Quality assurance calibration of 125I rapid strand in a sterile environment

125I seeds encased in stiffened absorbable suture material, commercially available from Amersham Healthcare as 125I Rapid Strand, are not readily calibrated because of the necessity of maintaining the sterility and integrity of the Rapid Strand. A method is proposed to verify the activity of 125I seeds in Rapid Strand in a sterile environment and to provide quality assurance of the resultant loading by autoradiographing loaded needles.

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. The 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 holding a batch of 30 needles was designed for autoradiographing the implant set.

Position-dependent calibration factors for each of the seed wells in the jig were determined; then these individual factors were combined into a single chamber factor of 21.6 pA/mCi. Starting from the most distal spacing jig slot, relative position factors were 0.99, 1.00, 0.99, 0.93, 0.73, and 0.13 for the six positions which produced a nonnegligible signal. Anisotropy in the chamber factor was determined by rotating the well in 30 degrees increments about the seeds in the jig. The chamber factor showed only a 0.2% variation with rotational 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 orientation. In the operating room, charge was collected from sterile Rapid Strands for a fixed time between 30 and 50 s and the measured ion chamber current was compared with the value predicted from Amersham's nominal activity. The average deviation between nominal and measured activity of 73 Rapid Strands tested was +0.5% +/- 2.2%. For single measurements, 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 loadings.

The Rapid Strand calibration procedure described maintains the sterility and integrity of 125I Rapid Strands and verifies that the manufacturer's stated activity is accurate to within 5%.