Abstract
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 (VRx) 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 VRx of PTV <95% (the planning goal) with 91.4% as the lowest, while VRx of the GTV and ITV remained 100% for all 16 patients. For B2B alignments, 14 out
of 16 (87.5%) patients have VRx of PTV <95%; 5 patients (31.3%) had VRx of ITV <95%; and 4 patients (25.0%) had VRx 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.
Keywords
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Article info
Publication history
Published online: April 02, 2022
Accepted:
February 23,
2022
Received in revised form:
February 18,
2022
Received:
November 22,
2021
Identification
Copyright
© 2022 American Association of Medical Dosimetrists. Published by Elsevier Inc. All rights reserved.
