QUALITY ASSURANCE CALIBRATION OF I-125 RAPID STRAND IN A STERILE ENVIRONMENT

Purpose: I-125 seeds encased in stiffened absorbable suture material, commercially available from Amersham Healthcare as I-125 Rapid Strand, are not readily calibrated because of the necessity of maintaining th e sterility and integrity of the Rapid Strand. A method is proposed to verify the activity of I-125 seeds in Rapid Strand in a sterile envir onment and to provide quality assurance of the resultant loading by au toradiographing loaded needles. Methods and Materials: A sterilizable insert for a Standard Imaging HDR 1000 Plus well chamber was designed to accommodate Amersham's plastic spacing jig that holds the strand. T he insert has a cylindrical lead sleeve allowing five of the 10 seeds in the jig to be exposed within the well chamber. A grooved tray for h olding a batch of 30 needles was designed for autoradiographing the im plant set. Results: Position-dependent calibration factors for each of the seed wells in the jig were determined; then these individual fact ors were combined into a single chamber factor of 21.6 pA/mCi, Startin g from the most distal spacing jig slot, relative position factors wer e 0.99, 1.00, 0.99, 0.93, 0.73, and 0.13 for the six positions which p roduced a nonnegligible signal, Anisotropy in the chamber factor was d etermined by rotating the well in 30 degrees increments about the seed s in the jig. The chamber factor showed only a 0.2% variation with rot ational angle. Attenuation due to the Vicryl suture cladding material was 0.2% and was measured by stripping the Vicryl from the strand and remeasuring the chamber factor with the seeds in their original orient ation. In the operating room, charge was collected from sterile Rapid Strands for a fixed time between 30 and 50 s and the measured ion cham ber current was compared with the value predicted from Amersham's nomi nal activity, The average deviation between nominal and measured activ ity of 73 Rapid Strands tested was +0.5% +/- 2.2%. For single measurem ents, the maximum and minimum deviations observed were +4.8% and -3.7% , respectively, Autoradiographs taken of the entire implant set on an aluminum tray milled to hold the needles confirmed the actual seed loa dings. Conclusions: The Rapid Strand calibration procedure described m aintains the sterility and integrity of I-125 Rapid Strands and verifi es that the manufacturer's stated activity is accurate to within 5%. ( C) 1998 Elsevier Science Inc.