Adrenal Scintigraphy

8.7.1 Radiopharmaceutical

• [¹³¹I]6ß-iodomethyl-19-norcholesterol (¹³¹I-Norcholesterol).

8.7.2 Uptake mechanism / biology of the tracer

Functioning adrenal cortex tissue can be visualized by ‘trapping’ a radiolabelled cholesterol precursor. Once taken up in the adrenal cortex, the radiopharmaceutical is not further metabolized. Since the tracer is taken up by the normal cortisol-producing adrenal tissue, this examination should ideally be performed under dexamethasone suppression when aldosterone or androgen-producing lesions are suspected.

8.7.3 Indications

• Usually, the adrenal cortex scintigraphy is a complementary procedure for localizing abnormal adrenal cortex tissue after the (biochemical) diagnosis of adrenal dysfunction.
• The main indication is the differentiation between bilateral hyperplasia and a tumour in patients with hypercortisonism (Cushing’s syndrome), hyperaldosteronism or hyperandrogenism.

8.7.4 Contra-indications

• Pregnancy is a relative contra-indication.
• Cessation of breastfeeding is recommended [3].

8.7.5 Clinical performances

The accuracy of planar scintigraphy with [¹³¹I]6ß-iodomethyl-19-norcholesterol for differentiating adenoma from hyperplasia varies from 47% to 94%. Recent data show significantly improved diagnostic accuracy using SPECT/CT [166].

8.7.6 Activities to administer

The suggested activity to administer is

• [¹³¹I]6ß-iodomethyl-19-norcholesterol: 20 MBq

No recommendations are given for paediatric nuclear medicine

No recommendations are given for paediatric nuclear medicine.

8.7.7 Dosimetry

The effective dose for [¹³¹I]6ß-iodomethyl-19-norcholesterol is 1.8 mSv/MBq [109]. The organ with the highest absorbed dose is the thyroid: 29 mGy/MBq.

The effective dose for [¹³¹I]6ß-iodomethyl-19-norcholesterol is: 36 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.".

8.7.8 Interpretation criteria/major pitfalls

Standard Scintigraphy

Symmetrical distribution of activity:

• Normal pattern.
• In Cushing’s syndrome or hyperaldosteronism, one sees symmetrically increased uptake as a result of bilateral hyperplasia and ectopic production of adrenocorticotropic hormone.
• Rarely: adrenogenital syndrome secondary to 17[g- or 11ß-hydroxylase deficiency.

Asymmetrical distribution of activity:

• Normal pattern: the right adrenal gland can be somewhat larger than the left (the normal size of the right adrenal gland is between 0.9 and 1.2 times that of the left adrenal gland). Also, there might appear to be more activity in the right adrenal due to over projection of the liver (especially in the first days).
• Macronodular hyperplasia (hyperaldosteronism).
• Small aldosteronoma.
• An aldosterone or androgen-producing carcinoma.
• Micronodular hyperplasia (hyperaldosteronism).
• Residue after unilateral adrenalectomy.

Unilateral uptake:

• Adrenal cortex adenoma (Cushing’s syndrome).
• Post adrenalectomy.
• Aldosterone or androgen-producing carcinoma.
• Adrenal infarction.
• Gallbladder visualization

Bilateral absence of uptake:

• Adrenal cortex carcinoma (Cushing’s syndrome).
• Hormonal therapy.
• Hyperlipidaemia, hypercholesterolemia.
• Poor labelling.
• Scintigraphy under dexamethasone suppression

Bilateral absence of uptake:

• Normal.
• Essential (low renin) hypertension.

Unilateral uptake:

• Aldosteronoma.
• Adrenal cortex adenoma (hyperandrogenism).

Symmetrical distribution of activity:

• Normal from the 5th day onwards.
• Macro- or micronodular hyperplasia (hyperaldosteronism).
• Secondary aldosteronism, e.g. due to stenosis of the renal artery.
• Medication (oral contraceptives, diuretics).
• Dexamethasone administration stopped too early.

8.7.9 Patient preparation

Block unintentional radioactive iodine uptake in the thyroid with sodium or potassium iodide (100-150 mg per day) or sodium or potassium perchlorate (200-400 mg per day) for 5-10 days, starting the day before administration of the radiopharmaceutical.

Stop all interacting drugs, such as oral contraceptives, dexamethasone, diuretics, propranolol, ketoconazole, cholestyramine, some hypolipemic agents and corticosteroids at least 48 h prior to the investigation. If the clinical question is whether there is an aldosteronoma, spironolactone must be stopped at least 6 weeks in advance, because spironolactone can lead to increased radiopharmaceutical uptake in the adrenal glands.

If dexamethasone suppression is required, prescribe 4 mg/day starting one week prior to the administration of the radiopharmaceutical and continue this for as long as the investigation lasts.

Laxatives are not routinely necessary but can be considered if there appears to be stasis of the radiopharmaceutical in the bowel. In that case, give laxatives for several days between scintigraphies.

8.7.10 Methods

There are no EANM guidelines at the present time.

The patient is positioned sitting or supine for posterior acquisition with the γ camera centred at the level of the twelfth thoracic vertebra.

For a standard adrenal cortex scintigraphy, two recordings are made. These take place on the fifth and eighth day post radiopharmaceutical administration (the day of administration is day 0).

If the examination is performed under dexamethasone suppression, daily recordings should be made from day 2 until the adrenal glands become clearly visible. Sometimes recordings are made only on the 3rd and 5th days after injection.

A SPECT(-CT) may be added to improve accuracy and localization.

If only static images are made, imaging of the kidneys ([^99mTc]Tc-DTPA or [^99mTc]Tc-DMSA) could lead to better localization of abnormalities.

Lateral images are not routinely required. They should be made when radiopharmaceutical uptake is quantified, or when determining the depth of residual adrenal tissue. The depth can be measured by placing a marker on the skin.

Consuming a high fat content meal might be helpful for differentiating between adrenal and gall bladder activity.