I-131 radioiodine blood activity measurement using quantitative SPECT/CT versus blood sample gamma counting in pre-therapeutic dosimetry for thyroid cancer

Introduction: I-131 radioiodine therapy is commonly used in the treatment of well-differentiated thyroid cancer with multiple metastases, and bone marrow toxicity has been predicted by blood-based dosimetry, requiring gamma counting of blood samples. Recent advancement of quantitative SPECT/CT holds promise for image-based dosimetry because blood radioactivity of I-131 can be accurately measured by drawing volume-of-interest (VOI) over the blood pool of SPECT/CT images. In this study we investigated the agreements of VOI drawing for I-131 radioiodine blood pool activity upon quantitative SPECT/CT and compared the I-131 activity between SPECT/CT and gold standard blood sample gamma counting.I-131 radioiodine therapy is commonly used in the treatment of well-differentiated thyroid cancer with multiple metastases, and bone marrow toxicity has been predicted by blood-based dosimetry, requiring gamma counting of blood samples. Recent advancement of quantitative SPECT/CT holds promise for image-based dosimetry because blood radioactivity of I-131 can be accurately measured by drawing volume-of-interest (VOI) over the blood pool of SPECT/CT images. In this study we investigated the agreements of VOI drawing for I-131 radioiodine blood pool activity upon quantitative SPECT/CT and compared the I-131 activity between SPECT/CT and gold standard blood sample gamma counting.

Methods: I-131 pre-therapeutic dosimetry studies using quantitative I-131 SPECT/CT with blood sampling were performed in 5 patients diagnosed with well-differentiated papillary thyroid cancer patients with multiple bone metastases from Mar 2022 to Nov 2023 at our hospital. After the oral administration of I-131 (3mCi), planar scan, blood sampling (2.5~3mL per venipuncture), and quantitative SPECT/CT (NMCT 670 or NMCT 670 pro, General Electronics) were sequentially performed for 8 times, at 1, 2, 4, 24, 48, 72, 96 and 120 hours post-administration. The typical quantitative SPECT images were reconstructed using attenuation correction, scatter correction, and resolution recovery (Q.volumetrix MI or Q.volumetrix AI, Xeleris version 5.0, General Electronics). The SPECT/CT scanners were equipped with high energy parallel collimators, and the system sensitivity values were 57.47 cpm/µCi for NMCT670 or 57.68 cpm/µCi for NMCT 670 pro scanners. Whole blood radioactivity, as gold standard, was counted for 30 minutes per sample using a gamma counter (Hidex Automatic Gamma Counter, Hidex Oy). Spherical VOIs were drawn at the center of ascending aorta on SPECT/CT images by 3 trained nuclear medicine specialists. The agreements of VOI drawings among the 3 specialists were tested using intra-class correlation coefficient (ICC). The comparison of I-131 radioactivity between SPECT/CT and blood sampling was performed by residence time calculated from exponential curve-fitting of time-activity curves.

Results: Among the 5 cases (female:male=3:2, mean age = 66.2 [range : 54-84], one patient skipped 2 out of 8 planned blood sampling and consecutive SPECT/CT scans (at 72 and 96 hours after I-131 administration), total of 38 blood sampling measurements, paired with respective quantitative SPECT/CT scans, were collected for analysis. The agreements of VOI drawings using SPECT/CT were extremely high with ICC of 0.977 (95% confidence interval=0.960-0.987, p<0.001). The residence time of the 5 patients was not significantly different between the SPECT/CT (0.00127±0.00073hrs, average of 3 specialists) and blood sample gamma counting (0.00147±0.00091hrs) (Wilcoxon Rank-sum test, P>0.05).

Conclusions: I-131 radioiodine blood activity measurement using quantitative SPECT/CT is precise and reproducible. Image-based pre-therapeutic I-131 dosimetry is promising, and further study is warranted for not only blood activity measurement but also for tumor activity assessment.