A Phase II Study of Stereotactic Body Radiotherapy with Hydrogel Spacer for Prostate Cancer; Dosimetric Comparison, Acute Toxicity and Quality of Life

Volumetric modulated arc therapy (VMAT) stereotactic body radiotherapy (SBRT) with flattening filter-free (FFF) beam can achieve faster dose delivery and shorter treatment time for prostate cancer. A hydrogel spacer is used to reduce the rectal toxicity, but the clinical data of SBRT with hydrogel spacer are limited. The aim of this prospective single-center phase II study was to determine the efficacy and safety of FFF-VMAT SBRT combined with hydrogel spacer for localized prostate cancer. Men with localized prostate cancer were eligible for the study. A hydrogel spacer was inserted into the perirectal space between prostate and rectum before the initiation of SBRT. All patients underwent the planning CT scans before and after the spacer placement. Patients received 36.25 Gy to 95% of the PTV in 5 fractions every other day excluding weekends with 6MV single arc FFF-VMAT. The primary endpoint was acute toxicities during and within 3 months after the completion of SBRT. Toxicities were assessed using Common Terminology Criteria for Adverse Events v4.03. The secondary endpoints were International Prostate Symptom Score (IPSS) and quality of life (QOL) assessed by the Expanded Prostate Cancer Index Composite (EPIC) questionnaire. Target and normal tissue dosimetry were compared before and after the hydrogel spacer insertion. Between February 2017 and July 2018, forty patients with localized prostate cancer were enrolled. The median age was 70 years (55-79). Three, 25, 6, and 6 patients had low-, intermediate-, high-, and very high-risk prostate cancer according to the National Comprehensive Cancer Network (NCCN) risk classification. Twenty-three patients (58%) received concomitant androgen deprivation therapy. The hydrogel spacer placement was successful in 39 cases (98%). Seven (18%) and eighteen patients (45%) developed grade 2 acute gastrointestinal and genitourinary toxicities, respectively. No grade 3 acute toxicities were observed. Median IPSS temporarily increased at 2 weeks and 1 month after RT when compared to the pretreatment baseline (P < 0.05, 0.05, respectively), and returned to the baseline value in 3 months. EPIC urinary and bowel scores significantly declined at 2 weeks and 1 month after RT (P <0.05, 0.05, respectively), and no difference was observed at 3 months post-RT. Spacer use was associated with lower rectum dose for mean dose, maximum dose, and V100% to V50% (all P < 0.05). FFF-VMAT SBRT with a hydrogel spacer can be safely administered with acceptable acute toxicity. Hydrogel spacer insertion significantly reduced the dose to the rectum.