November 2025
11 DOSIMETRY
The table below shows the dosimetry as calculated according to the Publication 80 of the ICRP (International Commission on Radiological Protection, Radiation Dose to Patients from Radiopharmaceuticals, Pergamon Press 1998).
| Absorbed dose per unit activity administered (mGy/MBq) |
| Organ | Adult | 15 years | 10 years | 5 years | 1 year |
| Adrenals | 3.2E-01 | 4.1E-01 | 6.2E-01 | 9.4E-01 | 1.5E+00 |
| Bladder | 3.3E-01 | 4.2E-01 | 6.7E-01 | 1.0E+00 | 1.7E+00 |
| Bone surfaces | 2.3E-01 | 3.0E-01 | 4.3E-01 | 6.4E-01 | 1.2E+00 |
| Brain | 4.8E-02 | 5.6E-02 | 7.9E-02 | 1.2E-01 | 2.0E-01 |
| Breast | 7.7E-02 | 9.6E-02 | 1.8E-01 | 2.8E-01 | 5.2E-01 |
| Gall bladder | 6.4E+00 | 7.1E+00 | 9.0E+00 | 1.5E+01 | 4.8E+01 |
| GI-tract Stomach SI Colon ULI LLI | 4.2E-01 1.9E+00 2.0E+00 1.9E+00 2.1E+00 | 5.5E-01 2.4E-+00 2.4E-+00 2.3E-+00 2.6E-+00 | 9.3E-01 3.8E+00 3.8E+00 3.5E+00 4.2E-00 | 1.5E+00 5.9E+00 5.8E+00 5.3E+00 6.5E+00 | 2.5E+00 1.0E+01 1.0E+01 9.1E+00 2.1E+01 |
| Heart | 3.3E-01 | 4.3E-01 | 6.4E-01 | 9.6E-01 | 1.6E+00 |
| Kidneys | 5.0E-01 | 6.1E-01 | 8.9E-01 | 1.3E+00 | 2.0E+00 |
| Liver | 6.9E-01 | 8.7E-01 | 1.3E+00 | 1.8E-00 | 3.2E+00 |
| Lungs | 2.4E-01 | 3.3E-01 | 4.7E-01 | 7.2E-01 | 1.3E+00 |
| Muscles | 2.0E-01 | 2.5E-01 | 3.7E-01 | 5.5E-01 | 9.8E-01 |
| Oesophagus | 1.1E-01 | 1.4E-01 | 1.9E-01 | 2.9E-01 | 4.8E-01 |
| Ovaries | 1.0E+00 | 1.3E+00 | 2.0E+00 | 2.9E+00 | 4.9E+00 |
| Pancreas | 4.5E-01 | 5.8E-01 | 1.1E+00 | 1.7E+00 | 2.6E+00 |
| Red marrow | 2.9E-01 | 3.4E-01 | 4.6E-01 | 6.0E-01 | 8.3E-01 |
| Skin | 7.5E-02 | 9.1E-02 | 1.4E-01 | 2.2E-01 | 4.2E-01 |
| Spleen | 3.0E-01 | 4.1E-01 | 6.6E-01 | 1.0E+00 | 1.7E+00 |
| Testes | 9.2E-02 | 1.3E-01 | 2.2E-01 | 3.7E-01 | 7.0E-01 |
| Thymus | 1.1E-01 | 1.4E-01 | 1.9E-01 | 2.9E-01 | 4.8E-01 |
| Thyroid | 6.9E-02 | 9.6E-02 | 1.5E-01 | 2.7E-01 | 5.2E-01 |
| Uterus | 7.5E-01 | 9.4E-01 | 1.5E+00 | 2.3E+00 | 3.8E+00 |
| Remaining Organs | 2.6E-01 | 3.4E-01 | 5.3E-01 | 8.3E-01 | 1.3E+00 |
| Effective Dose (mSv/MBq) | 6.9E-01 | 8.6E-01 | 1.3E+00 | 2.0E+00 | 3.9E+00 |
For this product the Effective Dose to a healthy adult resulting from the administration of a 370 kBq capsule is typically 0.26mSv.
