Dosimetry, Toxicity and Stability Associated With a Hyaluronic Acid‐Based Prostate‐Rectal Spacing Device During Prostate Radiation Therapy: A Retrospective Comparative Analysis

ABSTRACT Introduction Hypofractionated radiation therapy (HFRT) for localised prostate cancer may increase gastrointestinal (GI) toxicity. This study evaluated spacer geometry, dosimetric and clinical outcomes associated with the use of a stabilised hyaluronic acid (sHA)‐based rectal spacer, when inserted prior to HFRT, compared to a non‐spacer cohort. Method This was a retrospective comparative cohort study of patients receiving HFRT (60 Gy in 20 fractions) between July 2019 and December 2023 at two centres from a single institution. Prostate‐rectum spacing was measured at apex, mid‐gland, and base levels at simulation as baseline and on five cone beam CT scans during treatment. Rectal dose‐volume metrics and planning target volume (PTV) coverage were compared. Toxicities were retrospectively graded using CTCAE Version 5.0 for participants with a minimum follow‐up of 3 months. Results Ninety‐one patients were included in this study; 64 with sHA‐based spacers and 27 without spacing. The mean baseline spacing was 0.8, 1.0 and 0.9 cm at apex, mid‐gland and base, respectively, and this separation remained stable throughout treatment. The spacer cohort demonstrated significantly lower rectal doses across all metrics except V60Gy and achieved slightly higher PTV coverage (D98%: 57.8 vs. 57.3 Gy, p < 0.001). Acute GI toxicity (grade ≥ 1) was lower in the spacer cohort (23.5% vs. 48.0%; OR = 0.33, p = 0.05), with grade ≥ 2 toxicity in 2.9% versus 20.0%. Genitourinary toxicity rates were comparable between cohorts. No complications associated with spacer insertion were recorded. Conclusions sHA ‐based rectal spacers provided physically stable spacing throughout treatment, facilitating significant reductions in rectal dose and a favourable toxicity profile. This study adds further evidence to support the use of sHA‐based spacers in prostate HFRT.