90Y SIR-Spheres Activity Measurement with the New SIROS D-Vial Delivery Kit

Introduction: Successful treatment delivery and accurate dosimetry in ⁹⁰Y-microsphere radioembolization of liver tumors requires accurate pre- and post-treatment ⁹⁰Y activity assays. Currently at our institution, SIR-Spheres doses in traditional V-Vials for patient administration are verified with dose calibrators: Capintec CRC-15R (calibration number, Cal. #, 63x10) or CRC-55tR (Cal. # 65x10). As additional verification, the exposure rate (with Fluke 451B ionization chamber) from the V-Vial in acrylic shield at 30 cm is converted to activity using a calibration factor of 0.0606 mR/hr/mCi. Sirtex has introduced a new SIR-Spheres delivery kit that includes a SIROS D-Vial and acrylic shield – both with different form factors and geometries from the traditional V-Vial setup. The purpose of this work is to establish the appropriate CRC-15R and CRC-55tR calibration numbers and exposure rate conversion factor to accurately assay ⁹⁰Y activity in the new SIROS D-Vial and acrylic shield form factor.

Methods: Five D-Vials were prepared according to manufacturer instructions with initial ⁹⁰Y-microspheres activities of 32.1, 34.1, 58.1, 78.9, and 171.3 mCi. Agitated D-Vials within their acrylic shields were each assayed in triplicate over several days in 2 separate CRC-55tR and 2 separate CRC-15R dose calibrators using Cal. #s ranging from 54-65x10 until activity levels reached approximately 10 mCi. Likewise, exposure rates of the agitated D-Vials within their acrylic shields were measured multiple times over this time using 2 separate Fluke 451B meters. Given the asymmetrical form factor of the D-Vial and acrylic shield, exposure rates at 30 cm were measured with the D-Vials tubing facing away, toward, and orthogonal the ion chamber. The best Cal. # for each dose calibrator model was identified as the value resulting in the lowest median relative error across all vials. The exposure rate calibration factor was calculated as the best-fit slope of a line (y-intercept = 0) in a plot of exposure rate (mR/hr) versus known activity (mCi).

Results: The lowest ⁹⁰Y activity measurement errors in agitated D-Vials were achieved on CRC-15R dose calibrators using Cal. # 60x10 (-0.2% median, -1.2% to 1.0% interquartile range IQR, -3.9% to 2.4% range) and on CRC-55tR dose calibrators using Cal. # 59x10 (0.3% median, -0.4% to 1.5% IQR, -1.9% to 4.1% range). The errors were relatively constant between activities 40-160 mCi but did increase by ~2% as activity decayed below 20 mCi. For comparison, using the V-Vial Cal. #s resulted in errors of -1.7% median (-2.9% to -1.5% IQR) with CRC-15R and errors of -3.8% median (-4.8% to -1.5% IQR) with CRC-55tR. In 4 of the 5 D-Vials, the measured exposure rates were relatively consistent regardless of the positioning of the D-Vial tubing relative to the ion chamber. In the 5^th D-Vial, some measurements at activities above 80 mCi differed by up to 50% with both Fluke 541b meters. Therefore, only the 4 D-Vial data were used to determine the exposure rate conversion factor, as we investigate the source of the discrepancy in the 5^th D-Vial (e.g., specifications for thickness consistency in acrylic shield manufacturing). The resulting best-fit slope was 0.0546 mR/hr/mCi, 10% lower than the V-Vial exposure rate conversion factor.

Conclusions: For accurate SIR-Spheres assays in the new SIROS D-Vial delivery set, ⁹⁰Y-microsphere activities in agitated D-Vials should be measured in dose calibrators with calibration numbers 60x10 for CRC-15R and 59x10 for CRC-55tR. If assaying the activity indirectly through an exposure rate measurement, a conversion factor of 0.0546 mR/hr/mCi should be used when measuring with Fluke 451B ionization chambers at 30 cm. Incorrectly implemented V-Vial calibration numbers or exposure rate conversion factors will introduce errors ranging from 2-10%.

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