2303 - Sculpting Dose Inhomogeneity with Consideration of the Dominant Intraprostatic Lesion in Prostate SBRT: A Planning Feasibility Study

E. S. Hollis¹, M. V. Lawrence², E. C. Schreiber³, S. Sud⁴, N. A. Wijetunga⁵, and M. C. Repka⁴; ¹University of North Carolina Hospitals, Chapel Hill, NC, ²UNC Hospital, Chapel Hill, NC, ³Department of Radiation Oncology, University of North Carolina School of Medicine, Chapel Hill, NC, ⁴Department of Radiation Oncology, University of North Carolina, Chapel Hill, NC, ⁵University of North Carolina School of Medicine, Chapel Hill, NC

Purpose/Objective(s): While prostate cancer is often multifocal, a randomized trial demonstrated that a focal integrated boost to a dominant intraprostatic lesion (DIL) with conventionally fractionated external beam radiotherapy (EBRT) improves oncologic outcomes. However, strict adherence to organ-at-risk (OAR) constraints limits optimal DIL dosimetry with this approach. Stereotactic body radiotherapy (SBRT) with highly inhomogeneous, HDR-like dosimetry has been shown to confer lower post-treatment PSA nadirs than EBRT or typical homogeneous SBRT planning. In this study, we aim to determine feasibility of strategically manipulating dose inhomogeneity during SBRT planning with HDR-like dosimetry to maximize dose to the DIL while prioritizing OAR sparing. Materials/

Methods: Patients with prostate cancer treated to 3625 cGY in 5 fractions with robotic SBRT using inhomogeneous, HDR-like dosimetry with a planned hot spot = 150% prescription dose were identified from an IRB-approved institutional database. DILs were MRI PIRADS 4 or 5 lesions that were subsequently confirmed pathologically. All patients had a hydrogel rectal spacer placed. Patients on ADT were excluded due to potential for DIL changes by time of simulation. DIL contours were reviewed by two physicians for accuracy; no clinical or planning margin was added. New “re-plans” were developed with the goal of achieving DIL V50Gy >95% while prioritizing normal dose constraints. Planning metrics for PTV, DIL, urethra, rectum and bladder were compared using the Wilcoxon Signed-Rank test.
Results: 11 patients were re-planned. On closest axial distance, 8 DILs were <5 mm to the urethra, and 2 DILs were <5 mm to the rectum. In the re-plans, DIL V50GY was >90% in all but 1 plan. The average coverage of DIL V50Gy significantly improved in the re-plans (97.3% vs 21.7%, p=0.003). There was a significant difference between mean and max dose (cGy) to the DIL in re-plans compared to original plans (p=0.003, p=0.006, respectively). The average mean and max dose to the DIL was 5382 vs 4434 cGy and 5685 vs 4832 cGy in the re-plans and original plans, respectively. The mean PTV V3625cGy did not significantly change with re-plans (p= 0.429) and coverage was maintained (V3625cGy =95%) for all plans. Dose objectives to OARs were met in all plans with no significant difference in max dose to the urethra (3803 vs 3786 cGy, p=0.505) or rectum (3744 vs 3720 cGy, p=0.929). However, re-plans had a significant increase in bladder V20Gy (p=0.017) with a mean of 18.0% and 16.6% for the re-plans and original plans, respectively.
Conclusion: SBRT with highly inhomogeneous, HDR-like dosimetry may better escalate DIL dose compared to EBRT or more common SBRT techniques. Strategically manipulating dose inhomogeneity to increase DIL dose while maintaining normal dose constraints to OARS appears feasible with minimal compromise.