The use of radiochromic film to measure dose distributions resulting from high dose rate ¹⁹²iridium single catheter treatments

Purpose: Radiochromic film was used to measure and compare the dose distributions parallel to a high dose rate (HDR) ¹⁹²Iridium (¹⁹²Ir) brachytherapy afterloading catheter that resulted from optimized treatment plans using various combinations of prescribed dose magnitude and location as well as source spacing. Methods and Materials: Differences exist among clinical investigators for specification of the magnitude and location of prescribed treatment dose for brachytherapy irradiations using HDR ¹⁹²Ir afterloading. Typical prescriptions for endobronchial irradiation include 5 to 10 Gy at 10 mm or 15 Gy at 6 mm measured from the center of the afterloading catheter. The dose distributions that result from these irradiations are very difficult to quantify by conventional dosimetry methods. This study used radiochromic film to measure the dose distributions resulting from optimized treatment plans for source dwell position separations of 2.5 or 5.0 mm and for a prescribed treatment dose of either 15 Gy at 6 mm or 5 Gy at 10 mm, conditions that have been used at M. D. Anderson Cancer Center for the treatment of endobronchial lesions. An acrylic phantom was designed to allow for measurement of the dose distributions at 0.95 mm (catheter surface), 6 mm, and 10 mm from and parallel to the catheter for sources positioned along either 20 or 80 mm of the catheter. Results: Radiochromic film is shown to be a suitable quality assurance and dosimetry modality for the measurement of the dose distribution along an afterloading catheter resulting from an HDR ¹⁹²Ir source. Each of the treatment plans was about equally effective in being able to produce a uniform dose distribution at their respective planned target distances. Differences were more apparent when comparing the dose distributions at nontargeted distances. On the catheter surface the dose was very nonuniform and in the case of 2.5 mm source spacing along 20 mm of catheter with target dose planned to 10 mm, the central minimum dose was only 13 to 24% of the dose opposite to the most proximal and distal sources. The absolute doses measured at equivalent distances for the 15 Gy planned to 6 mm treatments are about 1.3 to 1.5 times higher than those measured for the 5 Gy planned to 10 mm treatments. It was also observed that the lateral positioning of the encapsulated source within the afterloading catheter can contribute to dose differences about the catheter that are greatest for measurements made in contact with the catheter surface (24 to 40%) but may also be large at the treatment planning distances of 6 (0 to 15%) and 10 mm (0 to 9%). Conclusion: At their respective treatment planning distances of 6 or 10 mm, each of the treatment plans produced dose distributions of comparable uniformity. Against the catheter, relatively more uniform dose distributions with higher minimum doses were obtained for (a) dose prescription at 6 mm, rather than at 10 mm; (b) source separation of 2.5 mm, rather than 5.0 mm (except for a 20 mm active catheter length with dose planned to 10 mm); and (c) longer active length of the catheter of 80 mm, rather than 20 mm.