Dosimetric evaluation of inclusion of proximal seminal vesicle in target volume in low-risk prostate cancer treated with stereotactic body radiotherapy

Aims: Ultra hypofractionation using stereotactic body radiotherapy (SBRT) for low-risk PCa is considered a viable treatment option. The target volume for ultra hypofractionated RT was determined as prostate and/or proximal seminal vesicles; however, there are no clear guidelines on when to add a proximal seminal vesicle to the target volume. We aimed to dosimetrically assess the effect of inclusion of the proximal seminal vesicle in the planning target volume (PTV) on the dose distribution of organ at risk (OAR) when SBRT is administered to patients with low-risk PCa. Methods: Low-risk PCa cases who underwent SBRT with CyberKnife were retrospectively screened, and 20 random cases were included. The contours of OARs and target volumes were checked as recommended in international contouring atlases by the same radiation oncologist. Two treatment plans by determining two different PTV (prostate alone in plan 1 and prostate with proximal seminal vesicles in plan 2) were made by the same specialist physicist. 5×7.25 Gy was chosen as the dose schedule defined for both plans. Results: Regarding coverage, homogeneity index, and new conformity index (nCI), there was no significant difference between the two plans (p=0.397, p=0.452, p=0.225). The plan 2 had a greater PTV Dmax (p