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Musculoskeletal Pathology
Assoc. Professor Jan Laco, MD, PhD
Bone diseases
1. Metabolic bone diseases
• Osteoporosis
• Osteomalacia and rickets
• Fibrous osteodystrophy
• Renal osteopathy
• Paget’s disease
2. Fractures
3. Inflammations
• Osteomyelitis
• Tuberculous osteomyelitis
4. Tumours
• Primary
• Secondary (metastatic)
• = absolute decline in the bone mass (bone atrophy)
• minimal trauma  fractures
• excessive amount of osteoclastic resorption
• impairment of osteoblast-mediated bone formation
• both
Type I osteoporosis (postmenopausal):
• increased rate of bone resorption (estrogen loss   in
number + activity of osteoclasts)
Type II osteoporosis (senile, over 70 Y):
• impairment of bone formation (failure to fill the normalsized resorption cavities with new bone)
Secondary osteoporosis (other disorders + drugs)
• Cushing’s syndrome
• Hyperthyreoidism
• Hypogonadism (early oophorectomy)
• Malnutrition, subtotal gastrectomy
• Immobilisation, hemiplegia, paraplegia
• Glucocorticoids
• thinning of individual trabecula x mineralisation is normal
Clinical features:
• hip fractures
• compressive fractures of vertebral bodies – kyphosis
• fractures of distal radius (Colles’ fracture)
Osteomalacia and Rickets
Defective mineralisation of osteoid matrix
Lack of vitamin D
• low dietary intake, lack of sunlight, small gut malabsorption
• chronic liver and/or renal diseases (impaired hydroxylation of
vitamin D precursors)
Hypophosphataemia (resistance to treatment with vitamin D)
• hereditary (abnormality of phosphate transport by renal
tubular epithelium)
• tumor-associated (effect of certain mesenchymal neoplasms)
Osteomalacia and Rickets
Osteomalacia - adults with closed epiphyses
•  amount of osteoid (> 20% of trabecular surface)
• NO bone loss
• deformities
• kyphoscoliosis, contracted pelvis, limb curvature,
Rickets - young individuals, epiphyses are not closed
• disturbance of endochondral ossification + failure of
matrix mineralisation
• deformities
• craniotabes rhachitica
• caput quadratum (frontal and parietal bossing)
• bulging of the costochondral junctions (“rickety rosary“)
Fibrous osteodystrophy
= osteitis fibrosa cystica generalisata von Recklinghausen
• pathogenesis
severe hyperparathyreoidism (esp. primary)   PTH
  osteoclastic resorption of bone
Mi: outline of bony trabeculae shows deep indentations filled with
numerous osteoclasts and connective tissue
• severe cases  brown tumors (a misnomer)
= areas in which trabecular bone has been completely eroded away
and replaced by vascular fibrous tissue with many osteoclasts
and hemosiderin depositions (brown color)
• clinical features: bone pain, pathological fractures,
Paget’s disease of bone
= osteitis deformans
Sir James Paget (1882)
enlargement + deformities of bones
> 45 years
one bone x many bones
sites: skull, pelvis, spine
etiology ??? (viral infection)
Paget’s disease of bone
• Gross:
• bones enlarged and thickened (thick calvarium) and softer
(deformities – bowing of lower limbs)
• skull deformities (narrowing of foramina of cranial nerves neuropathies, deafness)
• Micro:
• episodes of bone resorption + subsequent reparation
• 1. osteolytic phase – prominent osteoclastic resorption, giant
multinucleated osteoclasts
• 2. mixed / osteoblastic phase – continuing resorption and
osteoblastic new bone formation
• 3. osteosclerotic or burned-out phase – large irregular trabeculae
with mosaic-like appearance due to numerous cement lines
!!! greater risk of developing osteosarcoma !!!
= breaks in continuity of bone
• infraction = incomplete fracture
• pathological fracture:
• bone disease (osteoporosis, cysts, tumours)  minor trauma or
spontaneously  fracture
• Fracture healing:
hematoma between ends of bone + necrosis of BM 
organization of hematoma - granulation tissue 
connection of bone ends by fibrous tissue (fibrous callus)
 woven bone + cartilage in fibrous tissue  provisional
callus  remodelling to mature lamellar bone  definite
• Impaired fracture healing
• infection, poor immobilization, non-vital bone ends
• exuberant callus  pseudo-arthrosis
= infective inflammation of bone (Staphylococci, Streptococci...)
