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PET-Based Radiation Dosimetry in Man of ¹⁸F-Fluorodihydrotestosterone, a New Radiotracer for Imaging Prostate Cancer

¹ Memorial Sloan-Kettering Cancer Center, New York, New York
² Washington University, St. Louis, Missouri

16ß-fluoro-5-dihydrotestosterone (FDHT) is a promising new PET radiopharmaceutical for the imaging of prostate cancer. A recent clinical trial provided the opportunity for refinement of normal-tissue radiation-absorbed dose estimates based on quantitative PET. The objective of the current study was to derive estimates of normal-tissue absorbed doses for ¹⁸F-FDHT administered to patients with advanced prostate cancer. Methods: Absorbed dose estimates were derived from 10 ¹⁸F-FDHT PET studies (administered activity, 111–407 MBq) of 7 prostate cancer patients. Activity concentrations in plasma and red marrow (assuming a plasmacrit of 0.58, an extracellular fluid fraction of 0.40, and equilibration of activity between plasma and marrow extracellular fluid) were measured ex vivo from a peripheral blood sample. Liver, spleen, urinary bladder contents, and total-body activities were measured by region-of-interest analysis of quantitative whole-body studies acquired with a dedicated PET scanner. Total organ activities and residence times were calculated from the respective PET scan-derived activity concentrations assuming standard (70 kg) man organ masses. Urinary excretion was corrected for hepatobiliary excretion (liver activity), and a first-order adjustment was made for the bladder-wall mass based on the patient’s total-body mass. Mean organ absorbed doses were calculated with the MIRD formalism and the standard man model using the MIRDOSE3 software program. Results: The absorbed doses (mean ± SD) ranged from 0.00057 ± 0.000281 cGy/MBq (to skin) to 0.00868 ± 0.00481 cGy/MBq (to bladder wall) (voiding intervals, 1–2 h), and the effective dose equivalent was 0.00177 ± 0.000152 cSv/MBq. Conclusion: The maximum absorbed dose among all tissues in all 10 studies, 0.0151 cGy/MBq, occurred for the urinary bladder wall (with hydration and 1- to 2-h voiding intervals). To ensure that the maximum normal-tissue absorbed dose is kept below the recommended maximum permissible dose of 5 cGy per single administration, a maximum administered activity of 331 MBq (5 cGy/[0.0151 cGy/MBq]) is recommended for ¹⁸F-FDHT.

Key Words: FDHT • dosimetry • prostate cancer • ¹⁸F • androgens

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