SU‐E‐T‐408: RapidArc Dose Verification Using Ferrous Xylenol‐Orange Gels and Optical CT

Abstract

Purpose: To perform a 3D dosimetric verification of a RapidArc™ treatment using Ferrous Xylenol‐orange (FX) gels and optical CT. Methods: Two FX gels from the same batch were prepared using 0.3 mM ferrous‐ammonium‐ sulphate, 0.05 mM xylenol‐orange, 65 mM sulfuric acid and 6% by mass porcine gelatin dissolved in distilled water. Upon preparation, the FX gel solution was poured into two separate 0.5 mm thick PFA Teflon‐walled cylinders (9.6 cm diameter, 10 cm height). One FX gel was used for RapidArc™ verification. The RapidArc™ treatment plan created with Eclipse v.8.9 software (Varian Medical Systems), consisted of 6 MV x‐ray beams delivered with a single 360° gantry rotation by Varian Clinac 21IX, and a target dose of 237 cGy. A second FX gel was irradiated with a 12 MeV electron beam to independently calibrate the dose response. Both FX gels were read out using a commercial optical cone‐beam CT scanner at a central wavelength frequency of 590 nm (Vista, Modus Medical Devices). Optical CT reconstructions at 0.5 mm3 voxels were averaged to provide 1 mm3 voxels for comparison with the planned data. Results: In the target volume of comparable slices, planned and measured dose distributions (in units of cGy) are in agreement within 2–3%. In the low dose volume region, the planned dose data is generally less compared to the measured gel data by about 5%. Contrast is slightly reduced in the cone‐beam CT gel images which may be due to residual stray light. The data reveals that larger volumes of gel (>2 L) will be needed for the verification of large target and low dose volumes. Conclusions: FX gel dosimetry with optical CT readout provides accurate volumetric dose verification and quality assurance that will lead to the safe clinical implementation of complex radiotherapy applications such as RapidArc™.