SPECT Imaging for Pharmacokinetics and dosimEtry Towards TREATment Optimization

Radiopharmaceutical therapy (RPT) is an emerging systemic treatment modality that delivers radiation to targeted cells. Recently FDA approved RPT agents include Radium-223 for prostate cancer, 177Lu-DOTATATE for neuroendocrine cancers, and 177Lu-PSMA-617 for prostate cancer. RPT is currently being administered as weight-based or fixed activities for 4-6 cycles, and there is concern that this standardized regimen compromises the potential efficacy of this treatment modality. In large part, this is because rigorous, validated dosimetry methods are not in standard clinical use, especially for alpha emitter RPTs. Such methods using SPECT would make it possible to predict potential normal organ toxicity and tumor response for individual patients. The multidisciplinary Johns Hopkins RPT research group has focused on development of such SPECT dosimetry methods and has an active NIH P01 grant for this work. The ability to image and understand where the RPT distributes in patients makes it possible to estimate the radiation delivered to tissues. The study team of medical physics experts is developing quantitative 3-D, single-photon emission computed tomography (SPECT) imaging for dosimetry of beta and alpha emitters 1-9. Recognizing that such imaging must be made convenient to be widely adopted, the investigators are also examining the trade-off between simplifying imaging (shorter imaging times, fewer imaging sessions) and the accuracy of the absorbed dose calculations. Key barriers to implementing dosimetry for alpha emitters include the low count-rate and sub-optimal photon emissions and the emission of multiple daughter radionuclides. Currently available reconstruction methods in clinical SPECT systems cannot handle such complex imaging physics. The investigators have pioneered the development of simultaneous multiple radionuclide reconstruction methods for diagnostic applications. The overall goal of this project is to develop imaging methodologies that may be used to perform accurate RPT dosimetry and treatment planning, especially for alpha emitters. Within this context, the SEE-to-TREAT protocol will provide clinical data and images to validate quantitative reconstruction methods for SPECT imaging.