LAUSR

BACKGROUND Methods for accurate absolute dose calibration are essential for the proper functioning of radiotherapy treatment machines. Many systems do not conform to TG-51 calibration standards, and modifications are required. TG-21 calibration is also a viable methodology for these situations with the appropriate setup, equipment, and factors. It has been shown that both these methods result in minimal errors. A similar approach has been taken in calibrating the dose for a recent vault-free radiosurgery system. PURPOSE To evaluate modified TG-21 and TG-51 protocols for absolute dose (AD) calibrations of the ZAP-X radiosurgery system using ion chambers, film, and thermoluminescent dosimeters (TLDs). METHODS The current treatment planning system for ZAP-X requires absolute dose calibration at dmax (7 mm) and 450 mm SAD. Both N D , w 60 C o [ G y / C ] $N_{D,w}^{{60}Co}[ {Gy/C} ]$ and Nx [R/C] calibration coefficients were provided by an accredited dosimetry calibration laboratory for a PTW 31010 chamber (0.125cc). The vendor provides an f-bracket that can be mounted on the collimator. Various phantoms can then be attached to the f-bracket. A custom acrylic phantom was designed based on recommendations from TG-21 and TRS-398 that places the chamber at 500mm from the source with a depth of 44 mm acrylic and 456 mm SSD. Nx along with other TG-21 parameters were used to calculate the absolute dose. Measurements using a PTW MP3-XS water tank and the same chamber were used to calculate absolute dose using N D , w 60 C o $N_{D,w}^{{60}Co}$ and TG-51 factors. Dose verification was performed using Gafchromic film and 3rd party TLDs. RESULTS Measurements from TG-51, TG-21 (utilizing the custom acrylic phantom), film, and TLDs agreed to within ±2%. CONCLUSIONS A modified TG-51 absolute dose calculation in water is preferred, but may not be practical due to the difficulty in tank setup. The TG-21 modified protocol using a custom acrylic phantom is an accurate alternative option for dose calibration. Both of these methods are within acceptable agreement and provide confidence in the system's absolute dose calibration. This article is protected by copyright. All rights reserved.