Room: Exhibit Hall | Forum 5
Purpose: To investigate the use of electromagnetic guidance (Calypso) for motion management during gated radiotherapy of tumors in lung.
Methods: Patients received gated radiation treatment while freely breathing, guided by electromagnetic tracking of three anchored transponders bronchoscopically implanted in airways near the lung tumor under an IRB-approved protocol. Constancy of transponder positions relative to tumor was verified with respiratory-corrected cone-beam CT (CBCT). Treatment was gated at end-exhalation with the beam-on gating window of Â± 2-4 mm from planned position in S-I, A-P and L-R directions. Couch shifts were made during treatment when persistent deviation of the target position outside the window was observed. Calypso and machine log files were analyzed for transponder motion during treatment and reconstruction of motion-included delivered dose in gated and retrospectively simulated non-gated treatment.
Results: Retrospective motion analysis was performed on seven patients treated with SBRT. Target motion was reduced mainly in the S-I direction from an average 3.7 mm non-gated to 1.7 mm gated. Calypso-guided couch shifts up to 5 mm were applied remotely in 22 out of 26 fractions with up to 5 shifts per fraction. Without couch shifts, significant baseline (respiration-averaged) target drift occurred from CBCT setup to treatment delivery, with maximum values of 9.1 mm (S-I), 14.0 mm (L-R), and 4.1 mm (A-P). Compared to the planned values, the reconstructed dose in two patients showed 0.5% and <0.1% reduction in CTV D95% for gated delivery which were 5.7% and 0.7% respectively in the simulated non-gated delivery. V100% of CTV was improved with gating by 6.0% and 7.3%, respectively, compared to non-gated treatment.
Conclusion: Real-time 3D internal motion monitoring and rapid intra-treatment couch adjustment were achieved using Calypso transponders to guide gated radiotherapy in lungs. Motion-induced geometric uncertainty was significantly reduced which improved the target coverage compared to non-gated treatment.
Funding Support, Disclosures, and Conflict of Interest: This work was funded by Varian Medical Systems (Palo Alto, CA) and Memorial Sloan Kettering Cancer Center Support Grant (P30 CA008748). Dr. Andreas Rimner receives research support from Varian Medical Systems.