Total skin electron therapy vertical profiles measured using radiochromic film.

Abstract

Vertical dose profiles of Total Skin Electron Therapy (TSET) electron fields are often measured using ionization chambers (ICs); however, resulting protocols are tedious and time consuming due to complex gantry arrangements, numerous point dose measurements and extra-cameral corrections. This inefficiency is reduced when using radiochromic film (RCF) dosimetry through simultaneous dose sampling and the elimination of IC-related measurement corrections. To investigate the feasibility of RCF dosimetry for TSET vertical profile measurements and establish a novel RCF based vertical profile quality assurance protocol. Thirty-one vertical profiles were measured using GAFChromic® EBT-XD RCF on two matched linear accelerators (linacs) over 1.5 years. Absolute dose was quantified using a triple channel calibration method. Two IC profiles were collected for comparison to RCF profiles. Twenty-one archived IC measured profiles from two different matched linacs from 2006 to 2011 were analyzed. Inter- and intra-profile dose variability was compared between dosimeters. The time required for the RCF and IC protocols was compared. RCF measured inter-profile variability ranged from 0.66%-5.16% and 1.30%-3.86% for the two linacs. A 0.2%-5.4% inter-profile variability was observed for archived IC measured profiles. RCF measured intra-profile variability ranged from 10.0%-15.8%; six of 31 profiles exceeded the EORTC±10% limit. Archived IC measured profiles exhibited lower intra-profile variability (4.5%-10.4%). RCF and IC measured profiles agreed in the center of the field; however, RCF doses measured 170-179cm above the TSET treatment box base were ∼7% greater. Modification to the RCF phantom eliminated this discrepancy, resulting in comparable intra-profile variability and agreeance with the ±10% limit. Measurement times were reduced from 3h (IC protocol) to 30min (RCF protocol). RCF dosimetry improves protocol efficiency. RCF has been established as a valuable dosimeter for TSET vertical profile quantification when compared to ICs as the gold standard.