Nucleolar organizer regions (NORs) in simple and proliferating trichilemmal cysts (pilar cysts and pilar tumors).

Simple trichilemmal cysts or pilar cysts and pilar tumors are relatively rare entities often under-reported by pathologists. The pilar cysts are thought to proliferate and progress to pilar tumors. These pilar tumors can further undergo malignant change. We analyzed 25 pilar cysts and eight pilar tumors, including three with atypia and one malignant pilar tumor, with a view to study the above progression and assess the degree of cell proliferation using the Nucleolar Organizer Regions (AgNORs). There was a progressive increase in the AgNOR count from one dot per nucleus in pilar cysts to 1.5-2 in benign pilar tumors. AgNORs in pilar tumors with atypia (2.8) was more than the benign pilar tumors but were definitely less than the malignant pilar tumors (3.5). The malignant pilar tumor showed bizarre AgNORs and cells with as many as eight to nine AgNORs. Thus AgNOR counts suggested that there is a progressive increase in the degree of cell proliferation and thereby the AgNOR staining from pilar cysts to pilar tumors. This AgNOR staining could also be used to assess the cell proliferation in case of pilar tumors with atypia where it is difficult to rule out malignancy.