the fourth ventricle, which was dark on ADC maps. It was hyper intense on T1 weighted images and ISO index to the gray matter on T2 weighted images and showed only mild post-contrast enhancement. Screening of the entire spine did not reveal any drop metastasis. So in a child showing central or midline tumor, which is showing restricted diffusion and his ISO index to the gray matter on T2 weighted images with or without any significant post-contrast enhancement, possibility of group 4 medialoblastoma needs to be considered. Group 4 medialoblastomas are the most common form of medialoblastomas comprising about 40% of these neoplasms. These can be encountered in all age groups and are thrice more common in boys compared to girls. Prognosis is intermediate to poor. Classically, these are located in the midline or in the fourth ventricle. Enhancement is none to minimal. Frequently, these medialoblastomas are associated with metastasis and these are known to have nodular leptomaninjel metastasis. A classic site of disease deposition or metastatic deposition is the anterior third ventricle or the supracillus system. Group 3 medialoblastoma is another medialoblastoma which has a very poor prognosis. These comprise about 20% of medialoblastomas and are seen in young children. These are twice as common in boys compared to girls. Prognosis is poor with these medialoblastomas. Location again similar to group 4 medialoblastomas is in the midline involving the vermus and or the fourth ventricle. Enhancement tends to be variable but more commonly it can be peripheral or ring enhancing. Metastasis are frequent. A feature of group 3 medialoblastomas is that these can have a small primary tumor with fluorid or extensive de-seminated leptomaninjel disease. Classically the metastasis have a laminar or sugar coating appearance. So medialoblastomas are the most common malignant neoplasms in the posterior fossa and pediatric age group. And based on the locations these can be wingless or WNT subgroup of medialoblastomas which are classically located along the lateral recess of fourth ventricle or in the CP angle. The SHH or sonic hedgehog subgroup of medialoblastomas are surfacing neoplasms seen involving lateral aspect of the cerebellar hemispheres which can often abut the tentorium. Group 3 and group 4 medialoblastomas are midline neoplasms involving the vermus and or the fourth ventricle. This was a child who was treated for group 4 medialoblastoma and had presented with cerebellar signs. That is the child had positive thrombostast and had inability to perform tandem walking. And on post-contrast studies one can appreciate extensive leptomaninjel enhancement which is seen involving the cerebellar fissures as well as the fourth ventricle, pineal region as well as involving the surface of the brainstem. In addition one can also appreciate a deposit in the anterior part of third ventricle or the supracellar system which is a characteristic appearance of metastasis of group 4 medialoblastomas. As one scrolls through one can also see leptomaninjel enhancement in the temporal sulci. On axial images again there is extensive leptomaninjel enhancement identified in the cerebellar fissures which is seen extending in the fourth ventricle and along the surface of the brainstem. In addition one can also see the metastatic deposit in the anterior third ventricle region or the supracellar system. This is the diffusion weighted MRI of the spine of same child wherein one can see nodular areas of restricted diffusion seen involving the entire length of the spinal cord. These regions are seen as nodular areas which are isointest to the gray matter seen on the surface of the spinal cord and show post-contrast nodular enhancement. These are also seen involving the corda echoena nerve roots. So classically group 4 medialoblastoma metastasis tend to have nodular metastasis which can involve the brain as well as the spinal cord. Similar to the primary neoblasm these metastasis tend to show restricted diffusion. However unlike the primary neoblasms which seldom enhance the leptomaninjel metastasis tend to enhance avidly. So metastasis are quite rare with wingless and SHH subgroup of medialoblastomas. However they are quite frequently associated with group 3 medialoblastomas which can have this linear enhancement along the length of the spinal cord which can extend on the corda echoena nerve roots giving the sugar coating or laminar appearance. And group 4 medialoblastomas as alluded to earlier can present with nodular leptomaninjel metastasis or even deposits involving the anterior third ventricle or the supracellar systems. This