MSK practical exam

• Avascular necrosis (osteonecrosis)


  infarction of the bone and marrow; secondary to fx or corticosteroids ; trabecular bone and marrow; cortex not affected, subchondral infarcts->triangular/wedge-shaped tissue

  
• Pyogenic Osteomyelitis


  • Necrotic bone with empty osteocyte lacunae; may see brisk osteoclast activity • Bone often appears “moth eaten” (irregular erosions)caused by bacteria from hematogenous spread metaphysis of long bones in kids or in spine of adults

  
• Fibrous Cortical Defect/Non-ossifying Fibroma


  Benign, generally self-limited • Skeletally immature patients in cortex of metaphysis in long bones, often near the knee • Storiform proliferation of plump spindled cells with giant cells • Scattered hemosiderophages and foamy macrophages

  
• Fibrous Dysplasia


  Benign, often medullary, in the craniofacial bones or femur • Can be monostotic (one bone, more common) or polyostotic (multiple bones, often presenting younger) • Irregular, curvilinear woven bone (“Chinese letters”) without conspicuous osteoblastic rimming

  
• Osteoid Osteoma


  Benign, often in the cortex of long bones • <2 cm central nidus composed of microtrabecular woven bone rimmed by plump; • FOS gene; present with night pain relieved by NSAIDs

  
• Osteosarcoma


  • High-grade sarcoma in which the tumor cells produce bone; metaphysis of long bones near the knee; permeative bone destruction with fluffy immature mineralization and tumor ossification: Codman’s Triangle; teens; Infiltrative growth; local growth and rapid hematogenous dissemination, most frequently to the lungs

  
• Osteosarcoma


  local growth and rapid hematogenous dissemination, most frequently to the lungs; • Frequent TP53 and RB1 mutaƟons → increased risk in Li-Fraumeni syndrome and hereditary retinoblastoma

  
• Osteochondroma


  Benign • Cartilage cap with growth plate-like architecture and an underlying stalk with medullary and cortical bone • Continuous with medullary cavity and cortex of bone • Often on the metaphysis of long bones near the knee • Biallelic inactivation of EXT1 or EXT2

  
• Enchondroma/Chondroma 


  Benign, relatively common • Hyaline cartilage neoplasm arising within the medullary cavity; metaphysis of the short tubular bones of the hands; Hypocellular; chondrocytes in sharpedged lacunae with bland lymphocytelike nuclei; IDH1 and IDH2 mutations; treated with curettage;

  
• Chondrosarcoma


  Locally aggressive and can progress, but does not metastasize; Abundant cartilaginous matrix (usually hyaline, but sometimes partially myxoid), with lobular growth and entrapment of preexisting bone; increased cellularity, but bland condensed nuclei; No significant nuclear atypia or mitotic activity

  
• Chondrosarcoma


  Abundant cartilaginous matrix (usually hyaline, but sometimes partially myxoid), with lobular growth and entrapment of preexisting bone; increased cellularity, but bland condensed nuclei; Binucleated cells common

  
• Atypical Lipomatous Tumor (Well differentiated tumor)


  adult sarcoma, usually elderly • Non-metastasizing (but can recur/grow); • Extremity, retroperitoneum; • Variable amount of lipoblasts • Can be lipoma-like, sclerotic, or inflammatory • Contain amplified regions of 12q including MDM2 and CDK4

  
• Dedifferentiated Liposarcoma


  ALT/WDL component plus an abrupt transition to another component, which is usually an undifferentiated pleomorphic sarcoma (sometimes fibrosarcoma, or lower grade sarcoma); • With metastatic potential; more locally aggressive

  
• Embryonal Rhabdomyosarcoma


  “Small round blue cells tumor”; rhabdoid cells with abundant, eccentric eosinophilic cytoplasm, “tadpole cells,” and “strap cells; Botryoid type:

  Malignant; most common soft tissue sarcoma in kids <4y/o • Usually in the head and neck or genitourinary system

  
• Alveolar Rhabdomyosarcoma


  Malignant; • Often adolescents and young adults in deep soft tissue of extremities and sinuses; “Small round blue cell” tumor; often nest-like arrangement; FOXO1 fusions with either PAX3 or PAX7

  
• Leiomyoma 


  Benign; commonly in the dermis • uncommon in deep soft tissue; Cytologically bland spindled cells with cigar-shaped nuclei, in a fascicular architecture; No nuclear atypia

  
• Leiomyosarcoma


  • Malignant; poor prognosis • Usual location: retroperitoneum of adults • Often arise from veins • Fascicular architecture with notable pleomorphism • Mitotic activity, often atypical mitoses; necrosis • Complex karyotype

  
• Hemangioma 


  • Benign; composed of small or large caliber thin-walled blood vessels • Single layer of non-atypical endothelial cells • Often asymptomatic

  
• Angiosarcoma 


  High-grade malignant neoplasm with endothelial differentiation • Vasoformative architecture and/or expression of endothelial markers • Numerous extravasated RBCs and variable inflammation • Prominent nuclear atypia and readily observed mitoses

