Application of radiochromic gel dosimetry to commissioning of a megavoltage research linear accelerator for small‐field animal irradiation studies

Abstract

Purpose To develop and implement an efficient and accurate commissioning procedure for small‐field static beam animal irradiation studies on an MV research linear accelerator (Linatron‐M9) using radiochromic gel dosimetry. Materials The research linear accelerator (Linatron‐M9) is a 9 MV linac with a static fixed collimator opening of 5.08 cm diameter. Lead collimators were manually placed to create smaller fields of 2 × 2 cm2, 1 × 1 cm2, and 0.5 × 0.5 cm2. Relative dosimetry measurements were performed, including profiles, percent depth dose (PDD) curves, beam divergence, and relative output factors using various dosimetry tools, including a small volume ionization chamber (A14), GAFCHROMIC™ EBT3 film, and Clearview gel dosimeters. The gel dosimeter was used to provide a 3D volumetric reference of the irradiated fields. The Linatron profiles and relative output factors were extracted at a reference depth of 2 cm with the output factor measured relative to the 2 × 2 cm2 reference field. Absolute dosimetry was performed using A14 ionization chamber measurements, which were verified using a national standards laboratory remote dosimetry service. Results Absolute dosimetry measurements were confirmed within 1.4% (k = 2, 95% confidence = 5%). The relative output factor of the small fields measured with films and gels agreed with a maximum relative percent error difference between the two methods of 1.1 % for the 1 × 1 cm2 field and 4.3 % for the 0.5 × 0.5 cm2 field. These relative errors were primarily due to the variability in the collimator positioning. The measured beam profiles demonstrated excellent agreement for beam size (measured as FWHM), within approximately 0.8 mm (or less). Film measurements were more accurate in the penumbra region due to the film’s finer resolution compared with the gel dosimeter. Following the van Dyk criteria, the PDD values of the film and gel measurements agree within 11% in the buildup region starting from 0.5 cm depth and within 2.6 % beyond maximum dose and into the fall‐off region for depths up to 5 cm. The 2D beam profile isodose lines agree within 0.5 mm in all regions for the 0.5 × 0.5 cm2 and the 1 × 1 cm2 fields and within 1 mm for the larger field of 2 × 2 cm2. The 2D PDD curves agree within approximately 2% of the maximum in the typical therapy region (1–4 cm) for the 1 × 1 cm2 and 2 × 2 cm2 and within 5% for the 0.5 × 0.5 cm2 field. Conclusion This work provides a commissioning process to measure the beam characteristics of a fixed beam MV accelerator with detailed dosimetric evaluation for its implementation in megavoltage small animal irradiation studies. Radiochromic gel dosimeters are efficient small‐field relative dosimetry tools providing 3D dose measurements allowing for full representation of dose, dosimeter misalignment corrections and high reproducibility with low inter‐dosimeter variability. Overall, radiochromic gels are valuable for fast, full relative dosimetry commissioning in comparison to films for application in high‐energy small‐field animal irradiatio...

Optimization of the Dose Rate Effect in Tetrazolium Gellan Gel Dosimeters

“…Whereas the depth-dose curves were well-fit with the Monte-Carlo calculation below the depth of maximum dose ( d max ), there was a noticeable over-response in the dose build-up region close to the top of the gel, which may be attributed to the effect of the dissolved oxygen near the surface of the gel. A similar effect was previously reported in fresh samples of ClearView [ 24 ] but was absent in well-sealed and aged gels [ 16 , 19 ]. To avoid uncertain dose sensitivity near the top surface of the gel, the jars used in the follow-up tests were sealed with heat-adhesive aluminum foil and aged for a week before use.…”

“…However, as initially developed, tetrazolium salt gel dosimeters tend to have lower sensitivity compared to other radiochromic 3D dosimeters [ 3 ], as well as an appreciable dose rate effect at dose rates below approximately 600 cGy/min [ 12 ]. Developed in our laboratory, a new gel dosimeter has been commercialized under the name ClearView by Modus Medical Devices Inc. (Modus QA), (London, ON, Canada) and primarily used for geometric registration of small-beam irradiation [ 14 , 15 , 16 ] and end-to-end dosimetric evaluation of single isocenter multi-target treatment and prostate plan validation [ 17 ]. The initial results indicated that the relatively low sensitivity to dose and the dose rate effect of the sensitivity may cause misregistration of the delivered dose plan.…”

Investigation of a 3D dosimetry system utilizing radio‐chromic gel and telecentric optical‐CT readout

“…ClearView (ModusQA) is a novel single use radiochromic gel dosimeter designed for use with optical CT for relative dosimetry measurements with several published accounts of use 1–6 . When exposed to ionizing MV radiation, ClearView gel changes color from optically clear to purple through the reduction of a water‐soluble tetrazolium salt into an insoluble formazan dye.…”

“…INTRODUCTION ClearView (ModusQA) is a novel single use radiochromic gel dosimeter designed for use with optical CT for relative dosimetry measurements with several published accounts of use. [1][2][3][4][5][6] When exposed to ionizing MV radiation, ClearView gel changes color from optically clear to purple through the reduction of a water-soluble tetrazolium salt into an insoluble formazan dye. Independent studies 1,2,4 and the User's Guide 7 have reported the change in optical attenuation to be linear with dose between 0 and 80 Gy, with negligible signal diffusion and change with time (aging of irradiated gel ∼3 × 10 −4 cm −1 /day), low optical scattering component, and near tissue equivalence (electron density 1.015 ± 0.3% relative to water).…”