Establishing action levels for EPID-based QA for IMRT

Abstract

Although portal dosimetry is used to provide quality assurance (QA) for intensity-modulated radiation therapy (IMRT) treatment plans, trends in agreement between the portal dose prediction (PDP) and the measured dose have not been clarified. In this work, we evaluated three scalar parameters of agreement for 152 treatment plans (1152 treatment fields): maximum gamma (gamma max), average gamma (gamma avg), and percentage of the field area with a gamma value greater than 1.0 (gamma % > 1). These data were then used to set clinical action levels based on the institutional mean and standard deviations. We found that agreement between measured dose and PDP was improved by recalculating the fields at lower dose rates. We conclude that action levels are a useful tool for standardizing the evaluation of EPID-based IMRT QA.

Figure 1. Scatter plot of γmax, γavg, and γ%>1 for each field evaluated in this study. The institutional mean values for each parameter are shown on the right side of the plot with 1 SD and 2 SDs. Of the 1152 fields studied, 34 (3%) had one or more values greater than 3 SDs from the mean.

Figure 2. Portal dosimetry gamma evaluation showing good agreement between the measured dose and calculated PDP. (a) Overlay of measured image response and PDP. (b) Gamma map. Most of the field has gamma values less than or equal to 1.0 (green). The small area in the lower right quadrant of the field with gamma values greater than 1.0 (orange) corresponds to a high dose‐gradient region (shown in 2a).

Figure 3. Portal dosimetry for a split field. As in areas that have gamma values greater than 1.0 are shown in orange. Most areas with a gamma value greater than 1.0 are located in regions where the dose was not modified by the MLC and also not shielded by the dimensions of the upper and lower jaws. Such regions would normally be expected to have low dose gradients, but in these regions, small differences in dose (due to, for example, MLC leakage) could cause large gamma values, as most likely occurred here.

Figure 4. Portal dosimetry gamma evaluation for a split field with high γmax values. (a) Portal image with superimposed isodose lines from PDP. (b) Area with gamma values greater than 4.0 (orange). This shaded area corresponds to a high dose‐gradient region.

Figure 5. IMRT QA report template. This document is attached to the electronic treatment record for each patient after IMRT QA is completed. The document is also printed and signed by the medical physicist who completed the QA and is added to the patient's paper chart.