In-vivo proton range verification for reducing the risk of permanent alopecia in medulloblastoma treatment

Abstract

Objective

The purpose of this work is the clinical commissioning of a recently developed in-vivo range verification system for the head treatment of pediatric medulloblastoma patients. Inaccurate beam range for such treatment could lead to either inadequate dose coverage of the target volume or excessive dose to the head skin resulting permanent alopecia.

Methods

The in-vivo range verification system is designed to perform pre-treatment range verification and adjustment. An array of Si-diode detectors is to be placed on the patient immobilization mask in the exit direction of a whole-brain field; signal is analyzed, and the extracted water equivalent path length (WEPL) is compared to the expected one, revealing if a range correction is needed. The method was tested in solid water and anthropomorphic head phantom, with validation based on independent WEPL measurements. The measured WEPL were compared to those computed by the treatment planning system (TPS).

Results

The accuracy for the WEPL measurements by the diode system in both solid water and anthropomorphic head phantom were on average within a millimeter from more accurate measurement by the dose-extinction technique, with the error range for the two phantoms as (0–1 ​mm) and (0–1.3 ​mm), respectively. When compared to the WEPL calculated by the treatment planning system, the measured values were on average within 1 ​% (range 0–3%) of the beam range. The accuracy of dose measurements by the diodes in the fall-off part of the depth dose profile was validated against the reference Markus chamber. No need for further correction (due to different beam parameters and detector dose ageing effects) was found.

Conclusions

The range verification workflow was successfully tested in the anthropomorphic head phantom. The performance of the in-vivo range verification system and related workflow meet the clinical requirements in terms of the needed WEPL accuracy for pretreatment range verification.