Feasibility of single-time-point dosimetry for radiopharmaceutical therapies.

Due to challenges in performing routine personalized dosimetry in radiopharmaceutical therapies, interest in single-time-point (STP) dosimetry, particularly utilizing only one SPECT scan, is on the rise. Meanwhile, there are questions about reliability of STP dosimetry, with limited independent validations. In the present work, we analyze two STP dosimetry methods and evaluate dose errors for a number of radiopharmaceuticals based on effective half-life distributions. Method: We first challenged the common assumption that radiopharmaceutical effective half-lives across the population are Gaussian (normal) distributed. Then, dose accuracy was estimated based on two STP dosimetry methods for a wide range of potential scan-times post-injection (p.i.), for different radiopharmaceuticals applied to neuroendocrine tumors and prostate cancer. The accuracy and limitations of using each of the STP methods were discussed. Results: Log-normal distribution was shown as more appropriate to capture effective half-life distributions. The STP framework was shown as promising for dosimetry of 177Lu-DOTATATE, and for kidney dosimetry of different radiopharmaceuticals (errors<30%). Meanwhile, for some radiopharmaceuticals, STP accuracy is compromised (e.g. in bone marrow and tumors for 177Lu-PSMA therapies). Optimal SPECT scanning time for 177Lu-DOTATATE is at ~72 h p.i., while 48 h p.i. would be better for 177Lu-PSMA compounds. Conclusion: Our results demonstrate that simplified STP dosimetry methods may compromise the accuracy of dose estimates, with some exceptions such as for 177Lu-DOTATATE and for kidney dosimetry in different radiopharmaceuticals. Simplified personalized dosimetry in the clinic continues to be a challenging task. Based on these results, we make suggestions and recommendations for improved personalized dosimetry using simplified imaging schemes.