Sarcomas with BCOR genetic alterations or BCOR sarcomas are uncommon malignant undifferentiated small round cell tumours of soft tissue and bone characterised by BCOR genetic alterations and comprise sarcomas with BCOR-related gene fusions as BCOR-CCNB3 sarcomas and other BCOR-rearranged sarcomas as well as sarcomas showing an internal tandem duplication (BCOR-ITD) such as infantile undifferentiated round cell sarcomas and primitive myxoid mesenchymal tumours of infancy.
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Epidemiology
Sarcomas with BCOR genetic alterations are uncommon. BCOR-CCNB3 sarcomas are most commonly seen in children and adolescents with a male predilection 1,2 whereas BCOR-rearranged soft tissue sarcomas have a wider age range 1. Sarcomas with BCOR-internal tandem duplication are usually seen in the first year of life or at birth 1.
Diagnosis
The diagnosis of sarcomas with BCOR genetic alterations is made by molecular pathology.
Diagnostic criteria
Diagnostic criteria according to the WHO classification of bone tumours (2020 - blue book) 1:
- primitive round and spindle cells organised in sheets, nests or fascicles
- variable myxoid matrix with fine vasculature
- BCOR, SATB2 and cyclin D1 positivity on immunohistochemistry
In selected cases molecular confirmation of the BCOR genetic alteration is desirable.
Clinical presentation
Tumours often present with swelling and/or pain 1.
Pathology
Sarcomas with BCOR genetic alterations are small round cell tumours with substantial morphological overlap between the specific entities but diffuse BCOR positivity on immunohistochemistry 1.
Aetiology
The aetiology of sarcomas with BCOR genetic alterations is unknown.
Location
BCOR-CCNB3 sarcomas are more common in bone than in soft tissue with an estimated ratio of 3:2 and tend to be found in the following locations 1:
- pelvis
- lower extremity
- paraspinal region
Uncommon sites of occurrence include the head and neck area, kidney and lungs.
Infantile undifferentiated round cell sarcomas and primitive myxoid mesenchymal tumours of infancy are mainly seen in the soft tissues of the trunk, retroperitoneum or head and neck region 1.
Macroscopic appearance
Macroscopically sarcomas with BCOR genetic alterations have a tannish-grey appearance with interspersed areas of necrosis. They are typically soft and fleshy to touch and bone lesions usually feature cortical destruction and soft tissue extension 1.
Microscopic appearance
BCOR-CNNB3 sarcomas are characterised by the following histological features 1-3:
- a mixture of small round and ovoid spindle cells aligned in sheets, nests in a rich capillary network
- hypocellular areas within a myxoid matrix
- arrangement in short fascicles
- variable mitotic activity
Sarcomas with BCOR-ITD abnormalities show the following microscopic features 1,2:
- variable degrees of cellularity
- small primitive cells arranged in solid sheets or dispersed spindle cells in a myxoid matrix
- delicate vessels
Immunophenotype
Immunohistochemistry stains show a diffuse and strong BCOR positivity and expression of SATB2, cyclin D1 and TLE1. There is CD99 expression in about half of the cases 1,2. BCOR-CNNB3 are also positive for cyclin B3, which is negative in other BCOR positive tumours 1.
Radiographic features
Plain radiograph
On plain radiographs BCOR sarcomas seem to show a variable appearance with most being described as lytic or mixed lytic-sclerotic and less often being mainly sclerotic 4. In long bones, they were found to have an aggressive periosteal reaction 4.
CT
On CT BCOR sarcomas have been described as well-defined tumours of soft tissue density and heterogeneous enhancement 4 and with signs of cortical destruction.
MRI
On MRI BCOR sarcomas have been described as well-delineated soft tissue masses sometimes with necrosis or haemorrhage, cortical destruction and extensive and often circumferential soft tissue extension if present 4.
Less commonly described features include flow voids and perilesional oedema 4.
Signal characteristics have been mostly described as follows 4:
- T1: intermediate intensity
- T2: heterogeneously high signal intensity
- T1 C+ (Gd): intense heterogeneous enhancement
Nuclear medicine
High uptake of FDG has been reported on PET-CT 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
Currently, BCOR-CCNB3 tumours are treated with an Ewing sarcoma based treatment regimen with a similar response and survival rates of 72-80% after 5 years 1,4. Patients often present with metastatic disease to lungs, bones soft tissues, brain and pancreas 1,4. The prognosis of the other BCOR tumour types is variable and not well defined 1.
History and etymology
BCOR-CCNB3 sarcomas were first described by Gaëlle Pierron et al. in 2012 3-5.
Differential diagnosis
Conditions that can mimic the appearance of sarcomas with BCOR genetic alterations include 1-3:
- Ewing sarcoma (BCOR and CCNB3 immunohistochemistry is usually negative)
- CIC-rearranged sarcoma
- small cell osteosarcoma
- clear-cell sarcoma of the kidney
- synovial sarcoma
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