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

Abstract

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<0.001). There was better conformity index at plan 1, as well as lower monitor unit and beam on time (p<0.05). The plan 1 had statistically lower values for each treatment parameter assessed for bladder (p<0.05). The treatment parameters evaluated for the rectum were statistically lower in the plan 1, except for V32.625 (p<0.05). The plan 1 was statistically better in terms of V29.5 for the penile bulb and V37.5 and V38 for neurovascular bundles (p<0.05). There was no significant difference between the two plans in terms of femoral heads and bowel (p=0.180, p=0.209, p=0.398, p=0.726, p=0.053, p=0.068). In addition, regardless of plan type, a majority of treatment parameters for bladder were statistically significantly affected in plans with a PTV volume greater than 100 cc (p<0.05). Conclusion: In low-risk PCa, inclusion of the proximal seminal vesicle in the target volume may be overlooked as quantitatively insignificant increases in high-dose OAR volumes as they do not exceed dose constraints in routine clinical practice, and these high-dose OAR volumes are likely to be important in the development of toxicity. We recommend that special attention be paid to the high doses exposed in OARs in low-risk PCa. In addition, it should be kept in mind that bladder toxicity may increase with increasing PTV volume, especially above 100 cc.