A microscopic specimen of a tumour may show a small round blue cell tumour. These tumours are characterised by the presence of cells that are:
- Small (similar to lymphocyte in size)
- Round (round cells)
- Blue (blue staining due to high nuclear/cytoplasmic ratios
It can be a challenge to determine the exact origin of these tumours. The typical small round blue cell tumours in humans are:
- Poorly differentiated neuroendocrine tumours (e.g. small cell lung cancer, Merkel cell carcinoma)
- Indolent B cell lymphoma (chronic lymphocytic leukaemia, follicular lymphoma with no centroblasts)
- Melanoma (some variants)
- Paediatric age tumours
- Ewing sarcoma
- Alveolar rhabdomyosarcoma
- Nephroblastoma
- Medulloblastoma
- Desmoplastic small round cell tumour (often in the peritoneum)
- Olfactory neuroblastoma
The likelihood of each diagnosis depends on the site and age of the patient as well as, in some cases, risk factors.
Type | Site | Age | Risk Factors | Architecture | Immunohistochemistry |
---|---|---|---|---|---|
Small Cell Lung Cancer | Lung | Mean age 50-60 | Smoking | Neuroendocrine | TTF1+, CK7+, CK20-, Synatophysin +, Chromogranin + |
Merkel Cell Carcinoma | Skin | Mean age 60-70 | Sun exposure | Nests, sheets | TTF-, CK20+, Synaptophysin +, Chromogranin + |
Lymphoma | Nodes | Mean age 50-60 | None | Variable; follicular | CD79a+, CD20+ |
Ewing Sarcoma | Bone | Mean age 10 | None | Sheets, occasional rosettes | CD99+, vimentin |
Alveolar Rhabdomyosarcoma | Extremities, other sites | Mean age 10-20 | None | Nests, fibrovascular stroma | Myo-D1 +ve |
Nephroblastoma | Kidney | Mean age 3-4 | WAGR, Beckwith-Weidermann Denys-Drash |
Triphasic (blastemal, epithelial, stromal) |
Vimentin +, WT1 + |
Desmoplastic SRCT | Peritoneum | Unknown | Large nests, Desmoplastic stroma |
Cytokeratin, NSE + | |
Medulloblastoma |
Approach
The age and site of the tumour may provide sufficient diagnostic information in most cases; for instance, in a 65 year old smoke with a lung mass with biopsy showing small round blue cells, the likelihood of small cell carcinoma of the lung is incredibly high. An important differential is in children with a bony lesion, where it may be due to a primary Ewing sarcoma or alternatively a metastasis from nephroblastoma, rhabdomyosarcoma or other embryonal tumour. Immunohistochemistry is especially helpful at differentiating the neuroendocrine tumours from the embryonal tumours, and the embryonal tumours from each other.