Toxicity profile and clinical outcomes of stereotactic body radiotherapy with a focal boost without fiducials or perirectal hydrogel spacer for localized prostate cancer

Purpose: Whole-prostate dose escalation in stereotactic body radiotherapy (SBRT) for localized prostate cancer (PCa) can improve oncological outcomes, albeit at the cost of increased toxicity. A focal boost to the dominant intraprostatic lesion (DIL) is gaining interest as an alternative approach. Herein, we investigate the safety and efficacy of this approach.

Methods: This retrospective study enrolled patients with localized PCa who underwent five-fraction SBRT with a focal boost to the DIL at our institution between May 2016 and August 2021. The prescription doses to the whole prostate were 35 and 36.25 Gy for low- to favorable intermediate-risk PCa and unfavorable intermediate- to high-risk PCa, respectively. The focal boost to the DIL was up to 115-140% of the prescribed dose. None of the patients underwent pretreatment fiducial or perirectal hydrogel spacer placement. Acute and late genitourinary (GU) and gastrointestinal (GI) toxicities and oncological outcomes were assessed.

Results: Among the 520 patients, 44% were categorized as patients with high-risk PCa. The median follow-up period was 42.9 months. No acute or late grade ≥3 toxicities were observed. Acute and late grade 2 GU toxicities were observed in 22.3 and 6.1%, respectively, while GI toxicities were observed in 2.1 and 0.8% of the patients. The 4‑year relapse-free survival rate was 94.8% among all patients.

Conclusion: Our results indicate that SBRT with a focal boost without fiducials or perirectal hydrogel spacer for localized PCa has a promising toxicity profile and oncological outcomes. Longer follow-up studies are necessary to adequately evaluate late toxicities and efficacy.