What constitutes invasion in tumor pathology?

• A. Tumor cells penetrating beyond the basement membrane or infiltrating surrounding normal tissue.
• B. Tumor cells penetrating only within a capsule.
• C. Metastasis to distant organs.
• D. Angiogenesis.

Answer: (A)

Invasion is the local spread of tumor cells beyond the basement membrane into the surrounding normal tissue. This breach of the basement membrane and infiltration into adjacent stroma define malignant invasion and differentiate invasive cancer from noninvasive or benign growth, which remains confined, even if it forms a mass within a capsule. Metastasis is a separate process involving spread to distant organs, not the local invasion itself. Angiogenesis refers to new blood vessel formation to support tumor growth, not the act of invading neighboring tissues. Therefore, the key feature is tumor cells penetrating beyond the basement membrane and infiltrating surrounding tissue.