Monte Carlo investigation of dose distribution of uniformly and non-uniformly loaded standard and notched eye plaques

Abstract

To investigate the effect of using non-uniform loading and notched plaques on dose distribution for eye plaques. Using EGSnrc Monte Carlo (MC) simulations, we investigate eye plaque dose distributions in water and in an anatomically representative eye phantom. Simulations were performed in accordance with TG43 formalism and compared against full MC simulations which account for inter-seed and inhomogeneity effects. For standard plaque configurations, uniformly and non-uniformly loaded plaque dose distributions in water showed virtually no difference between each other. For standard plaque, the MC calculated dose distribution in planes parallel to the plaque is narrower than the TG43 calculation due to attenuation at the periphery of the plaque by the modulay. MC calculated the dose behind the plaque is fully attenuated. Similar results were found for the notched plaque, with asymmetric attenuation along the plane of the notch. Cumulative dose volume histograms showed significant reductions in the calculated MC doses for both tumor and eye structures, compared to TG43 calculations. The effect was most pronounced for the notch plaque where the MC dose to the optic nerve was greatly attenuated by the modulay surrounding the optic nerve compared to the TG43. Thus, a reduction of optic nerve D95% from 14 to 0.2 Gy was observed, when comparing the TG43 calculation to the MC result. The tumor D95% reduced from 89.2 to 79.95 Gy for TG43 and MC calculations, respectively. TG43 calculations overestimate the absolute dose and the lateral dose distribution of both standard and notched eye plaques, leading to the dose overestimation for the target and organs at risk. The dose matching along the central axis for the non-uniformly loaded plaques to that of uniformly loaded ones was found to be sufficient for providing comparable coverage and can be clinically used in eye-cancer-busy centers.

Keywords: Monte Carlo dosimetry; eye plaque brachytherapy; non-uniform loading; notched eye plaques.

FIGURE 1

The standard 16 mm COMS (a) and 20 mm notched (b) plaque designs. The inset to the picture shows the cross‐sectional view of the plaque. For the notched plaque in (b), the radius of the notch, r = 4.0 mm and offset a = 9 mm. (c) MC rendering of the eye model with the laterally located tumor and standard 16 mm COMS plaque; (d) The eye model with the tumor abutting the optic nerve and a 20 mm notched plaque. The source strength are given in Table 1. COMS, Collaborative Ocular Melanoma Study; MC, Monte Carlo.

FIGURE 2

Dose distributions in water for uniformly and non‐uniformly loaded 16 mm COMS eye plaques along x—(a), y—(b) and z—(c) z axis of the plaque (see Figure 1a) with the relative dose scaled to TG43 calculation. Cross‐sections are through the prescription point at (x,y,z) = (0,0,5.0) mm. The effect of plaque shielding is seen on (c). Note that the scale in (c) is restricted primarily to show the dose within the eye, and hence doesn't show the dose further along –z values. The source strengths are given in Table 1. COMS, Collaborative Ocular Melanoma Study.

FIGURE 3

Dose distributions in water for non‐uniformly loaded 20 mm eye plaques along x—(a), y—(b) and z—(c) z axis of the plaque (see Figure 1b). The effect of plaque shielding is seen on (c). Note that the scale in (c) is restricted primarily to show the dose within the eye, and hence doesn't show the dose further along –z values. The source strengths are given in Table 1. COMS, Collaborative Ocular Melanoma Study.

FIGURE 4

Dose distribution in the patient with (a) uniformly loaded standard 16 mm; (b) non‐uniformly loaded 16 mm; (c) uniformly loaded notched 20 mm; (d) non‐uniformly loaded notched 20 mm plaques in TG43 and MC calculations. The central slice of the eye was selected for visualization. The normalization point, lying 5 mm away from the plaque center (coinciding with tumor apex), corresponds to 85 Gy (100% iso dose line) in the TG43 simulation is shown with green circle. The slice containing the optic nerve was selected for better visualization of sparing effect of the notch. MC, Monte Carlo.

FIGURE 5

cDVH curves in the patient with (a) uniformly loaded standard 16 mm; (b) non‐uniformly loaded 16 mm; (c) uniformly loaded notched 20 mm; (d) non‐uniformly loaded notched 20 mm plaques. cDVH, cumulative dose‐volume histograms.