Radioactive131 iodine (RAI) body burden is estimated in thyroid cancer patients by a) exposure rate
meter and b) external probe counting. A calibration factor of 301 cpm/MBq (d = 16
cm) is used for the probe for estimates of whole body activity. Patients sit in a
rotating stool with their center corresponding to the field of view for estimation
of whole body RAI. Radioactive counts are obtained foranterio posterior (AP) and postero
anterior (PA) geometries. Whole body retention factor is expressed as a ratio against
assayed activity administered to the patient on day 1. With exposure rate measurement,
for off-thyroxin (hypothyriod) patients, the retention factors were 0.148 ± 0.12 (n
= 211) and 0.07 ± 0.08 (n = 68) at 48 h and 72 h, respectively. For recombinant TSH
(rhTSH) (euthyroid) group, the retention factors were 0.089 ± 0.06 (n = 24) and 0.05
± 0.05 (n = 19) at 48 h and 72 h, respectively. By probe counting method, the obtained
retention factors were 0.081 ± 0.013 (range: 0.071-0.096) (off-thyroxine group) and
0.039 ± 0.03 (range: 0.008-0.089) for the rhTSH group at 48 h. The 72 h retentions
in the off thyroxine (hypothyriod) group and the rhTSH (euthyroid) group were 0.048
± 0.024 (range: 0.016-0.076) and 0.005, respectively. The radioactive body burdens
at 48 h were in the range of 290-315 MBq (7.8-8.5 mCi) for the off-thyroxine group
and 44-286 MBq (1.2-7.7 mCi) for the rhTSH group. The calculated residence times in
whole body were 21.97 ± 3.8 h (range: 17.1-27.1) for off-thyroxine group and 14.28
± 2.75 h (range: 9.97-19.46) showing high statistical significance (P < 0.001). The
specific blood doses were 0.118 ± 0.025 mGy/MBq (range: 0.083-0.172) for the off-thyroxine
group (females n = 23); 0.87 ± 0.028 mGy/MBq (range: 0.057-0.130) (females n = 13),
0.080 ± 0.013 mGy/MBq (range: 0.069-0.098) (males n = 5) and 0.080 ± 0.028 (range:
0.059-0.118 for rhTSH patients (males n = 4). The mean mGy/MBq for blood was higher
in females--about 10% for the rhTSH group compared to 40% higher for the off thyroxine
(THW) group. The dose to blood was "inversely proportional" to the body weight, both
in the off-thyroxine group and the rhTSH group but the dependence was not very strong
with rhTSH administration because of less residence time of the radioactive iodine.
There was no correlation between administered activity and retained body burden at
48 h or 72 h.
Keywords
Blood dose in iodine
131
- differentiated thyroid cancer - iodine kinetics - radioactive iodine - whole body
retention
