Hui-Lung Tai md, Division of Urology, department of Surgery, Changhua Christian Hospital, 135 Nanhsiao Street, Changhua City, Taiwan. Email: firstname.lastname@example.org
Abstract We report the case of a huge right renal tumor in a 17-year-old girl. Absence of fat on preoperative magnetic resonance imaging suggested renal cell carcinoma, and surgery was performed. Pathologic evaluation with HMB-45 immunohistochemical staining conﬁrmed the diagnosis of epithelioid angiomyolipoma. The tumor consisted predominantly of epithelioid cells, and it could easily be misidentiﬁed as a renal cell carcinoma due to the paucity of the fat component. Previous reports have suggested that epithelioid angiomyolipomas have the potential to be malignant, and thus regular postoperative surveillance is recommended. Our patient had no signs of recurrence at her most recent follow up, 12 months after surgery.
Angiomyolipoma (AML) is a common soft tissue tumor that often involves the kidney or liver. Renal AML is traditionally considered benign due to lack of recurrence or metastases; conservative therapy is recommended.1,2 We report the case of a 17-year-old girl with a huge right renal tumor (15 cm × 11 cm × 5 cm) that radiographically suggested renal cell carcinoma (RCC) due to minimal intratumoral fat content. After nephrectomy for suspected malignancy, pathologic examination using immunohistochemical staining with HMB-45 conﬁrmed the diagnosis of epithelioid AML.
A 17-year-old healthy girl was referred for evaluation of a right renal tumor that she had identiﬁed as a palpable mass in the right upper abdomen. She was asymptomatic, and there was no family history of cancer or genetic disorders. Renal sonography revealed a huge right renal mass with heterogeneous echogenicity. A magnetic resonance image (MRI) conﬁrmed a right renal tumor of roughly 16 cm × 10 cm × 9 cm in size with no other intra-abdominal abnormality. The mass had cystic and solid components with calciﬁcation. With intravenous contrast medium, only the solid component was enhanced. The mass had well-deﬁned margins but no evidence of a fatty component (Fig. 1a,b). The renal collecting system was distorted due to external compression. Laboratory data were unremarkable. Excluding fat-containing AML, the presence of enhancement within a solid renal mass is characteristic of renal cell carcinoma (RCC). Right nephrectomy was performed. The patient recovered uneventfully and was discharged 6 days later. The patient was well, without evidence of recurrence, at her most recent follow up (12 months).
Grossly, the well-encapsulated tumor measured 15 cm × 11 cm × 5 cm, demarcated by adjacent kidney and thin layers of connective tissue from perirenal fat. Microscopically, the tumor was composed of polygonal epithelioid cells with densely eosinophilic cytoplasm, mild nuclear atypia, and dispersed multinucleated giant cells. Mitotic ﬁgures were absent. Only a minimal number of adipocytes and thick-walled vessels were noted (Fig. 2a–d). Immunohistochemistry strongly reacted with HMB-45 (Fig. 3) but was negative for S-100 protein, desmin, epithelial membrane antigen, cytokeratin, and RCC antigen. Overall, the ﬁndings ﬁt diagnostic criteria for epithelioid AML.
Classic AML has fat, vessels, and myogenic cells in varying proportions. However, a subtype is often misidentiﬁed as a sarcomatoid RCC due to a paucity of fat. This AML subtype consists predominantly of epithelioid cells that stain strongly for melanoma-associated markers (HMB-45).3 HMB-45, named for the material used to elicit immunological response (human melanoma, black), is speciﬁc for different epitopes of the protein product of the gene gp100-cl.4 Reactivity for HMB-45 protein provides a powerful tool with atypical tumors for distinguishing AML from RCC or other primary renal neoplasms.5
Angiomyolipoma can usually be diagnosed accurately due to its unique ultrasound and computed tomography (CT) appearance. The sonographic appearance is signiﬁcant for its marked hyperechoic signal mostly likely due to a combination of factors, including high fat content, multiple tissue interfaces and vascular tissues. The CT diagnosis of angiomyolipoma also depends on identifying fat in a renal lesion. On MRI the fat component of angiomyolipoma had high signal intensity on unenhanced T1-weighted images and lower intensity on T2-weighted images. Fat suppression techniques can also be helpful.2 Unfortunately some lesions contain only minimal fat and possibly a relatively large muscular component. These patients often undergo surgery due to diagnostic uncertainty. In our case, absence of fat on preoperative MRI suggested RCC, and surgery was performed. Pathologic evaluation with HMB-45 immunohistochemical staining conﬁrmed the diagnosis of epithelioid AML.6 Previous reports have suggested that epithelioid AML may be malignant,7 with documentation of local invasion,8 distant metastases,9 and death.10 Thus, regular radiographic surveillance is appropriate for patients with epithelioid AML.