[P206] Gafchromic and alanine detectors for verification of field junctions in craniospinal proton irradiation

Purpose Radiotherapy is one of essential methods of treatment for medulloblastoma – a primitive neuroectodermal tumor growing in cerebellum. Proton craniospinal irradiations are recommended because of sparing of intermediate-to-low-dose to healthy tissues and reduction of side effects, including secondary cancers. Two laterally opposed cranial fields and several posterior spine fields are usually used for irradiation in medulloblastoma as well in conventional radiotherapy as in proton one. In this field configuration a potential dose inhomogeneity at the field junctions can appear. We have investigated the dose homogeneity in the junction areas using Gafchromic films and alanine detectors. Methods A clinical plan for an anthropomorphic phantom of a 5-year-old child have been prepared. There were two opposed lateral brain fields and 2 posterior spine fields. Therefore two field junctions were investigated. Then the phantom was equipped with alanine detectors and Gafchromic films at chosen positions mainly in junction areas and irradiated. Alanine doses were assessed using proton beam calibration (5–20 Gy). The calibration curve was designed upon developed protocol based on ISO/WD 15566.1 document. Alanine doses were then compared with doses in the treatment plan. Gafchromic films were analyzed using in-house developed AnalyseGafchromic software allowing to calculate dose based on triple channel calibration and apply corrections on scanner spatial inhomogeneity and scan-to-scan instability. Finally, the measured 2D dose distributions were compared with dose planes exported from TPS (Treatment Planning System) using gamma index analysis. Results The compatibility of alanine doses and TPS doses was at the level of 1%. For all the dose planes measured with Gafchromic passing rates of gamma index test (3%/3 mm) was higher than 95%. Conclusions Patients with medulloblastoma may benefit greatly from proton radiotherapy, however the main condition for a successful treatment is a careful and accurate field junctions, where all the uncertainties of beam model in TPS, CT calibration curve and Patient Positioning may affect the dose delivered to the patient. We have proved that Gafchromic films and alanine detectors can be successfully used for complex system check for ability to irradiate the target with sufficient accuracy.