In most clinical investigations for which this substance is used (e.g. Crohn's disease) the effects of impaired ileal absorption and shorter gastrointestinal transit time tend to reduce the dose commitment compared with the normal case. However, in patients with severe cholestatic jaundice, the liver dose has been estimated to be about 100 times the normal value.
12 INSTRUCTIONS FOR PREPARATION OF RADIOPHARMACEUTICALS
PROCEDURE FOR USE
Measurement of bile pool loss
Measurement of the rate of bile loss from the endogenous pool using Tauroselcholic Acid (75Se) may be achieved either by determining the retention of activity in the body over a period of days or by determining the excretion of activity in faeces. The results may be expressed as a rate of loss if several measurements are taken, or more simply as a retained percentage after a fixed period (7 days is convenient). A whole body counter or other counter or other counting techniques may be used.
For some investigations scintigraphic studies may be appropriate.
Measurement of retained activity whole body counter
A 370 kBq (10 μCi) capsule is administered to the patient together with a drink of water. Using conventional whole body counting techniques an initial count of the patient provides, after background subtraction, a zero- time or 100% value.
After 7 days the patient is counted again, and the retained activity expressed as a percentage of the original value.
Alternative techniques
If a whole body counter is not available, other counting techniques may be used successfully. Since the activity is confined to the abdominal region, a counter with a field of view encompassing the abdomen can be employed.
A gamma camera is used extensively and single crystal probes have also been used.
A gamma camera with its collimator removed has proved successful and is the most commonly used technique. Collimated cameras have also shown to be useful. However, the use of this technique may require an increase in acquisition time to reduce uncertainties in Tauroselcholic Acid (75Se) retention measurements.
A standard axial positioning of the patient along the centreline of the counter should be maintained. The centre of the crystal should be positioned midway between the umbilicus and the base of the sternum.
To avoid excessive background interference from sources of technetium-99m, it is recommended that the camera window be set at the 265 keV photon peak of selenium-75 (20% window).
It is important that same technique is used at day 0 and day 7 to avoid any errors in the final calculation of the retention rate.
It is also important to keep the positioning of the patient constant at each measurement.
Procedure
1. The patient should be given at least 15 ml of water to drink prior to taking the capsule. A similar drink of water should be taken with the capsule and again afterwards to encourage rapid transit of the capsule to the stomach and subsequent dispersion of the contents.
2. Allow 1-3 hours for physiological equilibration.
3. Measure the background before the first count of the patient. A preset time may be used.
4. Place the patient on the couch as described above. AP Count for preset time (300 seconds suggested and record the counts).
5. Turn the patient and repeat the count from the other view, PA.
6. Measure the background again.
7. After background subtraction, calculate the geometric mean of the two patient counts √(PA x AP).
8. Repeat steps 3-7 after 7 days.
9. Correct the day 7 value for radioactive decay by multiplying by 1.04
10. Express day 7 value as percentage of day 0 value.
Measurement of excreted activity
The alternative method of estimating bile acid loss is by scintillation counting of total faecal samples collected over a period (e.g. 7 days). A dosage of 370 kBq (10 μCi) (orange and yellow capsule) is recommended. It is important to ensure that standard geometry is monitored and that total collection of faeces is achieved. Samples from patients undergoing two simultaneous radionuclide is known investigations should not be counted unless faecal excretion of the other radionuclide is known to be insignificant, or unless the counting equipment can be selectively set to accumulate only selenium-75 photon emissions.
Counting of the faecal γ activity using a sodium iodide crystal detector in a well counter or other suitable instrument is the counting method of choice.
The procedure for the administration of the capsule of Tauroselcholic Acid (75Se) is the same as when measuring retained activity.
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