High precision film dosimetry with GAFCHROMIC^® films for quality assurance especially when using small fields

Abstract

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.

Dosimetry tools and techniques for IMRT

“…Because of its nearly tissue-equivalent characteristics and well-matched dose response, radiochromic film can be used to acquire accurate relative dose distribution measurements. 95,105,[131][132][133][134] For IMRT commissioning, radiochromic film can be used to acquire beam profiles that will be used for modeling beam penumbra. 99,133 It can also be used to measure relative output factors for very small fields 100,133,[135][136][137] and can be used to measure phantombased IMRT dose distributions, for example, dose distribution measurements that will be used to verify the treatment planning system's programming of the linear accelerator delivery parameters ͑"monitor unit outputs"͒.…”

Scanning orientation effects on Gafchromic EBT film dosimetry

“…Gafchromic film products for diagnostic radiology [1,2,3,4] and radiotherapy applications [5,6,7,8,9,10,11] have expanded other the last few years with films specifically designed for niche areas becoming commercially available. Some of these films include the Gafchromic film range such as MD-55-2 and HS.…”

Quality Assurance in Stereotactic Radiosurgery/Radiotherapy according to DIN 6875-1

“…The output factors for the 4-, 8-and 14-mm collimator helmet relative to the 18-mm collimator helmet have been measured ( table 1 ) with different detectors which in principle fulfi ll the requirements for narrow photon beam dosimetry [3,4] . The detectors were placed with an uncertainty of approximately 0.1 mm in the centre of a homo- The fi gures in parentheses for the pin point chamber were not taken into account for calculating the mean values due to systematic errors [5].…”

“…There are four collimator helmets (4,8,14 and 18 mm); these are geometrical projections of the collimator aperture at the unit centre point of the Gamma Knife. The superposition of all 201 beams then produces approximately spherically shaped dose distributions, with full width at half maximum dimensions (in x and y directions) of 6, 11, 19 and 24 mm, respectively, at the isocentre [2] .…”

“…The output factor has to be determined for the 4-, 8-and 14-mm collimator helmet relative to the 18-mm colliPmator helmet, which produces the largest fi eld size using a spherical phantom [3,4] .…”