Modification and validation of an analytical source model for external beam radiotherapy Monte Carlo dose calculations

Abstract

Purpose: A dose calculation tool, which combines the accuracy of the dose planning method (DPM) Monte Carlo code and the versatility of a practical analytical multisource model, which was previously reported has been improved and validated for the Varian 6 and 10 MV linear accelerators (linacs). The calculation tool can be used to calculate doses in advanced clinical application studies. One shortcoming of current clinical trials that report dose from patient plans is the lack of a standardized dose calculation methodology. Because commercial treatment planning systems (TPSs) have their own dose calculation algorithms and the clinical trial participant who uses these systems is responsible for commissioning the beam model, variation exists in the reported calculated dose distributions. Today's modern linac is manufactured to tight specifications so that variability within a linac model is quite low. The expectation is that a single dose calculation tool for a specific linac model can be used to accurately recalculate dose from patient plans that have been submitted to the clinical trial community from any institution. The calculation tool would provide for a more meaningful outcome analysis.

Methods: The analytical source model was described by a primary point source, a secondary extra-focal source, and a contaminant electron source. Off-axis energy softening and fluence effects were also included. The additions of hyperbolic functions have been incorporated into the model to correct for the changes in output and in electron contamination with field size. A multileaf collimator (MLC) model is included to facilitate phantom and patient dose calculations. An offset to the MLC leaf positions was used to correct for the rudimentary assumed primary point source.

Results: Dose calculations of the depth dose and profiles for field sizes 4 × 4 to 40 × 40 cm agree with measurement within 2% of the maximum dose or 2 mm distance to agreement (DTA) for 95% of the data points tested. The model was capable of predicting the depth of the maximum dose within 1 mm. Anthropomorphic phantom benchmark testing of modulated and patterned MLCs treatment plans showed agreement to measurement within 3% in target regions using thermoluminescent dosimeters (TLD). Using radiochromic film normalized to TLD, a gamma criteria of 3% of maximum dose and 2 mm DTA was applied with a pass rate of least 85% in the high dose, high gradient, and low dose regions. Finally, recalculations of patient plans using DPM showed good agreement relative to a commercial TPS when comparing dose volume histograms and 2D dose distributions.

Conclusions: A unique analytical source model coupled to the dose planning method Monte Carlo dose calculation code has been modified and validated using basic beam data and anthropomorphic phantom measurement. While this tool can be applied in general use for a particular linac model, specifically it was developed to provide a singular methodology to independently assess treatment plan dose distributions from those clinical institutions participating in National Cancer Institute trials.

FIG. 1.

(a) The IROC Houston head and neck phantom, (b) The IROC Houston thorax phantom, (c) CT images of head and neck phantom with targets and critical structure contoured, and (d) CT images of thorax phantom with target and critical structures contoured.

FIG. 2.

(a) Energy spectrum comparison between spectrum calculated with the BEAM code and the spectrum computed from the analytical model for the 6 MV photon beam. (b) Energy spectrum comparison between spectrum calculated with the BEAM code and the spectrum computed from the analytical model for the 10 MV photon beam.

FIG. 3.

The 6-MV model: Output factor at d_max versus field size for the measured, calculated, and corrected output. A hyperbola curve was determined to correct the calculated values.

FIG. 4.

10 MV percent depth dose of 40 × 40 cm field size showing build-up region for the measured and calculated data sets. The plot includes the change in the electron contamination with field size (corrected) versus a constant contribution, regardless of field size (uncorrected).

FIG. 5.

Calculated and measured percent depth dose curves for 6 MV, 4 × 4 cm field and 10 MV, 40 × 40 cm field.

FIG. 6.

Calculated and measured dose profiles at 6 MV from a 4 × 4 cm field at depths of 1.5 cm (d_max), 6, 12.5, and 22 cm. The profiles are shown in order with depth where the profile at the depth of 2.4 cm is shown at the top.

FIG. 7.

Calculated and measured dose profiles at 10 MV from a 40 × 40 cm field at depths of 2.4 cm (d_max), 5, 10, and 20 cm. The profiles are shown in order with depth where the profile at the depth of 2.4 cm is shown at the top.

FIG. 8.

6 MV IMRT head and neck: Gamma maps (3%/2 mm) [(a) and (b)] and lateral dose profiles [(c) and (d)] from the same irradiation, but before [(a) and (c)] and after [(b) and (d)] changes to the model; implementation of a 0.4 mm MLC leaf bank offset to improve the penumbra. The primary and secondary targets are outlined in white while the critical structure is outlined in red. Note: The lower left corner the film has been clipped and is not part of the gamma analysis.

FIG. 9.

6 MV SBRT lung delivery: (a) gamma (b and c) profiles 10 MV IMRT lung delivery: (d) gamma (e and f) profiles.

FIG. 10.

DVH of the 10 MV IMRT lung patient plan for the PTV, carina, esophagus, and the lungs.

FIG. 11.

Gamma map (5%, 3 mm) of the 10 MV IMRT lung patient plan and associated axial CT image with DPM calculated dose distribution. The black arrow shows the dose line profile in the CT image for Fig. 12.

FIG. 12.

Dose (cGy) line profile comparing TPS (gold) and DPM (green) calculations. Red line denotes relative CT number. Location of profile is shown in the CT image of Fig. 10. (See color online version.)