POTENTIAL ROLE OF PROTON THERAPY IN THE TREATMENT OF PEDIATRIC MEDULLOBLASTOMA PRIMITIVE NEUROECTODERMAL TUMORS - REDUCTION OF THE SUPRATENTORIAL TARGET VOLUME

Purpose: One of the components of radiotherapy (RT) in medulloblastoma /primitive neuroectodermal tumors is the prophylactic irradiation of t he whole brain (WBI), With the aim of reducing late neuropsychologic m orbidity a CT-scan-based dosimetric study was undertaken in which trea tment was confined mainly or exclusively to supratentorial sites consi dered at high risk for disease recurrence. Methods and Materials: A co mparative dosimetric study is presented in which a three field (two la terals and one posterior) proton plan(spot scanning method) is compare d with a two-field conventional WBI 6 MV x-ray plan, to a 6-field ''ha nd-made'' 6 MV x-ray plan, and to a computer-optimized 9-field ''inver se'' 15 MV x-ray plan, For favorable patients, 30 Gy were delivered to the ventricles and main cisterns, the subfrontal and subtemporal regi ons, and the posterior fossa, For the unfavorable patients, 10 Gy WBI preceeded a boost to 30 Gy to the same treatment volume chosen for fav orable patients, The dose distribution was evaluated with dose-volume histograms to examine the coverage of the targets as well as the dose to the nontarget brain and optical structures, In addition, the risks of radiation-related late neuropsychologic effects after WBI were coll ected from the literature and used to predict normal tissue complicati on probabilities (NTCPs) for an intelligence quotient deficit after tr eatment with photon or proton beams. Results: Proton beams succeeded b etter in reducing the dose to the brain hemispheres and eye than any o f the photon plans, A 25.1% risk of an IQ score <90 was predicted afte r 30 Gy WBI, Almost a 10% drop in the predicted risk was observed when using proton beams in both favorable and unfavorable patients, Howeve r, predicted NTCPs for both optimized photon plans (''hand made'' and ''inverse'') were only slightly higher (0.3-2.5%) than those of proton beams, An age-modifying factor was introduced in the predictive NTCP model to assess for IQ differences in relation with age at irradiation , Children with ages between age 4 to 8 benefitted most from the dose reduction in this exercise (similar NTCP predictions for both proton a nd ''inverse'' plans). Conclusion: Modulated proton beams may help to significantly reduce the irradiation of normal brain while optimally t reating the supratentorial subsites at higher risk for relapse, A decr ease in morbidity can be expected from protons and both optimized prot on plans compared to WBI. (C) 1997 Elsevier Science Inc.