Primary cutaneous ALCL is a rare lymphoma of the skin. Although the cells responsible for primary cutaneous ALCL have many similar features to the cells responsible for systemic ALCL, the disease is typically less serious. The treatment is normally different and the prognosis is usually good. Importantly, primary cutaneous ALCL only occurs on the skin or (less commonly) also in a lymph node close to the part of skin affected (known as a "regional node"). ALCL is referred to as systemic if there is evidence of disease anywhere else in the body beyond the skin and an area or regional node. Primary cutaneous ALCL does not usually progress to systemic ALCL.

Primary cutaneous ALCL belongs to a group of diseases called cutaneous T-cell lymphomas. This is an umbrella term used to include a group of conditions that have some similar features but which differ in detail. Though they share some treatments in common, as more is learned about these conditions, the treatment is likely to become more specific for each type of cutaneous T-cell lymphoma.