Monte Carlo modeling of a 6 and 18 MV Varian Clinac medical accelerator for in-field and out-of-field dose calculations: development and validation

Abstract

There is a serious and growing concern about the increased risk of radiation-induced second cancers and late tissue injuries associated with radiation treatment. To better understand and to more accurately quantify non-target organ doses due to scatter and leakage radiation from medical accelerators, a detailed Monte Carlo model of the medical linear accelerator is needed. This paper describes the development and validation of a detailed accelerator model of the Varian Clinac operating at 6 and 18 MV beam energies. Over 100 accelerator components have been defined and integrated using the Monte Carlo code MCNPX. A series of in-field and out-of-field dose validation studies were performed. In-field dose distributions calculated using the accelerator models were tuned to match measurement data that are considered the de facto 'gold standard' for the Varian Clinac accelerator provided by the manufacturer. Field sizes of 4 cm x 4 cm, 10 cm x 10 cm, 20 cm x 20 cm and 40 cm x 40 cm were considered. The local difference between calculated and measured dose on the percent depth dose curve was less than 2% for all locations. The local difference between calculated and measured dose on the dose profile curve was less than 2% in the plateau region and less than 2 mm in the penumbra region for all locations. Out-of-field dose profiles were calculated and compared to measurement data for both beam energies for field sizes of 4 cm x 4 cm, 10 cm x 10 cm and 20 cm x 20 cm. For all field sizes considered in this study, the average local difference between calculated and measured dose for the 6 and 18 MV beams was 14 and 16%, respectively. In addition, a method for determining neutron contamination in the 18 MV operating model was validated by comparing calculated in-air neutron fluence with reported calculations and measurements. The average difference between calculated and measured neutron fluence was 20%. As one of the most detailed accelerator models for both in-field and out-of-field dose calculations, the model will be combined with anatomically realistic computational patient phantoms into a computational framework to calculate non-target organ doses to patients from various radiation treatment plans.

Figure 1

Plot of the Varian Clinac accelerator model using the plotting feature in the MCNPX code. All secondary shielding components are shaded in gray. The MLC was modeled although not plotted here.

Figure 2

The PDD curves from the 6 MV beam for the 4 cm × 4 cm, 10 cm × 10 cm and 40 cm × 40 cm fields. The 10 cm × 10 cm field was scaled by 80% and the 40 cm × 40 cm field was scaled by 60% for clarity. Measurement data were produced by the machine’s manufacturer as the ‘gold standard’ for this particular accelerator model (Kry 2008). For each tally the relative error was less than 2%. The relative error is the standard deviation of the mean dose value divided by the mean.

Figure 3

6 MV lateral dose profiles for (a) 4 cm × 4 cm, (b) 10 cm × 10 cm and (c) 20 cm × 20 cm fields at depths of 5, 10 and 20 cm. All curves are normalized to dose at d_max. For each tally the relative error was less than 2%. Measurement data were produced by the machine’s manufacturer as the de facto ‘gold standard’ for this particular accelerator model (Kry 2008).

Figure 4

The PDD curves from the 18 MV beam for 4 cm × 4 cm, 10 cm × 10 cm and 40 cm × 40 cm fields. The 10 cm × 10 cm field was scaled by 80% and the 40 cm × 40 cm field was scaled by 60% for clarity. For each tally the relative error was less than 2%. Measurement data were produced by the machine’s manufacturer as the de facto ‘gold standard’ for this particular accelerator model (Kry 2008).

Figure 5

The 18 MV lateral dose profiles for (a) 10 cm × 10 cm and (b) 40 cm × 40 cm fields at depths of d_max, 5 and 10 cm. No plot is provided for the 40 cm × 40 cm field at a depth of d_max since no measurement data were available at the time of the study. All curves are normalized to dose at d_max. In order to improve clarity in (a), the calculated and measured data for depths of 5 and 10 cm are reduced by 20 and 40%, respectively. Measurement data were produced by the machine’s manufacturer as the de facto ‘gold standard’ for this particular accelerator model (Kry 2008).

Figure 6

Comparison of measured and calculated out-of-field dose profile data from the 6 MV beam for field sizes of 4 cm × 4 cm, 10 cm × 10 cm and 20 cm × 20 cm at a depth of 3.75 cm in an acrylic phantom. The measurement data shown in the figure were taken from Kry et al (2006).

Figure 7

Comparison of measured and calculated out-of-field absolute dose data from the 18 MV beam for field sizes of 4 cm × 4 cm, 10 cm × 10 cm and 25 cm × 25 cm at a depth of 3.75 cm in an acrylic phantom. The measurement data plotted in the figure were taken from Kry et al (2007).

Figure 8

Neutron fluence spectrum from direct neutrons at a distance of +21 cm from the central axis in the in-plane direction. The fluence is normalized per unit lethargy, u, and MU.