3315 - Dosimetric Impact of Online MR-Guided Adaptive Radiotherapy for Focal-Boosted Prostate Stereotactic Body Radiation Therapy

12:45pm - 02:00pm PT

Hall F

Screen: 21

POSTER

Presenter(s)

Jonathan Pham, PhD - UCLA, Los Angeles, CA

J. Pham¹, M. Ghafarian², A. Krupien³, D. O'Connell¹, X. Qi⁴, P. T. Courtney¹, L. Valle¹, M. Casado¹, T. C. Wu¹, M. L. Steinberg¹, M. Cao⁵, and A. U. Kishan¹; ¹Department of Radiation Oncology, University of California, Los Angeles, Los Angeles, CA, ²University of California Los Angeles, Los Angeles, CA, United States, ³UCLA, Los Angeles, CA, ⁴Department of Radiation Oncology, University of California, Los Angeles, CA, ⁵Department of Radiation Oncology, University of California San Francisco, San Francisco, CA

Purpose/Objective(s): To investigate the frequency and dosimetric impact of online MR-guided adaptive dose painted stereotactic body radiotherapy (SBRT) with a simultaneous integrated boost for prostate cancer.

Materials/Methods: 21 prostate patients treated on a prospective phase II clinical trial were included in this post-hoc exploratory analysis. All patients were treated on a 0.35T MR-LINAC with a prescription of 36.25 Gy/5Fx to prostate & proximal seminal vesicle PTV, with simultaneous integrated focal-boost prescription of 50 Gy/5Fx to the dominant intraprostatic lesion (DIL) GTV identified on multi-parametric MRI and PSMA-PET. 2mm isotropic PTV margins were employed. All patients began apalutamide monotherapy =1 week prior to SBRT. Plan optimization priority was 1st designated to meet organ-at-risk (OAR) constraints for bladder, rectum, and urethra, followed by ensuring CTV/PTV coverage and lastly DIL GTV coverage. Prior to each treatment delivery, on-board MRI was acquired to assess the need for online adaptive radiotherapy. CTV and OARs were recontoured and a predicted plan was evaluated by recalculating the initial plan onto the daily anatomy. Adaptive plan reoptimization was performed due to either inadequate predicted target coverage and/or OAR constraint violation. Plan adaptation frequency & dosimetric differences between predicted & adapted plans were evaluated.

Results: All initial plans met OAR constraints. Among 21 patients, 15 had one DIL, 5 had two DILs, and 1 had three DILs. For each patient, daily CTV volume remained stable (average volume change of 2.6±4.7%). Across 105 total fractions, 81 predicted fractions (77%) did not meet planning objectives, among which 21% did not meet PTV/CTV coverage, 25% did not meet OAR constraints, and 31% did not meet both PTV/CTV coverage & OAR constraints. Plan adaptation was performed in 97 fractions (92%). All patients dosimetrically improved to compliance for at least 1 adapted fraction. Target coverage, OAR constraint, and fraction compliance for initial, predicted, and adapted plans are summarized in the accompanying Table.

Conclusion: Online adaptive radiotherapy improved daily target coverage and OAR sparing for dose painted SBRT with a simultaneous integrated boost, reducing dosimetric fraction variability. The impact of this on oncologic efficacy and toxicity will be evident with longer follow-up.

Abstract 3315 - Table 1

Constraint	Initial	Predicted (Fx Pass %)	Adapted (Fx Pass %)	(Adapted-Predicted)
DIL GTV D95% (Gy)	49.2 ± 2.9	48.8 ± 2.6	49.0 ± 2.8	0.2 ± 0.9
CTV V38Gy = 97% (%)	98.7 ± 0.7	95.4 ± 3.4 (50%)	97.8 ± 1.6 (86%)	2.4 ± 4.0 (+36%)
PTV V36.25Gy = 97% (%)	98.5 ± 0.6	95.9 ± 3.3 (57%)	98.8 ± 0.6 (96%)	2.9 ± 3.2 (+39%)
Urethra D0.03cc = 42Gy (Gy)	40.8 ± 0.5	41.5 ± 0.6 (77%)	40.8 ± 0.4 (100%)	-0.6 ± 0.5 (+23%)
Bladder V20Gy = 30% (%)	10.2 ± 5.3	17.6 ± 8.8 (82%)	13.2 ± 5.8 (98%)	-4.4 ± 4.7 (+17%)
Bladder D0.03cc = 42Gy (Gy)	39.1 ± 1.5	39.5 ± 1.7 (87%)	37.9 ± 1.7 (98%)	-1.6 ± 0.9 (+11%)
Rectum D0.03cc = 42Gy (Gy)	34.5 ± 5.0	35.3 ± 4.7 (83%)	33.6 ± 4.5 (96%)	-1.7 ± 2.4 (+13%)