Accuracy of out-of-field dose calculations by a commercial treatment planning system

Abstract

The dosimetric accuracy of treatment planning systems (TPSs) decreases for locations outside the treatment field borders. However, the true accuracy of specific TPSs for locations beyond the treatment field borders is not well documented. Our objective was to quantify the accuracy of out-of-field dose predicted by the commercially available Eclipse version 8.6 TPS (Varian Medical Systems, Palo Alto, CA) for a clinical treatment delivered on a Varian Clinac 2100. We calculated (in the TPS) and determined (with thermoluminescent dosimeters) doses at a total of 238 points of measurement (with distance from the field edge ranging from 3.75 to 11.25 cm). Our comparisons determined that the Eclipse TPS underestimated out-of-field doses by an average of 40% over the range of distances examined. As the distance from the treatment field increased, the TPS underestimated the dose with increasing magnitude--up to 55% at 11.25 cm from the treatment field border. These data confirm that accuracy beyond the treatment border is inadequate, and out-of-field data from TPSs should be used only with a clear understanding of this limitation. Studies that require accurate out-of-field dose should use other dose reconstruction methods, such as direct measurements or Monte Carlo calculations.

Figure 1

Phantom used for TLD measurements. The inferior edge of the treatment field abutted the superior border of slice 20 of the phantom. TLD capsules were loaded in slices 21 through slice 24 of the phantom, corresponding to distances of 3.75 cm to 11.25 cm from the field edge.

Figure 2

Out-of-field dose reported by the treatment planning system. Dose scale is set to show 5% to 0.1% of the prescription dose. Dose color wash distributions are shown for (a) sagittal view and axial plane at the center of phantom slices: (b) slice 21, (c) slice 22, (d) slice 23 and (e) slice 24. Note that all dose values above 5% are at the center of the phantom and are shown in dark pink.

Figure 3

Axial image for slice 23 of the phantom corresponding to the center of the phantom, with the data point grid shown by red squares. Within the TPS, point doses were determined at each location on the grid using the point dose measurement tool. Each point within the point dose grid corresponds to a single TLD capsule lying at the equivalent position in the phantom.

Figure 4

Plots of mean measured and calculated doses. Mean dose (μ ± σ) was calculated for all TLDs in each phantom slice (corresponding to specific distances from the field edge). Also shown are mean measured and calculated doses along the mid-plane (depth = 9.5 cm) for each phantom slice. The uncertainty bars on each data point represent one standard deviation of the mean. This value is dominated by the spread of doses across each phantom slice as compared to an additional standard uncertainty in each TLD measurement of ≤3%.

Figure 5

Mean measured and calculated absorbed doses at various depths in the phantom at a constant distance of 11.25 cm from the edge of the treatment field (i.e. in phantom slice 24). The uncertainty bars on each data point represent one standard deviation of the mean. This value is dominated by the spread of doses across each phantom slice as compared to an additional standard uncertainty in each TLD measurement of ≤3%.