Angiomyolipoma is a benign clonal neoplasm consisting of varying amounts of mature adipose tissue, smooth muscle, and tick-walled vessels.

Although a benign status is well accepted for angiomyolipomas, extrarenal occurrences have been reported in the hilar lymph nodes, retroperitoneum, and liver, and direct extension into the venous system has been reported. A uniformly benign clinical course in such cases has argued in favor of multicentric origin rather than metastasis.

Many angiomyolipomas exhibit regions of cellular atypia, and the pathologic differential diagnosis can exclude a number of subtypes of sarcoma, including fibrosarcoma, leiomyosarcoma, and liposarcoma, depending on the relative amounts of adipose, vascular, or smooth muscle tissue present. Positive immunoreactivity for HMB-45, a monoclonal antibody raised against a melanoma-associated antigen, is characteristic for angiomyolipoma and can be used to differentiate it from sarcoma in such cases.

Nevertheless, one should be aware that there have been reports of high-grade and eventually lethal leiomyosarcoma arising within an angiomyolipoma, and Christiano et al. (1999) have described a highly pleomorphic angiomyolipoma that was associated with the development of multiple pulmonary nodules, the majority of which stained positive for HMB-45. They believed that his case represented a malignant transformation of angiomyolipoma.

More recently, a malignant epithelioid variant of angiomyolipoma has been described that can metastasize. Such malignant variants of angiomyolipoma are rare, and the majority of them pursue a benign course.

In summary, angiomyolipoma is a benign neoplasm that enters the differential diagnosis of several malignant neoplasms. If malignant transformation occurs, it must be exceedingly rare, and some cases of reported malignization may be due to erroneous initial diagnosis of other sarcomatous tumors.

REFERENCE

CAMPBELL, S. C.; NOVICK, A. C.; BUKOWSKI, R. M. Renal tumors. In: WEIN, A. J. et al. (Ed.). Campbell-Walsh urology. 9th ed. Philadelphia: Saunders Elsevier, 2007. p. 1578-1579.

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So called "epithelioid angiomyolipoma" or "pure epithelioid perivascular epithelioid cell tumor/PEComa" is thought to be a manifestation of AML/PEComa with at least some malignant potential, although the rate of aggressive behavior varies somewhat in the literature. Such tumors are often likely to be confused with a carcinoma microscopically rather than a usual angiomyolipoma. My impression is that these tumors usually do not develop from a preexisting benign AML if ever but rather develop de novo as such, although I have not carefully looked at the literature on that aspect.

Article Pure Epithelioid PEComas (So-Called Epithelioid Angiomyolipo...

Another recent paper

Article Epithelioid angiomyolipoma of the kidney: Pathological featu...

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It can be also tuberose sclerose and involve other organs and first of all cns.