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Rectal hydrogel spacer is often injected at mid-gland to hopefully get a symmetric distribution from base to apex. However, the prostate and rectum often closely approximate from mid-gland to apex. This study compared spacer distribution and rectal dosimetry among three cohorts of patients who 1) didn’t have spacer, 2) had spacer injected standardly at mid-gland, and 3) had spacer injected just caudal to mid-gland. They measured prostate/rectum separation at multiple levels of the base, mid-gland, and apex. Among patients who had standard injection at mid-gland, they found that the extent of separation at mid-gland and apex (not at the base) were associated with the greatest reductions in rectal dosimetry. As expected, injection slightly caudal to mid-gland (4/10ths the from apex to base to be exact) resulted in greater separation at mid-gland and apex compared to standard injection technique and significantly lower rectal V30-V50. | Fukumitsu, Pract Radiat Oncol 2021