This video presents a pathology case conference focusing on an anterior thigh mass in a 15-year-old female. The discussion quickly identifies the lesion as Alveolar Soft Part Sarcoma (ASPS) based on classic microscopic features, including a cellular mass separated by thick fibrous septa, prominent nesting ("alveolar") architecture, and large cells with distinct eosinophilic cytoplasm and prominent nucleoli. The confirmed molecular abnormality is the characteristic translocation t(X;17) involving the TFE3 gene. The pathologists emphasize key diagnostic differentiators, particularly distinguishing ASPS from metastatic renal cell carcinoma and Alveolar Rhabdomyosarcoma. Unique teaching points include the presence of scattered pleomorphism (unusual for a translocation sarcoma) and a helpful visual clue: the notched or "Pac-Man"-like appearance of the nuclei. Finally, the video notes the rarity of ASPS and its aggressive biological potential, despite potentially slow growth.
Summarizing a Pathology Case: Alveolar Soft Part Sarcoma (ASPS)
0:08 Case Introduction: The case involves a mass in the anterior thigh of a 15-year-old female.
0:22 Microscopic Findings: The mass exhibits thick fibrous septa surrounding cellular nests, creating an alveolar pattern. The cells possess prominent nucleoli and show slight pleomorphism, often adopting a rhabdoid or plasmacytoid appearance with pink cytoplasm.
1:27 Diagnosis: The final diagnosis is Alveolar Soft Part Sarcoma (ASPS).
1:36 Molecular Abnormality: ASPS is defined by the translocation t(X;17), which involves the TFE3 gene. Immunohistochemistry typically shows nuclear positivity for TFE3.
2:16 Differential Diagnosis (Ddx): The primary differential diagnosis is often metastatic renal cell carcinoma (RCC), as some RCCs can look similar.
2:36 Differentiating ASPS from Alveolar Rhabdomyosarcoma (ARMS): A crucial distinction is made: ASPS consists of big, pink/clear cells, while ARMS is a "round blue cell tumor" characterized by multinucleation and wreath cells.
3:38 Key Diagnostic Clue (The "Pac-Man" Sign): A helpful visual feature of ASPS nuclei is the presence of little divots, notches, or crinkled edges, likened to an "apple someone took a bite out of" or "Pac-Man."
4:12 ASPS Pleomorphism: Unlike most translocation sarcomas (which are typically uniform/monotonous), ASPS is an exception that often exhibits scattered pleomorphism.
4:59 Non-Alveolar Patterns (Pitfall): ASPS can also appear in solid nests or trabeculae (a "zellballen" appearance), which can be confused with paraganglioma or RCC, particularly in small biopsies. Pathologists must keep ASPS in the differential when encountering this pattern in unusual soft tissue sites.
6:03 Rarity and Prognosis: ASPS is extremely rare. Although they can be slow-growing, these tumors unfortunately tend to metastasize, including spread to unusual sites like the brain, marking it as an aggressive tumor.