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.
Alveolar soft part sarcoma (ASPS) with lung metastasis
1. Ultrasound images: An approximately 6 cm × 4 cm oval-shaped mass is observed in the buttock region, with relatively well-defined margins and heterogeneous internal echogenicity.
2. MRI images: The mass is located near the muscle or fascial compartment of the buttock. On T1-weighted images, the signal is intermediate or slightly high. On T2-weighted images, the signal is relatively high. Prominent and concentrated enhancement is observed on contrast-enhanced scans. There is limited involvement of surrounding soft tissue structures, and no obvious bone invasion or changes are noted.
3. CT scan: A solid soft tissue mass with heterogeneous density is seen in the same region, showing marked enhancement, along with imaging characteristics suggestive of rich blood supply in certain areas.
4. Chest CT may suggest multiple slightly low-density or small nodular lesions in the lungs, raising concerns about potential metastatic risk.
5. Pathological sections indicate cells arranged in sheets or foam-like clusters with a characteristic alveolar structure. PAS staining shows glycogen positivity, and immunohistochemistry reveals TFE3 positivity in tumor cells.
Based on the patient’s age, sex, clinical history (slowly enlarging, palpable, relatively firm buttock mass in a young patient), imaging findings (MRI showing a highly vascularized soft tissue tumor with pronounced enhancement), and immunohistochemistry (TFE3 positivity, histology resembling alveolar structures), the most likely diagnosis is:
Alveolar Soft Part Sarcoma (ASPS).
If any uncertainty remains, further imaging (e.g., PET-CT) or more extensive pathological and immunohistochemical evaluation may be performed for confirmation. Additionally, regular chest CT follow-up is crucial to rule out or monitor metastatic spread.
During treatment or after surgical recovery, exercise should be introduced gradually based on the patient’s postoperative status, physical capacity, and functional level. Below is an example:
This report is based solely on the available imaging and pathological data, providing preliminary analysis to assist clinical evaluation. It does not replace in-person medical consultation or the professional judgment of clinicians. Patients should make final diagnostic and treatment decisions under the guidance of specialist physicians, taking into account their specific conditions.
Alveolar soft part sarcoma (ASPS) with lung metastasis
