Pathology MSK tumors

Question 1

Lipoma

Answer 1

• Well circumscribed, encapsulated adipose tissue w/o any areas of necrosis or hemorrhage (slow growing and painless)
• Cells look the same as normal adipose tissue, only difference is the fibrous capsule
• One variation: angiolipoma, these have variably-sized blood vessels mixed throughout the tumor
• These are benign lesions, most common soft tissue mass of adults

Question 2

Liposarcoma

Answer 2

• Encapsulated mass that is fast growing and painless, usually has central area of necrosis
• Often is nodular and can have irregular margins
• Histologically can see lipoblasts (dividing adipose cells w/ hyperchromatic nuclei, key finding) in high numbers, evidence of malignancy
• Lipoblasts are variable in size and abnormal looking (strangely shaped and sized)
• Another key finding is plexiform blood cells (capillary anastamoses) and a high degree vascularization
• Found usually in retropreitoneum and deep soft tissue of proximal extremities
• Malignant lesions, very rare in children

Question 3

Nodular faciitis

Answer 3

• Develops over a few months at the site of an injury (usually forearm), often in young adults
• Is semi-encapsulated, irregularly shaped
• Histologically there is spindle-cell growth, indicating fibroblasts
• At high power can see many fibroblasts, vascular proliferation and lymphocytes
• This is a benign process, reactive to the injury
• Malignant lesions from this process generally are slower growing than a few months

Question 4

Fibrosarcoma

Answer 4

• Circumscribed mass, may contain some calcification
• Has ill-defined septae and capsule, which can be eroding or infiltrating adjacent bone
• Lesion often occurs in the thigh and around the knee (adult) or in muscles of distal limbs (infantile)
• Pleomorphic cell population including fibroblasts (secreting collagen), plumper cells (have vacuolated cytoplasm), and occasional giant cells
• Abnormal mitotic figures can be seen, angulated densely interlacing bundles of fibroblasts (herring-bone pattern)
• This pleomorphic fibrosarcoma is malignant and has a poor prognosis

Question 5

Rhabdomyosarcoma

Answer 5

• Usually in children under 20, diagnostic cell type is the rhabdomyoblast
• Rhabdomyoblasts look like small blue round cells w/ eccentric abundant eosinophilic cytoplasm
• There can also be elongated cells that appear strap-shaped (strap cells: very long and thin). Presence of strap cells indicates a more differentiated and thus less severe disease
• Is a round blue cell tumor, see basophilic cells w/ hyperchromatic nuclei which are differentiated rhabdomyoblasts. These indicate a less differentiated and thus more severe disease
• t(2;13) associated, myoD1/myogenin positive
• These are malignant tumors and prognosis is poor

Question 6

Osteoid osteoma

Answer 6

• Pts present w/ severe focal bone pain that worsens at night and w/ activities but is greatly relieved w/ aspirin (pain due to prostaglandin E, which aspirin inhibits production of)
• Usually appears before age 20 (usually boys but bimodal age distribution), often in LE
• On Xray there is a gray oval mass (nidus: lucent middle) surrounded by dense reactive bone
• The nidus is the tumor, which has disorganized interconnected trabeculae w/ high osteoblastic activity and abundant osteoid in some areas but all of it is highly vascularized
• The nidus can be seen expanding into the bone causing the reactive process in adjacent bone
• These are benign neoplasms of osteocytes/osteoblasts

Question 7

Osteosarcoma

Answer 7

• Usually has metaphyseal origins and infiltrates into surrounding soft tissue (mostly affects men and knees/long bones)
• Bimodal incidence (older ppl usually due to sequellae of paget’s disease)
• Associated w/ mutations in Rb, p53 genes
• Serum alk phos levels increased
• Can see malignant osteoblasts w/ little cytoplasm and large, hyper chromatic pleomorphic nuclei surrounded w/ osteoid (non-mineralized)
• Lesions are usually much larger than osteoid osteoma
• Malignant tumors

Question 8

Osteochrondroma

Answer 8

• Usually in young adults (mostly boys), occurs at site after injury due to defect in growth plate particularly around knee
• Has smooth but nodular surface, w/ cartilaginous cap overlying cancellous bone (bone is highly vascularized)
• Histologically there are normal, uniform chondrocytes from the cap
• Lesion is made of both bone and cartilage, typically occurs at long bones, often around the knee
• Is a benign process

Question 9

Ewing sarcoma 1

Answer 9

• Slowly growing bone mass, that is expansive and destructive (causes typical B symptoms). Mostly found in children, boys (malignant and rare)
• Causes pain and prominent periosteal reaction, is a round blue cell tumor
• Neoplasm arises form bone and extends into soft tissue, is made of friable soft tissue w/ areas of hemorrhage and necrosis

Question 10

Ewing sarcoma 2

Answer 10

• There is infiltration of the medullary space by many uniform, small, round blue cells (neuroectoderm-derived)
• This lesion destroys bone but doesn’t produce osteoid, often in the diaphyses of long bones
• The cells have indistinct boundaries and hyperchromatic nuclei, looks similar to lymphomas
• These lesions are positive for FLI1, the result of a t(11;22) translocation (fusion of EWS w/ FLI1)

Question 11

Pleiomorphic undifferentiated sarcoma (PUS)

Answer 11

• Can be found in retroperitoneum/deep soft tissues
• Most often affects men >60 yo, is Dx of exclusion where other fibrohistiocyte lesions are ruled out (fibrosarcoma, fibromatosis, ect)
• Rarely can present as a bone lesion
• Presents as an enlarging painful mass in proximal extremity muscles or retroperitoneum, necrosis is typical
• Histologically see plump, pleomorphic large round spindle cells arranged in whirling pattern
• Can see giant cells and some inflammation, positive for vimentin
• Are malignant, and thus there isn’t much ECM b/c its filled w/ cells

Question 12

Desmoid tumor (deep-seated fibromatosis)

Answer 12

• Borders btwn non-aggressive tumors and low-grade fibrosarcomas, usually in teens-thirties
• Deep intramuscular or retroperitoneal mass that invades but doesn’t metastasize
• Micro: plump long sweeping fascicles of fibroblasts infiltrating adjacent tissue
• Cells less pleomorphic than PUS, look more like fibrosarcoma cells
• Fewer cells than PUS or fibrosarcoma b/c is not malignant