Rounded leaf end effect of multileaf collimator on penumbra width and radiation field offset: an analytical and numerical study

Abstract

Background. Penumbra characteristics play a significant role in dose delivery accuracy for radiation therapy. For treatment planning, penumbra width and radiation field offset strongly influence target dose conformity and organ at risk sparing.

Methods. In this study, we present an analytical and numerical approach for evaluation of the rounded leaf end effect on penumbra characteristics. Based on the rule of half-value layer, algorithms for leaf position calculation and radiation field offset correction are developed, which are advantageous particularly in dealing with large radius leaf end. Computer simulation is performed based on the Monte Carlo codes of EGSnrc/BEAMnrc, with groups of leaf end radii and source sizes. Data processing technique of curve fitting is employed for deriving penumbra width and radiation field offset.

Results. Results show that penumbra width increases with source size. Penumbra width curves for large radius leaf end are U-shaped. This observation is probably related to the fact that radiation beams penetrate through the proximal and distal leaf sides. In contrast, source size has negligible impact on radiation field offset. Radiation field offsets are found to be constant both for analytical method and numerical simulation. However, the overall resulting values of radiation field offset obtained by analytical method are slightly smaller compared with Monte Carlo simulation.

Conclusion. The method we proposed could provide insight into the investigation of rounded leaf end effect on penumbra characteristics. Penumbra width and radiation field offset calibration should be carefully performed to commission multileaf collimator for intensity modulated radiotherapy.