TH‐E‐BRE‐10: Independent Verification of Monitor Units for Volumetric Modulated Arc Therapy (VMAT)

Abstract

Purpose:A challenging problem in VMAT is how to independently validate the monitor units (MUs) of the treatment plan for a patient generated by the treatment planning system (TPS). In this work we establish a novel framework for independent MU verification of VMAT.Methods:The proposed MU verification technique is based on the relationship between the absorbed dose and the weighed sequence of station points. In VMAT, the dose at a point can be expressed as a sum of contributions from all the station points (or control points). To proceed, we first obtain the matrix elements which characterize the dosimetric contribution of the involved station points by computing the doses at a series of voxels, typically on the prescription surface of the VMAT treatment plan, with unit MU setting for all the station points. This dose matrix is calculated using an in‐house Monte Carlo (MC) software. The MUs of the station points are then derived by minimizing the least‐squares difference between doses computed by the TPS and that of the MC for the selected set of voxels. The technique is applied to sixteen clinical cases with a variety of energies, disease sites and TPS dose calculation algorithms.Results:For cases without tissue density inhomogeneity, the independently computed MUs agree with that of TPS to within 2.7% for all the station points. For lung cases, the MC‐calculated MUs differ significantly from that of the treatment plan computed using AAA. However, the discrepancies are reduced to within 3% when the TPS dose calculation algorithm is switched to a transport equation‐based technique (AcurosTM).Conclusion:An independent MU calculation method is developed, for the first time, for patient specific VMAT treatment plan QA.