In vivo dosimetry with alanine/electron spin resonance dosimetry to evaluate the urethra dose during high-dose-rate brachytherapy.

Purpose: To determine the dose of the urethra during Ir-192 high-dose-rate brachytherapy in 15 patients, a new detector form developed for in vivo measurements was used as described by Anton et al. 2009.

Methods And Materials: The detector consists of alanine powder with paraffin as binder which was filled in a shrinkable tube for measurements. The alanine strand so produced was inserted in the foley catheter during high-dose-rate brachytherapy of the prostate. The measured dose was compared with the dose calculated by the treatment planning system SWIFT for 15 patients. After 8 patients, a marker was introduced for better positioning of the alanine strand using transrectal ultrasound images.

Results: The measurements of the dose of the urethra agreed very well with the dose calculated by SWIFT within the estimated standard uncertainty of the method for 8 patients with a mean absolute deviation of 0.08 Gy. However, for the other 7 patients, a mean absolute deviation between delivered and measured dose of -5.13 Gy was seen. For these patients, the active volume of the alanine dosimeter could not properly be reconstructed on the ultrasound images.

Conclusion: The method presented in this study is useful for quality control of irradiations in vivo. To reconstruct the active volume on the corresponding ultrasound images, the correct application of the alanine strand in the urethra is very important. This procedure needs a well-trained physician.