ESTRO 2020 Abstract book

S782 ESTRO 2020

To compare the incidental testicular doses during volumetric-modulated arc therapy (VMAT) using the Monte Carlo algorithm-based treatment planning system (TPS) in patients receiving prostate-only and pelvic lymphatic irradiation. Material and Methods Testicular doses in 40 intermediate- and high-risk prostate cancer patients were determined on treatment planning system (TPS) using the VMAT technique at 6MV. Scattered testicular doses were also measured by MOSFET detectors placed on testis surface. A dose of 78 Gy was prescribed for the prostate and seminal vesicles in the intermediate- risk group, whereas an additional 54 Gy was prescribed for the pelvic lymphatics in the high-risk group; the doses were delivered in 39 fractions. The testicular doses of patients treated with prostate-only and pelvic field irradiation were compared. Results The median mid-prostate-to-mid-testes distance was 13.4 cm (9.0–17.6 cm). The median testicular doses measured per 200 cGy fraction by TPS and MOSFET detectors were 1.7 cGy (0.7–4.1 cGy) and 4.8 cGy (3.6–8.8 cGy), respectively. The TPS doses and MOSFET readings showed a significant strong correlation (Pearson r = 0.848, p < 0.001). The testicular doses measured by TPS (1.34 ± 0.36 cGy vs. 2.60 ± 0.95 cGy; p < 0.001) and MOSFET (4.52 ± 0.64 cGy vs. 6.56 ± 1.23 cGy; p < 0.001) were significantly lower in patients with prostate-only irradiation than in those with pelvic field irradiation. The mean cumulative scattered dose for prostate-only field delivering 78 Gy was 1.8 Gy and that for pelvic field irradiation was 2.6 Gy, consistent with the reported findings. A negative significant correlation was observed between the testicular doses and the distance from the isocenter for both the TPS doses (r = –0.430; p < 0.001) and MOSFET readings (r = –0.491; p < 0.001) (Figure 3). The testicular doses measured in TPS at ≤12, 12–14, and >14 cm distances were 3.11 ± 1.10, 2.07 ± 0.89, and 1.45 ± 0.43 cGy, respectively, and a significant difference was found between each dose according to distance (p < 0.001). Similarly, the MOSFET readings significantly differed at ≤12, 12–14, and >14 cm. In anthropomorphic phantom measurements, the median testicular doses were 2.12 cGy (0.70–4.11 cGy) and 5.71 cGy (3.03–8.86 cGy) as measured by TPS and MOSFET, respectively. For scattered doses, the percent difference between the measured and the calculated doses at 13 cm distance was 169.3%. Conclusion The patients with prostate-only irradiation received higher testicular doses than those with additional pelvic field irradiation possibly due to the increased scattered doses in large field irradiation using the VMAT technique. A negative significant correlation was demonstrated between testicular doses and the distance from the isocenter. The clinical response to an increased incidental testicular doses due to pelvic field irradiation remains unknown, and it warrants further investigation. PO‐1383 Comparing the error detection performance of Portal Dosimetry & PerFraction in pre‐treatment VMAT QA S. Moloney 1 1 Poole Hospital NHS Foundation Trust, Radiotherapy Physics, Poole, United Kingdom Purpose or Objective The purpose of pre-treatment VMAT QA is to ensure correct delivery of a treatment plan. In our centre, where the TPS (Varian Eclipse) and R&V system (Elekta Mosaiq) do not share a database, these checks are especially important. We intend to switch from Varian Portal Dosimetry to Sun Nuclear PerFraction for these checks. These systems are conceptually similar; a predicted dose plane is compared to a measured dose plane derived from an EPID image. We

Boxplots in figure 2 show the accuracy (positioning error at isocenter distance) of the MLC grouped by layer and bank, as obtained from the analyzed MPC checks. Repeatability (not shown) was better than 0.5 mm and 0.6 mm for the distal and proximal layers, respectively. The obtained values are far below the tolerances suggested by Varian (accuracy: 0.6 mm for the distal layer, and 0.55 for the proximal layer; repeatability: 0.8 mm for distal layer, and 0.9 mm for proximal layer). No temporal trends were observed in the MLC MPC results along the studied period.

No MLC-related machine breakdowns occurred during the 7-month analyzed period. Conclusion The MLC of the Halcyon 2.0 performed far below the tolerances recommended by the TG-142 report and by the manufacturer, along a period of 7 months, with no MLC- related machine breakdowns. These results suggest that more stringent tolerances can be applied for the quality assurance of the dual-layer MLC from the Halcyon 2.0. PO‐1382 Incidental testicular doses during volumetric‐ modulated ARC radiotherapy in prostate cancer patients C. Onal 1 , R. Bozca 1 , Y. Dolek 1 , O.C. Guler 1 , G. Arslan 1 1 Baskent University Faculty of Medicine Adana Dr Turgut Noyan Research and Treatment Center, Department of Radiation Oncology, Adana, Turkey

Purpose or Objective