A 58-year-old woman with colon cancer presents with 3 months of increasing shoness of breath. A chest X-ray reveals numerous, bilateral, round masses in both lungs. Histologic examination of an open-lung biopsy discloses malignant gland-like structures, which are nearly identical to the colon primary. Which of the following changes in cell behavior was the first step in the process leading to tumor metastasis from the colon to the lung in this patient?
The first event in tumor cell invasion is a breach of the basement membrane that separates an epithelium from the underlying mesenchyme. After invading the interstitial tissue, malignant cells penetrate lymphatic or vascular channels (choice D). In the lymph nodes, communications between the lymphatics and venous tributaries allow malignant cells access to the systemic circulation.
Arrest within the circulating blood or lymph
Exit from the circulation into a new tissue
Invasion of the underlying basement membrane
Penetration of vascular or lymphatic channels
General Concepts