Manual calculation of treatment time for high dose rate brachytherapy with a flexible intraoperative template (FIT)

A method is presented for estimating the total treatment time for a brachytherapy radiation fraction with a planar flexible intraoperative template (FIT), using an 192Ir high dose rate afterloading device. The FIT can be rectangular or irregularly shaped. The manual calculation serves as an independent check of the treatment time calculated by the treatment planning system for applications with varying sizes, shapes and dose prescription depths. The parameters required for the calculation are the number of active dwell positions, the catheter spacing and dwell position spacing, the source strength, the applied dose and the depth of dose prescription. For a fixed depth of dose prescription (1.25 cm) and fixed dwell position and catheter spacing (0.5 and 1 cm respectively) the manual calculation accurately predicts (usually within 2%) the total treatment time as calculated by the treatment planning system. For varying catheter and dwell position spacings and dose prescription depths the accuracy is still within 7%. An action threshold of 5% allows detection of errors made in the number of active dwell positions (+/-9), catheter spacing (+/-1 mm) and dose prescription depth (+/-1 mm). Errors in dwell position spacing (0.25 cm or more) could also be accurately detected.