Carbon ion therapy for ocular melanoma: planning orthogonal two-port treatment

We recently started orthogonal two-port carbon ion therapy for choroidal melanoma with the intent to reduce the incidence of radiation complications that occur with mono-port therapy. Treatment planning techniques involving therapeutic beam characteristics are described here. The vertical (140 MeV/u) and horizontal (170 MeV/u) carbon ion beams from the synchrotron at the NIRS were shaped, using the passive beam delivery system, to irradiate the target volume. The range modulating ridge filters were designed to produce spread-out Bragg peaks (SOBPs) with a region of uniform HMV-I cell killing. The apertures and range compensators were designed for individual patients. A commercial treatment planning system, which was customized to our general carbon ion therapy, was tested for applicability to this treatment. Dose distributions were calculated with either a broad beam or a pencil beam algorithm using parameters determined by measurements and calculations. We evaluated the accuracy of the system software features, and replaced or added some other features to the software. The system was used for 12 patients during the past year. For nine patients two-port treatment was assessed to be more effective than mono-port therapy and these patients were treated with two fractions of vertical beams and three fractions of horizontal beams.