1099 - Implementing a Simulation-Free Workflow in Palliative Radiotherapy for Bone Metastases: Dosimetric Evaluation of Diagnostic CT-Based Planning Using 3D-CRT and VMAT

Purpose/Objective(s):

Simulation-free radiotherapy using diagnostic CT (dCT) has emerged as a promising strategy to accelerate treatment delivery in the palliative setting; however, dosimetric validation, particularly for VMAT, remains limited. This study aims to evaluate the dosimetric accuracy of dCT-based planning compared with standard simulation CT (simCT) and to assess its suitability for supporting a simulation-free workflow in palliative radiotherapy for bone metastases using 3D-CRT and VMAT techniques.

Materials/Methods:

Sixty-six patients with thoracic, lumbar, or pelvic bone metastases were retrospectively analyzed. Patients receiving SBRT, those with metallic implants, or those with >28 days between simCT and dCT were excluded. Diagnostic CT images from prior PET/CT scans were calibrated using HU-electron density conversion, and a flat-couch model was applied to replicate treatment geometry. Rigid registration was performed at the target level. The entire involved vertebra was delineated on both CTs. For each patient, 3D-CRT plans with AP/PA fields and VMAT plans prescribing 30 Gy in 10 fractions were generated on both dCT and simCT. Plans optimized on dCT were transferred to simCT and recalculated without modifying beam parameters, simulating a simulation-free workflow in which the patient is planned with dCT but treatment is delivered using simCT geometry. These were compared with standard simCT plans. The primary endpoint was PTV D_95%. Secondary analyses included target coverage and OAR doses. Dosimetric differences were evaluated using paired tests, and equivalence was assessed using “Two One-Sided Tests” (TOST) with a ±5% margin and a 95% CI.

Results:

PTV D_95% differences between dCT- and simCT-based plans were statistically significant but small in magnitude, with mean variations of 0.94% ± 1.92% for 3D-CRT and -1.39% ± 2.42% for VMAT (both p<0.01). Despite statistical significance, TOST analysis confirmed dosimetric equivalence for both techniques within the predefined ±5% margin (p<0.01). Regional analyses showed the best agreement in the pelvic region for 3D-CRT (0.26% ± 1.82%) and in the lumbar region for VMAT (-0.33% ± 1.55%). The largest deviations were observed in lumbar 3D-CRT plans (1.73% ± 1.79%) and pelvic VMAT plans (-2.72% ± 3.14%), although these remained within acceptable limits. Mean Body-PTV D_max differences were 1.70% ± 2.37% for 3D-CRT and 2.67% ± 3.10% for VMAT. Similarly, spinal cord Dmax differences were small (1.75% ± 2.32% and 2.85% ± 3.42%, respectively). Overall, all observed differences remained within the predefined equivalence margin.

Conclusion:

Diagnostic CT–based planning achieved dosimetrically equivalent target coverage and OAR doses compared with conventional simulation CT for both 3D-CRT and VMAT techniques. These results support the dosimetric feasibility of a simulation-free workflow using both planning techniques for palliative radiotherapy of bone metastases, potentially improving treatment access and patient comfort.