Optimized point dose measurement: An effective tool for QA in intensity-modulated radiotherapy

In some cases of Intensity-modulated radiotherapy (IMRT) point dose measurement, there exists significant deviation between calculated and measured dose at isocenter, sometimes greater than ±3%. This may be because IMRT fields generate complex profiles at the reference point. The deviation arises due to lack of lateral electronic equilibrium for small fields, and other factors such as leakage and scatter contribution. Measurements were done using 0.125-cc ion chamber and Universal IMRT phantom (both from PTW-Freiburg). The aim is to find a suitable point of measurement for the chamber to avoid discrepancy between calculated and measured dose. Various beam profiles were generated in the plane of the chamber for each field by implementing patient plan on the IMRT phantom. The profiles show that for the fields which are showing deviation, the ion chamber lies in the steep-gradient region. To rectify the problem, the TPS (Treatment Planning System) calculated dose is found out at various points in the measurement plane of the chamber at isocenter. The necessary displacement to the chamber, as noted from the TPS, was given to obtain the optimum result. Twenty cases were studied for optimization, whose percentage deviation was more than ±3%. The results were well within tolerance criteria of ±3% after optimization. The mean percentage deviation value for the 20 cases studied, with standard deviation of 2.33 under 95% confidence interval, was found out to be 2.10% ± 1.14. Those cases that have significant variation even after optimization are further studied with film dosimetry.