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Mohammad Mahfujur Rahman, Evercare Hospital Dhaka, Bangladesh
We developed new LINAC machine quality assurance (QA), which is based on volumetric modulated arc therapy (VMAT). The new machine QA overcomes the shortcomings of current intensity modulated radiation therapy (IMRT) based LINAC machine QA. American Association of Physicists in Medicine (AAPM) task group (TG) 142 report is one of the most used LINAC machine QA guidelines. This report suggested periodic QA of multileaf collimator (MLC) of LINACs that are used for IMRT delivery, because IMRT modulates only MLC movement for beam modulation. However, VMAT modulates beam by simultaneous modulation of MLC movement, gantry movement and dose rate. Machine QA as recommended in other LINAC machine QA guidelines like International Commission on Radiation Units and Measurements (ICRU) report 83, European Society for Radiotherapy and Oncology (ESTRO) booklet 9, Institute of Physics and Engineering in Medicine (IPEM) report 81 (2nd edition) etc. are similar to that in AAPM TG 142 report. Also, in current clinical practice, plan QA for any VMAT treatment plan is performed once before the first treatment session. Such plan QA leaves a chance of missing VMAT delivery error in any VMAT treatment session of the full course of treatment. Though it is ideal to perform plan QA before every treatment day, this is unrealistic because of unbearable workload. Therefore, new machine QA is needed for LINACs such that it can evaluate the LINAC performance in delivering VMAT and it can supplement the requirement of daily plan QA. In this research, we investigated, how to evaluate the short- and medium-term performance of the medical LINAC in delivering real VMAT treatment plans and how daily treatment results are changed by performing QA over the entire treatment period in the case of actual VMAT treatment. Also, the effect of gantry rotation angle on the overall dose difference was evaluated from segmented delivery of full arcs of the VMAT. Based on result of these studies, we developed a treatment plan for QA of medical linear accelerators, dedicated to VMAT delivery. We named this as plan-class specific reference (pcsr) QA plan.
We divided the human body into five anatomical sites that define VMAT plan classes and we selected one real VMAT treatment plan per plan class to adopt VMAT plan in computational human phantom such that it can be used as representative VMAT treatment plan to surrogate the actual treatment plans of the plan class. These five pcsr QA plans was found effective for VMAT QA of the treatment unit. The QA, developed in this research is planned to be used for routine machine QA, while the current IMRT-related QA is performed only once a month as a monthly and the VMAT plan QA is performed only once before the treatment, and. Thus, pcsr QA would be useful to check the daily performance of the LINAC in implementing VMAT plans of different plan class. In this research, we introduced a LINAC machine QA for VMAT. This is the first attempt of this research and some quality management contents have not been done in depth. If in the near future, pcsr QA would be dealt in more detail, we think that the optimal machine quality assurance for VMAT can be achieved.