Pathology

Soft tissue sarcoma types

While soft tissue sarcomas seem to suggest one type of tumor, in reality they are made up of many subtypes which can carry different prognosis (Figure 1). The array of tumors under this heading are bewildering (Percentages are relative, with those not recorded indicating an incidence of less than 1%). No age is spared in this disease because of the various types of tumors. The incidence of soft tissue sarcomas (STS) increases with age. In childhood, rhabdomyosarcomas are most common. In adulthood, non-rhabdomyosarcoma soft tissue sarcomas (NRSTS) are more common. Remarkably, here at Limb Salvage and Revision Arthroplasty Surgery, we have experience in managing all of them (and a few more esoteric subtypes):

  1. Alveolar soft-part sarcoma
  2. Angiosarcoma (1%)
  3. Dermatofibrosarcoma protuberans
  4. Epithelioid sarcoma (5%)
  5. Extraskeletal chondrosarcoma
  6. Extraskeletal osteosarcoma
  7. Fibrosarcoma (10%)
  8. Leiomyosarcoma (10%)
  9. Liposarcoma (25%)
  10. Malignant fibrous histiocytoma (30%)
  11. Malignant hemangiopericytoma
  12. Malignant mesenchymoma
  13. Malignant schwannoma
  14. Malignant peripheral nerve sheath tumor (10%)
  15. Peripheral neuroectodermal tumors
  16. Rhabdomyosarcoma
  17. Synovial sarcoma (15%)
  18. Sarcoma, NOS (not otherwise specified)




Figure 1. The designation soft tissue sarcoma actually refers to many different subtypes. In the past, the prognosis of this group used to be thought to be uniform. Presently however we are beginning to appreciate the value of sub-typing these entities for prognostication and specific management. 

Pathophysiology

We will consider the pathophysiology of the more common sarcomas here.

Malignant fibrous histiocytoma

Ironically this is the most common diagnosis for high grade soft tissue sarcomas when it is in fact a misnomer. Originally it was felt to be histiocytic in 1963. However, it is now known to be more akin to fibroblasts. The diagnosis has fallen out of favour with pathologists recognizing a need to better indicate the cell type presently. To avoid confusion, this group of tumors are now more commonly referred to as pleomorphic high grade sarcoma (non-otherwise specified). This entity is not to be confused with malignant fibrous histiocytomas of bone which are an accepted diagnosis (Figure 2). Generally, they occur in adults and have non-specific differentiation. Collagen production should be demonstrated. Nevertheless it is often referred to as a “wastebasket” diagnosis where tumors are classified when they cant be classified elsewhere.

Types

  • Superficial
    • Fibroxanthoma
  • Deep
    • Storiform-pleomorphic, myxoid, giant cell, inflammatory

Figure 2. The malignant fibrous histiocytoma used to be the most diagnosed entity. Thi can reach huge proportions in the pelvis (a). Scans can help identify if bone is involved (b). Resections involving bone can be difficult to distinguish from the more chemosensitive malignant fibrous histiocytoma of bone (c). While in the past histology was difficult to separate from other sarcoma subtypes, today the tumor is virtually synonymous with high grade pleomorphic sarcomas (d).

Liposarcoma

With efforts to better characterise the cell type as discussed above, liposarcomas have now become the most commonly described sarcoma. Identifying lipoblasts are the key to their diagnosis (Figure 3). There are amazingly six variants of liposarcoma each with 1 of three different genetic signatures:

 

Histological subtypes

  • Low grade (well differentiated lipoma like, sclerosing)
    • c-myc, HMGC19, ring chromosomes
  • Intermediate grade (round cell, myxoid)
    • 12-16 TLS-CHOP fusion transcript
  • High grade (high grade, pleomorphic)
    • Heterogenous genetic aberrration

Figure 3. The liposarcoma is actually six different types of sarcomas under one heading. These all require the identification of the lipoblast (a). In the more well-differentiated forms they go by names like sclerosing (b). Intermediate forms have myxoid tissue (b). The high grade forms are very cellular (d).
 

Primitive neuroectodermal tumors

This perplexing group of tumors has gone by many names in the past (ie. Ewing’s sarcoma, Ewing’s Family Tumor, etc). Nevertheless, it has now been determined that they all have the characteristic 11-22, EWS translocation. These can be Fli-1 (good prognosis or non-Fli-1 (poor prognosis). They can occur in different locations and have historically different names based on location (Figure 4) but are all characterised by the same “small blue round cell” phenotype. Their name says it all. They are primitive stem cell tumours (“blastoma”) and therefore highly proliferative, largely undifferentiated with a high nuclear:cytoplasmic ratio. Being neuroectodermal, they are able to mature along one or more “pathways” available to the embryonic neuroectodermal anlage. Additional qualifiers are used if maturation is detectable hence the varied names (eg. ependymoblastoma, myomedulloblastoma etc).


Figure 4. The primitive neuroectodermal tumor (PNET) is a family of tumors that are characterised by small blue round cells (a). Depending on where they originate they can have various confusing names (b).

Synovial sarcoma

Synovial sarcomas typically lay dormant for a number of years before becoming active in the 20 to 50 year age group.

 

Histological types

  • Biphasic (may have better prognosis)
  • Monophasic (may have poorer prognosis)

 

Genotype

  • SYT-SSX1 associated with biphasic phenotype but may have poor prognosis
  • SYT-SSX2 associated with monophasic phenotype but may have good prognosis


Rhabdomyosarcoma

This group of tumors is the most common subgroup of solid tumors in childhood (Figure 5).

Types

  • Embryonal     (49%)                                   
    • 66% FYS (five-year survival)
  • Alveolar          (31%)                                               
    • 54% FYS
  • Botyroid
  • Pleomorphic

 

Staging and prognosis

I           Complete resection                           85% FYS

II         Microscopic residual disease           88% FYS

III        Gross residual                                    66% FYS

IV        Metastatic                                           26% FYS



Figure 5. Rhabdomyosarcomas are the most common of the paediatric solid tumors and are chemo- and radiosensitive. Embryonal (a,b) and Alveolar (c,d) are the main types other two other variants are known.