Verification of absolute dose rates for intravascular brachytherapy beta sources

The absolute dose rate around beta sources for intravascular brachytherapy was measured with thin thermoluminescent (TL) dosimeters and radiochromic film. The calibration of these dosimeters was performed at the dose maximum in a 6 MeV electron beam of a linear accelerator. Compared to the values given by the manufacturers, higher dose rates of about 20% for Sr-90/Y-90 source trains and 20 to 40% for Y-90 line sources were found. We also measured the dose rate for a planar Sr-90/Y-90 source, which was calibrated at NIST (National Institute of Standards and Technology). This source serves as a reference for the determination of the dose rates by the supplier of the Y-90 sources. For this source we measured a 13% higher dose rate compared to the value given in the calibration report by NIST. Our observation can have important consequences for the interpretation and comparison of the dose-related clinical results of intravascular brachytherapy with these and other beta sources. Also the comparison of the clinical results obtained with beta sources and gamma sources, i.e. Ir-192, depends largely on the accuracy of the absolute dosimetry of the beta sources.