The transit dose component of high dose rate brachytherapy: direct measurements and clinical implications

To measure the transit dose produced by a moving high dose rate brachytherapy source and assess its clinical significance.

The doses produced from source movement during Ir-192 HDR afterloading were measured using calibrated thermoluminescent dosimeter rods. Transit doses at distances of 0.5-4.0 cm from an endobronchial applicator were measured using a Lucite phantom accommodating 1 x 1 x 6 mm thermoluminescent rods. Surface transit dose measurements were made using esophageal and endobronchial catheters, a gynecologic tandem, and an interstitial needle.

No difference was detected in thermoluminescent dosimeter rod responses to 4 MV and Ir-192 spectra (427 nC/Gy) in a range of dose between 2 and 300 cGy. The transit dose at 0.5 cm from an endobronchial catheter was 0.31 cGy/(Curie-fraction) and followed an inverse square fall-off with increasing distance. Surface transit doses ranged from 0.38 cGy/(Curie-fraction) for an esophageal catheter to 1.03 cGy/(Curie-fraction) for an endobronchial catheter. Source velocity is dependent on the interdwell distance and varies between 220-452 mm/sec. A numeric algorithm was developed to calculate total transit dose, and was based on a dynamic point approximation for the moving high dose rate source. This algorithm reliably predicted the empirical transit doses and demonstrated that total transit dose is dependent on source velocity, number of fractions, and source activity. Surface transit doses are dependent on applicator diameter and wall material and thickness. Total transit doses within or outside the desired treatment volume are typically < 100 cGy, but may exceed 200 cGy when using a large number of fractions with a high activity source.

Current high dose rate brachytherapy treatment planning systems calculate dose only from source dwell positions and assume a negligible transit dose. Under certain clinical circumstances, however, the transit dose can exceed 200 cGy to tissues within and outside the prescribed treatment volume. These additional, unrecognized doses could increase potential late tissue complications, as predicted by the linear quadratic model. To enhance the clinical safety and accuracy of high dose rate brachytherapy, total transit dose should be included in calculated isodose distributions. Significant transit doses to tissues outside the treatment volume should be documented.