A 13-year-old male presents to your clinic with 2 to 3 months of progressive pain in the elbow, as well as swelling and loss of range of motion. He also has had low-grade fevers. X-rays are shown in Figure 8–21A and B. Of note, ESR, LDH, and WBC counts are elevated on laboratory studies.

Figure 8–21 A

Figure 8–21 B

What is the likely diagnosis?

A. Osteomyelitis

B. Chondrosarcoma

C. Ewing sarcoma

D. Osteosarcoma

Discussion

The correct answer is (C). Ewing sarcoma is a malignant bone tumor composed of small, round blue cells and is the second most common primary bone tumor in children. Pain is the most common symptom, sometimes accompanied by limp, swelling, and fever. Since laboratory abnormalities (elevated WBC, ESR, and LDH) are often seen, this can be mistaken for infection. Radiographs reveal a permeative bone lesion with layered periosteal reaction, sometimes called “onion skinning.” It is poorly marginated and permeative, accompanied often by an extensive soft-tissue mass that requires MRI to fully characterize.

What is the chromosomal translocation associated with this lesion?

A. t(11;22)

B. t(x;18)

C. t(12;16)

D. t(2;13)

Discussion

The correct answer is (A). The classic chromosomal translocation associated with Ewing sarcoma is t(11;22) which results in the EWS/FLI1 fusion gene. This fusion can be identified by polymerase chain reaction and differentiates Ewing sarcoma from other round cell lesions. The other answers are incorrect: t(x;18) is seen in synovial sarcoma, t(12;16) is associated with myxoid liposarcoma, and t(2;13) is seen in alveolar soft parts sarcoma.

Which of the following histologic slides (Fig. 8–22A–D) is most consistent with this diagnosis?

A. A

B. B

C. C

D. D

Figure 8–22 A–C

Figure 8–22 D

Discussion

The correct answer is slide (A). Histopathologic examination of Ewing sarcoma reveals small, round blue cells with prominent round or oval nuclei, and minimal cytoplasm. Slide B showed plasma cells, which in contrast to Ewing sarcoma cells, have abundant cytoplasm with eccentric clock faced nuclei. Slide C demonstrates a highly cellular synovial sarcoma, while slide D represents desmoid fibromatosis.

Which immunohistochemical stain is helpful to histologically differentiate this tumor from other marrow tumors?

A. CD31

B. CD99

C. CD34

D. S100

Discussion

The correct answer is (B). CD99 is a cell surface marker that is seen in Ewing sarcoma.

Objectives: Did you learn...?

To recognize Ewing sarcoma imaging?

To recognize Ewing sarcoma histology and genetics