Separation effect and development of implantation technique of hydrogel spacer for prostate cancers

Abstract Purpose The purpose was to improve the placement of a hydrogel spacer in prostate cancer patients receiving radiotherapy. Methods and Materials One hundred and sixty patients with prostate cancer were classified into 3 groups as follows: group 1, no spacer (n = 30); group 2, spacer placed using conventional technique (n = 100); and group 3, spacer placed using new technique (n = 30). When placing the spacer, the tip of the needle was placed at the middle of the prostate gland (group 2) or at a level corresponding to a cranial:caudal ratio of 6:4 and as close to the prostate gland as possible (group 3). The separation effect was then examined and compared among the groups. Results The separation in group 2 was larger than that in group 1 from the base to the apex (4 mm) level of the prostate, while the separation in group 3 was larger than that in group 2 from the middle to the apex (4 mm) level of the prostate. The separation values for the middle to the apex, the spacer thickness from the apex (10 mm) level to the apex, the rectal exclusion from the middle to the apex, and the laterality were correlated with the 50 and 60 Gray relative biological effectiveness (Gy(RBE)) rectal dose (p = 4.1 × 10−9 – 0.046). The separation vales were strongly correlated with the spacer thickness at the apex (10 mm) and apex (4 mm) (p = 1.1 × 10−18 – 1.8 × 10−17). The rectal volumes at 10–60 Gy(RBE) differed among the groups (p = 5.1 × 10−19 – 5.4 × 10−3). The rectal volumes in group 2 were smaller than those in group 1 at all dose levels, while those in group 3 were smaller than those in group 2 at dose levels of 30–50 Gy(RBE). Conclusions The separation, spacer thickness and rectal exclusion from the middle to the apex of the prostate and the laterality of the hydrogel spacer affected the reduction in the rectal dose. The rectal dose can be further reduced by implanting a spacer on the caudal and the prostate side.