In a series of 100 patients with confirmed ACTH-dependent Cushing syndrome, the most common site of the ACTH-secreting tumor is which of the following?

Explanation

## Most Common Site of ACTH Secretion in Cushing Syndrome **Key Point:** The pituitary gland is the most common site of ACTH-secreting tumors in Cushing syndrome, accounting for approximately 70–80% of all ACTH-dependent cases. ## Distribution of ACTH-Secreting Tumors | Site | Frequency | Tumor Type | Clinical Features | |------|-----------|-----------|-------------------| | **Pituitary** | **70–80%** | **Corticotroph adenoma (microadenoma)** | **Slow onset, female predominance, suppressible** | | Lung | 10–15% | Small cell carcinoma | Rapid onset, very high ACTH, non-suppressible | | Pancreas | 5–10% | Neuroendocrine tumor | Variable presentation, non-suppressible | | Thymus | 2–5% | Carcinoid tumor | Indolent course, non-suppressible | | Other (adrenal medulla, ovary) | <2% | Rare neuroendocrine tumors | Uncommon presentations | ## Anatomical and Pathological Basis **High-Yield:** The pituitary gland contains corticotroph cells (15–20% of anterior pituitary cells) that normally secrete ACTH in response to CRH from the hypothalamus. Adenomatous transformation of these cells is the most frequent endocrine tumor of the pituitary. **Clinical Pearl:** Pituitary ACTH-secreting adenomas are typically: - **Microadenomas** (<10 mm) in 80% of cases - **Benign** with low recurrence rate after surgical resection - **Responsive to dexamethasone suppression** (distinguishing feature from ectopic ACTH) - **More common in women** (female-to-male ratio ~3:1) - **Age of onset:** typically 30–50 years ## Ectopic ACTH Sources (Remaining 20–30%) When ACTH-dependent Cushing is NOT from the pituitary: 1. **Lung (small cell carcinoma)** — most common ectopic source (~50% of ectopic cases) - Aggressive, rapid onset - Very high ACTH levels (often >200 pg/mL) - Non-suppressible by dexamethasone 2. **Pancreatic neuroendocrine tumor** — second most common ectopic source - Often indolent - May be part of MEN-1 syndrome - Non-suppressible 3. **Thymic carcinoid** — less common but important - Often associated with MEN-1 - Slow growth - Non-suppressible **Mnemonic:** **PELT** — Pituitary (most common), Ectopic lung (second), Less common: pancreas and thymus. ## Diagnostic Approach to Localize ACTH Source ```mermaid flowchart TD A[ACTH-dependent Cushing confirmed]:::outcome --> B[Perform dexamethasone suppression test]:::action B --> C{Suppression at high dose?}:::decision C -->|Yes| D[Pituitary source]:::outcome C -->|No| E[Ectopic ACTH source]:::outcome D --> F[MRI pituitary]:::action E --> G[CT chest/abdomen/pelvis]:::action E --> H[Consider IPSS if imaging negative]:::action F --> I[Microadenoma visible?]:::decision I -->|Yes| J[Confirm pituitary source]:::outcome I -->|No| K[IPSS for localization]:::action ``` [cite:Harrison 21e Ch 375]