Journal of X-Ray Science and Technology 28 (2020) 71–82 DOI 10.3233/XST-190580 IOS Press 71 Assessment of an Elekta Versa HD linear accelerator for stereotactic radiosurgery with circular cone collimators Mutian Zhang a,∗ , Qiyong Fan b , Yu Lei c , Bishnu Thapa a and Gilbert Padula a a Radiation Therapy, Summa Health Cancer Institute, Akron, Ohio, USA b Department of Medical Physics, Memorial Sloan Kettering Cancer Center, New York, NY, USA c Department of Radiation Oncology, University of Nebraska Medical Center, Omaha, Nebraska, USA Received 8 August 2019 Revised 6 November 2019 Accepted 7 December 2019 Abstract. BACKGROUND: Versa HD linear accelerators (linacs) are used for stereotactic radiosurgery treatment. However, the mechanical accuracy of such systems remains a concern. OBJECTIVE: The purpose of this study was to evaluate the accuracy of an Elekta Versa HD linac. METHODS: We performed measurements with a ball bearing phantom to calculate the rotational isocenter radii of the linac’s gantry, collimator, and table, and determine the relative locations of those isocenters. We evaluated the accuracy of the cone-beam computed tomography (CBCT) guidance with a film-embedding head phantom and circular cone-collimated radiation beams. We also performed dosimetric simulations to study the effects of the linac mechanical uncertainties on non-coplanar cone arc delivery. RESULTS: The mechanical uncertainty of the linac gantry rotation was 0.78 mm in radius, whereas that of the collimator and the table was <0.1 mm and 0.33 mm, respectively. The axes of rotation of the collimator and the table were coinciding with and 0.13mm away from the gantry isocenter, respectively. Experiments with test plans demonstrated the limited dosimetric consequences on the circular arc delivery given the aforementioned mechanical uncertainties. End-to-end measurements determined that the uncertainty of the CBCT guidance was ≤1 mm in each direction with respect to the reference CT image. CONCLUSIONS: In arc delivery, the mechanical uncertainties associated with the gantry and the table do not require remarkable increases in geometric margins. If large enough, the residual setup errors following CBCT guidance will dominate the overall dosimetric consequence. Therefore, the Versa HD linac is a valid system for stereotactic radiosurgery using non-coplanar arc delivery. Keywords: Linear accelerator, stereotactic radiosurgery, mechanical uncertainty, cone-beam CT guidance 1. Introduction Linear accelerator (linac) based stereotactic radiosurgery (SRS) is an increasingly popular treat- ment modality for brain metastases and other similar types of brain lesions [1–4]. In SRS procedures, a very high radiation dose is delivered to a small target in a single fraction treatment. This is typ- ically performed in a non-coplanar arc setting and guided by an integrated imaging system (e.g., cone-beam computed tomography or CBCT). Therefore, a sufficiently high level of accuracy in both mechanical performance and image guidance is required to perform SRS procedures. Numerous recent ∗ Corresponding author: Mutian Zhang, PhD, Radiation Therapy, Summa Health Cancer Institute, 161N Forge Street, Akron, Ohio, OH 44304, USA. Tel.: +1 330 375 7868; E-mail: mutian zhang@hotmail.com. 0895-3996/20/$35.00 © 2020 – IOS Press and the authors. All rights reserved