Assessing Spatial Concordance Between Theranostic Pairs Using Phantom and Patient-Specific Acceptance Criteria: Application to ^99mTc-MAA SPECT/⁹⁰Y-Microsphere PET

Purpose: Predictive 3-dimensional dosimetry requires spatial concordance between diagnostic and therapeutic activity distributions. We assess similarity between theranostic pairs (^99mTc–macroaggregated albumin [MAA] single photon emission computed tomography [SPECT] and ⁹⁰Y microsphere positron emission tomography [PET]) in patients using criteria that account for spatial resolution differences and misregistration. Methods and Materials: Phantom-based acceptance criteria were determined using a liver phantom filled with ^99mTc and ⁹⁰YCl₃ and scanned with SPECT/computed tomography [CT] and PET/CT, respectively. Gaussian blurring was applied to PET to match ^99mTc phantom scan image quality. After rigid registration between SPECT/CT and PET/CT, perturbations up to ±3 voxels were applied to determine the similarity metric (SM) sensitivity. ^99mTc-MAA SPECT/CT and ⁹⁰Y microsphere PET/CT image pairs/patients (n = 23) were processed analogously. SMs calculated included the Pearson correlation coefficient (ρ_r), Lin's concordance correlation coefficient (ρ_c), Spearman's rank correlation coefficient (ρ_s), the mean squared difference, and the Dice similarity coefficient (DSC). Patient-specific acceptance criteria were determined by evaluating the SMs of the blurred PET compared with itself misregistered. Results: After transforming PET to SPECT resolution, high similarity was found in phantom, with ρ_c, ρ_r, ρ_s > 0.98 ± 0.01, a mean squared difference of (4.1 ± 0.3) × 10⁻⁴ and DSC > 0.85 ± 0.01 for investigated thresholds (5%, 30%, and 50%). SMs for patients varied from poor to good. A small percentage (13%-30%) of patient scans were acceptable using phantom-based acceptance criteria. The percentage increased slightly (17%-35%) using patient-specific acceptance criteria. DSC for most patients were substantially lower (average 0.95 vs 0.61 for 5% threshold) than phantom values. Conclusions: At best, 35% of patients had an SM within the acceptance criteria established to account for imaging-related effects impacting spatial concordance between ^99mTc-MAA SPECT and ⁹⁰Y PET. Additional clinical factors should be evaluated in the future. The procedure of accounting for image-related effects when assessing spatial concordance can be applied to other theranostic pairs.