Question

A diaphyseal tumor, probably small round blue cells. Which test/translocation is useful for diagnosis?

• A. t(11;22)
• B. t(15;17)
• C. —

Hint:

"Small round blue cell" tumor of the bone diaphysis in a young patient — think of the EWS-FLI1 fusion.

Answer 1

The Correct Option is A

Step 1: Identify the lesion. A tumor arising in the diaphysis (mid-shaft) of a long bone that is composed of small round blue cells on histology is the classic description of Ewing sarcoma (a member of the Ewing sarcoma family of tumors / primitive neuroectodermal tumor, PNET). Small round blue cell tumors are so named because the cells are small, uniform, have scant cytoplasm and large hyperchromatic (blue-staining) nuclei on H&E.

Step 2: Recall the defining molecular marker. Ewing sarcoma is defined by the reciprocal translocation t(11;22)(q24;q12), which fuses the EWSR1 gene on chromosome 22 with the FLI1 gene on chromosome 11, producing the EWS-FLI1 fusion oncoprotein. Detecting this translocation (by FISH or RT-PCR) is the most useful confirmatory test, so option (a) \(t(11;22)\) is correct.

Step 3: Why the distractor is wrong. Option (b) \(t(15;17)\) is the translocation of acute promyelocytic leukemia (APML, AML-M3) - it fuses PML (chr 15) with RARA (chr 17) and has nothing to do with a bone tumor. It is therefore incorrect.

Final answer: Option a - t(11;22).