Symmetry, separation, and stability: Physical properties for effective dosimetric space with a stabilized hyaluronic acid spacer

• Published in: Medical physics (Lancaster) 2024-09, Vol.51 (9), p.6231-6245
• Main Authors: Svatos, Michelle, Chell, Erik, Low, Daniel A., Pigrish, Vadim, Orio, Peter F., Miller, Katie, King, Martin T.
• Format: Article
• Language: English
• Subjects: Humans; hyaluronic acid; Hyaluronic Acid - chemistry; Male; Physical Phenomena; prostate radiation therapy; Prostatic Neoplasms - diagnostic imaging; Prostatic Neoplasms - radiotherapy; Radiometry; Radiotherapy Dosage; Radiotherapy Planning, Computer-Assisted - methods; Rectum - radiation effects; rectum spacer
• ISSN: 0094-2405; 2473-4209; 2473-4209
• DOI: 10.1002/mp.17292

Background The proximity of the rectum to the prostate in radiation therapy (RT) for prostate cancer presents a significant dosimetric challenge, leading to high rectal doses and resulting in detrimental side effects. Perirectal tissue spacing reduces rectal dose and gastrointestinal toxicities by mechanically separating these organs. A variety of materials have been explored for use as rectal spacers, most recently, a stabilized hyaluronic acid (HA) gel, which can be formed into deliberate a shape, and retains the definition of that shape, while remaining flexible, unlike polyethylene glycol (PEG) hydrogels. Purpose This study evaluates the dosimetric impact of the spacer, including shape symmetry, the degree of separation at different locations, and the temporal stability of the space. Our goal is to provide physics‐informed guidance on the optimal use of this sculptable spacer. Methods A secondary analysis was performed on data from a 13‐center prospective randomized trial (NCT 04189913), involving 136 patients with centrally‐reviewed treatment plans conducted on CT/MR simulation scans before and after receiving HA spacer implants. Patients were treated with 60 Gy in 20 fractions to the prostate. For this study, python software was utilized for automated processing of DICOM RTstruct and RTdose files, facilitating detailed analysis of the spacer's impact on anatomical displacement and dosimetric outcomes. Complete dose‐volume histograms (DVHs) were reconstructed, and combined into composite population DVHs before and after implant, verified against trial‐reported dose points. Patients were divided into similar groups of separation and symmetry, and differences in their composite DVHs were tested for significance. Stability of the spacer was studied by comparing serial MRI images and by computing the distance between contours at four axial planes, at simulation and 3‐month follow‐up, post RT. Results The introduction of the HA spacer significantly enhanced rectal sparing, as evidenced by a reduction in the mean rectal integral dose by over 6 Gy. High rates of implant symmetry (>95%) were observed, indicating nearly optimal lateral spacer placement. In superior–inferior coverage, this study like many others, saw the spacing largest at the superior extent but becoming more variable inferiorly at the level of the prostate apex. This allowed study of the apex as a specific area for dosimetric concern. Stability assessments confirmed that the spacer maintained