Dosimetric consideration of individual 125I source strength measurement and a large-scale comparison of that measured with a nominal value in permanent prostate implant brachytherapy

Purpose We investigated the difference between measured and manufacturer's nominal source strength in a large sample of a single model of ¹²⁵I seeds. Physical characteristics of single seed measurement by the well-type ionization chamber were also investigated to provide dosimetric data. Materials and methods A well-type ionization chamber with a single seed holder was used to measure source strength of all 1935 ¹²⁵I seeds implanted in the initial 28 patients in our hospital. Physical characteristics including linearity of readings for different integral time intervals, reproducibility, isotropy, and axial positional sensitivity were assessed. To calculate the source strength, the integral charge during 30 s was measured and converted to air kerma strength. The nominal activity stated by the manufacturer was compared with the measured value. Results Linearity, reproducibility, and isotropy of the well-type ionization chamber were within 0.2%. Measured source strength was on average 2.1% (range −7.6% to +7.2%), lower than the nominal value. Standard deviation of all measured seeds was 2.0%. The maximum difference between the measured and the manufacturer's nominal source strength in each patient was −3.7%. The standard deviation averaged 1.6%. Conclusion The nominal source strength of the ¹²⁵I seeds agreed well with the measured value. Our study can be helpful as guidance for individual ¹²⁵I seed source strength measurement.