FLUORINE-18-FLUORO-L-DOPA DOSIMETRY WITH CARBIDOPA PRETREATMENT

This article presents dosimetry based on the measurement of fluoro-DOP A activity in major tissues and in the bladder contents in humans afte r oral pretreatment with 100 mg carbidopa. Methods: Bladder activity w as measured continuously by external probe and calibrated using comple te urine collections. Quantitative dynamic PET scans provided time-act ivity curves for the major organs. Bladder wall dosimetry was calculat ed using the methods of MIRD Pamphlet No. 14. Effective dose was calcu lated as described in ICRP Publication 60. Results: Mean absorbed dose to the bladder wall surface per unit administered activity was 0.150 mGy/MBq (0.556 rad/mCi) with the realistic void schedule used in our s tudies. The dose was 0.027 mGy/MBq (0.101 rad/mCi) to the kidneys, 0.0 197 mGy/MBq (0.0728 rad/mCi) to the pancreas, and 0.0186 mGy/MBq (0.06 88 rad/mCi) to the uterus. Absorbed doses to other organs were an orde r of magnitude or more lower than the bladder, 0.009-0.015 mGy/MBq. Th e effective dose per unit administered activity was 0.0199 mSv/MBq (0. 0735 rem/mCi.) Conclusion: Urinary excretion of fluoro-DOPA was altere d significantly by pretreatment with carbidopa. In general, any manipu lation of tracer metabolism in the body should be expected to produce changes in biodistribution and dosimetry. The largest radiation dose w as to the bladder wall, for which our estimate was one-fifth of that f rom the original report. The methods used reflect realistic urinary ph ysiology and typical use of this tracer. The principles of MIRD Pamphl et No. 14 should be used in planning studies using tracers excreted in the urine to minimize the absorbed dose.