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).