Purpose: To assess the feasibility of three offline adaptive workflows for prostate radiotherapy (RT) through a retrospective study, potentially allowing future offline dose assessment and adaptive replanning for prostate RT.
Methods: We investigated an offline adaptive therapy workflow previously developed for head-and-neck patients on a small cohort of seven prostate patients over their first five fractions. Because this workflow was previously shown to have poor deformations in the pelvis, a lower-bound threshold contour was used to reduce bladder-volume variation. Three workflow variations were assessed for accuracy in estimated bladder volume and delivered dose-to-bladder: Velocity’s “Navigator” using full region-of-interest (fROI), Navigator with a reduced ROI (rROI), and structure-guided deformation (SGD). DICE coefficients were compared between deformed-bladder volumes and true day-of-treatment volumes. Workflow-estimated dose-to-bladder and NTCP were compared to ground-truth calculations.
Results: The average ratio of day-of-treatment bladder volume compared to simulation was 82±17% for 6 of the 7 patients. For these patients, the fROI and rROI workflows tended to over-estimate bladder volume, with average DICE-Similarity-Coefficients of 1.089±0.088 and 1.095±0.087 respectively, whereas the SGD workflow agreed with the true volume (1.010±0.027). Correspondingly, the fROI and rROI workflows tended to underestimate bladder dose (fractional dose differences up to 94%) and NTCP, with improved agreement using SGD. The seventh patient’s bladder tended to be extremely over-filled, with day-of-treatment volume up to 548% simulation volume. Due to the large volume difference, poor deformations resulted in unreliable day-of-treatment bladder volume, dose, and NTCP estimates for all workflows.
Conclusion: The SGD workflow with a lower-bound threshold for bladder fill at treatment is feasible for offline bladder-dose assessment for prostate treatments and may enable future adaptive replanning. All workflows were inaccurate for drastically over-filled bladder volumes, but since over-filling does not tend to result in increased bladder dose/NTCP, it is not of great concern for offline dose assessment.
Funding Support, Disclosures, and Conflict of Interest: Dr. Hyun has received an honorarium for presenting work related to this project in a webinar hosted by Varian.
Prostate Therapy, Deformation, NTCP