Optimal equations for describing the relationship between prostate volume, number of sources, and total activity in permanent prostate brachytherapy

Objectives: To determine whether there is an optimal type of mathematical equation for predicting seed and activity requirements for permanent prostate brachytherapy. Methods: Four institutions with extensive brachytherapy experience each submitted details of more than 40 implants. The data was used to generate power and linear equations to reflect the relationship between preimplant volume and the number of seeds implanted, and preimplant volume and the total implant activity. We compared the R and standard error of the generated equations to determine which type of equation better fit the data. Results: For the limited range of prostate volumes commonly implanted (20-60 mL), power and linear equations predict seed and activity requirements comparably well. Conclusions: Linear and power equations are equally suitable for generating institution-specific nomograms.