Purpose: The purpose of this work is to characterize the ZAP-X (ZAP Surgical Systems, Inc., San Carlos, CA) radiosurgical device beam quality using EGSnrc Monte Carlo k(Q) calculations of several ionization chambers. The TG-51 and TRS-398 clinical dosimetry protocols require k(Q) factors for calibrating external radiotherapy beams.
Methods: ZAP-X produces a 3 MV photon beam with varying narrow conical beam collimation settings. For the k(Q) calculations, the beam was collimated to a diameter of 25 mm at 45 cm SSD (source to surface distance). This is a small field set-up compared to reference calibration conditions of 10 x 10 cm² at 100 cm SSD, that were used for the reference Co-60 beam. No machine-specific reference field has yet been defined for the ZAP-X linac, so as a first step only an overall k(Q) factor was calculated, with the chamber measurement point set at 10 cm depth in a 30 x 30 x 30 cm³ tank of water. For the Co-60 beam, the chamber depth was 5 cm as usual. Three ionization chambers were considered: the PTW 31010, Exradin A28 and Exradin A1SL. The Monte Carlo models for these chambers were previously produced from detailed manufacturer specifications and validated.
Results: The overall k(Q) factors for the PTW 31010, Exradin A28 and Exradin A1SL under the 3 MV ZAP-X beam were calculated. Machine-specific reference k(Q) factors were then investigated based on additional simulations for various set-up conditions.
Conclusion: Beam quality of the ZAP-X radiosurgical device was investigated using EGSnrc Monte Carlo simulations. Despite the small field size and low beam energy, overall k(Q) factors seemed relatively insensitive to varying ionization chamber models, and machine-specific reference conditions are proposed for the new ZAP-X linac.