There are situations when the uptake of the tracer is gradual and the
clearance also is slow. In these cases,
=1.44f·AoTe(TQ/TU) (14.19)where TUis the biological uptake half-time,Teis the effective excretion half-
time [Eq. (3.12)] and TQis the effective uptake half-time.TQis calculated
by Eq. (3.12) using the physical half-life TPand the biological uptake half-
time TU.
Two other situations can occur when the uptake is instantaneous, but the
TPof the radionuclide is greater than the biological half-life TB, or TBis
greater than TP.
When TP>>TB, the cumulated activity is given by
=1.44f·AoTB (14.20)If the tracer is excreted by several excretion routes such as urinary excre-
tion, fecal excretion, etc., the fraction of activity excreted and the effective
halftime of each mode are used to calculate the fractional cumulated activ-
ity of each mode, which are then summed to calculate the total cumulated
activity.
When TB>>TP, the cumulated activity is calculated as
=1.44f·AoTP (14.21)In this case, there is no biological excretion.
The SValues
The mean absorbed dose per cumulated activity,S, is more appropriately
expressed as
(14.22)
where the symbols vand rrepresent the target and the source, respectively.
The calculation of these values is quite laborious. The MIRD Committee
of the Society of Nuclear Medicine calculates these values for radiophar-
maceuticals commonly used in nuclear medicine and publish them period-
ically. Table 14.3 includes a partial list of Svalues for 99mTc obtained from
MIRD pamphlet no. 11.
Problem 14.1
Calculate the absorbed dose to the lungs from the administration of 4 mCi
(148 MBq) 99mTc-MAA particles, assuming that 99% of the particles are
trapped in the lungs. The value of Sfor the lungs is 5.2 × 10 −^5 rad/mCi·hr.
Assume that the 99mTc activity is uniformly distributed in the lungs and 45%
of the activity is cleared from the lungs with a biological half-life of 3 hr and
55% with a biological half-life of 7 hr.
Sr
m
ii r
in
()← =←()
=∑
1
1ΔfA ̃
A ̃
A ̃
214 14. Internal Radiation Dosimetry