What are the two most common CNS tumours?
known as primitive neuroectodermal tumours
- pinealoblastoma
- medulloblastoma
What is the metastasis of PNET?
- highly invasive
- spread rapidly through CSF
- 20% have spread to SC before diagnosis
Where do medulloblastomas arise?
- in cerebrellum or posterior fossa
What is the most common and rare type of PNET?
Common - medulloblastoma (20% of all paediatric brain tumours)
Rare - pinealoblastoma
What is the epidemiology of medulloblastoma?
- rare in adults: most occur before 16
- more common in males (M:F, 2:1)
What causes the signs and symptoms for PNET?
- associated with raised intracranial pressure which can be caused by a blockage of the ventricles leading to a build up of CSF
- local swelling around the tumour itself
What are the signs and symptoms of medulloblastoma?
- headache
- vomiting
- irritability
- problems with motor function
What are the signs and symptoms of glioblastoma?
- headaches
- seizures
- vomitting
- trouble speaking
- blurred vision
What classifies an average risk patient?
- older then 3
- minimal or no residual tumour (<1.5cm max diameter)
- no evidence of metastatic spread
What classifies a high risk patient
- significant residual disease (>1.5cm max diameter)
- evidence of metastatic disease
- under 3
What are the three main treatment modalities for PNET?
- surgery
- craniospinal irradiation
- adjuvant chemotherapy
When is surgery used for PNET?
- essential as first line treatment
- should be as complete as possible without causing disability
How soon after surgery should CNS RT commence?
- as soon as recovery permits (2-3 weeks) because of risk of tumour seeding
- post foassa or primary tumour site to be boosted after whole CNS treatment
When should chemo be used?
- concurrent chemo followed by adjuvant mutli agent therapy for 1 year has proven improved survival
If child is under 3 is RT used?
- usually delayed because of associated risks, but control rates are higher when radiation is used
What is the typical RT fields for CNS treatment?
- lateral skull fields
- direct post fields to treat spine as far inf as the fourth sacral formina and including the sacral nerve roots
- number of spine fields depends on spine length
What is the technique for the lateral skull fields?
- isocentric and collimated to match the divergent sup edge of the adjoining posterior spine field at mid sagittal plane
- inf border of skull field will match the sup border of the spine field at midline on the post skin surface
- to get complete match need to rotate the floor 3-5 degrees or set skull inf jaw to zero if head is small enough
What is the CNS brain technique with VMAT?
- 1-2 arcs
- an extra arc using floor turn to achieve OAR may be needed
What are the important OAR to consider in skull fields?
- optic chiasm and nerves (54Gy), globes (45Gy)
- cochlea (20-30Gy)
- brainstem (54Gy)
- hypothalamus and pituitary dose to be minimised
What are some considerations for the lateral skull fields?
- inf border to finish C4-5
- primary site is to be posted so junction region should be well inf of this as there will be hot spot at the junction
- limiting factor is shoulder position
- mandible/chin position needs to be considered (divergent edge of upper spine but don’t want so up that neck folds are formed otherwise skin reaction)
What is the consideration with using extended SSD for spine field?
- increased divergence of beam
- could result in only needing one junction point
What is the point of moving junction and where are they located?
- to decrease hot and cold spots (dose over junction is smooth transition)
- each junction will have a set of at least three, 5mm junctions
Why is there a gap on the skin between the two spinal fields?
- so the hot spot or overlapping of beams occurs in the PTV and not on the skin
What is the advantage of treating prone?
- access to spine for palpation of set-up and field placement
- skin marks for junction
- general accuracy
