Dosimetric verification of IMRT treatments using ionization chamber, radiographic film and gamma function

This study aims to evaluate the method adopted for patient specific intensity modulated radiotherapy (IMRT) quality assurance through the verification of planned and measured dose distributions and analyzing the correlation of the results with the acceptance criteria previously established. In the planning system, the patient's complete plan is transferred to the computed tomography images of a pelvis or head and neck (HN) phantom composed of slabs of polymethylmethacrylate with an ionization chamber and radiographic film in the coronal plane. The dose measured with the ionization chamber and the dose distribution obtained with the film are compared with the planning. The comparison between measured and planned dose distributions was performed using the gamma function (γ), assessing the percentage of points that meet the criteria of 3% dose difference and 3 mm distance to agreement. By the previous analysis of 111 plans an acceptance criteria was defined based on the average results and after that they were applied on 30 new cases. The acceptance criteria were: 1.8%±1.4% (prostate), 2.7%±1.9% (HN) and 2.1%±1.7% (others) for the mean absolute errors between the doses provided by the planning system and those measured with the ionization chamber. The criteria for the average percentage of agreement (γ≤1) between the dose distributions was 97.2%±2.8% (prostate), 95.2%±3.3% (HN) and 92.3%±7.4% (others). The new analyzed cases showed mean absolute errors of 1.1%±1.2% (prostate), 2.2%±1.5% (HN) and 1.6%±1.2% (others) and average percentage of agreement of 98.3%±1.0% (prostate), 95.2%±2.3% (HN) and 96.7%±2.9% (others). The acceptance criteria agreed with other authors and were respected by the planning verified posteriorly. We conclude that the established acceptance criteria are appropriate for the reality of our institution, and we are able to treat the patients safely. (author)