Session Item

Silicone-based radiochromic dosimeters allow for 3D dose verification with high spatial resolution. Key dosimetric properties are often characterized using cuvette-sized dosimeters, since 3D experiments are time consuming and costly. However, dosimetric properties for larger samples must also be validated. The aim of this study was therefore to assess the dose response obtained from cuvette-sized dosimeters readout using a spectrophotometer versus larger cylindrical dosimeters readout using an optical 3D CT scanner.

Dosimeters were fabricated from silicone elastomer, curing agent, chloroform and leucomalachite green from a single batch and divided into two groups 1) 25 cuvettes (1 cm x 1 cm x 4.5 cm) and 2) 5 cylindrical dosimeters (height = 5 cm, diameter = 5 cm). Cuvettes were placed between a 4.5 cm solid water (SW) build-up slab and a 5 cm SW backscatter slab and irradiated to doses of 2, 5, 10, 15 and 20 Gy in a single fraction with a beam quality of 6 MV and dose rate of 8.0 Gy/min. Five cuvettes were irradiated simultaneously per dose level. The cylindrical dosimeters were immersed in a water tank (Figure 1) and irradiated with two opposing 10 cm x 10 cm fields to obtain a uniform dose distribution throughout the dosimeters to doses of 2, 4, 6, 8 and 10 Gy with 6 MV beam quality and a dose rate of 5.6 Gy/min. The number of monitor units needed to deliver a given dose was calculated based on a CT scan of the setup imported to the treatment planning system (Eclipse, Varian Medical Systems). One cylindrical dosimeter was irradiated per dose level. The dosimeters were readout prior to and after irradiation. Cuvettes were readout using a spectrophotometer (Spectroquant Pharo 100) at 625 nm. Cylindrical dosimeters were readout using a Vista^TM 16 optical CT scanner (Modus Medical Devices) at 635 nm and data reconstruction was performed with a resolution of 1.0 mm x 1.0 mm x 1.0 mm, using an OSC-TV algorithm provided as an integral part of the scanner. The dose response was given as the irradiation-induced change in attenuation coefficient (averaged over a (5 x 5 x 5)-voxel region at the center of the cylindrical dosimeters) plotted against dose. 

Linear dose responses were found in the investigated dose ranges for both cuvettes and cylindrical dosimeters (Figure 2). The dose response for the cuvettes was (0.022 ± 0.0003) Gy^-1cm^-1 while it for the cylindrical dosimeters was (0.044 ± 0.004) Gy^-1cm^-1. 

The dose response reported for larger samples measured using the optical CT scanner operating at 635 nm was greater than that for cuvettes as reported by spectrophotometry at 625 nm. Further studies are needed to determine whether the difference is related to variations in volume, shape, readout modality or irradiation conditions. Both optical CT of large samples and spectrophotometry of small samples provide high-quality relative measurements of absorbed dose. However, careful calibration is needed to relate these two distinct types of experiments.