POTENTIAL ROLE OF PROTON THERAPY IN THE TREATMENT OF PEDIATRIC MEDULLOBLASATOMA PRIMITIVE NEURO-ECTODERMAL TUMORS - SPINAL THECA IRRADIATION

Purpose: Conventional postoperative photon-beam radiotherapy to the sp ine in children with medulloblastoma/PNET is associated with severe la te effects, This morbidity (growth and developmental) is related to th e exit dose of the beams and is particularly severe in young children, With the purpose of reducing this toxicity, a dosimetric study was un dertaken 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 distributio n achievable with a posterior modulated 100 MeV proton beam (spot scan ning 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 propor tions of the vertebral body volume receiving greater than or equal to 50% of the prescribed dose were 100 and 20% for 6 MV x-rays and proton s, respectively, For 6 MV x-rays >60% of the dose prescribed to the ta rget was delivered to 44% of the heart volume, while the proton beam w as able to completely avoid the heart, the liver, and in all likelihoo d the thyroid and gonads as well. Conclusion: The present study demons trates 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 in tensive chemotherapy protocols and for very young children, Sparing th e thyroid gland, the posterior heart wall, and the gonads may be addit ional advantages in assuring a long-term posttreatment morbidity-free survival. (C) 1997 Elsevier Science Inc.