Fractionated stereotactic radiotherapy: A method to evaluate geometric and dosimetric uncertainties using radiochromic films

Abstract

In the authors' hospital, stereotactic radiotherapy treatments are performed with a Varian Clinac 600C equipped with a BrainLAB m3 micro-multileaf-collimator generally using the dynamic conformal arc technique. Patient immobilization during the treatment is achieved with a fixation mask supplied by BrainLAB, made with two reinforced thermoplastic sheets fitting the patient's head. With this work the authors propose a method to evaluate treatment geometric accuracy and, consequently, to determine the amount of the margin to keep in the CTV-PTV expansion during the treatment planning. The reproducibility of the isocenter position was tested by simulating a complete treatment on the anthropomorphic phantom Alderson Rando, inserting in between two phantom slices a high sensitivity Gafchromic EBT film, properly prepared and calibrated, and repeating several treatment sessions, each time removing the fixing mask and replacing the film inside the phantom. The comparison between the dose distributions measured on films and computed by TPS, after a precise image registration procedure performed by a commercial piece of software (FILMQA, 3cognition LLC (Division of ISP), Wayne, NJ), allowed the authors to measure the repositioning errors, obtaining about 0.5 mm in case of central spherical PTV and about 1.5 mm in case of peripheral irregular PTV. Moreover, an evaluation of the errors in the registration procedure was performed, giving negligible values with respect to the quantities to be measured. The above intrinsic two-dimensional estimate of treatment accuracy has to be increased for the error in the third dimension, but the 2 mm margin the authors generally use for the CTV-PTV expansion seems adequate anyway. Using the same EBT films, a dosimetric verification of the treatment planning system was done. Measured dose values are larger or smaller than the nominal ones depending on geometric irradiation conditions, but, in the authors' experimental conditions, always within 4%.

Evaluation of a Gafchromic EBT2 film dosimetry system for radiotherapy quality assurance

“…Since its introduction in 2004, Gafchromic's EBT radiochromic film (International Specialty Products, Wayne, USA) has been widely adopted for use in quality assurance testing of clinical radiotherapy treatments including 3D conformal [1], intensity modulated and stereotactic treatments [2][3][4][5] as well as arc therapies [6] including tomotherapy [7]. EBT has been rigorously investigated and found to be capable of producing reliable and reproducible dose measurements, with a high degree of spatial accuracy and uniformity of response [1,[8][9][10][11][12][13].…”

Calibration of EBT2 film using a red‐channel PDD method in combination with a modified three‐channel technique

“…5 EBT ® film is a radiochromic film produced by International Specialty Products (ISP) and had been widely used to address the requirements of quality assurance in radiotherapy. [6][7][8][9] It contains a radiation-sensitive dye organized into microcrystals, which is embedded in a gelatin binder to measure the dose of ionizing radiation. The EBT2 film (Ashland Inc.) is the second generation EBT film and uses the same active component but with a number of improvements, including a yellow marker dye to protect the active layer from exposure to ultraviolet and visible light, 10 a synthetic polymer-coated layer to reduce the energy dependence, 3,4 and a nearly waterequivalent, effective atomic number of 6.8.…”

A comprehensive evaluation of treatment accuracy, including end-to-end tests and clinical data, applied to intracranial stereotactic radiotherapy

“…Verellen et al assessed the dosimetric and positional accuracy of the radiation delivery to the target by an anthropomorphic phantom which allows the insertion of dosimeters and lead beads [11]. Coscia et al [12] evaluated the isocenter repositioning error during several treatment sessions comparing calculated dose distributions and corresponding film measurements. Schulz et al employed a Fricke-gel dosimeter to confirm the dosimetric accuracy of stereotactic radiation treatment delivery [13].…”