Define perineural invasion and its clinical significance.

• A. Tumor cells infiltrating nerves; associated with higher recurrence and worse prognosis.
• B. Tumor cells within blood vessels; associated with hematogenous spread.
• C. Tumor cells within lymphatics; associated with nodal metastasis.
• D. Tumor cells invading bone; associated with structural damage.

Answer: (A)

Perineural invasion is when tumor cells enter and spread along nerves, traveling through the nerve sheath and surrounding spaces. This provides a route for cancer to extend beyond the main tumor mass and into adjacent tissues that are hard to remove completely. Clinically, its presence signals more aggressive disease and is linked to a higher risk of local recurrence after treatment and a poorer prognosis. It also helps explain why some patients experience nerve-related symptoms or pain as the tumor interacts with neural pathways. In pathology, recognizing perineural invasion can influence treatment decisions, such as opting for wider surgical margins or adding adjuvant radiation in cancers where this feature is common and impactful.

Tumor cells within blood vessels indicate vascular (hematogenous) spread, not perineural invasion; tumor cells within lymphatics indicate lymphatic spread and nodal metastasis; and tumor cells invading bone indicate osseous invasion, which are distinct patterns of invasion.