Teaching Case

A 23-year-old female patient complained of pain and a mass in the buttock. Previous medical history was unremarkable. On physical examination an ovoid and swelling mass with a transversal diameter of 6 cm was discovered in the buttock.

Figure 1. US: Heterogeneous mass in the buttock with tubular structures corresponding to large blood vessels.
Figure 2. MRI of the pelvis.
2a. Sagittal T1-weighted image: Heterogenous mass in the buttock.
2b. The lesion in coronal T1-weighted image without gadolinium showed a sligthly more intense signal than the surrounding muscles.
2c. The mass presented high, heterogeneous signal intensity in STIR-weighted images. Tubular void flows within and around the mass are identified.
2e. In coronal T1-weighted image with gadolinium the lesion enhanced, predominantely in the periphery.
Figure 3. CT of the lesion: Heterogenous, peripheral enhancing mass after intravenous contrast administration. No bone erosion was found.
Figure 4. Chest CT shows a solitary pulmonary nodule of 5 mm of diameter.
Figure 5. Microscopic preparations
5a. H-E microscopic preparation: Polyhedral, epitheliomorphic cells forming solid nests. Nuclei were hyperchromatic with vesicles, the cytoplasms were eosinophilic with abundant glycogen.
5b. Characteristic intracellular protein carbohydrate crystals were visible with PAS staining.
5c. Citoplasmatic pattern vimentina positive.

Alveolar soft part sarcoma (ASPS) is a high-grade malignant soft tissue tumour of uncertain histogenesis and low incidence, less than 1% of soft tissue tumours. It usually occurs in young adults, second or third decade of life, and females. ASPS most commonly presents as a slowly growing mass in the lower extremity, arising in relation to muscle or musculofascial planes.
ASPS is of particular clinical significance because of the high prevalence of metastases: 24% patients at the diagnostic moment are found to have metastases. In the follow-up of patients without metastatic disease at the diagnosis, 40% develop these within 5 years, 62% within 10 years and 85% within 20 years [1]. Lung is the most commonly involved site by metastatic tumour followed by bone and brain.
In the differential diagnosis it is necessary to consider different aspects such as the location, the age and the imaging findings in order to narrow the possibilities.
This tumour most often affects the thigh and must be distinguished from malignant fibrous histiocytoma and liposarcoma, all of which most commonly occur in the thigh and the buttock [2]. Malignant fibrous histiocytoma occur most often in middle age and, unlike alveolar soft-part sarcoma, are uncommon before the age of 30. They appear as a malignant mass although their vascular pattern is variable.
According to imaging features the differential diagnosis must include highly vascular tumours such as clear cell sarcoma, some metastases, haemangioma, liposarcoma and soft-tissue tumour with haemorrhage. Haemangioma also has slightly high signal intensity on T1w images due to fibrofatty components, and has signal void within high flow vessels [2]. Other vascular origin tumours such as haemangioendothelioma, “spindle cell” haemangioendothelioma, haemangiopericytoma and angiosarcoma are solid tumours lacking venous channels found in ASPS [3].
Differentiation from arteriovenous malformation is possible thanks to demonstration of abundant solid tissue component and signal voids on MR imaging. Arteriovenous malformation has exclusively vascular components with scanty solid tissue components in the tumour and frequently has fibrofatty tissue with high signal on T1w images within the mass [4].
ASPS should be considered in the differential diagnosis of a hypervascular soft-tissue mass, particularly in the buttock or the thigh of a young female. Distinctive radiological features of ASPS allow the radiologist to suggest the diagnosis.