Room: AAPM ePoster Library
Purpose: For patients undergoing lung SBRT, their tumor positions may vary due to anatomical changes in lungs, e.g. the presence of pleural effusion. In the era of daily CBCT verification, this study is to investigate whether adaptive re-planning is necessary for patients with identified large shifts.
Methods: We studied fifteen fractions from fifteen patients in whom recorded treatment couch shifts were > 1.5 cm in any directions based on daily kV-CBCT guidance. The treatment positions for these CBCTs were reinstituted through applying 6D couch shifts after rigid tumor-to-tumor registration with the corresponding planning CTs. The tumor volumes, including PTVs (5 mm expansion to the ITVs), ITVs, and GTVs, were subsequently transferred to the CBCTs from the planning CTs. The contours for lungs and patient bodies were automatically drawn on CBCTs through thresholding. The transferred tumor volumes and normal structures were further visually verified. The CBCTs aligned with treatment positions were then imported back into the treatment planning system for dose calculations with the treated beam configurations. To minimize uncertainties in the kV-CBCT Hounsfield Unit (HU), uniformed HU values were manually assigned to body contours (1.0 g/cm³), ITVs (0.7 g/cm³) and lungs (0.3 g/cm³). The dose coverage to the tumor volumes was analyzed.
Results: With rigid tumor-to-tumor registration, D95% of PTVs in 7 of 15 fractions were 91.4%–94.6% of the prescription dose but 100% of GTVs and ITVs received 100% of the prescription dose for all 15 fractions. Assuming ITVs and PTVs were the targeted volumes in CBCTs during the treatment, it’s appropriate to proceed with treating these fractions without re-planning.
Conclusion: Directly aligning to tumor enables us to achieve the treatment goal with adequate dose coverage to internal target volumes with a 5 mm planning margin, even for tumors with large positional changes. Re-planning is not necessary under these scenarios.