Validation of a digital method for patient-specific verification of VMAT treatment using a 2D ionisation detector array

VMAT treatment involves the delivery of complex beam parameters including the dynamic multi-leaf collimators (MLC) from a linac. The use of a detector array has become a standard method to verify the treatment accuracy before the treatment planned is delivered to the patient with some limitations. This study aims to evaluate the accuracy of a high-resolution, digital method using linac log data for VMAT treatment verification. The log data was obtained from a Versa HD (Elekta ltd., Crawley, UK) linac during VMAT treatment. All the dynamic treatment parameters including MLC position, and MLC speed, were recorded at the rate of 4 Hz in the log data. The treatment delivery was also measured simultaneously using an Octavius 1500 2D ionisation chamber array (PTW, Freiburg, Germany) during the log data measurements. The parameters were extracted and analysed using algorithms written in MATLAB (MathWorks, Natick, MA). The results demonstrated that the log data can track MLC leaves with an accuracy of 1 mm at speeds ranging from 3.04 to 3.40 cm/s. Evaluation of the fluence generated for the VMAT delivery using log data was shown to agree well with the planned dose distribution measured in the 2D detector array, with an average gamma pass rate of 97.5% at 3%/3 mm. Log data obtained a higher gamma pass rate of 98.5% compared to the 2D detector array of 97.5%. The digital linac log data provides the basis for an essential high-resolution real-time verification tool that may be used for routine VMAT verification.