Assaying multiple (125)I seeds with the well-ionization chamber SourceCheck(4π) 33005 and a new insert

PURPOSE: To provide a practical solution that can be adopted in clinical routine to fulfill the AAPM-ESTRO recommendations regarding quality assurance of seeds used in prostate permanent brachytherapy. The aim is to design a new insert for the well-ionization chamber SourceCheck(4π) 33005 (PTW, Germany) that allows evaluating the mean air-kerma strength of up to ten (125)I seeds with one single measurement instead of measuring each seed individually. MATERIAL AND METHODS: The material required is: a) the SourceCheck(4π) 33005 well-ionization chamber provided with a PTW insert to measure the air-kerma strength S (K) of one single seed at a time; b) a newly designed insert that accommodates ten seeds in one column, which allows measuring the mean S (K) of the ten seeds in one single measurement; and c) a container with ten seeds from the same batch and class of the seeds used for the patient implant, and a set of nine non-radioactive seeds. The new insert is characterized by determining its calibration coefficient, used to convert the reading of the well-chamber when ten seeds are measured to their mean S (K). The proposed method is validated by comparing the mean S (K) of the ten seeds obtained from the new insert with the individual measurement of S (K) of each seed, evaluated with the PTW insert. RESULTS: The ratio between the calibration coefficient of the new insert and the calibration coefficient of the PTW insert for the SourceCheck(4π) 33005 is 1.135 ± 0.007 (k = 1). The mean S (K) of a set of ten seeds evaluated with this new system is in agreement with the mean value obtained from measuring independently the S (K) of each seed. CONCLUSIONS: The new insert and procedure allow evaluating the mean S (K) of ten seeds prior to the implant in a single measurement. The method is faster and more efficient from radiation protection point of view than measuring the individual S (K) of each seed.