A patch source model for treatment planning of ruthenium ophthalmic applicators

Abstract

Beta-ray emitting Ru-106/Rh-106 ophthalmic applicators have been used for close to 4 decades in the treatment of choroidal melanoma. The form factor of these applicators is a spherically concave silver bowl with an inner radius of curvature between 12 and 14 mm, and a total shell thickness of 1 mm. The radioactive nuclide is deposited in a layer 0.1 mm below the concave surface of the applicator. Calculation of dose distributions for clinical treatment planning purposes is complicated by the concave nature of the distributed source, the asymmetric shape of the active region of some applicators, imperfections in the manufacturing process which can result in an inhomogeneous distribution of activity across the active surface, and absorption and scatter in the 0.1 mm layer of silver which seals and protects the radioactive layer. A semi-empirical method of calculating dose distributions for these applicators is described which is fundamentally compatible with treatment planning systems that use the AAPM TG43 brachytherapy formalism. Dose to water is estimated by summing a "patch source" dose function over a discrete number of overlapping patches uniformly distributed over the active surface of the applicator. The patch source dose function differs conceptually from a point source dose function in that it is intended to represent the macroscopic behavior of a small, disk-like region of the applicator. The patch source dose function includes an anisotropy term to account for angular variation in absorption and scatter as particles traverse the 0.1 mm silver window. It geometrically models the nearfield of a patch with properties akin to both a small disk and infinite plane, and models the farfield as if the patch were a point. This allows a manageable number of discrete patches (300 to 1000) to provide accuracy appropriate for clinical treatment planning. This approach has the advantages of using familiar concepts and data structures, it is computationally quick, and it readily adapts to asymmetric applicator shapes and inhomogeneities in the radionuclide distribution. A method for optimizing the patch source dose function parameters is presented, and the dosimetric calculations are compared with published Monte Carlo calculations and measurements.

Monte Carlo Estimation of Absorbed Dose Distributions Obtained from Heterogeneous 106Ru Eye Plaques

“…Later, a point-kernel numerical method was used to calculate the dose distribution around a curved radiation source with the shape of a spherical shell [9] . Other researchers combined the point-kernel and the Monte Carlo method [10,11] . Finally, some studies have used Monte Carlo algorithms to simulate the radiation transport produced by eye applicators to determine the dose distribution [11][12][13][14][15][16][17] .…”

“…Other researchers combined the point-kernel and the Monte Carlo method [10,11] . Finally, some studies have used Monte Carlo algorithms to simulate the radiation transport produced by eye applicators to determine the dose distribution [11][12][13][14][15][16][17] . Until very recently, the commonly used treatment planning system for 106 Ru plaques was based on simplified physical models of radiation transport where the emitter substance was as-sumed to be homogeneously distributed over the surface of the plaques and the anatomy of the eye was approximated to a homogeneous water sphere.…”

“…This is particularly true in the presence of small radiation fields, like the ones used for eye irradiation, where radiation transport cannot be accurately handled by the conventional nonstochastic algorithms usually employed in treatment planning systems. Despite the accuracy of the Monte Carlo method, albeit limited by the underlying cross section models and by the statistical uncertainty inherent in the method, calculations of dose distributions are still difficult [11,14] . Most published articles consider the radioactive substance to be homogeneously distributed, although actual eye plaques present inhomogeneous distributions of the emitter substance.…”

Monte Carlo Simulation of the Treatment of Uveal Melanoma Using Measured Heterogeneous ¹⁰⁶Ru Plaques

“…Although the use of 106 Ru applicators on uveal melanomas has been clinically studied, there is still a lack of precise knowledge on how the absorbed dose distribution in the tumor volume and the surrounding structures at risk determines the clinical outcome [1-5]. Obviously, misplacement of the plaque can cause underdosing of the tumor and the inclusion of structures at risk into the high-dose volume, resulting in less than optimal tumor control probability and enhanced side effects.…”

Monte Carlo Simulation of the Treatment of Eye Tumors with ¹⁰⁶Ru Plaques: A Study on Maximum Tumor Height and Eccentric Placement

“…However, most published works base their conclusions on retrospective clinical studies. There is a lack of knowledge about the spatial absorbed dose distribution in the structures at risk and in the tumor volume [15]. Quantitative knowledge about the spatial dose distribution in the tumor and in the structures at risk is necessary, although not sufficient, for determining the reasons for the success or failure of the therapy in a given patient, and it is also desirable for improving the technique of placement of the plaque.…”