is an infant with posterior fossa mass which on actual CT scan images is predominantly hyper dense with multiple cystic or necrotic areas within. On this sagittal reformat of the CT scan one can see the lesion which is occupying the midline of the posterior fossa again showing hyper density because of the hemorrhage. The same patient on T2 aid actual images is seen to have a midline mass which is seen involving the wormus and the fourth ventricle which is quite heterogeneous showing multiple hypo intense areas likely due to hemorrhage within associated with multiple hyper intense cystic or necrotic areas. On susceptibility weighted imaging the mass is seen to have extensive areas of blooming that is areas which appear dark because of presence of hemorrhage and it shows heterogeneous but mild post contrast enhancement. So an infant having a hemorrhage highly heterogeneous neoplasm which is present in the cerebellum in the midline which can be associated with restricted diffusion. A possibility of atypical pterotodraptoid tumor needs to be considered in this age group. ATRT constitute about 1-2% of all pediatric brain tumors are seen with a peak under 2 years of age and have equal incidence in males and females. Prognosis is invariably poor most children dying within 6-7 months. 40% of the ATRTs are seen located in the cerebellum. These are heterogeneous neoplasm with areas of hemorrhage and necrosis and does show heterogeneous post contrast enhancement. This is an 8 years old girl who had presented with history of headache, diplopia and projectile vomiting and hence underwent imaging. And on this axial T1 weighted image we can see a mass which is hyper intense to the gray matter in the posterior fossa predominantly involving the midline cerebellar structures as well as the fourth ventricle. The mass is also strikingly hyper intense on T2 weighted images in this axial scan wherein we can see that the mass is occupying the fourth ventricle and extending through the foramina of Lashka into the cerebellar pontine angle cisterns. On diffusion weighted images the mass doesn't show any significant restricted diffusion and on corresponding ADC maps we do not see any significant dark areas to suggest true restricted diffusion. On post contrast studies the neoplasm is showing heterogeneous but avid post contrast enhancement and screening of the spine which included T2 sag as well as T1 sag post contrast images did not reveal any obvious drop metastasis. So a neoplasm in a young child which is seen in the posterior fossa which is hyper intense on T2 weighted images without any significant restricted diffusion and extending through the foramina of Lashka is an ependymoma. So ependymomas are third most common posterior fossa neoplasms encountered in children after phylocytic astrocytomas and medial blastomas. These can be seen in very young children as well as older children and adolescents. Ependymomas are classified into two types, posterior fossa A ependymomas and posterior fossa B ependymomas. The posterior fossa A ependymomas are usually encountered in children under three years of age whereas posterior fossa B ependymomas are seen in older children and adolescents. Ependymomas are typically more common in males than in females. Prognosis is poor for posterior fossa A ependymomas whereas posterior fossa B ependymomas carry a good prognosis. Classically posterior fossa A ependymomas are seen arising from the lateral recess of the fourth ventricle. The posterior fossa B A ependymomas are midline ependymomas which can be seen involving the obex that is the inferior most portion of the fourth ventricle. Both these ependymomas extend across the foramina of Lusca and Magendii and encase neurovascular structures. Enhancement is modded in either types of ependymomas. This is a six years old child who had presented with cerebellar symptoms and on MRI study on T1 weighted axial image we can see a predominantly cystic lesion involving the cerebellum with a mural nodule. Again on T2 weighted images we see a cystic lesion with a mural nodule and on flare images the cyst is partially suppressed contrast that with the CSF in the fourth ventricle and the temporal hauls which is more suppressed than the contents of the cyst and of course we again continue to see the mural nodule. So in a young child with a posterior fossa cystic neoplasm showing a mural nodule the appearance is quite characteristic of a pyelocytic astrocytoma. Pyelocytic astrocytomas are most common pediatric brain tumors. These constitute about 70-80% of neoplasms in the posterior fossa of which about 60% are located in the cerebellum. These