Validation of a GPU-Based 3D dose calculator for modulated beams

Abstract

A superposition/convolution GPU-accelerated dose computation algorithm (the Calculator) has been recently incorporated into commercial software. The algorithm requires validation prior to clinical use. Three photon energies were examined: conventional 6 MV and 15 MV, and 10 MV flattening filter free (10 MVFFF). For a set of IMRT and VMAT plans based on four of the five AAPM Practice Guideline 5a downloadable datasets, ion chamber (IC) measurements were performed on the water-equivalent phantoms. The average difference between the Calculator and IC was -0.3 ± 0.8% (1SD). The same plans were projected on a phantom containing a biplanar diode array. We used the forthcoming criteria for routine gamma analysis, 3% dose-error (global (G) normalization, 2 mm distance to agreement, and 10% low dose cutoff). The γ (3%G/2 mm) average passing rate was 98.9 ± 2.1%. Measurement-guided three-dimensional dose reconstruction on the patient CT dataset (excluding the Lung) resulted in a similar average agreement rate with the Calculator: 98.2 ± 2.0%. The mean γ (3%G/2 mm) passing rate comparing the Calculator to the TPS (again excluding the Lung) was 99.0 ± 1.0%. Because of the significant inhomogeneity, the Lung case was investigated separately. The calculator has an alternate heterogeneity correction mode that can change the results in the thorax for higher-energy beams (15 MV). As this correction is nonphysical and was optimized for simple slab geometries, its application leads to mixed results when compared to the TPS and independent Monte Carlo calculations, depending on the CT dataset and the plan. The Calculator vs. TPS 15 MV Guideline 5a IMRT and VMAT plans demonstrate 96.3% and 93.4% γ (3%G/2 mm) passing rates respectively. For the lower energies, which should be predominantly used in the thoracic region, the passing rates for the same plans and criteria range from 98.6 to 100%. Overall, the Calculator accuracy is sufficient for the intended use.

Keywords: GPU-accelerated calculations; convolution/superposition; dose calculations; segmented beams.

Figure 1

Gamma analysis (2%L/2 mm) error maps (inserts) between Pinnacle and SNC calculator, and normalized cross‐plane dose profiles at 20 cm depth for 10MVFFF (a) and 15 MV (b) 20 × 20 cm² fields. Ion chamber (IC) profiles are also included for comparison. Red and blue pixels are where the Calculator dose is above and below Pinnacle respectively.

Figure 2

Central axis PDDs on the lung slab phantom for the 2 × 2 cm² field: 15MV (a), 10 MVFFF (b), and 6MV (c).

Figure 3

Frequency distribution of the γ‐analysis passing rates comparing the Calculator to the Delta4 for all Guideline 5a test cases and energies. N = 24.

Figure 4

Frequency distribution of the γ‐analysis passing rates comparing the Calculator to ACPDP measurement‐guided dose reconstruction on the patient datasets for Guideline 5a test cases excluding the Lung (all energies). N = 18.

Figure 5

Graphical representation of the γ(2%G/2 mm) comparison for the 6X Anal IMRT plan between the Calculator and ACPDP (left column) and Pinnacle vs. ACPDP (right column). The highlighted pixels where the gamma analysis fails are overlaid on the dose map. The targets are also shown. The transverse cuts (a), (b) are taken superiorly to demonstrate the areas of failure between the irradiated nodal chains. The coronal cuts (c), (d) are 2 cm posterior to the midline, where both the primary and secondary targets are prominently present.

Figure 6

Frequency distribution of the γ‐analysis passing rates comparing the Calculator to Pinnacle on the patient datasets for Guideline 5a test cases, excluding the Lung (all energies). N = 18.