Analytic IMRT dose calculations utilizing Monte Carlo to predict MLC fluence modulation

Abstract

A hybrid dose-computation method is designed which accurately accounts for multileaf collimator (MLC)-induced intensity modulation in intensity modulated radiation therapy (IMRT) dose calculations. The method employs Monte Carlo (MC) modeling to determine the fluence modulation caused by the delivery of dynamic or multisegmental (step-and-shoot) MLC fields, and a conventional dose-computation algorithm to estimate the delivered dose to a phantom or a patient. Thus, it determines the IMRT fluence prediction accuracy achievable by analytic methods in the limit that the analytic method includes all details of the MLC leaf transport and scatter. The hybrid method is validated and benchmarked by comparison with in-phantom film dose measurements, as well as dose calculations from two in-house, and two commercial treatment planning system analytic fluence estimation methods. All computation methods utilize the same dose algorithm to calculate dose to a phantom, varying only in the estimation of the MLC modulation of the incident photon energy fluence. Gamma analysis, with respect to measured two-dimensional (2D) dose planes, is used to benchmark each algorithm's performance. The analyzed fields include static and dynamic test patterns, as well as fields from ten DMLC IMRT treatment plans (79 fields) and five SMLC treatment plans (29 fields). The test fields (fully closed MLC, picket fence, sliding windows of different size, and leaf-tip profiles) cover the extremes of MLC usage during IMRT, while the patient fields represent realistic clinical conditions. Of the methods tested, the hybrid method most accurately reproduces measurements. For the hybrid method, 79 of 79 DMLC field calculations have gamma < 1 (3%/3 mm) for more than 95% of the points (per field) while for SMLC fields, 27 of 29 pass the same criteria. The analytic energy fluence estimation methods show inferior pass rates, with 76 of 79 DMLC and 24 of 29 SMLC fields having more than 95% of the test points with gamma < or = 1 (3%/3 mm). Paired one-way ANOVA tests of the gamma analysis results found that the hybrid method better predicts measurements in terms of both the fraction of points with gamma < or = 1 and the average gamma for both 2%/2 mm and 3%/3 mm criteria. These results quantify the enhancement in accuracy in IMRT dose calculations when MC is used to model the MLC field modulation.

Fig. 1

Flow diagram of the hybrid method used to determine the IMRT fluence modulation for a given MLC leaf sequence. See text for a detailed description of the process flow.

Fig. 2

Measured and calculated profiles for 6 MV beam for the test case when the leaf tips are positioned on the central axis.

Fig. 3

Measured and calculated profiles for selected picket-fence fields. The source-to-axis distance (SAD) of 100 cm is the same for both energies, while the depth for the 6 MV beam is 5 and 10 cm for the 18 MV beam.

Fig. 4

The measured and calculated profiles for a field completely blocked by the multi-leaf collimator (MLC). The source-to-axis distances (SADs) are the same as in Fig. 3. The depths for both energies are the same as the depths in Fig. 3.

Fig. 5

Measured and calculated leaf leakage for closed squared fields of different sizes. The leakage was averaged over the central 3×3 cm² and normalized to the open-field dose, averaged in the same way.

Fig. 6

The number of dynamic multileaf collimator (DMLC) fields with an observed $\stackrel{‒}{\gamma }$ for Analytic 1, Analytic 2, and the hybrid methods. Data were binned into 0.025 intervals for plotting purposes. The top panel is for gamma acceptance criteria of 2% /2 mm, while the bottom panel is for acceptance criteria of 3% /3 mm. With an increase in the rigor of the MLC modeling by the analytic methods, the difference between the analytic and Monte Carlo modeling of the MLC decreases.

Fig. 7

The number of segmented multileaf collimator (SMLC) fields with an observed $\stackrel{‒}{\gamma }$ for PINNACLE³ v6.2b, PINNACLE³ v7.6c, and the hybrid methods. Data were binned into 0.067 intervals for plotting purposes. PINNACLE³ v7.6c performs better than PINNACLE³ v6.2b, however, they hybrid method best reproduces the film measurements.

Fig. 8

The number of dynamic multileaf collimator (DMLC) fields with γ ≤1 for the same fields as in Fig. 6. Data were binned into 1.4% intervals for plotting purposes. As in Fig. 6 with an increase in the sophistication of the analytic MLC modeling methods, a greater fraction of the points (for each field) have γ ≤1.

Fig. 9

The number of segmented multileaf collimator (SMLC) fields with γ ≤1 for the same fields as in Fig. 7. Data were binned into 2% intervals for plotting purposes. PINNACLE³ v7.6c computed doses have more points per image with γ ≤1, than PINNACLE³ 6.2b computed doses. However, the hybrid method demonstrates closer agreement with measurements than either PINNACLE³ implementation.