By what three routes do internal malignancies metastasize to the skin?

They extend by local infiltration, lymphatic spread, or hematogenous spread. Breast carcinoma and oral cancer are the most likely to demonstrate direct extension into the skin. It is assumed that the fundamental mechanisms are similar to those of metastasis to parenchymal organs, but this has not been investigated. Rare cases have been inoculated by local procedures such as needle biopsies.

Jilani G, Mohamed D, Wadia H, et al: Cutaneous metastasis of renal cell carcinoma through percutaneous fine needle aspiration biopsy: case report, Dermatol Online J 15:10, 2010.