THE TRANSIT DOSE COMPONENT OF HIGH-DOSE-RATE BRACHYTHERAPY - DIRECT MEASUREMENTS AND CLINICAL IMPLICATIONS

Purpose: To measure the transit dose produced by a moving high dose ra te brachytherapy source and assess its clinical significance. Methods and Materials: The doses produced from source movement during Ir-192 H DR afterloading were measured using calibrated thermoluminescent dosim eter rods. Transit doses at distances of 0.5-4.0 cm from an endobronch ial applicator were measured using a Lucite phantom accommodating 1 x 1 x 6 mm thermoluminescent rods. Surface transit dose measurements wer e made using esophageal and endobronchial catheters, a gynecologic tan dem, and an interstitial needle. Results: No difference was detected i n 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 dos e at 0.5 cm from an endobronchial catheter was 0.31 cGy/(Curie-fractio n) and followed an inverse square fall-off with increasing distance. S urface transit doses ranged from 0.38 cGy/(Curie-fraction) for an esop hageal catheter to 1.03 cGy/(Curie-fraction) for an endobronchial cath eter. Source velocity is dependent on the interdwell distance and vari es between 220-452 mm/sec. A numeric algorithm was developed to calcul ate total transit dose, and was based on a dynamic point approximation for the moving high dose rate source. This algorithm reliably predict ed the empirical transit doses and demonstrated that total transit dos e is dependent on source velocity, number of fractions, and source act ivity. 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 . Conclusion: Current high dose rate brachytherapy treatment planning systems calculate dose only from source dwell positions and assume a n egligible 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 coul d increase potential late tissue complications, as predicted by the li near quadratic model. To enhance the clinical safety and accuracy of h igh 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.