[Comparison of calculation precision of photon dose between the two algorithms for the situation with air cavity and...

In order to evaluate the calculation precision of the pencil beam (PB) algorithm and convolution-superposition(CS) algorithm for the situation with air cavity and small fields, we built a water phantom with an air cavity slab, in which the depth dose (DD) and off-axis ratio (OAR) for field 1cm x 1cm to field 7cm x 7cm were calculated by PB algorithms, CS algorithms and Monte Carlo (MC) simulation. The evaluation of algorithms by MC simulation was achieved by comparisons of DD with the spread penumbras of OAR curve self-defined as the width between isodose lines of 10% and 90%. It was shown that PB algorithm and CS algorithm both overestimated the DD but the degree overestimated by PB algorithm was more serious. The CS algorithm showed a better agreement with the MC simulation for the OARs, which spread to both laterals, while that was not predicted accurately by PB algorithm. It was indicated that PB algorithm and CS algorithm do not have high calculation precision whereas CS algorithms is relatively better for the situation with air cavity and small fields.