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Gross Features: Angiomyolipoma a complex neoplasm typically composed of thick-walled
blood vessels, smooth muscle, and fat, the relative proportions of which vary widely
When fat predominates, angiomyolipoma is yellow and has a greasy appearance. When
smooth muscle and blood vessels predominate, angiomyolipoma is pink or tan.
Hemorrhage and degeneration may give a friable and mottled appearance.
Angiomyolipoma Extending into Vena Cava
21 cases, all but one female
7 with tuberous sclerosis
Mean age 44, range 16-75
Mean follow up 33 months (n=14) range 1-122
All alive and well
angiomyolipomas are single and tend to be larger than 4 cm in diameter since the
fat density on CT scan facilitates the radiologic diagnosis and it is recommended
that asymptomatic ones be removed only when they exceed 4 cm (rupture and
hemorrhage are uncommon in smaller ones). In tuberous sclerosis, there usually are
multiple angiomyolipomas (indeed multiple angiomyolipomas suffice for the diagnosis
of tuberous sclerosis). Angiomyolipomas tend to be well-circumscribed but
confluence of multiple angiomyolipomas in tuberous sclerosis can cause an
appearance of infiltration. Rarely, angiomyolipoma extends into the renal vein or
Microscopic Features: The microscopic diagnosis of angiomyolipoma usually is
straightforward because of the typical mixture of spindle-shaped smooth muscle
cells, fat, and thick-walled blood vessels. Nuclear pleomorphism and mitotic
activity are occasional present. The smooth muscle of angiomyolipoma frequently
consists of both spindle and epithelioid cells but it is rare for epithelioid
smooth muscle to predominate.
Diagnostic Problems with Epithelioid Angiomyolipoma
Epithelioid cells mimicking renal cell carcinoma
Predominance of spindle cells mimicking leiomyoma
Nuclear atypia and high cellularity mimicking leiomyosarcoma
Confluent angiomyolipomas mimicking an infiltrative process
Collision between angiomyolipoma and renal cell carcinoma
Recently, it has been recognized that some angiomyolipomas are composed mainly of
epithelioid smooth muscle of epithelioid type, producing a histological picture
distinctly different from the usual angiomyolipoma. These tumors are characterized
by cells with abundant eosinophilic cytoplasm. At first examination, tumors in
this category have some features which suggest carcinoma: polygonal cells with
abundant eosinophilic cytoplasm growing in sheets with areas of necrosis, mitotic
activity, and nuclear pleomorphism.
Differential Diagnosis: Among primary renal epithelial tumors, the principal
considerations are clear cell renal cell carcinoma with granular cytoplasm, the
eosinophil variant of chromophobe renal cell carcinoma, and renal oncocytoma.
Among mesenchymal tumors, benign fibrous histiocytoma is a consideration. Clear
cell renal cell carcinoma with granular cytoplasm differs from epithelioid
angiomyolipoma in having the characteristic vascular pattern and alveolar, tubular,
or microcystic architectural patterns, all of which are absent from epithelioid
angiomyolipoma. Immunohistochemistry can be helpful in the differential diagnosis.
Epithelioid angiomyolipoma uniformly fails to react with antibodies to cytokeratins
and epithelial membrane antigen, and reacts with antibodies to smooth muscle actin,
and to a lesser extent, desmin. That smooth muscle cells of angiomyolipoma,
especially those with epithelioid morphology, react with HMB-45 has been known
since 1991. Recently, HMB-45 has been shown to react with leiomyomas arising from
the renal capsule.