DISCUSSION:

SCO is a very rare type of bone malignant tumors arising usually from long bones (femur, tibia, and humerus). Average age is 20 years [6-83 years], but most of the patients are younger than 20 (2). Females were more affected than males, with a sex ratio M/F of 4/5 (3). This osteosarcoma rarely affects short\souts bones, and rib-localization is very uncommon. To the best of our knowledge, only two cases were described in the literature (4, 5). Table 1 summarizes the characteristics of these two cases comparing to our case.
Histopathological investigation plays an important role in the diagnosis. In fact, small round cell lesions of the bone encompass a large heterogeneous group of tumors and tumor-like lesions. The main differential diagnosis of SCO is Ewing sarcoma. In hematoxylin and eosin colored slides, osteoid production is the characteristic pathological feature of SCO, and may easily distinguish it from Ewing sarcoma. However, in small biopsies, osteoid production can be missing. Thus, immunohistochemistry as well as molecular studies are necessary to make the correct diagnosis. In our case, the tumor was positive for CD99, which is rare but possible in SCO. The immunohistochemical positivity along with the histological features and the cytoplasmic PAS positivity led to the erroneous diagnosis of Ewing sarcoma. A molecular study, excluding the existence of specific sarcoma translocation, like EWSR1-FLI1 fusion-gene could helped to establish the exact diagnosis. Unfortunately, molecular studies are unavailable in our institution. Differential diagnosis includes other tumors, like non-Hodgkin lymphoma, small cell lung carcinoma or rhabdomyosarcoma. Immunohistochemistry is sufficient to exclude these conditions (7).
Surgical resection is the unique curative treatment of SCO (6). Postoperative chemotherapy and radiotherapy may be administered. Vincristine, Adriamycin, Actinomycine D and cyclophosphamide are the most common administrated agents. However, chemo-radiotherapy together is not necessary without the evidence of any malignant cells on the surgical margin or the presence of distant metastasis (7). The 5 year-survival rate for the classic osteosarcoma is 77%, whereas it is 28% for small cell osteosarcoma (5, 8).