A 68-year-old woman with a history of breast cancer presents with bone pain and elevated serum calcium. Laboratory investigations reveal an elevated carcinoembryonic antigen (CEA) level. Which of the following is the most common primary malignancy associated with elevated CEA in clinical practice?

Explanation

## Tumor Marker: Carcinoembryonic Antigen (CEA) ### Overview CEA is a glycoprotein normally produced during fetal development and found in low levels in healthy adults (<5 ng/mL). Elevated levels indicate malignancy or benign inflammatory disease. ### Most Common Malignancy: Colorectal Adenocarcinoma **Key Point:** CEA is the most frequently elevated tumor marker in colorectal cancer and is used for screening, diagnosis, and surveillance. **High-Yield:** - **Sensitivity in CRC:** 50–80% (depends on stage; higher in advanced disease) - **Specificity:** ~90% when >10 ng/mL - **Clinical use:** Baseline CEA before surgery; post-operative surveillance for recurrence - **Prognostic value:** Elevated pre-operative CEA correlates with worse prognosis and higher recurrence risk ### CEA Elevation Frequency Across Malignancies | Malignancy | Frequency of Elevation | Sensitivity | Clinical Utility | |---|---|---|---| | **Colorectal cancer** | Most common | 50–80% | Gold standard marker; used for surveillance | | Breast cancer | Common | 40–50% | Indicates advanced/metastatic disease | | Lung cancer (adenocarcinoma) | Common | 40–60% | Useful in adenocarcinoma subtype | | Gastric cancer | Moderate | 30–50% | Less specific than in CRC | | Ovarian cancer | Rare | <20% | Not a primary marker; CA-125 preferred | | Pancreatic cancer | Moderate | 30–40% | CA 19-9 is more specific | **Clinical Pearl:** While CEA can be elevated in multiple malignancies and benign conditions (smoking, inflammatory bowel disease, cirrhosis), colorectal cancer remains the most common and clinically significant association. CEA is the standard tumor marker for CRC surveillance post-resection. ### Why Colorectal Cancer is the Answer Colorectal adenocarcinoma is the most common malignancy associated with elevated CEA. It is the primary indication for CEA measurement in clinical practice, and elevated levels are used for prognostication and post-operative surveillance.