Monte Carlo based independent monitor unit calculation in IMRT

Intensity Modulated RadioTherapy (IMRT) treatment plan verification by comparison with measured data is time consuming. In this paper, a method for monitor unit (MU) calculation and plan comparison for step and shoot IMRT is described. This procedure is based on the Monte Carlo code EGSnrc / BEAMnrc running on a Linux cluster. The beamlets of an IMRT treatment plan are simulated individually by Monte Carlo and converted in absorbed dose to water per MU. A matrix equation representing the dose delivered by each beamlet in the patient is formed assuming that the dose is a linear combination of the MU of every beamlets. This equation is solved by a Non-Negative Least Square Fit optimization algorithm (NNLS). The dose distribution is obtained by multiplying the Monte Carlo absorbed dose to water per MU with the NNLS MU. Various treatment plan localizations obtained with a commercial treatment planning system (TPS) are compared with the proposed method for validation. This independent method can be used as an IMRT QA and further development could allow this technique to be used for other radiotherapy techniques like tomotherapy or volumetric modulated arc therapy.