PET-based radiation dosimetry in man of 18F-fluorodihydrotestosterone, a new radiotracer for imaging prostate cancer.
16 beta-fluoro-5 alpha-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 (18)F-FDHT administered to patients with advanced prostate cancer.


METHODS
Absorbed dose estimates were derived from 10 (18)F-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.


CONCLUSIONS
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 (18)F-FDHT.
Zanzonico
Pat B
PB
Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA.
Finn
Ronald
R
Pentlow
Keith S
KS
Erdi
Yusuf
Y
Beattie
Bradley
B
Akhurst
Timothy
T
Squire
Olivia
O
Morris
Michael
M
Scher
Howard
H
McCarthy
Timothy
T
Welch
Michael
M
Larson
Steven M
SM
Humm
John L
JL
D001822:Body Burden; D013196:Dihydrotestosterone / Q000031:analogs & derivatives* / Q000493:pharmacokinetics*; D006801:Humans; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008297:Male; D008657:Metabolic Clearance Rate; D009928:Organ Specificity; D049268:Positron-Emission Tomography / Q000379:methods*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000378:metabolism*; D011829:Radiation Dosage; D011835:Radiation Protection / Q000379:methods*; D011874:Radiometry / Q000379:methods*; D012062:Relative Biological Effectiveness; D018570:Risk Assessment / Q000379:methods; D012307:Risk Factors; D014018:Tissue Distribution; D014915:Whole-Body Counting / Q000379:methods*
C494901:16-fluorodihydrotestosterone; D013196:Dihydrotestosterone