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?

Published:January 11, 2022DOI:


      • The penile bulb (PB) is an important organ-at-risk(OAR) to consider for sexual morbidity when delivering prostate cancer RT
      • Inter-fraction changes to PB during MRI-guided RT in the post-radical prostatectomy (RP) setting were evaluated
      • There were no significant differences in PB volume, positional change, or mean dose during RT
      • Findings indicate that PB organ motion may not need to be accounted for in radiation treatment planning
      • Validation study with larger sample size is warranted



      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).


      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).


      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.



      RP (radical prostatectomy), PB (penile bulb), PCa (prostate cancer), Fx (fraction of radiation)
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