Objective: Stereotactic body radiation therapy (SBRT) delivers ablative doses in one or a few fractions to extracranial sites of disease. The Halcyon (Varian Medical Systems, Palo Alto, CA) is a modern linear accelerator (linac) that has been designed to provide faster treatment delivery. The Halcyon linac is mainly designed for intensity-modulated radiation therapy/volumetric modulated arc therapy (IMRT/VMAT) delivery due to unique features like single 6-MV flattening-filter-free beam, dose rate of 800 MU/min, fast-moving multileaf collimators, and automated daily image-guided radiotherapy (IGRT) workflow. We sought to report our initial clinical experience delivering SBRT on the Halcyon linac.

Methods: A retrospective review was conducted on patients who received SBRT on Halcyon between September 2018 to June 2019. Estimated delivery time was calculated from total monitor units (MUs) by using Halcyon-specific maximal dose rate of 800 MU/min. Mean treatment delivery time for IMRT and VMAT were compared with the Mann-Whitney U test.

Results: A total of 51 patients were identified, with a median age of 71 years (interquartile range, [IQR]: 62-77). Fifty-seven consecutive sites were treated on the Halcyon unit. The most common location treated was lung (77.2%) followed by bone (10.6%), mediastinum (5.3%), abdomen (3.5%), pelvis (1.8%), and neck (1.8%). The median treatment volume was 19.0 cc (IQR: 11.8-41.4). The most common total dose (Gy)/fraction number was 48/4 (n=20, 35.1%), followed by 54/3 (n=12, 21.1%). IMRT and VMAT were utilized for 36 and 21 deliveries, respectively. The median estimated treatment delivery time was 3.95 minutes (IQR: 3.28-5.13). The mean treatment delivery time was 9.4 minutes, when accounting for automated IGRT acquisition, verification process, and setup. The mean treatment delivery time for IMRT and VMAT was 9.91 minutes and 8.51 minutes, respectively (p<0.01).

Conclusion: SBRT delivery is feasible on the Halcyon unit. Treatment delivery time on the Halcyon compares favorably to treatment times reported in prior SBRT series using older-generation linacs. To our knowledge, this is the largest reported series of patients treated with SBRT utilizing the Halcyon linac.