Dose rate and energy dependence study of methacrylic acid gelatin tetrakis (hydroxymethyl) phosphonium chloride gel with flattened and unflattened photon beams.

AIMS: The aim of this study is to evaluate the dose rate and energy dependency of methacrylic acid gelatin tetrakis (hydroxymethyl) phosphonium chloride (MAGAT) gel in unflattened photon beam using X-ray computed tomography (CT) and ultraviolet (UV)-visible spectroscopy. MATERIALS AND METHODS: MAGAT gel was prepared and it was exposed to 6 MV flattened and unflattened beams. The dose selected for irradiation was ranging from 3 to 15 Gy with an increment of 3 Gy. The dose rate dependency of the gel was investigated by exposing the gel to three different dose rates of 250, 500, and 1500 cGy/min for flattening filter free (FFF). To verify the energy dependency of the gel, it was exposed by both FFF and flattening filter (FF) for constant dose rate (250 cGy/min) and different energy (6 and 10 MV X-ray photons). The exposed gels were scanned by X-ray CT and UV-visible spectrophotometer. RESULTS: The change in dose sensitivity observed over the dose rate from 250 cGy/min to 1500 cGy/min was 58.00% and 57.89% using a UV-visible spectrophotometer and X-ray CT analysis method. Energy dependency was evaluated with respect to dose sensitivity and the variation between 6 MV FF and FFF photon beams was found to be 2.20% and 2.21% using UV-visible spectrophotometer analysis and X-ray CT, respectively. Similarly, the variation noticed with 10 MV FF and FFF was 2.30% using UV-visible spectrophotometer analysis and 2.22% using X-ray CT analysis. CONCLUSIONS: The results clearly show that the MAGAT gel was highly dose rate-dependent and less dependent on energy. The beam quality variation between FF and FFF was less. The similar results obtained using X-ray CT scanner and UV-visible spectrophotometer indicate that this study can be recommended for polymer gel scanning procedure.

Impact of tumour motion compensation and delineation methods on FDG PET-based dose painting plan quality for NSCLC radiation therapy.

INTRODUCTION: To quantitatively estimate the impact of different methods for both boost volume delineation and respiratory motion compensation of [18F] FDG PET/CT images on the fidelity of planned non-uniform 'dose painting' plans to the prescribed boost dose distribution. METHODS: Six locally advanced non-small cell lung cancer (NSCLC) patients were retrospectively reviewed. To assess the impact of respiratory motion, time-averaged (3D AVG), respiratory phase-gated (4D GATED) and motion-encompassing (4D MIP) PET images were used. The boost volumes were defined using manual contour (MANUAL), fixed threshold (FIXED) and gradient search algorithm (GRADIENT). The dose painting prescription of 60 Gy base dose to the planning target volume and an integral dose of 14 Gy (total 74 Gy) was discretized into seven treatment planning substructures and linearly redistributed according to the relative SUV at every voxel in the boost volume. Fifty-four dose painting plan combinations were generated and conformity was evaluated using quality index VQ0.95-1.05, which represents the sum of planned dose voxels within 5% deviation from the prescribed dose. Trends in plan quality and magnitude of achievable dose escalation were recorded. RESULTS: Different segmentation techniques produced statistically significant variations in maximum planned dose (P < 0.02), as well as plan quality between segmentation methods for 4D GATED and 4D MIP PET images (P < 0.05). No statistically significant differences in plan quality and maximum dose were observed between motion-compensated PET-based plans (P > 0.75). Low variability in plan quality was observed for FIXED threshold plans, while MANUAL and GRADIENT plans achieved higher dose with lower plan quality indices. CONCLUSIONS: The dose painting plans were more sensitive to segmentation of boost volumes than PET motion compensation in this study sample. Careful consideration of boost target delineation and motion compensation strategies should guide the design of NSCLC dose painting trials.