Ross et al. Journal of Medical Case Reports 2011, 5:383 http://www.jmedicalcasereports.com/content/5/1/383 Page 3 of 6 enabling the ureteric stents to be removed. Since this time there has only been one minor episode of urinary tract infection, but pain continues to be a problem. A recent MRI scan (see Figures 1 and 2) shows that over the entire treatment period with sirolimus, a sub- stantial degree of tumor shrinkage has occurred, from a maximum diameter of 92 mm a year after initial presen- tation (prior to starting sirolimus) to 53 mm following two years of sirolimus treatment. The patient has con- tinued with sirolimus treatment, currently at a dose of 4 mg daily, 5 mg being associated with mouth ulceration. Discussion A diagnosis of PEComa is made on the basis of histolo- gical and immunohistochemical findings. Basic charac- teristic features of perivascular epithelioid cells include abundant clear to eosinophilic granular cytoplasm with an immunoprofile of HMB45 positivity and lack of expression of S-100 protein, though the latter is positive in a minority of cases. Other features include expression of another melanoma-associated antigen melan A, coex- pression of muscle cell markers without cytokeratin expression and the presence of melanosomes or pre- Figure 1 MRI scan of the pelvis prior to starting sirolimus, following total abdominal hysterectomy and bilateral salpingo- oophorectomy (TAH BSO), demonstrating the perivascular epithelioid cell tumor (PEComa) measuring 92 mm.