Hyaluronic acid rectal spacer in EBRT: Usability, safety and symmetry related to user experience

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

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 (NCT04189913), 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 its position and dimensions between the simulation and the 3-month post-RT, implying stable geometry during treatment, with only minimal separation changes observed. Statistical analysis using the Kruskal-Wallis test revealed significant correlations of larger separations at the inferior and apical planes with improved dosimetric outcomes, including rV30Gy.

CONCLUSION

The use of a stabilized HA spacer in prostate RT effectively enhances prostate-rectum separation, leading to significant rectal sparing without undesirable dose compromises. This study underscores the role of strategic placement and shape, specifically including > 1 cm separation from the base down to the prostate apex. When combined with the treatment planning techniques used in the trial to create a steep dosimetric gradient across the spacer, these findings elucidate the dosimetric outcomes that can be expected in the clinical implementation of HA spacer. This is particularly relevant in the evolution of hypofractionated treatment regimens for prostate cancer therapy.

Use of rectal balloon spacer in patients with localized prostate cancer receiving external beam radiotherapy

Objective

To evaluate the efficacy of the balloon spacer when used to reduce the radiation dose delivered to the rectum in prostate cancer patients undergoing external beam radiotherapy.

Method

A single center retrospective analysis including 75 PC patients with localized T1-T3a disease who received balloon spacer followed by EBRT. Pre- and post-implantation computed tomography (CT) scans were utilized for treatment planning for standard EBRT (78-81 Gy in 1.8-2 Gy fractions). Rectal dosimetry was assessed using DVHs, and toxicities were graded with CTCAE v.4.

Results

A median (IQR) prostate-rectum separation resulted in 1.6 cm (1.4-2.0) post balloon spacer implantation. Overall, 90.6 % (68/75) of patients had a clinically significant 25 % relative reduction in the rectal with a median relative reduction of 91.8 % (71.2-98.6 %) at rV70. Three (4.0 %) patients reported mild procedural adverse events, anal discomfort and dysuria. Within 90 days post-implantation, five patients (6.67 %) and 1 patient (1.33 %) reported grade 1 and grade 2 rectal toxicities (anal pain, constipation, diarrhea and hemorrhoids). Genitourinary (GU) grade 1 toxicity was reported in 37 patients (49.33 %), with only one patient (1.33 %) experiencing grade 2 GU toxicity. No grade ≥ 3 toxicity was reported.

Conclusion

Balloon spacer implantation effectively increased prostate-rectum separation and associated with dosimetric gains EBRT for PC stage T1-T3a. Further controlled studies are required to ascertain whether this spacer allows for radiotherapy dose escalation and minimizes gastrointestinal (GI) toxicity.