Technical note: Absolute dose measurements of a vault-free radiosurgery system

Background: Methods for accurate absolute dose (AD) 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 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 AD calibration at d_max (7 mm) and 450 mm source-to-axis distance. Both $N_{D,w}^{60Co}\left[Gy/C\right]$ and N_x [R/C] calibration coefficients were provided by an accredited dosimetry calibration laboratory for a physikalisch technische werkstatten (PTW) 31010 chamber (0.125 cc). 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 technical report series-398 that places the chamber at 500 mm from the source with a depth of 44-mm acrylic and 456-mm SSD. N_x along with other TG-21 parameters was used to calculate the AD. Measurements using a PTW MP3-XS water tank and the same chamber were used to calculate AD using $N_{D,w}^{60Co}$ 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 AD 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 AD calibration.