Capillary vessels in Ewing's sarcoma: Overlooked?

Sir,

We describe an interesting cytomorphologic observation in a case of extraskeletal Ewing's sarcoma that may prove to be a useful clue to cytological diagnosis.

A 10-year-old male presented with a recurrent tumor on the back. The tumor was excised at a private hospital three months back and had rapidly regrown to the present size of 9 × 4 cm. Before excision, the tumor had been present for one year. Computerised tomography and radiography examination confirmed that the tumor was localized to the soft tissue, with no bony involvement.

On aspiration, the tumor was vascular and yielded a blood-mixed aspirate. Smears were cellular and showed small round cells arranged in loosely cohesive clusters. These clusters showed an abundance of capillary-sized vascular channels around which these cells were arranged [Figure 1]. The small round cells were of two different types. One cell type showed large cells with a high N/C ratio, a rim of finely vacuolated cytoplasm and finely dispersed chromatin. The second cell type was smaller in size and showed a more condensed chromatin. The cytoplasmic vacuoles were stained for glycogen using periodic acid Schiff reagent.

Figure 1

Small round cells adherent to vascular channels, against a tigroid background (Giemsa, ×400)

The slides and blocks of the previous excision were procured. Sections showed sheets of monomorphous cells divided into lobules by fibrovascular septae. Tumor cells showed ill-defined margins and scant amounts of pale to clear cytoplasm. The nuclei were round to oval to spindle shaped and showed finely dispersed chromatin and inconspicuous nucleoli. Mitotic activity was inconspicuous. On immunohistochemistry, tumor cells were strongly reactive with vimentin, CD99 and EWS antibody. A diagnosis of extraskeletal Ewing's sarcoma was considered.

Extraskeletal Ewing's sarcoma is the soft tissue counterpart of Ewing's sarcoma, first described by Angervall and Enzinger in 1975. Reports on cytomorphology are scant, but describe morphologic features similar to Ewing's sarcoma.[1-3] The presence of vascular endothelial channels in these clusters has not been previously described in cytological aspirates.

Ewing's sarcoma is richly vascular and shows an abundance of thin-walled vessels within the fibrovascular septae, producing a lobular pattern.[4] The vessels tend to get compressed and obscured by the closely packed tumor cell population. Immunostaining against CD31 was used in the present case to highlight these vascular channels. The rich vascularity becomes conspicuous in areas of degeneration and necrosis of tumor cells, producing the classical “filigree” pattern. The vascular nature of the tumor was probably responsible for the designation of this tumor as "diffuse endothelioma" in the past. Interestingly, an endothelial origin had been proposed for this tumor initially, which was later disproved using immunohistochemical markers. Recently, experimental studies have demonstrated angiogenesis as well as vasculogenesis in these tumors.[5] Therapeutic measures to suppress tumor growth by counteracting this process are being investigated.

In addition to the presence of intracytoplasmic glycogen and a two-cell population, these vascular channels with adherent tumor cells may be used as an additional diagnostic criterion to help in differential diagnosis of small round cell tumors on cytology.