Small cell osteosarcoma

Small cell osteosarcomas (SCOS) are a rare subtype of osteosarcoma characterised by the production of small round cells.

Epidemiology

Small cell osteosarcomasaccount 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.

Diagnosis

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

Diagnostic criteria according to the WHO classification of soft tissue and bone tumours (2020 - blue book(5^th edition) ¹:

• imaging features of a bone tumour

• 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 tumour cells and frequent atypical mitotic figures are desirable features ¹.

Clinical presentation

Clinical signs and symptoms are similar to those of conventional osteosarcoma and include pain and swelling ^1,2.

Pathology

Small cell osteosarcoma is characterised by sheets of small round cells with focal neoplastic bone formation ^1,2.

Aetiology

The aetiology is unknown ¹.

Location

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.

Macroscopic appearance

Macroscopically small cell osteosarcoma looks like conventional osteosarcoma ¹.

Microscopic appearance

Microscopically small cell osteosarcomas are characterised by the following histological features ^1,2:

• small round to spindled tumour cells with scarce cytoplasm (high nuclear to cytoplasmatic ratio)

• focal, frequently lace-like osteoid formation

• round and oval nuclei

• mitoses readily present

Immunophenotype

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.

Radiographic features

Small cell osteosarcoma displays typical imaging features of an aggressive bone tumour 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 tumour osteoid calcification in the metaphysis and a non-mineralised more lytic appearance in a diaphyseal location resembling Ewing sarcoma ^1-4.

Radiology report

The radiological report should include a description of the following:

• form and location

• tumour margins and transition zone

• cortical destruction

• 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 FHthe American orthopaedist Franklin H Sim and his colleagues pathologists K Krishnan Unni, John W Beabout and David Carl Dahlin in 1979^4,5.

Differential diagnosis

Small cell osteosarcoma mimics the appearance of conventional osteosarcoma, beyond that, it can look like the following tumours ^1-3:

• Ewing sarcoma (no new bone formation)

• BCOR sarcoma

• lymphoma

• mesenchymal chondrosarcoma