Prostatic intraepithelial pagetoid histiocyte: a potential diagnostic pitfall

To the Editor: Intraepithelial pagetoid foam cell (IPFC) of the mammary duct has been well documented. However, IPFC another organ has never been reported. As far as we know, no article has been published on prostatic IPFC. We herein report a case of IPFC found in the prostate biopsy of a 67-year-old man who had complained of bilateral leg paralysis. X-ray and CT studies revealed multiple bone metastases, his serum prostate specific antigen (PSA) was 1414 ng/mL. A sextant prostate biopsy showed adenocarcinoma of Gleason score of 4 + 5 = 9. Intraepithelial pagetoid cells were present in a prostatic duct (Fig. 1) in the middle part of the left lobe, but not in an acinar gland. The urologist was not aware whether the location of the biopsied material was from close to the seminal vesicle or not. Pagetoid cells have large foamy cytoplasm and small round nuclei. These cells have light gray or brown pigment. The pigment was positive for PAS reaction and consistent with lipofuscin. Immunohistochemically, the pagetoid cells reacted to PG-M1 (Fig. 2) and KP1 antibodies. They were negative for AE1/3, 34bE12, P63 and PSA. We concluded the prostatic IPFC was a histiocyte, thus intraepithelial pagetoid histiocyte (IPH) seems a better designation for this case. Mammary IPFC may be confused with lobular neoplasia or ductal carcinoma. Prostatic IPFC/IPH also may be confused with prostatic intraepithelial neoplasia, pagetoid urothelial carcinoma, or foamy gland prostatic ductal adenocarcinoma. Prostatic IPFC/IPH has little nuclear atypia and a low nuclear–cytoplasmic ratio. The nuclei have fine chromatin and no prominent nucleoli. Immunohistochemistry for PG-M1 and/or KP1 should be considered for difficult cases.