ICOS and primary cutaneous T-cell non-Hodgkin lymphoma: The pcPTCL-NOS are a heterogeneous group of CTCL showing variable clinical (often aggressive) and immunohistochemical features giving rise to some difficulties with regard to classification among the CTCL variants, and may even exhibit T-cell helper follicular (THF) markers such as PD-1, CD10, CXCL13, BCL6, and ICOS and share pathologic characteristics with angioimmunoblastic T-cell and THF lymphomas [61].