Cushing's syndrome and ectopic ACTH search
The radiological presentation of the adrenal glands in Cushing's syndrome depends on the pathogenesis of the disease.
- ACTH-independent form: characterized by a unilateral lesion presented as an adenoma or carcinoma.
- ACTH-dependent form: manifests as bilateral enlargement of the glands. In Cushing's disease (pituitary adenoma), the contours of the adrenal glands remain smooth. In cases of ectopic ACTH production, marked macronodular transformation and significant enlargement of the glands occur, which may even mask the primary tumor process.
To search for the source of ectopic ACTH, contrast-enhanced CT of the chest, abdomen, and pelvis is optimal. The difficulty lies in the fact that bronchial carcinoids are often very small (3–15 mm), and ACTH-producing small cell lung cancer is radiologically indistinguishable from non-secreting tumors. If CT does not provide answers, MRI comes to the rescue, capable of revealing tiny islet cell tumors of the pancreas.
Primary aldosteronism (Conn's syndrome)
In Conn's syndrome, it is critically important to differentiate aldosterone-producing adenoma (APA), which requires surgery, from bilateral adrenal hyperplasia (BAH), which is subject to medical therapy.
- Aldosteronomas on CT: have low X-ray attenuation (less than 10 HU), show poor contrast enhancement, and are extremely rarely calcified. Their average size is about 2 cm. Thin-slice multidetector CT (1–2 mm slices) is mandatory for reliable detection.
- Bilateral hyperplasia on CT: is diagnosed if the thickness of the adrenal limbs reaches 5 mm or more.
- MRI: thanks to the chemical-shift imaging technique (chemical-shift imaging), signal drop is visualized in out-of-phase images, confirming an abundance of intracellular lipid.
If imaging data are equivocal (e.g., a non-functioning nodule on one side and a small adenoma on the other), selective adrenal venous sampling (AVS) is used. The technique is technically challenging for catheterizing the right vein, but provides lateralization accuracy of over 95%.
Congenital adrenal hyperplasia (CAH) and virilization
Virilization occurs due to androgen excess. In adults, this is most often the consequence of adrenocortical carcinoma (usually greater than 2 cm in diameter). In children, the leading cause is CAH, which in 95% of cases is caused by 21-hydroxylase deficiency.
On CT and MRI, CAH shows bilateral, marked diffuse or nodular enlargement of the glands. A specific sign is preservation of the normal adeniform configuration of the organ despite hyperplasia. Without adequate treatment, secondary adenomas may form, which after the initiation of replacement therapy either regress or are replaced by adipose tissue (lipomatous replacement).
Important: in children and young patients, preference is given to ultrasound and MRI to avoid exposure to ionizing radiation.
Medullary tumors: Pheochromocytomas
Although pheochromocytoma originates from the adrenal medulla, it is a major neuroendocrine tumor of the adrenal gland. By secreting catecholamines, it causes paroxysmal crises with hypertension and tachycardia.
Sporadic pheochromocytomas are usually large at the time of detection (in 90% of cases greater than 2 cm). On unenhanced CT, this is a soft-tissue attenuation mass, containing punctate calcifications in 12% of cases. Upon administration of non-ionic contrast (which is absolutely safe for such patients), the tumor demonstrates avid enhancement. Due to frequent necrosis, the center of the mass may appear cystic, but the thick peripheral rim actively enhances.
Briefly: answers to topical questions
What MRI characteristics are typical for adrenocortical carcinoma?
Adrenocortical carcinoma on MRI is typically characterized by a large heterogeneous mass with signs of hemorrhage, necrosis, and vascular invasion.
- T2-weighted images (T2-weighted images) — a large heterogeneous mass is visualized, and a tumor thrombus invading the inferior vena cava may be detected.
- T1-weighted fat-suppressed images (fat saturation) — areas with high signal intensity are identified, indicating the presence of hemorrhage zones.
- Contrast-enhanced T1-weighted images — heterogeneous enhancement of the contrast agent with non-enhancing areas corresponding to zones of necrosis is noted.
Which tumors are most frequently the source of ectopic ACTH production syndrome?
The cited sources mention tumors of the chest and abdomen, including tumors of the lungs, pancreas, colon, and thymus, as potential sources of ectopic ACTH production.
- Bronchial carcinoids — usually small in size (3–15 mm), difficult to distinguish from granulomas and hamartomas.
- ACTH-producing small cell lung cancer (SCLC) — radiologically looks identical to non-ACTH-producing tumors.
- Neuroendocrine tumors of the pancreas and colon — radiologically look identical to non-ACTH-producing tumors; abdominal MRI can reveal small pancreatic islet cell tumors not visible on CT.
- Thymic carcinoid (Thymic Carcinoid) — causes Cushing's syndrome due to ectopic ACTH secretion in approximately 40% of patients.
Which radionuclide imaging methods are used for specific visualization of pheochromocytoma?
Scintigraphy with radioiodinated guanethidine analogues and positron emission tomography are used for specific visualization of pheochromocytoma.
- MIBG scintigraphy (Meta-iodobenzylguanidine) — uses 131I-MIBG and 123I-MIBG isotopes for whole-body scanning to detect foci of radiotracer accumulation in chromaffin cells.
- PET/CT — the most reliable radiopharmaceutical is 6-[18F]-fluorodopamine, which localizes adrenal and extra-adrenal lesions. [11C]-hydroxyephedrine, [11C]-epinephrine, and [18F]-fluorodihydroxyphenylalanine are also used. 18FDG PET has relatively low sensitivity and is used mainly for metastatic disease.
The "rule of tens" for pheochromocytomas: 10% are multiple, 10% are malignant (especially when size >6 cm), 10% are non-functioning, 10% are associated with genetic syndromes (neurofibromatosis, von Hippel-Lindau disease, multiple endocrine neoplasia).
Class and exam questions
At what point in the diagnostic workup should adrenal CT or MRI be ordered?
Strictly after clinical and laboratory confirmation of hormonal activity. Imaging is needed to localize the source, not to establish the presence of the endocrine syndrome itself.
Why might a CT scan "miss" an aldosteronoma?
Aldosteronomas are small in size (sometimes 1 cm). If standard CT with thick slices is performed, the tumor is easily missed. Thin-slice CT (1–2 mm) is required.
What is [11C]-metomidate PET/CT?
This is a reserve method for searching for aldosteronomas. A tumor-to-normal adrenal SUVmax ratio greater than 1.25 has high specificity, but the method is limited due to the need for a cyclotron in the clinic.
What else to study on this topic
- Technique and pitfalls of selective adrenal venous sampling (AVS).
- Chemical-shift MRI protocols for detecting intracellular lipid.
- Differential diagnosis of adrenal incidentalomas.
- Features of the radiological pattern of adrenocortical carcinoma (ACC).
- Algorithm for searching for the source of ectopic ACTH production in negative CT findings.
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