Author: Guang-Pei Chen, Renae Conlin, Juan A. Garcia-Alvarez, Kirk Morris, Haidy G. Nasief, Eenas A. Omari, Eric S. Paulson 👨🔬
Affiliation: Department of Radiation Oncology, Medical College of Wisconsin 🌍
Purpose: Cumulative dose limits make replacing and calibrating Metal Oxide Semiconductor Field Effect dosimeters (MOSFET-d) used for in-vivo dosimetry a common clinical task. Calibration protocols for Radixact machines are scarce and challenging to develop due to their non-flat beam. We present a workflow that streamlines MOSFET-d calibration for Radixact machines.
Methods: The protocol was developed for a five-MOSFET-d array using the vendor-provided calibration jig (CJ) embedded in solid water blocks. A CT scan was acquired and imported into the Precision Treatment Planning System (TPS), and a helical Intensity Modulated Radiotherapy (h-IMRT) plan was generated to deliver a uniform dose at the CJ MOSFET-d locations. This plan allows the convolution of the MOSFET-d angular dependence into the calibration factors (CFs). The delivery of the plan was verified with an ion chamber and 3D diode array. Post-calibration verification was conducted by inserting water-equivalent 3D-printed pegs containing the MOSFET-d into the Radixact cheese phantom and measuring the dose from the rotational output procedure. CFs from the helical protocol were compared with those obtained using static delivery protocols (SDP, gantry at 0o, MOSFET-d at dmax) on Radixact and a C-arm linac flat 6 MV beam.
Results: MOSFET-d CFs from the h-IMRT plan significantly differed (t-test, p<0.01) from Radixact-SDP and C-arm-SDP CFs, with means (±SD) 1.114±0.010, 1.087±0.008, and 1.071±0.004 mV/cGy, respectively. The MOSFET-d angular dependence impacted the CFs more than variations in the energy spectrum (6 MV, flat vs. non-flat). During the verification procedure, the average deviations from the verified TPS doses were -0.14±1.32 (helical), 2.40±1.32 (Radixact-SDP), and 3.82±0.68% (C-arm-SDP).
Conclusion: The calibration protocol implemented for Radixact in vivo dosimetry with MOSFET-d enables the simultaneous calibration of all detectors, addressing the challenges posed by the Radixact transverse profile, and integrates the angular dependence in the CFs, reducing systematic measurement errors that SDP would otherwise introduce.