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Electron-beam treatment planning for retinoblastoma was investigated and an optimal treatment plan was devised for a particular case using a new three-dimensional Monte Carlo-based treatment planning system known to be capable of correctly predicting dose perturbations caused by body surface obliquities and tissue heterogeneities. Computed tomography (CT) data files were used to construct a three-dimensional eye phantom representing the anatomy of a child's orbit. Dose distributions in sagittal, transverse, and coronal planes were predicted with 1-mm resolution. Study of these distributions led to an optimal treatment plan consisting of an anterior-lateral pair, with the anterior field being a 10-MeV, 30-mm-diam circular field, centrally blocked by a 10-mm-diam lucite lens shield and the lateral field being a 16-MeV, 30 x 25-mm D-shaped field. The anterior field delivers a therapeutic dose to the ora serrata, but it underdoses the posterior retinal surface behind the lens shield; the lateral field provides the necessary boost dose to the posterior retinal surface. An equally weighted combination of the two fields produces a dose distribution in which the entire retinal surface receives a therapeutic dose, with less than 10% of that dose being delivered to the lens, brain, and the contralateral orbit.

A Monte Carlo electron transport code for the desktop computer is applied to the problem of determining the radiation dose throughout an oblique-surface heterogeneous medium. Absorbed-dose distributions are obtained for 10 MeV electrons incident upon flat-surface and oblique-surface water phantoms, as well as flat-surface water phantoms containing an air cavity and containing a bone heterogeneity.

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An improved electron multiple-scattering distribution is presented in the form of a composite function which combines three expressions valid over different scattering angle regions: a modified relativistic Mott single-scattering term for large angle scattering, a modified Moliere Gaussian term for small angle scattering, and an exponential term for the intermediate angle scattering region. The exponential term has two adjustable parameters which make possible the smooth transition from the large to the small scattering angle regions. The proposed distribution exhibits better agreement with experiment than other multiple-scattering distributions commonly used in Monte Carlo electron transport codes, is amenable to direct sampling over a continuous range of electron energies and step sizes, can be used for elements of any atomic number, and is particularly suitable for use on small memory computers.