“Dark Paget” Cells in Extramammary Paget Disease: A Staining Artifact and Diagnostic Pitfall

To the Editor: Extramammary Paget disease (EMPD) is characterized histologically by pale intraepidermal neoplastic cells. However, we have noted several lesions (of which 3 are listed below) with a possible staining artifact where the pagetoid cells were not pale, but darker than expected. The only clues were the anatomic location, a high index of suspicion, and scattered cells that stood out from the keratinocytes, including many that were darker than the surrounding keratinocytes. Immunohistochemical studies confirmed the diagnosis of EMPD. Case 1 is a vulvar lesion of a 66-year-old woman with a remote history of rectal carcinoma. She had undergone excision and had received chemotherapy and radiation therapy. She presented with a chronic vulvar rash that was diagnosed as EMPD. She experienced local recurrence of EMPD 18 months after diagnosis. Case 2 is a left pubic lesion of a 73-year-old man. Case 3 is a vulvar lesion of a 79-year-old woman. Additional clinical information was not available for Cases 2 and 3. On histologic examination, all 3 biopsies shared the following feature: single or clustered cells that were darker than the surrounding epidermis (Fig. 1). Immunohistochemistry revealed that the lesional cells were CK 7- and Cam 5.2-positive (not shown) (case 1) and CK7-positive and CK 5/6-negative (not shown) (cases 2 and 3). Comparison of the immunohistochemical studies with the H&E findings illustrates that although some Paget cells are darker and stand out on H&E, many are not easily evident or distinguishable from the surrounding keratinocytes.FIGURE 1.: A–C, Left vulvar lesion (case 1) showing pagetoid cells with relatively darker cytoplasm extending to the stratum corneum (A and B, H&E, ×40 and ×100 respectively; C, CK7, ×40). D–F, Left pubic lesion (case 2) showing pagetoid cells with relatively darker cytoplasm in single cells (D and E, H&E, ×100 and ×200 respectively; F, CK7, ×40). G–I, vulvar lesion (case 3) showing clusters of darker pagetoid cells (G and H, H&E, ×100 and ×200 respectively; I, CK7, ×40).Paget disease was first described by James Paget in 1874 (mammary Paget1's disease) (MPD). Following that in 1889, EMPD was described by Radcliffe Crocker.2 Paget disease is a rare form of intraepithelial adenocarcinoma. The origin of primary EMPD is still debated. An origin from Toker cells, pluripotent stem cells, or intraepidermal cells of apocrine gland ducts have been suggested.3,4 Historically, Paget disease is divided into MPD and EMPD based on the anatomic location. Most cases of MPD are associated with underlying breast carcinoma5 whereas most cases of EMPD are primary and hence called primary EMPD. However, some EMPD cases are associated with an underlying malignancy (secondary EMPD). The most common underlying malignancies associated with secondary EMPD include rectal carcinoma, perianal carcinoma, urothelial carcinoma, endometrial, and endocervical neoplasms.6,7 In the largest recently published cohort of patients with EMPD, patients with anal Paget had a much higher risk of both proximal and local neoplasms compared with patients with genital Paget.8 Paget cells contain cytoplasmic mucin and hence stain positive for mucicarmine, Alcian blue, and PAS.9,10 Interestingly, MPD and EMPD express different mucin genes. MUC5AC and MUC1 are expressed in all cases of primary EMPD, whereas MUC2 is usually negative.11 In contrast, Paget cells in MPD are usually positive for MUC1 and negative for MUC5AC.11,12 Most cases of both MPD and EMPD also stain positive for EMA and carcinoembryonic antigen.13,14 Some studies show that carcinoembryonic antigen is only positive in 35% of the cases of MPD and positive in most cases of EMPD.11,15 Among the cytokeratin family, CK7 is a highly sensitive marker for Paget cells,16,17 but it can be expressed by pagetoid Bowen disease, sebaceous carcinoma in situ, and Toker cells.18–20 Low molecular weight cytokeratins such as Cam5.2 are usually positive, whereas high molecular weight cytokeratins are negative.21,22 CAM 5.2 is preferred in our practice, because it usually shows diffuse and strong positivity in both MPD and EMPD as it is negative in pagetoid Bowen disease.22 Melanocytic markers are negative, which is a very helpful feature in evaluating cases of where pigmentation is present.23–26 The confounding cases presented highlight the importance of noting scattered, darker cells in the appropriate context as a clue to EMPD, and performing further workup to avoid misdiagnosis. It is possible that these “dark Paget” cells are the result of a staining artifact, but it is one that we have noted in several cases from different practices/laboratories (as highlighted by the cases in this report from 3 different practices). It is not clear why this staining pattern occurs. It could be speculated that darkly stained nuclei standing out more than the cytoplasm may be contributing to a “darker” overall appearance of the cells. However, even the cytoplasm does not appear to be as pale as typically seen in Paget cells. Therefore, the cause of this staining pattern is not known, but it is helpful to be aware of, because it may lead to a consequential diagnostic pitfall.