Radiotherapy to the entire craniospinal axis with a boost to the site of the original disease is standard of care for children over 5 years old.
Without radiotherapy very few patients survive (Cushing's original series 1 out of 61 patients were alive at 3 years).
In one series (Landberg 1980), there was 5% survival at 10 years with RT to the posterior fossa alone and 50% survival at 10 years if additional craniospinal RT given.
Doses of greater than 50 Gy to the posterior fossa are necessary to reduce the risk of local relapse.
Delay between surgery and the start of RT (of more than 4 weeks) is associated with a poorer survival outcome particularly in children over age 5
Chemotherapy combined with cranio-spinal RT to 35 - 36 Gy yields 5 year survivals of about 60% - 70% overall. However there is inadequate information about the optimum cranio-spinal dose, especially in patients who have no disease beyond the posterior fossa.
POSTERIOR FOSSA BOOST
- This should be planned with image guidance (CT/MR fusion software) to deliver 3D conformal therapy.
- There is debate about whether the extent of the original tumor should be treated with margins or the entire posterior fossa should be treated to a high dose. There is a randomization within the current Children's Oncology Group (COG) investigational study (ACNS0331) in an attempt to look at this question – in this study the total dose to the posterior fossa or the tumor bed with margins is 5400 cGy in 180 cGy fractions.
Off study most radiation oncologists would treat the tumor with margins:
GTV = the original Gross Tumor Volume – that is the original extent of dsease prior to any surgery or treatment. The original extent of disease is identified using MR/CT fusion software.
CTV = Clinical Target Volume and is the Gross tumor volume plus an additional margin to treat subclinical microscopic disease. This is usually the GTV plus 1.5 cm margin - except at a bone or tentorial interface – where it remains within the confines of the posterior fossa.
PTV = Planning Target Volume and is institution defined according to immoblization techniques and inherent set up uncertainties. This is usually an additional 0.3 to 0.5 cms around the CTV.
PTV should be covered by 95% isodose.
3D conformal RT used (multiple beam arrangement) - Energy 6 or 10mV
Example of 3D conformal posterior fossa boost using multiple 6 mV fields:
Portal imaging - Industrial or check films should be taken on the machine to verify field position.
NB The junction between the cranial and spinal fields must be outside the posterior fossa boost volume.
Spinal metastases are boosted - usually to a total of 45 Gy.
Normal Tissue Tolerance
No more than 50% of the cord between C1 and C2 should receive more than 54 Gy.
