Small cell osteosarcoma
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Small cell osteosarcomas (SCOS) are a rare subtype of osteosarcoma characterized by the production of small round cells.
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Small cell osteosarcomas account for approximately 1.5% of osteosarcomas. They occur mainly in young adolescents with a mild female predilection but have been found in a wide age range 1-4.
Small cell osteosarcoma is a subtype of osteosarcoma. The diagnosis is based on a combination of pathological and typical imaging features like conventional osteosarcoma 1.
Diagnostic criteria according to the WHO classification of soft tissue and bone tumors (5th edition) 1:
imaging features of a bone tumor
osteoid matrix with neoplastic bone formation
permeative and destructive growth pattern
and in addition, the following is required:
small blue round cell morphology
focal osteoblastic bone formation
As with conventional osteosarcoma, high-grade atypia of tumor cells and frequent atypical mitotic figures are desirable features 1.
Clinical signs and symptoms are similar to those of conventional osteosarcoma and include pain and swelling 1,2.
Small cell osteosarcoma is characterized by sheets of small round cells with focal neoplastic bone formation 1,2.
The etiology is unknown 1.
Small cell osteosarcoma has a similar distribution as conventional osteosarcoma but is more frequently seen in the diaphysis of long bones (up to 15%) 1.
Macroscopically small cell osteosarcoma looks like conventional osteosarcoma 1.
Microscopically small cell osteosarcomas are characterized by the following histological features 1,2:
small round to spindled tumor cells with scarce cytoplasm (high nuclear to cytoplasmatic ratio)
focal, frequently lace-like osteoid formation
round and oval nuclei
mitoses readily present
Immunohistochemistry is similar to conventional osteosarcoma with an expression of SATB2, which might help in the differentiation from Ewing sarcoma 1,4. CD99 might be positive in some cases 3,4.
Small cell osteosarcoma displays typical imaging features of an aggressive bone tumor with a mixed lytic and blastic and/or permeative destruction pattern periosteal reaction and soft tissue extension often similar to conventional osteosarcoma. However, they often display tumor osteoid calcification in the metaphysis and a non-mineralized more lytic appearance in a diaphyseal location resembling Ewing sarcoma 1-4.
The radiological report should include a description of the following:
form and location
tumor margins and transition zone
soft tissue extension
Treatment and prognosis
Treatment of small cell osteosarcomas is most frequently similar to conventional osteosarcoma consisting of neoadjuvant chemotherapy and surgical resection with wide margins 1-4. Small cell osteosarcomas do not respond well to radiotherapy 4.
Prognosis of small cell osteosarcomas is slightly worse than the conventional form 1 with poor prognostic factors being a poor response to chemotherapy, positive surgical margins and metastatic disease 4.
History and etymology
Small cell osteosarcomas were first described by the American orthopedist Franklin H Sim and his colleagues pathologists K Krishnan Unni, John W Beabout and David Carl Dahlin in 1979 4,5.
Small cell osteosarcoma mimics the appearance of conventional osteosarcoma, beyond that, it can look like the following tumors 1-3:
Ewing sarcoma (no new bone formation)
- 1. Baumhoer D, Böhling TO, Cates JM, Cleton-Janson AM, Hogendoorn PC, O’Donell PG, Rosenberg AE. Osteosarcoma. In: WHO Classification of Tumours Editorial Board. Soft tissue and bone tumours. Lyon (France): International Agency for Research on Cancer; 2020. (WHO classification of tumours series, 5th ed.; vol. 3). https://publications.iarc.fr
- 2. Edeiken J, Raymond A, Ayala A, Benjamin R, Murray J, Carrasco H. Small-Cell Osteosarcoma. Skeletal Radiol. 1987;16(8):621-8. doi:10.1007/bf00357110 - Pubmed
- 3. Bishop J, Shum C, Sheth S, Wakely P, Ali S. Small Cell Osteosarcoma. Am J Clin Pathol. 2010;133(5):756-61. doi:10.1309/ajcpo07vgdzcbrjf - Pubmed
- 4. Zhong J, Hu Y, Si L et al. Clarifying Prognostic Factors of Small Cell Osteosarcoma: A Pooled Analysis of 20 Cases and the Literature. Journal of Bone Oncology. 2020;24:100305. doi:10.1016/j.jbo.2020.100305 - Pubmed
- 5. Sim F, Unni K, Beabout J, Dahlin D. Osteosarcoma with Small Cells Simulating Ewing's Tumor. J Bone Joint Surg Am. 1979;61(2):207-15. - Pubmed