Alveolar soft part sarcoma

Definition

A rare tumour, usually of adolescents and young adults, consisting of large eosinophilic cells with granular cytoplasm, arranged in nests / alveolar structures.

Epidemiology

This rare tumour accounts for 0.5% to 0.9% of all soft tissue sarcomas. It most commonly occurs between the ages of 15 and 35 years, rarely before 5 or after 50 years.

Clinical features

The most common site is the thigh/buttock (40% of cases), except in children, where the orbit and tongue are the most common sites. Occasional cases have been reported from the female genital tract, mediastinum, lung, stomach and bone.

Presentation is usually with a slow-growing mass. Because metastasis occurs early, some cases present with pulmonary or cerebral lesions.

Radiology

The high degree of vascularity results in contrast-enhancement on CT and a high signal intensity on T1 and T2 weighted images on MRI.

Macroscopic appearances

The tumour is poorly circumscribed. It is soft, grey or yellow. Necrosis and haemorrhage are common.

Histopathology

Cells form nests or, due to loss of cohesion, pseudo-alveolar structures. These are separated by thin fibrous septa with prominent sinusoidal vessels. Paediatric cases may lack nesting. The tumour cells are uniformly large and round/polygonal. There are well defined cell border and finely granular eosinophilic cytoplasm, less often clear or vacuolated. Very variable numbers of rod-like crystals are better seen with a diastase PAS stain. They contain one or two, sometimes up to five, nuclei. Mitoses are infrequent. Vascular invasion is almost universal.

Immunohistochemistry

There are no consistently positive findings:

	Desmin	sometimes positive
	Myo-D1	often cytoplasmic but not nuclear staining
	Myogenin	consistently negative
	S-100	about 25% of cases
	Synaptophysin	negative
	Chromogranin	negative
	NSE	about 25% of cases
	Neurofilament	negative
	Carboxy terminal of TFE3	nuclear positivity
	MCT1 monocarboxylate transporter	crystals positive
	CD147 (chaperone to MCT1)	crystals positive
	Cytokeratin	negative
	EMA	negative

Ultrastructure

Cells are surrounded by a discontinuous basal lamina. The cytoplasm contains numerous mitochondria, abundant ER and prominent Golgi. There are membrane-bound or free rhomboid crystals with a periodicity of 10 nm. These may sometimes be within secretory granules.

Cytogenetics

A consistent translocation, der(17)t(X;17)(p11;q25) results in a chimeric fusion protein, TFE3/ASPL (also known as ASP-SCR1, RCC17). The same translocation occurs in some paediatric/young adult renal adenocarcinomas.

Differential diagnosis

Management

Prognosis

Local recurrence after resection is uncommon, but metastasis is the rule, sometimes occurring many years later. In those without apparent metastases at the time of initial surgery, the survival is 60% at 5 years, 38% at 10 years and 15% at 20 years. Patient age (metastases more likely with increasing age), tumour size and stage, but not histological features influence survival. Metastases occur to lung, bone and brain but rarely to lymph nodes.

References

WHO classification of tumours: Tumours of soft tissue and bone.

This page last revised 19.1.2005.