Point dose calculations using an analytical pencil beam kernel for IMRT plan checking.

A method to verify the monitor units for a treatment plan is to calculate point doses, possibly at the isocentre, by using a simple calculation method. This verification is recommended to find mistakes in the treatment plan. Treatment plans for intensity modulated radiation therapy are no exception. The method should employ a simple physical model and a dose calculation algorithm, which is different from the method used for the treatment plan. Our approach uses a convolution algorithm and an analytical pencil beam kernel with eight parameters. The model is intuitive and simple. At the same time, the method is so general that it can be applied to both step-and-shoot and sliding-window techniques. The results of applications to actual treatment plans show that the calculated total isocentre doses are accurate within +/-2% of planned doses for six-field prostate plans when calculation points are in a uniform dose region. Head and neck cases show a slightly larger difference than prostate cases. When calculation points are located in a region of high dose gradient, however, the difference could be greater than 5%.