Dosimetric control of ~(90)Sr-~(90)Y source trains for endovascular brachytherapy by radiochromic film

摘要

We examine the dosimetric control of ~(90)Sr-~(90)Y source trains used for the prevention of restenosis by endovascular irradiation. Beta sources have many advantages in this respect, but their dosimetric control is not easy, because of the very steep dose gradients. Radiochromic films are exposed to the 3 radiation in a phantom, at the reference distance of 2 mm. Exposure to ~(60)Co gamma-rays and 4.5 MeV electron linac beam are used for dose calibration. No significant difference is found between ~(60)Co and electron calibrations. A bad reproducibility (up to +-8%) is observed in dose measurements with the source train, attributed to fluctuations of the position of the individual sources in the catheter through which the sources are conveyed. This problem is solved by simultaneously exposing two films, on the two sides of a 4-mm thick phantom, with the catheter at the centre. After film digitisation and conversion to dose, the geometrical mean of the corresponding doses at 2 mm in the two images is calculated. A much better reproducibility is obtained (+-2%). A software has been written for the analysis and averaging of the images. The results are consistent with Monte-Carlo calculations for a source of the same activity. They disagree with the initial dose calibration of the source train, although traceable to NIST.