  
• Schwannoma 


  • Benign nerve tumor ; usually adults; Usually solitary and sporadic; Alternating compact spindle cells (Antoni A) and hypocellular less orderly areas (Antoni B); Rows of nuclear palisading → Verocay bodies; • Hyalinized blood vessels and lymphoid aggregates common

  
• Neurofibroma 


  Benign, most commonly solitary and sporadic; • Can be cutaneous (most common), intraneural, or diffuse; Randomly oriented spindled cells with wavy, hyperchromatic nuclei.; hypocellular and variably myxoid • Thin and thick collagen strands (“shredded carrot collagen”)

  
• Malignant Peripheral Nerve Sheath Tumor


  Malignant, usually in adults with NF1; Variable appearance, can resemble undifferentiated pleomorphic sarcoma or fibrosarcoma.; • IHC: Patchy S-100 and SOX10.

  
• Malignant Triton Tumor


  wala

  
• Nodular Fasciitis 


  Benign and self-limited • Rapidly growing, mass-forming subcutaneous lesion, sometimes after trauma, that self-regresses • Often upper extremity or head and neck of kids or young adults • Can be misdiagnosed as a sarcoma because of rapid growth and mitotic activity • Bland spindled to stellate cells with variably cellular “tissue culturelike” pattern, extravasated RBCs • MYH9-USP6 gene fusions

  
• Fibromatosis


  Benign (never metastasizes!), but infiltrative and with a strong tendency to recur; • Infiltrative growth by broad, sweeping fascicles. • Uniform spindled cells with small, pale nuclei with pinpoint nucleoli; • IHC: nuclear β-catenin

  
• Undifferentiated Pleomorphic Sarcoma


  malignant mesenchymal tumors with high-grade, pleomorphic cells ; large, grey-white fleshy masses (10-20cm) with necrosis and hemorrhage; sheets of large anaplastic spindled to polygonal cells with hyoerchromatic, irregular and sometimes bizarre nuclei

  
• Giant Cell tumor of the tendon sheath


  • Benign; can be locally destructive; Mixture of bland mononuclear cells, foamy macrophages, hemosiderin-laden macrophages, and osteoclast-like giant cells

  
• Giant Cell Tumor of the tendon sheath


  • Localized-type • Discrete, rounded proliferation; usually occurs in digits • Less aggressive. Often cure with excision. • Diffuse-type (Pigmented Villonodular Synovitis [PVNS]) • Grows in expansive sheets, often in or around knee • Often intraarticular, large, destructive • Need to treat more aggressively

  
• Ewing Sarcoma


  Malignant; most common in children and young adults • Most common in the diaphysis of long bones; causes a multilaminated periosteal reaction; • Small round blue cell morphology.; IHC: CD99

  
• Giant Cell Tumor of Bone


  Locally aggressive with rare metastases (often to the lung); • Proliferation of numerous (reactive) large osteoclasts together with a mononuclear neoplastic component without atypia; hemorrhage, hemosiderin, and macrophages; skeletally mature individuals, involving the epiphysis; treated with the RANK ligand inhibitor Denosumab

  
• Synovial Sarcoma


  • Malignant, usually young adults • Soft tissue, thorax, head and neck • Fairly uniform spindled cells with relatively little cytoplasm with ovoid, “stubby,” nuclei with hyperchromatic granular chromatin and small nucleoli; SS18-SSX gene fusions [t(X;18)]

  
• Aneurysmal Bone Cyst


  • Benign • Multiloculated blood-filled cystic spaces • Cyst wall is composed of fibroblasts, osteoclast-like giant cells, hemosiderin pigment, and new bone formation. • Most often in the metaphysis of long bones and posterior vertebrae • Most common in kids • USP6 gene rearrangements

  
• Osteoarthritis


  Degenerative disorder of articular cartilage in which matrix breakdown exceeds synthesis • Inflammation is minimal and typically secondary • Locally produced cytokines may contribute to progression of joint degeneration

  
• Osteoarthritis 


  Old, degenerative disease

  
• Rheumatoid arthritis


  • Histopathologic findings: • Chronic synovitis • Pannus; Impacts all joints, but worst impacted are small joints of hands (MCP & PIP), feet, and cervical spine

  
• Gout


  middle-aged males; Needle-shaped” yellow, negatively birefringent crystals; Joint fluid often neutrophil-rich

  
• Pseudogout Calcium Pyrophosphate Dihydrate (CPPD)


  Appear as rhomboidal purple positively birefringent crystals; • H&E: well-demarcated basophilic material with virtually no inflammatory response; associated with degenerative joint disease • Often incidental finding in arthroplasty finding

  
• Traumatic Neuroma


  Non-neoplastic nerve proliferation after trauma; Haphazard proliferation of nerve fascicles including axons and perineurial cells; Damaged nerve often easily identified

  
• plasmapheresis, IVIg


  Treatment for GBS
• Myasthenia Gravis


  Autoimmune disease usually associated with autoantibodies directed against ACh receptors  weakness of the skeletal muscle • Autoantibodies: • Postsynaptic ACh receptors • Sarcolemmal protein musclespecific receptor tyrosine kinase • ~10% of patients have thymoma