Development of an Independent Monitor Unit Verification Program for Photon Beams in 3-Dimensional Conformal Treatment Planning System

Résumé

This study focuses on the development of an independent monitor unit verification program for photon beams in 3-dimensional conformal radiotherapy as patient specific QA for isocentric treatment. An in-house MS-Excel spreadsheet program was developed to validate monitor units generated by the treatment planning system, with sole intention of ensuring an accurate dose calculation. A total of 3339 treatment fields generated from over 667 patient data for different treatment sites treated with either 6MV or 15MV or both energies were analyzed. The treatment plans were generated with Oncentra Masterplan V4.3 running on collapsed cone algorithm with heterogeneity correction. The planning system's generated monitor units for each field was based on prescribed dose per fractionation. The independent monitor unit verification program however was built with appropriate beam data obtained from ion chamber measurements. The planning system's dose at isocentre were juxtaposed with, and tested for accuracy with that calculated by the independent verification program. Variations between the two dose sets as well as dose at Dmax for a given monitor unit were analyzed. For homogenous medium, there was a good agreement between the treatment planning system's dose at isocentre and the Excel-developed program; with variations in dose at isocentre ranging between -0.15% and -1.73% (against a ±7% set-tolerance). Disparities in Dmax dose ranged between -0.34% and -1.4%. However, in analyzing heterogeneous patient media, disparity in dose at isocentre and Dmax were estimated as -2.09% and -2.12% respectively.

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