Examkin

Question

Difficulty: HardInterpreting Imaging Results

A 62-year-old man comes to the clinic for follow-up after an emergency department visit for left flank pain. His pain has resolved, and he successfully passed a small ureteral stone at home. His medical history is notable for stage I non-small cell lung cancer, for which he underwent a right lower lobectomy 3 years ago without signs of recurrence on subsequent surveillance. During his recent emergency department visit, a non-contrast computed tomography (CT) scan of the abdomen and pelvis was performed. The scan confirmed a 3-mm calculus in the distal left ureter and incidentally revealed a 2.5-cm, smooth, well-circumscribed nodule in the left adrenal gland. The radiologist notes that the region of interest within this adrenal nodule demonstrates an average attenuation of +4 Hounsfield units (HU). Vital signs are within normal limits, and physical examination is unremarkable. Based on the specific imaging characteristics of this adrenal mass, which of the following is the most likely diagnosis?

  1. Lipid-rich adrenocortical adenomaAnswer
  2. B
    Metastatic non-small cell lung cancer
  3. C
    Lipid-poor adrenocortical adenoma
  4. D
    Adrenal myelolipoma
  5. E
    Pheochromocytoma

Answer

Lipid-rich adrenocortical adenoma
The correct diagnosis is a lipid-rich adrenocortical adenoma. The initial evaluation of an incidentally discovered adrenal mass (incidentaloma) relies heavily on unenhanced CT imaging. The Hounsfield unit (HU) scale measures tissue radiodensity. Because adrenal adenomas frequently contain abundant intracellular lipid (cholesterol), they have low radiodensity. An unenhanced attenuation threshold of less than 10 HU is approximately 70% sensitive and 98% specific for a benign lipid-rich adrenocortical adenoma. Because this patient's mass has an attenuation of +4 HU, it can be definitively diagnosed as a benign lipid-rich adenoma based on imaging alone, avoiding the need for further costly or invasive diagnostic imaging like contrast-washout studies, MRI, or biopsy.

Step-by-Step Solution

1
Identify the key imaging finding on the non-contrast CT.
The incidental adrenal nodule has a measured attenuation of +4 Hounsfield units (HU).
Attenuation values (measured in HU) quantify tissue radiodensity and are the primary initial data point used to characterize adrenal incidentalomas.
2
Apply the diagnostic criteria for adrenal adenomas based on radiographic attenuation.
An unenhanced attenuation of less than 10 HU is diagnostic for a lipid-rich adrenocortical adenoma.
Abundant microscopic intracellular lipid significantly lowers the average attenuation, distinguishing benign adenomas from most malignant and functional lesions.
3
Synthesize the objective imaging findings with the patient's clinical history.
Despite the patient's prior history of non-small cell lung cancer, the objective imaging finding (< 10 HU) establishes a diagnosis of a benign adenoma and effectively rules out metastasis.
Adrenal metastases almost universally have attenuation values > 20 HU. In this scenario, interpreting the precise imaging characteristics supersedes the clinical anchoring on the oncologic history.

Key Concept

On a non-contrast CT scan, an adrenal mass with an attenuation of less than 10 Hounsfield units (HU) is diagnostic of a benign lipid-rich adrenocortical adenoma, requiring no further advanced imaging.
Rate this question

Topics

Interpreting Imaging Results

More questions from this topic