Is adaptive planning necessary for patients with large tumor position displacements observed on daily image guidance during lung SBRT?

For patients undergoing stereotactic body radiation therapy for lung cancer, their tumor positions may vary due to anatomical changes. This study is to investigate whether adaptive re-planning is necessary for patients with large tumor position displacements observed from daily kV-cone-beam computed tomography (kV-CBCT). We selected 16 fractions from 16 patients with recorded treatment couch shifts greater than 1.5 cm under kV-CBCT guidance. The treatment positions for these patients were manually restored in kV-CBCTs via bone-to-bone alignments (B2B) and tumor-to-tumor alignments (T2T) with corresponding planning CTs. The tumor volumes, including PTVs, ITVs, and GTVs, were transferred from the planning CTs to these kV-CBCTs. With the planned beam configurations and treatment isocenters, kV-CBCTs were imported into the treatment planning system for dose recalculations. To minimize uncertainties of the Hounsfield Unit (HU) in kV-CBCTs, uniformed HU values were assigned to the externals, ITVs, and lungs. The percentage volumes of GTVs, ITVs, and PTVs receiving the prescription dose (V) and the dose to the normal structures were analyzed. Seven out of the 16 patients were identified with >5mm tumor position displacements after subtracting the recorded couch shifts from the shifts of B2B alignment. For T2T alignments, 9 out of 16 (56.3%) patients had V of PTV <95% (the planning goal) with 91.4% as the lowest, while V of the GTV and ITV remained 100% for all 16 patients. For B2B alignments, 14 out of 16 (87.5%) patients have V of PTV <95%; 5 patients (31.3%) had V of ITV <95%; and 4 patients (25.0%) had V of GTV <99%. T2T alignment with 5 mm PTV margin was found superior to B2B alignment, resulting in adequate dose coverage to the ITVs, even for tumors with large positional changes. Adaptive re-planning may not be necessary under these scenarios.