Monte Carlo and analytic modeling of an Elekta Infinity linac with Agility MLC: Investigating the significance of accurate model parameters for small radiation fields

Abstract

Purpose: To explain the deviation observed between measured and Monaco calculated dose profiles for a small field (i.e., alternating open-closed MLC pattern). A Monte Carlo (MC) model of an Elekta Infinity linac with Agility MLC was created and validated against measurements. In addition, an analytic model which predicts the fluence at the isocenter plane was used to study the impact of multiple beam parameters on the accuracy of dose calculations for small fields.

Methods: A detailed MC model of a 6 MV Elekta Infinity linac with Agility MLC was created in EGSnrc/BEAMnrc and validated against measurements. An analytic model using primary and secondary virtual photon sources was created and benchmarked against the MC simulations and the impact of multiple beam parameters on the accuracy of the model for a small field was investigated. Both models were used to explain discrepancies observed between measured/EGSnrc simulated and Monaco calculated dose profiles for alternating open-closed MLC leaves.

Results: MC-simulated dose profiles (PDDs, cross- and in-line profiles, etc.) were found to be in very good agreements with measurements. The best fit for the leaf bank rotation was found to be 9 mrad to model the defocusing of Agility MLC. Moreover, a very good agreement was observed between results from the analytic model and MC simulations for a small field. Modifying the radial size of the incident electron beam in the BEAMnrc model improved the agreement between Monaco and EGSnrc calculated dose profiles by approximately 16% and 30% in the position of maxima and minima, respectively.

Conclusion: Accurate modeling of the full-width-half-maximum (FWHM) of the primary photon source as well as the MLC leaf design (leaf bank rotation, etc.) is essential for accurate calculations of dose delivered by small radiation fields when using virtual source or MC models of the beam.

Keywords: Agility; Elekta; Monaco; Monte Carlo; analytic model; virtual source.

Figure 1

BEAMnrc preview of the Elekta Infinity linac model with Agility MLC showing the various component modules.

Figure 2

BEV of the fields constructed to evaluate the LBROT value. The fields consist of a small field size (1‐open leaf) to verify leaf bank rotation and one larger field size (5‐open leaves) for dosimetric verification.

Figure 3

(a) Primary and (b) secondary virtual photon sources used in the analytic model. The primary and secondary photon sources are placed at Z = 1.1 cm and Z = 15.9 cm from the reference plane (Z = 0 cm), respectively.

Figure 4

Ray diagram illustrating the photon fluence calculation process of the analytic model. The fluence at each point along the in‐line position on the isocenter plane is the integral of the source. The source boundaries are shown by the photon rays tracing from the isocenter to the source plane.

Figure 5

(a) Comparison of the measured and calculated PDD curves for a 5 × 5 cm² field at SSD = 100 cm normalized at 10 cm depth, (b) Percent dose differences for calculated point doses against measurements. Error bars represent statistical uncertainty from MC simulations (0.1%).

Figure 6

Comparison of the measured and calculated (a) cross‐line and (b) in‐line profiles for various field sizes at 5 cm depth and 100 cm SSD.

Figure 7

Comparison of the measured and calculated relative output factors for various field sizes at 5 cm depth and 100 cm SSD.

Figure 8

Comparison of dose profiles between EBT3 film measurements (solid) and MC simulations (dotted) for the field shown in Fig. 2 for LBROT values of (a) 0, (b) 6, (c) 9, and (d) 12 mrad. The best fit parameter was found to be LBROT = 9 mrad.

Figure 9

Variation of leaf transmission by increasing the interleaf air gap. All transmission values are normalized to the transmission corresponding to the nominal interleaf air gap.

Figure 10

Photon fluence at the isocenter plane from MC simulations and analytic model calculations for LBROT = 9 mrad. Fluence curves are normalized to their integral so that the integral of the resultant curve is equal to 1.

Figure 11

Impact of changes in (a) leaf bank rotation (normalized to nominal value of leaf bank rotation or LBROT = 9 mrad), (b) leaf attenuation (normalized to full MLC leaf attenuation), and (c) primary source size (normalized to nominal source size of 1 mm) on the relative fluence integral, maximum fluence, and average DTA of the fluence in analytic model.

Figure 12

Comparison of Monaco calculations against (a) EBT3 film measurements and (b) EGSnrc simulations for the field shown in Fig. 2.

Figure 13

Comparison between Monaco calculations and EGSnrc simulations with modified parameters: FWHM = 2 mm in the in‐line position and inclusion of tongue and groove. Dose differences were reduced to approximately 1% at the maxima and 2% at the minima.