Main Session
Sep 29
PQA 06 - Genitourinary Cancer, Gynecological Cancer, and Health Care Access and Engagement

3323 - Differences In Rectal Dosimetry between Implantable Rectal Spacer Options for Prostate Cancer Radiotherapy

02:15pm - 03:30pm ET
Poster Hall - Exhibit Hall A
Screen: 14
POSTER

Presenter(s)

Sean McCauley, MD - Mayo Clinic Arizona, Phonix, AZ

S. T. McCauley1, M. F. Sischka2, D. M. Frendl2, Y. Ding1, J. Shen1, J. Holmes1, H. J. Gunn3, A. W. Bogan4, M. J. Susen1, P. R. Lara1, S. E. James1, B. Laughlin1, J. C. M. Rwigema1, N. Y. Yu1, A. A. Khan2, M. Humphreys2, C. E. Vargas1, and J. R. Niska1; 1Department of Radiation Oncology, Mayo Clinic, Phoenix, AZ, 2Mayo Clinic Arizona Department of Urology, Phoenix, AZ, 3Department of Quantitative Health Sciences, Mayo Clinic, Rochester, MN, 4Department of Qualitative Health Sciences, Section of Biostatistics, Mayo Clinic, Scottsdale, AZ

Purpose/Objective(s):

Prostate cancer radiotherapy (RT) can cause rectal toxicity. To reduce rectal side effect, implantable rectal spacers (IRSs) may be used. The 3 FDA-approved biodegradable IRSs are hydrogel (SpaceOAR), hyaluronic acid (Barrigel), and rectal balloon implant (BioProtect). Few studies have compared all 3 rectal spacing options. In the last quarter of 2025, we completed the first 11 cases of BioProtect at our institution. Following this, we reviewed the differences in rectal dose from prostate RT between SpaceOAR Vue (SO), Barrigel (BG), and BioProtect (BP).

Materials/Methods:

We analyzed all intact prostate cancer cases treated with RT after IRS placement at our institution in the last quarter of 2025. Dose–volume histogram (DVH) data for the percentage of rectal volume receiving a specified percentage of prescription dose (Vxx%) were extracted, including V50%, V75%, V80%, V90%, V95%, and V100%. Mean doses for each DVH parameter were compared using independent t-tests between the 3 different IRS options, the 3 different IRS options by photon or proton, and photon vs proton for all IRS cases. Significance was assessed at the 0.05 level, and 95% confidence intervals of the differences were estimated.

Results:

Among 65 IRS cases, we placed 32 SO, 22 BG, and 11 BP. By modality, we treated 27 were treated with protons (15 SO, 8 BG, 4 BP) and 38 were treated with photons (17 SO, 14 BG, 7 BP). Pairwise comparisons of IRSs within all modalities identified a statistically significant difference favoring BP compared to BG for V75% (p= 0.036) and V80% (p=0.047). When comparing BP to both BG and SO with proton cases, there was a statistically significant difference favoring BP for V50% (p=0.028). When comparing all proton versus photon cases, there was a statistically significant difference favoring photons for V75% (p=0.0003), V80% (p=0.0003), V90% (p=0.001), and V95% (p=0.0096).

Conclusion:

We report the rectal dosimetry for the first 11 cases using BP at our institution in comparison to contemporaneous prostate RT cases planned with SO and BG. Rectal dosimetry may improve with BP compared to SO or BG. At rectum V75% or greater, photon plans had lower rectal dose than proton plans but not with BP. Larger studies are needed to assess if these trends persist to better guide IRS selection for prostate cancer RT.