Radiation dose estimates in humans for (11)C-acetate whole-body PET

11C-Acetate is currently being investigated as a new tracer for imaging neoplasms, most notably prostate cancer and its metastases. Previously reported dose estimates for (11)C-acetate prepared by the Oak Ridge Institute for Science and Education (ORISE) were based on a simple 3-compartment model in which all activity not measured in blood or excretion via breath was assumed to reside in the heart. Because all organs are involved in acetate metabolism to some extent, these estimates might overestimate heart and underestimate other organ dosimetry. Dynamic whole-body (11)C-acetate PET was therefore performed on 6 healthy human volunteers. Measured dose estimates for all target organs were compared with the existing ORISE values.

After transmission scanning had been performed for measured attenuation, 525 MBq of (11)C-acetate were injected intravenously, and 5 sequential whole-body emission scans were obtained from the head to mid thighs. Regions of interest were drawn to encompass the entire activity in all visible organs at each time point. Time-activity data were fit in a least-squares sense to obtain residence times. Absorbed dose estimates were determined using MIRDOSE3.1 software.

The effective dose was 0.0049 mSv/MBq. The organs receiving the highest absorbed doses were the pancreas (0.017 mGy/MBq), bowel (0.011 mGy/MBq), kidneys (0.0092 mGy/MBq), and spleen (0.0092 mGy/MBq). No urinary excretion of tracer was measurable.

Using these new estimates for (11)C-acetate dosimetry, the maximum injected activity under Radioactive Drug Research Committee limits can be raised up to 5-fold over the limit imposed by the previous ORISE estimates. A higher injected activity would improve counting statistics and, it is hoped, overall image quality and tumor detection with whole-body (11)C-acetate PET.