A simple method for converting point coordinates into along/away values for use in brachytherapy calculations.

Brachytherapy, the use of radioactive sources for the treatment of tumors, is an important tool in radiation oncology. Accurate calculations of the dose delivered to malignant and normal tissues are the responsibility of the medical physics staff. Independent calculations, to verify the results of a commercial treatment planning system, are an important part of a good quality-management system for brachytherapy, a Nuclear Regulatory Commission requirement. While the commercial treatment system has the capacity to perform calculations to thousands of points within the volume of the patient's body, it is sufficient for the purposes of a check to perform dose calculations for a few points. Using the procedure outlined in this presentation, coordinates, in three dimensions, of a linear source tip and end and an interest point can be converted into along and away values. These along and away values can then be used to retrieve dose rates from along/away tables in the literature. Calculation results from both a commercial treatment planning system and a locally prepared computer program using this method are compared.