A Reusable Proton Range Compensator and Verification of Its Functionality

Abstract

changes (STC) following PRT and investigated any correlation with Monte Carlo (MC) calculated LET and RBE values. Materials/Methods: We performed a retrospective review of 111 medulloblastoma patients treated with craniospinal passive scattering PRT followed by posterior fossa (PF) or involved field boost at our institution between 2002 and 2011. The locations of STC consistent with possible radiation injury were documented on magnetic resonance image scans of symptomatic patients and, after image registration, contoured on the original planning computed tomography scans. Dose and LET distributions were calculated using MC (TOPAS) for patient and beam geometry of the treated plans. RBE values were estimated by two models, based on LET and dose distributions: the first (Wilkens) highlights relative RBE variations, given an a-priori defined average RBE value; the second (Carabe) estimates absolute RBE values. Results: At median follow up of 4 years, the 5-year cumulative incidence of STC was 4%. A total of 10 STC areas were found in 4 patients, involving the brainstem(3), resection cavity(1), cervical spine(1), cerebellum (2) and inner acoustic canal(1). 8 STC areas had higher LET value than the target, with average LET value of all STC areas being 2.7 versus 2.4 keV/mm in the PF boost area (paired student t-test p-value Z 0.05). Carabe model estimated an average RBE value of 1.202 for the STC areas versus 1.185 for the target (p-value Z 0.06). Conclusions: Cumulative incidence of STC for in patients treated for medulloblastoma with PRTwas comparable to that reported for photon RT. The brainstem was the most common site. Although STC areas had higher LET values, the corresponding RBE difference was less than 2%. Similar variations were present in all treatment plans including patients that did not develop STC. The etiology of RT changes is likely dependent upon other contributing and unknown factors, such as biological and genetic differences in our patients. Author Disclosure: D. Giantsoudi: None. R.V. Sethi: None. B. Yeap: None. T.I. Yock: None. N.J. Tarbell: None. H. Paganetti: None. S.M. MacDonald: None.