Dosimetric feasibility of dose-painting radiotherapy for targeting hypoxia in prostate cancer on a novel ring gantry radiotherapy system.

73 Background: This study aimed at demonstrating the dosimetric feasibility of tumoral and hypoxia-guided dose escalated radiation therapy in prostate cancer on a novel ring gantry system TPS, based on conventionally fractionated and hypofractionated regimens. Methods: Three patients who underwent external-beam RT for intermediate-risk prostate cancer in the PAIR prostate study and had pre-therapeutic F-MISO PET uptake in the tumor, were selected. The GTV and the hypoxic region (biological tumor volume [BTV]) within the PTV on F-MISO PET uptake were delineated. IMRT planning based on three different prescriptions was performed: 1) Standard fractionation (SF)- 77Gy/35 fractions (fx) to PTV, with a boost to 95Gy and 118Gy in 35 fx to the GTV and to the hypoxic region defined by F-MISO PET, respectively, 2) moderate hypofractionation (MH)- 60 Gy/20 fx with a boost to 67Gy and 91Gy in 20 fx to the GTV and to the hypoxic region, respectively, and 3) high hypofractionation (HH)- 40 Gy/5 fx with a boost to 50Gy in 5 fx to the GTV and as high as possible to the hypoxic region (SBRT). Planning was performed on the research version of the RefleXion treatment planning system (X1-TPS). Results: The average size of the GTV and BTV was 6.03±2.05 cc, and 1.90±1.64 cc, respectively. For the SF regimen, the average D98% and D2% to the PTV, GTV and BTV were 71.33±0.15 Gy and 110.43±8.57 Gy; 91.43±1.12 Gy and 108.53± 10.29 Gy; and 112.33±3.54 Gy, and 121.37±1.31 Gy, respectively. For the MH and HH techniques, the average D98% to the PTV was 56.13±0.51 Gy, and 37.03± 1.81 Gy, and the average D2% was 85.53±8.52 Gy, and 58.47±1.90 Gy. In addition, the average D98%, and D2% to the GTV was 65.67±0.15 Gy, and 80.27±10.20 Gy using the MH treatment technique, while the average D98%, and D2% to the BTV was 85.87±5.00 Gy, and 94.9±1.15 Gy, respectively. Also, for the HH treatment planning method, the average D98%, and D2% to the GTV was 50.33±3.79 Gy, and 57.23±4.75 Gy, while the average D98%, and D2% to the BTV was 54.57±3.67 Gy and 61.00±2.17 Gy, respectively. The average D50% of Bladder, Rectum, and Femoral heads was 12.28±11.39 Gy, 26.09±5.89 Gy, and 11.44±2.92 Gy, while the average V15Gy was 43.07±18.68%, 72.39±8.81 %, and 23.43±20.44%, respectively. Conclusions: This study showed the dosimetric feasibility of dose-escalating the tumor and the hypoxic region in patients with prostate cancer on the RefleXion X1 TPS without compromising the OARs dose limits.