Stereotactic Radiotherapy to the Prostate and Pelvic Lymph Nodes for High-Risk and Very High-Risk Prostate Cancer in a Setting with a Hydrogel Spacer: A Toxicity Report

<b>Background/Objectives</b>: Stereotactic radiotherapy (SABR) is a recognized standard treatment modality for localized prostate cancer, though data is limited regarding the risk of increased toxicity when including the elective nodes (ENI) for high-risk disease. Placement of a peri-rectal spacer can decrease the risk of toxicity to the rectum when administering high-dose prostate radiotherapy. Herein we present toxicity findings for patients who underwent five-fraction prostate SABR with ENI in a setting with peri-rectal spacing. <b>Methods</b>: Genitourinary (GU) and gastrointestinal (GI) toxicity data was analyzed for patients with ≥12 months of follow-up who were treated with curative-intent five-fraction SABR with ENI. A radiopaque hydrogel spacer was placed for all eligible patients. The primary endpoints were the three-month toxicity, which was measured using CTCAEv5, and quality of life (QoL), which was measured using EPIC 26. Secondary endpoints included intermediate-term GU and GI toxicity between 6 and 12 months. Univariable logistic regression was used to assess associations between baseline patient characteristics and the presence of a peri-rectal hydrogel spacer and GU and GI toxicity. <b>Results</b>: Among the 100 patients treated, 69 had grade group 4/5 disease and 40 had evidence of T3a/3b extension. The ENI dose was 25 Gy/5, and 78.9% of the patients received 40 Gy to the prostate, while the remainder were given 36.25-37.5 Gy. A total of 70% underwent placement of a radiopaque hydrogel spacer. GU toxicities of grades 1, 2, and 3 were reported in 28/22/1% of the patients, respectively, at three months; in 18/11/0% at six months; in 11/9/0% at nine months; and in 5/3/0% at twelve months. GI toxicities of grades 1 and 2 were reported in 14/0% of the patients at three months and 8/1% at six months, with all cases resolving by nine months. MCICs in the urinary incontinence, urinary obstructive, and bowel domains were reported in 5%, 18%, and 4% at three months; by twelve months, these values decreased to 2%, 2%, and 0%, respectively. The presence of a hydrogel spacer resulted in reductions in high and intermediate doses to the rectum and had a significant inverse association with short-term GI toxicity (HR: 0.09, CI: 0.27-0.35, <i>p:</i> 0.0004). <b>Conclusions</b>: In this prospective series, five-fraction SABR including ENI was well tolerated, and the presence of a hydrogel spacer was associated with a lower risk of rectal toxicity.