Assessing Inter-Fraction Changes in The Size and Position of The Penile Bulb During Daily MR-Guided Radiation Therapy to The Prostate Bed: Do We Need to Adjust How We Plan Radiation in The Post-Radical Prostatectomy Setting to Reduce Risk of Erectile Dysfunction?

Abstract

Background: We evaluated inter-fraction penile bulb (PB) changes in prostate cancer (PCa) patients undergoing MR-guided RT in the post-radical prostatectomy (RP) setting.

Materials and methods: 10 patients with PCa status-post RP received MR-guided RT from 2017-2019. Patients received daily setup volumetric MRI scans prior to RT delivery for alignment and target localization. Setup MRI datasets from Fx 1, Fx 19, and Fx 37 were fused for each patient based on soft tissue anatomy. The PB was contoured on each MRI. Data on volume (cc), superior/inferior positional change (cm), and mean dose (Gy) was collected. Differences were assessed by Student's t-test (sig. p<0.05).

Results: The mean PB volume change from Fx 1→ 19 was +0.34 ± 0.34 cc (p=0.11) and from Fx 1→ 37 was +0.22 ± 0.28 cc (p=0.31). The mean positional change from Fx 1→ 19 was +0.08±0.26 cm (p=0.37) and from Fx 1→ 37 was +0.05 ±0.25 cm (p=0.57). The mean change in mean PB dose from Fx 1→ 19 was +0.19±4.86 Gy (p=0.98) and from Fx 1→ 37 was -1.51≖7.46 Gy (p=0.88).

Conclusion: We present the first study evaluating inter-fraction changes to the PB during MR-guided RT. We found no clinically meaningful difference in the volume, positional change, or mean dose during RT in the post-prostatectomy setting, suggesting that PB organ motion may not need to be accounted for in radiation treatment planning.

Trial registration: ClinicalTrials.gov NCT04025372 NCT04861194.

Keywords: MRI-guided radiotherapy; Organ motion; Penile bulb; Prostate-bed radiotherapy; Sexual toxicity.

Conflict of interest statement

CONFLICTS OF INTEREST STATEMENT

Dr. Baumann reports consulting work with Mevion, Regeneron/Sanofi, Boston Scientific, and Galera Therapeutics as well as research funding from Varian, all outside of the submitted work. The remaining authors have no disclosures.

Copyright © 2022 Elsevier Inc. All rights reserved.

Figures

Figure 1:

Penile Bulb Volume (A), Displacement (B), and Mean Dose (C) for all 10 patients from treatment fractions 1, 19, and 37. The differences did not reach statistical significance. Box and whisker plot for volume change (D) and displacement (E) from treatment fractions 1 → 19 and fractions 1 → 37.