PURPOSE: Conventional postoperative photon-beam radiotherapy to the spine in 
children with medulloblastoma/PNET is associated with severe late effects. This 
morbidity (growth and developmental) is related to the exit dose of the beams 
and is particularly severe in young children. With the purpose of reducing this 
toxicity, a dosimetric study was undertaken in which proton therapy was compared 
to standard megavoltage photon treatment.
METHODS AND MATERIALS: The results of a comparative dosimetric study are 
presented in such a way that the dose distribution achievable with a posterior 
modulated 100 MeV proton beam (spot scanning method) is compared with that of a 
standard set of posterior 6 MV x-ray fields. The potential improvements with 
protons are evaluated, using dose-volume histograms to examine the coverage of 
the target as well as the dose to the vertebral bodies (growth plates), lungs, 
heart, and liver.
RESULTS: The target (i.e., the spinal dural sac) received the full prescribed 
dose in both treatment plans. However, the proportions of the vertebral body 
volume receiving > or = 50% of the prescribed dose were 100 and 20% for 6 MV 
x-rays and protons, respectively. For 6 MV x-rays > 60% of the dose prescribed 
to the target was delivered to 44% of the heart volume, while the proton beam 
was able to completely avoid the heart, the liver, and in all likelihood the 
thyroid and gonads as well.
CONCLUSION: The present study demonstrates a potential role of proton therapy in 
decreasing the dose (and toxicity) to the critical structures in the irradiation 
of the spinal neuraxis in medulloblastoma/PNET. The potential bone marrow and 
growth arrest sparing effects make this approach specially attractive for 
intensive chemotherapy protocols and for very young children. Sparing the 
thyroid gland, the posterior heart wall, and the gonads may be additional 
advantages in assuring a long-term posttreatment morbidity-free survival.