Abstract
The measurement of dose distributions in clinical brachytherapy, for the purpose of quality control, commissioning or dosimetric audit, is challenging and requires development. Radiochromic film dosimetry with a commercial flatbed scanner may be suitable, but careful methodologies are required to control various sources of uncertainty. Triple‐channel dosimetry has recently been utilized in external beam radiotherapy to improve the accuracy of film dosimetry, but its use in brachytherapy, with characteristic high maximum doses, steep dose gradients, and small scales, has been less well researched. We investigate the use of advanced film dosimetry techniques for brachytherapy dosimetry, evaluating uncertainties and assessing the mitigation afforded by triple‐channel dosimetry. We present results on postirradiation film darkening, lateral scanner effect, film surface perturbation, film active layer thickness, film curling, and examples of the measurement of clinical brachytherapy dose distributions. The lateral scanner effect in brachytherapy film dosimetry can be very significant, up to 23% dose increase at 14 Gy, at ± 9 cm lateral from the scanner axis for simple single‐channel dosimetry. Triple‐channel dosimetry mitigates the effect, but still limits the useable width of a typical scanner to less than 8 cm at high dose levels to give dose uncertainty to within 1%. Triple‐channel dosimetry separates dose and dose‐independent signal components, and effectively removes disturbances caused by film thickness variation and surface perturbations in the examples considered in this work. The use of reference dose films scanned simultaneously with brachytherapy test films is recommended to account for scanner variations from calibration conditions. Postirradiation darkening, which is a continual logarithmic function with time, must be taken into account between the reference and test films. Finally, films must be flat when scanned to avoid the Callier‐like effects and to provide reliable dosimetric results. We have demonstrated that radiochromic film dosimetry with GAFCHROMIC EBT3 film and a commercial flatbed scanner is a viable method for brachytherapy dose distribution measurement, and uncertainties may be reduced with triple‐channel dosimetry and specific film scan and evaluation methodologies.PACS numbers: 87.55.Qr, 87.56.bg, 87.55.km
Highlights
The measurement of dose distributions produced by clinical brachytherapy treatment equipment is challenging due to large dose ranges, high dose gradients, and small spatial scales
Radiochromic film dosimetry is often employed for dose distribution measurement in radiotherapy, with a number of advantages over other dosimetry methods, including high spatial resolution, low energy dependence, and near water equivalence,(1,2,3,4) and relative ease of signal readout with a desktop scanner.[5,6] the resulting pixel value in a scanner-produced image of a radiochromic film is a complex convolution of scanning lamp emission, absorption of the film, sensitivity of CCD array and, importantly, optical properties of the scanner along the light path influenced by polarization caused by the film
The use of triple-channel film dosimetry in brachytherapy has been evaluated in both test cases and clinical dose distribution measurements
Summary
The measurement of dose distributions produced by clinical brachytherapy treatment equipment is challenging due to large dose ranges, high dose gradients, and small spatial scales. Radiochromic film dosimetry is often employed for dose distribution measurement in radiotherapy, with a number of advantages over other dosimetry methods, including high spatial resolution, low energy dependence, and near water equivalence,(1,2,3,4) and relative ease of signal readout with a desktop scanner.[5,6] the resulting pixel value in a scanner-produced image of a radiochromic film is a complex convolution of scanning lamp emission, absorption of the film, sensitivity of CCD array and, importantly, optical properties of the scanner along the light path influenced by polarization caused by the film. Triple-channel film techniques have been studied in external beam radiotherapy applications, typically up to 2 to 3 Gy,(10,11) but their application in brachytherapy, with prescription doses of 7 to 8 Gy and peak doses that are significantly higher, as well as different dose distribution and different energy spectrum, needs further investigation
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