Session Item

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.

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.

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%.

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.