Purpose: Radiopharmaceuticals for diagnostic and therapeutic purposes have rapidly emerged in recent years. With recent FDA approval of ⁶⁴Cu-DOTATATE (t(1/2):12.7h; β+:17.6%), there has been excitement over the possibility of using this imaging agent to predict the dosimetry of ¹⁷⁷Lu-DOTATATE (t1/2:6.6d) or ⁶⁷Cu-analogs (t(1/2):2.6d). Pre-treatment PET/CT imaging is considered standard practice, so the possibilities of pre-treatment planning and quantitative assessment of potential benefit from therapy are highly attractive. The purpose of this work is to establish the qualitative and quantitative limitations of late-timepoint ⁶⁴Cu PET imaging.
Methods: An anthropomorphic chest phantom (SNMMI-CTN) containing spherical-‘tumors’ was filled with ⁶⁴Cu-DTPA. Initial tumor-to-background ratio was 1.07:9.61μCi/mL, for a total fill activity of 11.02mCi. PET/CT images were acquired (GE Discovery MI PET/CT;10min/bed position;2-bed-positions/scan) and reconstructed (VPFX-4i16s) every 12hours for 7days and then every 24hours for 3days. Reconstructed images were assessed qualitatively to determine tumor detectability as a function of tumor size and time. Quantitative activity recovery-coefficients (RC-SUVmean) were calculated as a function of sphere-size and decay-time. Additionally, a radioactive half-life was calculated from activity measurements over-time to assess system linearity over a range of activity levels.
Results: All lesions (7–37mm) were visualized 4days (~8 half-lives) after injection. Spheres with diameter range 7-17mm were gradually lost to background noise during days 5–7, with only the 22, 28, and 37mm spheres being visualized on day 8. Quantitative analysis of measured radioactivity vs. time indicated excellent PET response linearity even under low-count-density conditions. A ⁶⁴Cu half-life of 12.79±0.03hours was measured, which is ~0.7% higher than accepted value. RC-curves demonstrated an increase in variability at late time-points, however, generally the measured RC-curve shape was maintained out to 6days post-injection.
Conclusion: Copper-64 appears to be an excellent diagnostic pair for theranostic applications. This study has confirmed that late-timepoint imaging following administration is possible, thus demonstrating promise for pre-treatment radiopharmaceutical dosimetry.
PET, Quantitative Imaging, Nuclear Medicine