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High precision film dosimetry with GAFCHROMIC ® films for quality assurance especially when using small fields
Author(s) -
Mack Andreas,
Mack Günther,
Weltz Dirk,
Scheib Stefan G.,
Böttcher Heinz D.,
Seifert Volker
Publication year - 2003
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1593634
Subject(s) - dosimetry , quality assurance , scanner , materials science , optics , brachytherapy , radiation treatment planning , computer science , medical physics , physics , nuclear medicine , radiation therapy , engineering , medicine , operations management , external quality assessment
Treatment units for radiosurgery, brachytherapy, implementation of seeds, and IMRT generate small high dose regions together with steep dose gradients of up to 30%–50% per mm. Such devices are used to treat small complex‐shaped lesions, often located close to critical structures, by superimposing several single high dose regions. In order to test and verify these treatment techniques, to perform quality assurance tasks and to simulate treatment conditions as well as to collect input data for treatment planning, a GAFCHROMIC® film based dosimetry system for measuring two‐dimensional (2‐D) and three‐dimensional (3‐D) dose distributions was developed. The nearly tissue‐equivalent radiochromic GAFCHROMIC® film was used to measure dose distributions. A drum scanner was investigated and modified. The spectral emission of the light source and the filters together with the efficiency of the CCD filters for the red color were matched and balanced with the absorption spectra of the film. Models based on refined studies have been developed to characterize theoretically the physics of film exposure and to calibrate the film. Mathematical descriptions are given to calculate optical densities from spectral data. The effect of darkening has been investigated and is described with a mathematical model. The influence of the scan temperature has been observed and described. In order to cope with the problem of individual film inhomogeneities, a double irradiation technique is introduced and implemented that yields dose accuracies as good as 2%–3%. Special software routines have been implemented for evaluating and handling the film data.