It is located in the posterior fossa and develops at the expense of the cerebellum, usually in its middle section. It is very aggressive and quickly induces balance disorders, followed by compression of the cranial nerves, located further forward, in the brainstem (the continuation of the spinal cord within the brain, leading to a risk of paralysis of the eyes and face, and hearing loss). In the final stages, tumour cells may invade the entire CSF (cerebrospinal fluid), bathing the brain and spinal cord.
Intensive treatment usually begins with surgical removal, followed by a combination of chemotherapy and RT, spread over several months. The peculiarity of radiotherapy lies in the need to “cover” the entire cranio-spinal axis (brain + marrow + cauda equina), but with a higher dose of radiation in the most at-risk area: the cerebellum.
The long-term survival of localized forms is around 70%.

Craniospinal irradiation is very recent in France because of its complexity and the fact that it is almost indispensable to use the new “spot scanning” technology. It will certainly make it possible to significantly limit the doses diffused into the surrounding tissues (eyes, ears, facial bones, teeth, thyroid gland, organs of the thorax, abdomen and pelvis), or even into the spinal column itself, in some of them.
The superimposed dose delivered at the end of treatment, to the posterior fossa alone is, on the other hand, simpler and has been widely used since protons started to be used in paediatrics in the 2000s.
Radiation schedule: daily sessions, spread over approximately 6 weeks.