G. S. Gevorkyan, D. A. S. Toesca, B. Laughlin, R. C. Tegtmeier, M. J. Flakus, L. Zhu, E. L. Clouser, C. E. Vargas, J. C. M. Rwigema, S. E. James, Q. Chen, Y. Rong, and N. Y. Yu; Department of Radiation Oncology, Mayo Clinic, Phoenix, AZ
Purpose/Objective(s): To assess the per-fraction target coverage and organ-at-risk (OAR) sparing effects with application of three distinct planning target volume (PTV) margins for cone beam computed tomography (CBCT)-based dose calculation and evaluation of intact prostate radiotherapy. Materials/
Methods: A retrospective evaluation was performed on daily CBCT data of intact prostate cancer patients with four implanted fiducials who underwent either stereotactic body radiotherapy (SBRT – 35Gy in 5 fractions, n=15) or conventional fractionation radiotherapy (CFRT – 60 or 70Gy in 20 or 28 fractions, n=16). Clinical target volume (CTV) and relevant OAR contours (bladder and rectum) were generated using an in-house deep learning-based CBCT auto-segmentation model and validated by physician assessment. A trained and clinically validated knowledge-based planning model was utilized to generate treatment plans on the planning CT data for each patient with uniform PTV margins of 0, 2, and 3 mm (expanded from the CTV), and these plans were then applied to CBCT images corresponding to select fractions based on the treatment type (SBRT vs. CRFT). Relative dose metrics for the CTV and the bladder and rectum OAR volumes were assessed for plans generated with each PTV margin. Statistical analysis was performed via an ANOVA single factor test (with an alpha value of 0.05). Results: CTV D90% > 90% was achieved 97.67% of patients treated with SBRT and 99.36% with CFRT when utilizing 2 mm and 3 mm margins, and coverage was lower on 0 mm PTV margins (93.02% SBRT, 96.15% CFRT). Maximum bladder dose in SBRT patients remained under the recommended limit of 105% for 88.75% of fractions with 0 mm, 79.38% for 2 mm, and 66.87% for 3 mm (p = NS). Whereas maximum rectum dose in SBRT patients remained under 105% in 90.63% of fractions for 0 mm, 85.63% for 2 mm, and 83.13% for 3 mm, with a difference that was statistically significant between patients with 0 versus 3 mm PTV margins (p < 0.01). Bladder and rectum V65% was under the recommended threshold (< 50%) in all CFRT cases irrespective of the PTV margins utilized. Rectum V65% among CFRT cases was lower when utilizing 0 mm PTV margins compared to 2 or 3 mm (p < 0.01). For SBRT, bladder V90% was lower than 10% in 97.78% with 0 mm PTV, 96.67% with 2 mm, and 94.44% with 3 mm PTV margins. Whereas the V50% was lower than 50% in 100% of cases using 0 mm or 2 mm PTV, and in 98.89% when using 3 mm PTV margins (p = NS). Among SBRT patients, rectum V90% < 10% and V50% < 50% were achieved in 100% of cases irrespective of PTV margins, with a difference that was significant between 0 and 3 mm (p < 0.01). Conclusion: A 2.0 mm PTV margin demonstrated optimal balance, ensuring CTV coverage while minimizing OAR dose which helps to reduce radiation-related GU toxicity risks.
