Evaluation of a scalable qSPECT calibration method for radiopharmaceutical dosimetry

1433 Objectives: Accurate dosimetry-guided radiopharmaceutical therapy (RPT) requires quantitative SPECT (qSPECT) imaging to assess radiotracer accumulation in tissues of interest. To this end, two primary calibration metrics are needed beyond normal scanner operation: (1) system sensitivity, used to convert from counts to radioactivity, and (2) activity recovery coefficients used for partial volume correction (e.g. tumor dosimetry). Methods for collection of these data vary by institution and investigator, potentially leading to inter-observer variability in final dosimetric specifications. The objective of this work was to design and evaluate a novel calibration workflow that can be easily implemented clinically and in the setting of multi-center RPT clinical trials as a quantitative SPECT/CT scanner validation paradigm. Methods: All scanning was performed using a Siemens Intevo SPECT/CT system (180° non-circular orbit, 60 views/head, 30 s/view, 1.45x zoom, 128x128 matrix, 20% window on photopeak, 15% upper/lower scatter windows, ME collimators for 99mTc and 177Lu, HE collimators for 131I). For determination of camera sensitivity, a ~300 mL bottle was filled with 1 mCi of 99mTc, 177Lu, or 131I. The activity-filled bottle was scanned in three different scatter-inducing geometries: (A) suspended in air, (B) placed within a water-equivalent plastic cylinder (20 cm diameter, 20 cm length; CIRS Inc.), and (C) suspended within a water-filled IEC body phantom. Images were reconstructed using two commercially available OSEM3D algorithms (Flash3D, Siemens Inc.; SPECTRA Quant, MIM Software Inc.). Both algorithms utilize CT-based attenuation correction and collimator-detector response modeling. Images were reconstructed with 8 iterations, 12 subsets, and with no pre- or post-reconstruction filtering. Sensitivity measurements were performed using ROIs drawn by intensity thresholding at the 1% level on each set of images. For determination of recovery coefficients, the SNMMI CTN3 anthropomorphic chest phantom was filled with 99mTc or 177Lu with tumor-to-background activity concentration ratios of 20:1. CTN3 images were reconstructed as described above. Results: Despite a consistent source geometry (~300 mL bottle), measured sensitivity varied significantly with phantom geometry for 99mTc, 177Lu, and 131I. Apparent system sensitivity was highest in geometries with the most scatter (IEC body phantom w/ water) and lowest in geometries with minimal scatter (bottle suspended in air). Compared with sensitivity measured in water, sensitivity measurements with the bottle suspended in air had a deviation ranging from -8% for 131I to -24% for the 113 keV emission of 177Lu (average of all measurements: -16% ± 7%). By comparison, measurements acquired in the water-equivalent cylinder had smaller deviations, ranging from -2% for 131I to -5% for the 113 keV emission of 177Lu (average of all measurements: -3.3% ± 1.5%). Sensitivity discrepancies when determined using different proprietary reconstruction engines were minimal, with an average ratio (MIM / Siemens) of 1.008 ± 0.013. Measured recovery coefficients (SUVmean) for spheres in the CTN3 phantom were nearly identical when comparing 99mTc and 177Lu images acquired using the ME collimator, with an average absolute error of 3.0% (min: 0.6%, max: 6.1%) for sphere sizes 13 - 37 mm. Conclusions: Phantom size significantly impacts apparent SPECT sensitivity when using the triple-energy window scatter correction method. Sensitivity factors should therefore be determined under realistic scatter conditions (finite source size surrounded by a scatter-inducing medium). For this purpose, a water-equivalent plastic cylinder and activity-filled bottle is simple and appropriate, but may underestimate sensitivity by ~3%. Recovery coefficients obtained using 99mTc with a ME collimator may be suitable for 177Lu tumor dosimetry, thereby avoiding the consequence of long-lived phantom contamination.