Soft tissue sarcomas include all sarcomas except those that arise from bone, the pretext giving the tissue of origin thus: leiomyo-sarcoma (for sarcomas arising from smooth muscle), neurofibro-sarcoma (for those arising from neural sheaths), fibro-sarcoma (for those arising from fibrous cells – and malignant fibrous histiocytoma is a variant of this type), rhabdomyo-sarcoma (for those arising from voluntary muscle cells), lipo-sarcoma (for those arising from fat cells), synovial sarcoma (for those arising from the synovium of joints), angio-saarcoma (for those arising from blood vessel tissue) etc…
It is clear that the group is a very heterogenous one but enough similarity exists for us to consider the group together.
Sarcomas are malignant tumours of the mesenchyme - the background matrix tissues of the body (the third germ layer). Where the sarcoma is resembles the tissue of origin (e.g. long stranded cells, perhaps even demonstrating the cross striations of voluntary muscle and indicating rhabdomyosaarcoma) the diagnosis is easily reached by the histologist.
However, many sarcomas are poorly differentiated and down the microscope it is difficult for the histologist to tell whether such a tumour has a sarcomatous origin. Here the recent explosion of special staining methods and immunocytogenetics has assisted greatly. Thus sarcomas frequently stain for desmin and vimentin (whereas poorly differentiated melanomas stain for melanin and S-100 [although some sarcomas can also stain for S-100], lymphomas stain for leucocyte common antigen – LCA- and carcinomas stain for cytokeratin).
Furthermore, there are well recognised cytogenetic abnormalities such as the expression of t[X;18][p11.2;q11.2] in the tumour cells of synovial sarcoma, t[11:12]]q24;q12] in Ewing’s sarcoma, and t[12;16][q13;p11] in myxoid sarcoma – all of which may help the histologist to identify the tissue of origin of a poorly differentiated tumour.
Overall, the rhabdomyosarcomas predominate in childhood and in adults the distribution of frequency is: Malignant fibrous histiocytoma 20-25%, fibrosarcoma 15%, liposarcoma 15%, leiomyosarcoma 10%, rhabdomyosarcoma 8%, synovial sarcoma 7%, neurofibrosarcoma 7%, others 12%.
Lastly it is now the histologist’s job to grade the sarcoma, as it has been firmly established that the tumour’s grade is strongly related to overall outcome/prognosis.
The grading is based on tumour differentiation (a point we have already alluded to with regard to well differentiated tumours resembling the tissue of origin and poorly differentiated tumours having no such distinguishing foot-prints), pleomorphism, which refers to heterogeneity of appearance of different cells within the sarcoma, cellularity (density of cellular composition within the tumour), number of mitoses (i.e. the number of cells in division when viewed at the moment of preservation of the histological specimen – a modern method of assessing this is by the use of a special stain called the Ki-676 stain which picks up a nuclear antigen that is only present in mitosing cells) and various other factors such as vascular (blood vessel) formation and invasion of such vessels – all to suggest a scoring system which predicts the aggressiveness of behaviour of the tumour. The final score would segregate the sarcoma into a low, intermediate or high grade cancer (sometimes graded 1-4 respectively).
A low grade tumour would resemble the tissue of origin, most of the cells would conform to this picture; it would have a low number of mitoses per high power microscope field and no vascular invasion.