S liver liver 4 6 10 5 rad\u00b5 Ci hr S liver kidneys 3 9 106 rad\u00b5 Ci hr The liver

S ( liver ← liver ) = 4 . 6 × 10 5 rad /µ Ci-hr S ( liver ← kidneys ) = 3 . 9 × 10 6 rad /µ Ci-hr (The liver to liver S -value is slightly different than we had calculated, as MIRD 11 used slightly different decay data.) Assume A 0 = 1 mCi = 1000 µ Ci; then, Ã ( liver ) = 1 . 443 · 1000 µ Ci ( 0 . 3 · 0 . 5 hr + 0 . 1 · 5 . 5 hr ) = 1010 µ Ci-hr Ã ( kidneys ) = 1 . 443 · 1000 µ Ci · 0 . 2 · 1 . 2 hr = 350 µ Ci-hr D ( liver ) = 1010 µ Ci-hr · 4 . 6 × 10 − 5 rad /µ Ci-hr + 350 µ Ci-hr · 3 . 9 × 10 − 6 rad /µ Ci-hr D ( liver ) = 0 . 0465 rad + 0 . 0014 rad = 0 . 048 rad .

10.5 Internal Dose Calculations for Nuclear Medicine Patients 239 Note that the liver contributes 97% of its total dose. Dividing by the injected activity, the dose, given these input assumptions, is 0.048 rad/mCi. So, if we redesigned the study to use 3 mCi, the liver absorbed dose would be 3 mCi × 0.048 rad/mCi = 0.14 rad. Example 1 MBq of 99m Tc is given to a patient: 40% goes to the liver and has a 10 hr biological half-time; the other 60% goes to the spleen and has an infinite biological half-time (never leaves). A liver = 4 × 10 5 Bq × 1 . 443 × 6 × 10 h 6 + 10 x 3600 s h A liver = 7 . 79 × 10 9 Bq − s A spleen = 6 × 10 5 Bq × 1 . 443 × 6 h × 3600 s h A spleen = 1 . 87 × 10 10 Bq − s S liver ← liver = 4 . 1 x 10 − 5 rad /µ Ci − hr = 3 . 08 x 10 − 6 mGy / MBq − s S spleen ← spleen = 3 . 1 x 10 − 4 rad /µ Ci − hr = 2 . 33 x 10 − 5 mGy/MBq − s S spleen ← liver = 9 . 6 x 10 − 7 rad /µ Ci − hr = 7 . 2 x 10 − 8 mGy/MBq − s S liver ← spleen = 9 . 6 x 10 − 7 rad /µ Ci − hr = 7 . 2 x 10 − 8 mGy/MBq − s D liver = 7 . 79 x 10 3 MBq − s × 3 . 08 x 10 − 6 mGy/MBq − s + 1 . 87 x 10 4 MBq − s × 7 . 2 x 10 − 8 mGy / MBq − s = 0 . 025 mGy D spleen = 7 . 79 x 10 3 MBq − s × 7 . 2 x 10 − 8 mGy / MBq − s + 1 . 87 x 10 4 MBq − s × 2 . 33 x 10 − 5 mGy / MBq − s = 0 . 43 mGy Example: Dose to the Fetus MIRD Dose Estimate Report No. 13 29 gives the following numbers of disinte- grations for intravenous administration of 99m Tc MDP. Cortical bone 1.36 µ Ci-hr/ µ Ci administered Cancellous bone 1.36 µ Ci-hr/ µ Ci administered Kidneys 0.148 µ Ci-hr/ µ Ci administered Urinary bladder 0.782 µ Ci-hr/ µ Ci administered Remainder of body 1.64 µ Ci-hr/ µ Ci administered If 17 mCi of 99m Tc-MDP has been given to a woman who is two weeks pregnant, what is the likely absorbed dose to the fetus? In early pregnancy, the dose to the nongravid uterus is a reasonably good estimate of the fetal dose, because the size and shape of the uterus relative to other organs has not changed substantially. Therefore, we can use S -values for these source organs irradiating the uterus: S (Uterus ← Cortical bone) = 5.7 × 10 − 7 rad/ µ Ci-hr S (Uterus ← Cancellous bone) = 5.7 × 10 − 7 rad/ µ Ci-hr S (Uterus ← Kidneys) = 9.4 × 10 − 7 rad/ µ Ci-hr

240 Chapter 10 Internal Dose Assessment S (Uterus ← Urinary bladder) = 1.6 × 10 − 5 rad/ µ Ci-hr S (Uterus ← Total body) = 2.6 × 10 − 6 rad/ µ Ci-hr The last S-value is not exactly what we need. It is the S -value for an organ being irradiated by activity uniformly distributed in the whole body (i.e., including bone, kidneys, etc.). The formula for calculating the S -value for remainder of the body for a given configuration of other source organs is: 30 S ( r k ← RB ) = S ( r k ← T B ) m T B m RB − h S ( r k ← r h ) m h m RB where: S ( r k ← RB ) is the S -value for the remainder of the body irradiating target region r k S ( r k ← T B ) is the S -value for the total body irradiating target region r k S ( r k ←