Monte Carlo calculations and measurements of absorbed dose per monitor unit for the treatment of uveal melanoma with proton therapy

Abstract

The treatment of uveal melanoma with proton radiotherapy has provided excellent clinical outcomes. However, contemporary treatment planning systems use simplistic dose algorithms that limit the accuracy of relative dose distributions. Further, absolute predictions of absorbed dose per monitor unit are not yet available in these systems. The purpose of this study was to determine if Monte Carlo methods could predict dose per monitor unit (D/MU) value at the center of a proton spread-out Bragg peak (SOBP) to within 1% on measured values for a variety of treatment fields relevant to ocular proton therapy. The MCNPX Monte Carlo transport code, in combination with realistic models for the ocular beam delivery apparatus and a water phantom, was used to calculate dose distributions and D/MU values, which were verified by the measurements. Measured proton beam data included central-axis depth dose profiles, relative cross-field profiles and absolute D/MU measurements under several combinations of beam penetration ranges and range-modulation widths. The Monte Carlo method predicted D/MU values that agreed with measurement to within 1% and dose profiles that agreed with measurement to within 3% of peak dose or within 0.5 mm distance-to-agreement. Lastly, a demonstration of the clinical utility of this technique included calculations of dose distributions and D/MU values in a realistic model of the human eye. It is possible to predict D/MU values accurately for clinical relevant range-modulated proton beams for ocular therapy using the Monte Carlo method. It is thus feasible to use the Monte Carlo method as a routine absolute dose algorithm for ocular proton therapy.

Figure 1

Schematic drawing of the Monte Carlo model geometry representing the ocular nozzle used in this study. A 159 MeV proton beam enters the nozzle from the left. In the case of a range-modulated beam, the proton beam will start within the slab of polymethyl methacrylate located at (A) to simulate range modulation as described in the text. Next in the modeled nozzle is a thick slab of polycarbonate resin thermoplastic (B) that acts to adjust the beam range and laterally scatter the beam, producing a therapeutically useful proton field with a maximum energy of approximately 70 MeV. The monitor chambers (C) monitor the output of the nozzle as the beam exits through the final collimating aperture (D) and stops in the water phantom (E).

Figure 2

Cross-sectional view in the xz-plane of the eye model used in Monte Carlo simulations. All dimensions of the eye model were customizable to patient-specific anatomy. Selected anatomic features of the eye, which may also be transformed to arbitrary gaze angles to represent the eye's treatment position, are labeled. Since no tumor was the target of the proton beams considered in this work, none appears here.

Figure 3

The relative dose (D) as a function of depth (d) in water from measurements with an ionization chamber (open circles) and the Monte Carlo simulation (solid line). The figure shows the Bragg curve for the most penetrating beam available for the ocular nozzle.

Figure 4

The relative dose (D) as a function of depth in water (d) for the modulated beam, with a nominal range of 3 cm and a modulation width of 1.5 cm, as measured with an ionization chamber (open circles) and predicted by the Monte Carlo simulation (solid line). The two curves have been normalized to their interpolated values at the depth of 2.3 cm, which was the calibration depth for measurement and simulation.

Figure 5

Cross-field profile from measurements on film (open circles) and Monte Carlo simulations (solid line). Error bars on the film data points indicate the standard deviation from three repeat measurements of the same beam. For clarity of presentation, the measured points between –1 < x < 1 are plotted at half-resolution.

Figure 6

Absolute depth–dose profile in water from ionization chamber measurement (open circles) and Monte Carlo simulation (solid line). This profile shows the most penetrating beam available from the ocular nozzle, with a depth at the distal 90% dose level of approximately 4 cm in water.

Figure 7

Absolute depth–dose profile in water from ionization chamber measurement (open circles) and Monte Carlo simulation (solid line). This profile shows an SOBP with a depth to the distal 90% dose level equal to 2.9 cm and modulation width between the proximal and distal 90% dose levels equal to 1.4 cm.

Figure 8

Dose distribution of an SOBP incident upon a model of the human eye from Monte Carlo simulation in the treatment machine's coordinate system. Absolute values of the isodose contours are indicated. The two SOBPs are shown (a) a half-modulated beam that has a range of 20 mm and a modulation width of 10 mm and (b) a fully-modulated beam that has a range of 30 mm, i.e., the SOBP encompasses the entire penetration range of the beam.