Quality assurance of VMAT on flattened and flattening filter‐free accelerators using a high spatial resolution detector

Abstract

Purpose: This study investigated the use of high spatial resolution solid-state detectors (DUO and Octa) combined with an inclinometer for machine-based quality assurance (QA) of Volumetric Modulated Arc Therapy (VMAT) with flattened and flattening filter-free beams. Method: The proposed system was inserted in the accessory tray of the gantry head of a Varian 21iX Clinac and a Truebeam linear accelerator. Mutual dependence of the dose rate (DR) and gantry speed (GS) was assessed using the standard Varian customer acceptance plan (CAP). The multi-leaf collimator (MLC) leaf speed was evaluated under static gantry conditions in directions parallel and orthogonal to gravity as well as under dynamic gantry conditions. Measurements were compared to machine log files. Results: DR and GS as a function of gantry angle were reconstructed using the DUO/inclinometer and in agreement to within 1% with the machine log files in the sectors of constant DR and GS. The MLC leaf speeds agreed with the nominal speeds and those extracted from the machine log files to within 0.03 cm s −1. The effect of gravity on the leaf motion was only observed when the leaves traveled faster than the nominal maximum velocity stated by the vendor. Under dynamic gantry conditions, MLC leaf speeds ranging between 0.33 and 1.42 cm s −1 were evaluated. Comparing the average MLC leaf speeds with the machine log files found differences between 0.9% and 5.7%, with the largest discrepancy occurring under conditions of fastest leaf velocity, lowest DR and lowest detector signal. Conclusions: The investigation on the use of solid-state detectors in combination with an inclinometer has demonstrated the capability to provide efficient and independent verification of DR, GS, and MLC leaf speed during dynamic VMAT delivery. Good agreement with machine log files suggests the detector/inclinometer system is a useful tool for machine-specific VMAT QA.

Evaluation of silicon strip detectors in transmission mode for online beam monitoring in microbeam radiation therapy at the Australian Synchrotron

“…The IAEA states that QA must encompass all 'necessary procedures that ensure consistency of the medical prescription, and safe fulfilment of that prescription, as regards the dose to the target volume, together with minimal dose to normal tissue, minimal exposure of personnel and adequate patient monitoring aimed at determining the end result of the treatment' (IAEA, 1998). In mega-voltage (MV) applications on a CLINAC, this task of QA has inspired the development of new technologies and methods to extend the capability of medical physicists to monitor not only the intensity, but also the shape and size, which are of particular relevance in some of the more advanced techniques available in conventional radiotherapy, such as intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT), with notable examples by do Amaral et al (2015) and by Matar et al (2020). Both approaches utilize the LINAC accessory tray to support their chosen technology and each method is capable of performing personalized QA for patients, including the identification of possible sources of error during treatment.…”

“…Both approaches utilize the LINAC accessory tray to support their chosen technology and each method is capable of performing personalized QA for patients, including the identification of possible sources of error during treatment. The 2D array of silicon diodes described by Matar et al (2020) is representative of the parallel occurring between conventional radiotherapy and MRT, requiring small but important adjustments to be suitable to the context of MRT.…”

“…Like its conventional radiotherapy equivalent, outlined by Matar et al (2020) for MV applications, a detector operating in transmission mode for MRT should incorporate an array structure with multiple-channel readout to allow for simultaneous measurement of both peak and valley doses. To be suitable for MRT, the detector should incorporate a 1D or 2D array of single-strip detectors with an appropriate pitch to account for the fractionated nature of the field, i.e.…”