title: 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 were developed, which were advantageous particularly in dealing with large radius leaf end. Computer simulation was 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 was employed for deriving penumbra width and radiation field offset.
  
  
  RESULTS
  Results showed that penumbra width increased with source size. Penumbra width curves for large radius leaf end were U-shaped. This observation was probably related to the fact that radiation beams penetrated through the proximal and distal leaf sides. In contrast, source size had negligible impact on radiation field offset. Radiation field offsets were found to be constant both for analytical method and numerical simulation. However, the overall resulting values of radiation field offset obtained by analytical method were slightly smaller compared with Monte Carlo simulation.
  
  
  CONCLUSIONS
  The method we proposed could provide insight into the investigation of rounded leaf end effects on penumbra characteristics. Penumbra width and radiation field offset calibration should be carefully performed to commission multileaf collimator for intensity modulated radiotherapy.
authors.affiliation: Department of Mechanical Engineering, Tsinghua University, Beijing, China.
authors.forename: Dong
authors.initials: D
authors.lastname: Zhou
authors.affiliation: Department of Mechanical Engineering, Tsinghua University, Beijing, China.
authors.forename: Hui
authors.initials: H
authors.lastname: Zhang
authors.affiliation: Department of Mechanical Engineering, Tsinghua University, Beijing, China.
authors.forename: Peiqing
authors.initials: P
authors.lastname: Ye
keywords: Monte Carlo simulation; multileaf collimator; penumbra width; radiation field offset; rounded leaf end effect