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An AAPM Grand Challenge


AAPM is sponsoring a Challenge on the calculation of time-integrated activity in radiopharmaceutical therapy dosimetry. The results of the Challenge will be announced publicly at the Grand Challenge Symposium of the 2023 AAPM Annual Meeting in Houston, TX.


Dosimetry for radiopharmaceutical therapy (RPT) is a complex task. In contrast to external beam radiotherapy, where radiation is deposited through a well-defined external radiation source, the dose distribution in RPT depends on the time-varying distribution of the radiopharmaceutical inside the patient after administration, which is highly dependent on the radiopharmaceutical’s kinetics.

In addition to the geometry and composition of relevant source and target regions and S values describing the physical process of radiation deposition, it is therefore essential for RPT dosimetry to monitor the time course of the radiopharmaceutical distribution in each relevant region. In practice, this translates to a need for accurate activity measurements at different time points post-injection (e.g., by blood samples and/or SPECT/CT imaging) in combination with a fast and repeatable methodology for source or target region delineation, which is illustrated in Figure 1. This results in time-activity curves (TACs) for all relevant structures, based on which time-integrated activities (TIAs) can be calculated.

Goal of the Challenge

Results from the SNMMI 177Lu Dosimetry Challenge 2021 [Uribe et al., JNM 2021] have recently demonstrated that RPT dosimetry is overwhelmingly user- and software-dependent: there is a wide variation in the RPT dosimetry methodology, and understanding the contributions of individual steps within the workflow is crucial for standardization. The TACTIC challenge zooms in on one of the most important sub-aspects of RPT dosimetry: TAC determination. Participants will be presented with a series of snapshots of precisely known time-activity curves at clinically relevant sampling time points. Based on their submitted TAC functions and TIA values, the Challenge Team will assess the degree of variation between different means of TAC/TIA estimation. Among all submissions, we will identify the best-performing methods including their uncertainties and thus take a decisive step towards improved robustness of TAC/TIA determination for RPTs.

Important Dates


The Challenge dataset will be composed of 25 synthetic datasets of peri-therapeutic time-activity curves in kidneys, blood, and tumor after 177Lu-PSMA therapy. These data are generated based on randomly selected high-quality clinical datasets, with realistic uncertainties added.

Figure 2

Challenge Format

The TACTIC Challenge consists of 3 phases (see Figure 2):

Publication Plan

The results of this Grand Challenge will be summarized to reflect the variability and information-content dependence of TAC and TIA values in RPT dosimetry, including suggestions on how to harmonize their calculation. These findings will be published, with participants listed as co-authors, in suitable journals such as Medical Physics or the Journal of Nuclear Medicine, within 2 years from completion of the challenge.



For further information, please contact Sasha Ivashchenko.