Evaluation of a commercial Monte Carlo dose calculation algorithm for electron treatment planning

Abstract

The RayStation treatment planning system implements a Monte Carlo (MC) algorithm for electron dose calculations. For a TrueBeam accelerator, beam modeling was performed for four electron energies (6, 9, 12, and 15 MeV), and the dose calculation accuracy was tested for a range of geometries. The suite of validation tests included those tests recommended by AAPM's Medical Physics Practice Guideline 5.a, but extended beyond these tests in order to validate the MC algorithm in more challenging geometries. For MPPG 5.a testing, calculation accuracy was evaluated for square cutouts of various sizes, two custom cutout shapes, oblique incidence, and heterogenous media (cork). In general, agreement between ion chamber measurements and RayStation dose calculations was excellent and well within suggested tolerance limits. However, this testing did reveal calculation errors for the output of small cutouts. Of the 312 output factors evaluated for square cutouts, 20 (6.4%) were outside of 3% and 5 (1.6%) were outside of 5%, with these larger errors generally being for the smallest cutout sizes within a given applicator. Adjustment of beam modeling parameters did not fix these calculation errors, nor does the planning software allow the user to input correction factors as a function of field size. Additional validation tests included several complex phantom geometries (triangular nose phantom, lung phantom, curved breast phantom, and cortical bone phantom), designed to test the ability of the algorithm to handle high density heterogeneities and irregular surface contours. In comparison to measurements with radiochromic film, RayStation showed good agreement, with an average of 89.3% pixels passing for gamma analysis (3%/3mm) across four phantom geometries. The MC algorithm was able to accurately handle the presence of irregular surface contours (curved cylindrical phantom and a triangular nose phantom), as well as heterogeneities (cork and cortical bone).

Keywords: MPPG 5.a validation; Monte Carlo; electron treatment planning; heterogeneous dose calculation.

Figure 1

CT images of the four complex validation phantoms with RayStation‐calculated dose for the (a) nose phantom, (b) bone phantom, (c) breast phantom, and (d) lung phantom

Figure 2

Percent error between RayStation‐calculated output factors and measured output factors for various square cutout sizes, applicator sizes, electron energies, and source to surface distances. Errors > 3% but < 5% are highlighted in yellow, and those exceeding 5% are highlighted in red.

Figure 3

Comparison between RayStation‐calculated dose (thick line) and film‐measured dose (thin line) for (a) 6 MeV electrons for the nose phantom, (b) 6 MeV electrons for the bone phantom, (c) 6 MeV electrons for the lung phantom, and (d) 9 MeV electrons for the breast phantom.

Figure 4

(a) Pinnacle‐calculated dose distribution based on a point‐based prescription for a breast boost treatment plan (the 5th case in Tables 3 and 4), (b) RayStation‐calculated dose based on a volume‐based prescription, and (c) the DVH for the lumpectomy PTV (shown in yellow) for Pinnacle (solid line) and RayStation (dotted line)