W Harris*, E Yorke, H Li, C Czmielewski, M Chawla, R Lee, A Hotca-Cho, D Mcknight, A Rimner, D Lovelock, Memorial Sloan Kettering Cancer Center, New York, NY

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  W Harris

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TH-B-TRACK 6-5 (Thursday, 7/29/2021) 11:30 AM - 12:30 PM [Eastern Time (GMT-4)]

Purpose: To evaluate the efficacy of using implanted anchored electromagnetic transponders (EMTs) as surrogates for 1)tumor position and 2)repeatability of lung inflation during deep-inspiration breath-hold (DIBH) lung radiotherapy.

Methods: 40 patients were treated with either SBRT or conventional lung radiotherapy on an IRB approved prospective protocol using coached DIBH. Three anchored EMTs were bronchoscopically implanted into small airways near or within the tumor. DIBH treatment was gated with Calypso™ by tracking the EMT positions. Breath-hold cone-beam CTs (CBCTs) were acquired prior to every SBRT treatment or weekly for conventionally fractionated patients. Rigid registration between each CBCT and the patient’s breath-hold planning-CT was performed to match to 1)spine 2)EMTs and 3)tumor. Differences in registration between EMT-centroid and spine were analyzed to determine surrogacy of EMTs for lung inflation. Differences in registration between EMT-centroid and tumor were analyzed to determine surrogacy of EMTs for tumor position.

Results: 221 CBCTs were acquired and analyzed. The average differences between EMT-centroid and spine registration among all CBCTS were 0.05±0.62cm, -0.05±0.41cm, and 0.06±0.23cm in superior-inferior (SI), anterior-posterior (AP) and lateral directions, respectively. Only 58% of CBCTS had differences in registration <0.5cm for EMT-centroid compared to spine, indicating that lung inflation is not reproducible from simulation to treatment. The average differences between the EMT-centroid and tumor registration among all CBCTs were 0.06±0.18cm, 0.06±0.19cm, and 0.02±0.12cm in SI, AP and lateral directions, respectively. 95% of CBCTS resulted in <0.5cm change between EMT-centroid and tumor registration, indicating that EMT positions correspond well with tumor position during treatments.

Conclusion: The implanted anchored EMTs are good surrogates for tumor position and are reliable for monitoring and maintaining tumor position during DIBH treatment when tracking with Calypso™. Large differences in registration between EMT-centroid and spine for most treatments suggest that the level of lung inflation achieved at simulation is often not reproduced.

Funding Support, Disclosures, and Conflict of Interest: The authors gratefully acknowledge the financial support of Varian Medical Systems, Palo Alto, CA. The anchored electromagnetic transponders were supplied by Varian Medical Systems, Palo Alto, CA.

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