47. Tumours of adipose tissue and peripheral nerves, synovial sarcoma

From greek.doctor

Since writing this topic, "extraskeletal Ewing sarcoma" has been added to the topic.

Introduction

Tumors of the adipose tissues are either subtypes of lipoma or subtypes of liposarcoma. Here are some important types:

  • Lipoma
    • Conventional lipoma
    • Angiolipoma – comprised of adipocytes and blood vessels
    • Angiomyolipoma – comprised of adipocytes, smooth muscle cells and vessels
    • Intramuscular lipoma – arises from adipose tissue within muscle
    • Hibernoma – arises from brown adipose tissue
  • Liposarcoma
    • Well differentiated liposarcoma
    • Dedifferentiated liposarcoma
    • Myxoid liposarcoma
    • Pleomorphic liposarcoma

These are the important tumors of peripheral nerves:

  • Benign
    • Neurofibroma (Assocciated With NF1)
    • Schwannoma  (Assocciated With NF2)
    • Granular cell tumor
  • Malignant
    • Malignant peripheral nerve sheath tumor

Lastly, we’ll briefly discuss synovial sarcoma.

Lipoma

Lipomas are benign tumors of adipose tissue. They’re the most common soft tissue tumors in adults. Most lipomas are solitary lesions that occur under the skin of the extremities, but they can occur anywhere there is adipose tissue, basically anywhere in the body. These tumors are yellowish due to their fat content, and they’re “mobile”, meaning that they can be “moved around” under the skin.

The most common subtype is the conventional lipoma. They’re soft, “rubbery”, painless, well-demarcated masses. Histology shows that they’re composed of mature adipocytes with no pleomorphism.

Other types of lipomas include:

  • Angiolipoma – comprised of adipocytes and blood vessels
  • Angiomyolipoma – comprised of adipocytes, smooth muscle cells and vessels
  • Intramuscular lipoma – arises from adipose tissue within muscle
  • Hibernoma – arises from brown adipose tissue

Angiolipomas are similar to the conventional type, but painful.

Angiomyolipoma is most commonly found in the perirenal fat tissue. Some consider it the most common benign kidney tumor (others consider oncocytoma to be the most common).

While lipomas are usually solitary masses, they can be multiple in certain genetic conditions. Lipomatosis is a group of diseases where affected people develop multiple lipomas. These lipomas are often diffuse and non-encapsulated, in contrast to sporadic lipomas.

Lipomas can’t progress into liposarcomas and are therefore not necessary to remove unless they produce symptoms, but many prefer to remove them for cosmetic reasons.

Liposarcoma

Liposarcomas are malignant tumor with adipocyte differentiation. They occur most commonly in older adults around 50 – 60. Unlike lipomas, these tumors usually occur in deep soft tissue like the retroperitoneum, popliteal fossa and thigh. The prognosis depends on the histological subtype and the location. Deeper tumors are harder to excise completely. Those that occur in the retroperitoneum are hardest to remove due to the retroperitoneal structures.

Most liposarcomas are comprised of lipoblasts. These adipocyte-like cells have vacuolated cytoplasm and atypical nuclei.

The important subtypes are:

  • Well differentiated liposarcoma
  • Dedifferentiated liposarcoma
  • Myxoid liposarcoma
  • Pleomorphic liposarcoma

Well-differentiated liposarcoma is the subtype with the best prognosis. It can be regarded as being of intermediate dignity, lying between benign and malignant. The reason for this is that they can infiltrate locally but they virtually never metastasize. Due to their local infiltration they can be hard to remove completely, causing them to have a high recurrence rate. This increases the risk of this subtype developing into dedifferentiated liposarcoma.

A gene called Mdm2 is amplified in well-differentiated liposarcoma. Well-differentiated liposarcomas that occur in the subcutaneous tissues are called atypical lipomatous tumors.

Dedifferentiated liposarcoma has worse prognosis than the aforementioned type. Unlike the well-differentiated type this type can metastasize. These tumors are usually comprised of two parts, where one of the parts is the well-differentiated liposarcoma it developed from while the other part is the dedifferentiated liposarcoma itself. The larger the dedifferentiated part is compared to the well-differentiated part, the worse the prognosis.

Myxoid liposarcoma frequently occurs in the retroperitoneum and can locally infiltrate, which gives it a bad prognosis. It can also metastasize. It’s associated with a t(12;16) translocation that stops adipocytes from maturating.

Pleomorphic liposarcoma is the rarest subtype. It mostly affects deep soft tissues of the extremities. As its name suggests it’s characterized by severe pleomorphic lipoblasts and high malignancy.

Tumours of peripheral nerves

Most tumors of peripheral nerves, benign and malignant, develop from the nerve sheath and therefore show some degree of Schwann cell differentiation. The important peripheral nerve tumors are:

  • Benign
    • Neurofibroma
    • Schwannoma
    • Granular cell tumor
  • Malignant
    • Malignant peripheral nerve sheath tumor

Neurofibroma is a non-encapsulated benign tumor that can arise anywhere on the body. Most cases are sporadic and occur in adults, but a small percentage of cases are associated with multiple tumors and neurofibromatosis type 1 and occurs in children.

The three subtypes are:

  • Solitary/localized neurofibroma
  • Diffuse neurofibroma – is infiltrative
  • Plexiform neurofibroma – grows within large peripheral nerves. Occurs only in NF1

Schwannoma may also arise anywhere on the body, but they most commonly affect the vestibular nerve, in which case they’re called acoustic neuroma or vestibular Schwannomas. These types may impair hearing.

Schwannomas are encapsulated and most commonly sporadic, but they are associated with neurofibromatosis type 2 or schwannomatosis in 10% of cases.

Bilateral vestibular Schwannoma is pathognomic for NF2.

You can read more about NF type  1 and 2 in topic 46, Pathology 1.

Granular cell tumor may arise anywhere, but most commonly occurs in the tongue or elsewhere in the oral cavity.

Malignant peripheral nerve sheath tumor (MPNST) may develop from neurofibromas, especially the plexiform subtype. People with neurofibromatosis type 1 account for 25% of the cases of MPNST.

Synovial sarcoma

Despite their name, synovial sarcomas rarely occur inside joints (< 10% of cases) and the neoplastic cells don’t originate from synoviocytes. They most frequently affect deep soft tissue around the knees or other parts of the extremities. The name comes from the fact that the first cases were described around the joints and were believed to originate from there.

Synovial sarcomas show a characteristic t(X;18) translocation (translocation of the X chromosome and chromosome 18). Synovial sarcoma has a relatively poor prognosis; less than 30% survive more than 10 years.