Thyroid Scintigraphy (99mTc-sestamibi)

8.4.1 Radiopharmaceutical:

• 2- [^99mTc]Tc-methoxyisobutylisonitrile ([^99mTc]Tc-sestamibi)

8.4.2 Uptake mechanism / biology of the tracer

[^99mTc]Tc-sestamibi is a lipophilic cation that crosses the cell membrane and penetrates reversibly into the cytoplasm via thermodynamic driving forces and then irreversibly passes the mitochondrial membrane using a different electrical gradient regulated by a high negative inner membrane potential. The cancer cells, with their greater metabolic turn-over, are characterized by a higher electrical gradient of mitochondrial membrane, and thereby exhibit an increased accumulation of [^99mTc]Tc-sestamibi compared to normal cells.

8.4.3 Indications

• Approved by the EMA for differentiation of benign from malignant cold nodules.

8.4.4 Contra-indications

• The only absolute contra-indication is pregnancy.
• It is not recommended to interrupt breast feeding although an interruption of 4 h during which one meal is discarded can be advised to be on the safe side [3].

8.4.5 Clinical performances

Sensitivity 100%, specificity 96%, and accuracy 98%; PPV 88% and NPV 100%; positive likelihood ratio was 12.35 (95% CI 8.76-21.45) [149].

8.4.6 Activities to administer

The suggested activity to administer is

• [^99mTc]Tc-sestamibi: 370 MBq.

In paediatric nuclear medicine, the activities should be modified according to the EANM paediatric dosage card (https://www.eanm.org/publications/dosage-calculator/). The minimum recommended activity to administer is 80 MBq.

8.4.7 Dosimetry

The effective dose for [^99mTc]Tc-sestamibi is 9.0 µSv/MBq [3]. The organ with the highest absorbed dose are the kidneys: 36 µGy/MBq and the gallbladder wall: 39 µGy/MBq.

The effective dose for [^99mTc]Tc-sestamibi is 3.3 mSv per procedure.

Caveat:
“Effective Dose” is a protection quantity that provides a dose value related to the probability of health detriment to an adult reference person due to stochastic effects from exposure to low doses of ionizing radiation. It should not be used to quantify the radiation risk for a single individual associated with a particular nuclear medicine examination. It is used to characterize a certain examination in comparison to alternatives, but  it should be emphasized that if the actual risk to a certain patient population is to be assessed, it is mandatory to apply risk factors (per mSv) that are appropriate for the gender, the age distribution and the disease state of that population.".

8.4.8 Interpretation criteria/major pitfalls

A nodule cold in both Na[^99mTc]TcO₄ and [^99mTc]Tc-sestamibi is rated as benign. A [^99mTc]Tc-sestamibi positive nodule with a fast washout (as per semiquantitative analysis) is rated as benign.

8.4.9 Patient preparation

None (fasting state optional to reduce physiologic hepatic extraction ameliorating tracer availability).

8.4.10 Methods

Planar double-phase (semiquantitative analysis), eventually complementary SPECT(/CT).