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Necessity of 4D Cone-Beam CT Imaging in Patient Setup for Gated Stereotactic Body Radiation Therapy of Lung Cancer

J Wu

J Wu*, F Chen, J Ye, V Mehta, C Loiselle, J Spiegel, Swedish Cancer Institute, Seattle, WA

SU-E-J-6 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

To investigate the necessity of 4D-CBCT imaging in patient setup for gated lung SBRT, and provide alternative solutions if 4D-CBCT imaging is not available.

4D-CT images were acquired for five patients underwent lung SBRT. Average Intensity Projection (AIP) CTs were reconstructed. Gating plans were generated using the 50% end-exhalation (EE) CT with 30% duty cycle. PTV was created by adding a 5mm margin to the union of GTVs contoured on the 40%-60% CTs. Prior to each treatment, localization images (3D-CBCT and 4D-CBCT) were acquired and registered to their reference CTs for setup comparison. Six registration scenarios were investigated. For the first four scenarios, localization images were registered to the reference images by matching tumor centers: EE-CBCT to EE-CT, EE-CBCT to AIP-CT, 3D-CBCT to EE-CT, and 3D-CBCT to AIP-CT. For the intermediate two scenarios, two additional registrations were performed by matching tumor superior edges. The EE-CBCT to EE-CT registration was taken as the bench mark. Relative tumor offsets of the remaining registrations to the bench mark were analyzed.

For tumor center matching, the relative tumor offsets of the EE-CBCT to AIP-CT registration were 0.1±0.1(LR), 0.5±0.1(SI), 0.1±0.1(AP)cm. The offsets for the 3D-CBCT to EE-CT registration were 0.0±0.0, 0.5±0.1, 0.1±0.1cm. The offsets for the 3D-CBCT to AIP-CT registration were 0.1±0.0, 0.1±0.0, 0.0±0.0cm. For tumor superior edge matching, the offsets of the EE-CBCT to AIP-CT registration were 0.1±0.1, 0.1±0.0, 0.1±0.1cm. The offsets of the 3D-CBCT to EE-CT registration were 0.0±0.1, 0.0±0.1, 0.0±0.0cm.

4D-CBCT imaging is crucial to precisely localize the tumor within the gating window. If 4D-CBCT imaging is not available, registration of 3D-CBCT to AIP-CT is recommended by matching tumor centers. If EE-CT is used as the reference, tumor superior edges should be matched. Otherwise, either a larger PTV margin should be used or the tumor might be mistargeted.

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