are encountered in children between 5-15 years of age and have equal incidence amongst boys and girls. These are grade 1 neoplasms with excellent prognosis. They have a median overall survival exceeding 90% at 10 years. Classically lesions in the cerebellum are cystic which show a mural nodule with the nodule showing avid post contrast enhancement. The brain stem pyelocytic astrocytomas tend to be solid infiltrative and have ill-defined margins. This is another child who had presented with cerebellar symptoms and cranial neopalcies and on MRI scan this is a flare axial image wherein we can see a large mass which is seen involving the pons causing enlargement of the pons resulting in a plump ponder in appearance or a so-called fat pons. It is also seen extending in the right cerebellar hemisphere. The mass is hypo intense on T1 weighted images and on post contrast study this mass shows mild post contrast heterogeneous enhancement. On diffusion weighted imaging there is no restricted diffusion identified and on ADC maps there are no dark or hypo intense areas seen. So a diffusely infiltrative mass causing enlargement of pons resulting in plumpy pons which is encasing the basilar artery. This is a characteristic appearance of diffuse infiltrative pontine glioma. Diffuse infiltrating pontine gliomas constitute about 10% of childhood tumors which are seen in the brain stem. Of which 2 third are present in the pons which are seen as diffuse infiltrative pontine gliomas. Prognosis in this histone mutant gliomas is invariably poor with a median survival of approximately 6 to 7 months. Classically these are seen as diffuse infiltrating neoplasms expanding the pons giving it an appearance of a fat pons. Frequently these encase the basilar artery. These are hyper intense on T2 weighted images and show no restricted diffusion. These tumors also do not show any significant post contrast enhancement. The roof of the fourth ventricle is formed by what is known as the fastidium and the floor of the fourth ventricle is formed by the tegmentum. This is important because the origin of the tumors arising from these structures can help us narrow the differences. As in medulloblastoma is a tumor which arises from the roof of the fourth ventricle can involve the cerebellum as well as the fourth ventricle. Appendymoma is a tumor arising from the floor of the fourth ventricle which frequently extends to the foramina of Lysca and Magendi. And then we can have brain stem gliomas especially diffuse the infiltrating pontine gliomas which are seen as neoplasms which will be expanding the pons. To conclude we have this algorithmic approach for the posterior fossa neoplasms which can be helpful in narrowing the differential diagnosis. So first and foremost we look at the location of the tumor. If the tumor is cerebellar in location we look at diffusion characteristics whether the tumor is showing restricted diffusion or not. If the tumor is showing restricted diffusion and is located in cerebellar hemisphere which is surfacing or has nodular post contrast enhancement, possibility of SHH subgroup of medulloblastoma can be considered. If there is no restricted diffusion and the tumor shows solid and cystic areas or typically a cystic lesion with a mural nodule raises the possibility of pylocytic astrocytoma. The tumor is located in the fourth ventricle and it shows restricted diffusion then these can be medulloblastoma particularly group 3 or group 4 medulloblastomas. Our tumor without any restricted diffusion will usually be an appendymoma. If the tumor is located in the foramina of Lushka or the CP angle and shows restricted diffusion in a child under 2 years of age, possibility of ATRT needs to be considered. If the child is above 2 years of age then a wing less subgroup of medulloblastoma can be considered. However, if there is no restricted diffusion in a tumor located in foramina of Lushka or the CP angle, again a possibility of appendymoma needs to be kept higher in the differentials. Then we have tumors located in the brainstem. Again brainstem neoplasms can be located in the pons or in the medulla. So, pontine neoplasms showing restricted diffusion are usually high-grade tumors. Example, embryonal tumor with multi-layered rosettes. Those without any restricted diffusion usually constitute diffusely infiltrating pontine gliomas. Tumors located in other brainstem structures like brain or medulloblongata again may or may not have restricted diffusion. Tumors showing restricted diffusion are usually aggressive high-grade neoplasms whereas tumors without any restricted diffusion are low-grade glial neoplasms.