Error Analysis of IMRT Film QA and its clinical significances
Yijian Cao, Gary Gluckman, and Stephen Rush
Purpose: to analyze the dosimetric discrepancy using film QA techniques to verify the IMRT delivery and to find a practical method in evaluate errors from the IMRT QA process and its implications in clinic.
Principle and Method: IMRT delivers radiation completely determined by the computing software. It therefore is vitally important to verify that the dose or intensity profile delivered is indeed the one intended and planned. Among many 2D dose map measurement and verification techniques, the film dosimetry is still the widely used
method. However, disagreement always exits because combination of many factors such as film quality, processing conditions, and most important, the pattern of the variation of the dose profile. In our study, we focused on the discrepancies caused by the intensity profile of the beam, or modulation factor. The results of film measurements and calculations from treatment planning system for each treatment fields are compared and analyzed over large number of patients, while the differences are related to the modulation factors of the beam.
Results and Discussion: The percentage difference depended closely on modulation factor as expected. The uncertainties of the small field dosimetry in RTP system, and the limits of the dynamic range of the film, as well as the beam delivery system errors attribute most the disagreement in comparing the planer dose and dose profile of the film
dosimetry. The result provides a guideline to establish an acceptancelevel for these errors and access the clinical significance of the disagreements.