3197 - Delivered Dosimetry during Post-Prostatectomy Prostate Bed SBRT with MR-Guided Versus CT-Guided Radiotherapy
Presenter(s)
E. D. Yu1, L. M. Smith2, T. Romero3, T. M. M. Ma4, J. Nikitas5, J. E. Juarez Casillas6, Y. Gao7, S. Yoon8, T. Jiang9, K. Taparra6, L. Valle6, L. K. Ballas10, J. M. Lamb11, M. L. Steinberg12, M. Cao13, and A. U. Kishan6; 1UCLA David Geffen School of Medicine/UCLA Medical Center, Los Angeles, CA, 2UCLA, Los Angeles, CA, 3Department of Medicine, University of California, Los Angeles, Los Angeles, CA, 4University of Washington, Department of Radiation Oncology, Seattle, WA, 5Department of Radiation Oncology, University of Pennsylvania, Philadelphia, PA, 6Department of Radiation Oncology, University of California, Los Angeles, Los Angeles, CA, 7Department of Radiation Oncology, Stanford University, Stanford, CA, 8Department of Radiation Oncology, City of Hope National Medical Center, Duarte, CA, 9Department of Urology, Stanford University, Stanford, CA, 10Cedars-Sinai Medical Center, Los Angeles, CA, CA, 11UCLA, Department of Radiation Oncology, Los Angeles, CA, 12Department of Radiation Oncology, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, CA, 13Division of Physics, Department of Radiation Oncology, UCSF, San Francisco, CA
Purpose/Objective(s):
Organ and target deformation presents a significant challenge when delivering radiation to the prostate fossa, particularly with stereotactic body radiotherapy (SBRT). Therefore, evaluating delivered rather than planned dosimetry is critical. Post-prostatectomy SBRT is limited by target motion and proximity to radiosensitive organs at risk (OARs). Magnetic resonance–guided radiotherapy (MRgRT) may allow margin reduction and improved dosimetric accuracy compared with conventional computed tomography–guided radiotherapy (CTgRT). We performed a post-hoc analysis of SCIMITAR phase II trial of post-prostatectomy SBRT to compare delivered dosimetric accuracy between MRgRT and CTgRT.Materials/Methods:
One hundred patients received SBRT (30–34 Gy/5fx) to the prostate bed ± pelvic nodes on the SCIMITAR trial (2018–2021). Among patients treated with CTgRT (5-mm PTV margins), 41 of 69 (59%) had sufficient-quality onboard imaging for dosimetric analysis. All 31 patients treated with MRgRT (3-mm PTV margins) had evaluable imaging. Dosimetric analysis involved transferring the planned dose onto daily onboard images with all relevant structures re-contoured. Planned dose distributions were mapped to daily anatomy to estimate delivered dosimetry. Dosimetric endpoints and rates of constraint failure were compared between groups.Results:
MRgRT demonstrated significantly less deformation of the clinical target volume (CTV), rectum, and rectal wall compared with CTgRT (all p < 0.05). Delivered dosimetry favored MRgRT, with significantly higher target coverage (CTV V95%: 98.5% vs 93.6%; PTV V95%: 95.9% vs 82.7%; both p < 0.001) and lower rectal dose (rectum V32.5Gy: 5.5% vs 9.9%; rectal wall V24Gy: 20.8% vs 26.0%; p < 0.001). Despite the use of tighter margins, MRgRT was associated with significantly fewer instances of delivered target coverage failure. Fractions treated with CTgRT were substantially more likely to fail at least one target constraint compared with MRgRT (70% vs 13%).Conclusion:
MRI-guided SBRT to the prostate bed is associated with improved dose delivery compared to CT-guided SBRT, resulting in superior delivered target coverage (despite tighter margin) and improved rectal sparing. These dosimetric advantages support MRgRT as a preferred approach for post-prostatectomy SBRT, providing a mechanistic basis for improved adverse event profiles seen on the SCIMITAR clinical trial.