Soft tissue sarcomas encompass a broad range of tumours, some of which resemble known mesenchymal tissue (eg. liposarcoma, leiomyosarcoma) and others which do not (eg. desmoplastic small round cell tumour).
Soft Tissue Tumours
This ordering is the same as the WHO Blue Book on Soft Tissue and Bone Tumours
Macroscopically, these tumours are often encapsulated and yellow to white on the cut surface. White to tan areas represent fibrosis or areas of increased cellularity (dedifferentiated or round cell changes). There are several characteristic genetic changes for some of these tumours.
Lipoma is the most common soft tissue neoplasm, about 100 times more common than all malignant soft tissue sarcomas. It characteristically has translocation of 12q13 involving the HGMIC gene (2/3rds of cases).
Well differentiated and dedifferentiated liposarcomas likely represent the same disease, with some (5-10%) well differentiated cases progressing on to dedifferentiated disease. The characteristic finding on genetics are ring or giant marker chromosomes which contain amplification of the MDM2 gene; dedifferentiated areas may contain TP53 mutation.
Myxoid liposarcoma (about 10% of soft tissue sarcomas) contains a characteristic translocation t(12;16) which forms a chimeric transcription factor FUS-DDIT3 that likely causes overexpression of neoplastic genes.
Pleomorphic liposarcoma is the most uncommon type and contains pleomorphic spindle cells and adipocytes. Genetic features are complex, often with numerous additional chromosomes.
Rhabdomyosarcomas occur in three types:
- Embryonal rhabdomyosarcoma, which occurs in younger children and is the most common. It consists of variably differentiated rhabdomyoblasts, from oval rhabdomyoblasts to longer tadpole and spindle cells. Immunohistochemistry is positive for vimentin, MyoD1 and myogenin in most cells. Genetic changes are complex but variation in 11p15 are common. It frequently occurs in the head and neck and genitourinary systems.
- Alveolar rhabdomyosarcoma accounts for about 20% of cases. It is a small round blue cell tumour that occurs in nests with fibrovascular septa (typical), solid or mixed with embryonal variant. Most cases have a translocation involving chromosomes 13 and 1 or 2, creating a chimeric transcription factor. It more frequently occurs in the limbs.
- Pleomorphic rhabdomyosarcoma is a disease of older adults. It consists of pleomorphic spindle cells mixed with rhabdomyoblasts. Staining for rhabdomyosarcoma protein is required for diagnosis.
This is the mixed bag of unusual soft tissue tumours where the cell of origin is unclear.
- i) Desmoplastic Small Round Cell Tumour
- ii) Synovial Sarcoma
- iv: Alveolar Soft Part Sarcoma
- v) Epithelioid Sarcoma