• infection way:
• directly (open fractures, operations)
• hematogeneously
• pathogenesis
• acute pyogenic inflammation of BM   rise of interstitial pressure 
 blood supply  ischaemic necrosis
• inflammation  subperiosteal space (subperiosteal abscess) 
periosteal blood vessels shear off  ischemic necrosis
• perforation of periosteum  soft tissues and skin - sinus track, portions
of dead bone can be discharged onto the skin
• adults
• sites: mandible (periapical infection, fracture)
• complications
• sepsis, amyloidosis, pyogenic arthritis, skin squamous carcinoma
Tuberculous osteomyelitis
• children, blood-borne infection
• long bones, vertebrae
• bone destruction + granulomatous lesion with caseation
• Tuberculosis of the spine (Pott’s disease)
• sharp anterior angulation of the spine (kyphosis)
• caseation may spread to the paravertebral soft tissues and track down
along the psoas muscle to form an inguinal subcutaneous mass
Bone metastases
• Much more common than primary bone tumours
• Usually multiple
• Osteolytic (bone destruction)
• Osteoplastic (reactive bone formation) – prostatic carcinoma
• Tumors
• prostatic carcinoma, breast carcinoma, renal cell carcinoma,
carcinoma of stomach, thyroid carcinoma
Primary bone tumours – classification
less common, usually solitary
diagnosis: microscopy + age + site + RTG
1. Bone-forming tumours
• Osteoma
• Osteoid osteoma
• Osteoblastoma
• Osteosarcoma
2. Cartilage-forming
• Osteochondroma
• Chondroma
• Chondrosarcoma
3. Other tumours
• Giant cell tumor of bone
• Ewing’s sarcoma
4. Tumour-like lesions
• Fibrous dysplasia
• Bone cysts
• benign lesion, rather hamartoma
• formation of dense mature lamellar bone
• sites: skull, facial bones, paranasal sinuses, orbit, torus
palatinus et mandibularis
• may cause mechanical problems
• ! component of Gardner’s colonic polyposis syndrome
Osteoid osteoma
benign osteoblastic lesion
severe pain
young people (5-24 years), male predominance
site: long bones (femur, tibia), ends of the shafts
• small radiolucent area + radio-opaque rim of reactive bone
• Microscopic features:
• small central area (nidus) < 1cm + zone of dense sclerotic
bone (reaction to the nidus)
• nidus - interlacing bands of osteoid in haphazard fashion
• osteoid surrounded by normal osteoblasts
• vascular fibrous tissue in spaces between trabeculae
• most common primary malignant neoplasm of bone
• peak 10-25 Y, (elderly people - Paget’s disease), males
• site: metaphyseal region of the long bones
• lower end of femur, upper end of tibia, upper end of humerus, mandible
• from medullary cavity  invasion of cortical bone  elevate
periosteum (RTG sign - Codman’s triangle)  soft tissues
• microscopy:
• identification of bone or osteoid formation by the tumour cells
• irregular “ lace-like“ osteoid trabeculae surrounded by atypical
malignant osteoblasts
• malignant cartilage and sarcomatous spindle cell stroma also present
•    malignant with poor prognosis, metastases to lungs
Parosteal osteosarcoma
• variant of osteosarcoma in older people
juxtacortical position of metaphyseal region
forms large lobulated mass tending to encircle the shaft
very slow growth
microscopic features
• well-formed bone and osteoid + spindle cell stroma with only
scanty cytological signs of malignancy
• very good prognosis with adequate resection
Osteochondroma (exostosis)
very common, hamartoma ?
young people, males
site: long bones (femur, tibia, humerus), metaphyses,
mandible (condylar and coronoid process)
pedunculated or sessile lesions - bone + cap of cartilage
ordered columnar arrangement of chondrocytes and zone of
endochondral ossification (as in epiphyseal growth plate)
multiple osteochondromas, some risk of malignant transformation
(secondary chondrosarcoma)
• benign tumour - mature hyaline cartilage
• children, adolescents and young adults
• site: medullary cavity of small bones of hands and
feet (enchondroma), maxilla (rare)
• Ollier’s disease (multiple chondromas)
• radiolucent lesion expanding and thinning cortical bone
• Micro
• lobules of mature hyaline cartilage
• malignant cartilaginous tumor
• primary (de novo) x secondary (malignant transformation of
benign cartilaginous tumors – osteochondroma)
• middle aged and elderly (primary chondrosarcoma), younger
age group (secondary chondrosarcoma), M : F - 3:2
• site: central skeleton (ribs, pelvis, proximal end of femur and
humerus), anterior maxilla
• aggressive growth, destruction of cortical bone and extension into the
soft tissues
• Micro
• cartilaginous tissue with high cellularity, cytologic atypia of
chondrocytes, binucleated cells, mitotic activity, necrosis
• differential diagnosis low-grade chondrosarcoma x
• histological picture + clinical and radiological features!!!
Giant cell tumor of bone
• peak 3rd decade
• site: long bone epiphyses
• lytic and radiolucent lesion
• Gross
• brown with grey and red areas (fibrosis and hemorrhage)
• Micro
• stromal mononuclear cells + multinuclear giant cells ~
• locally aggressive, may recur locally, malignant
change in about 10% of cases (metastases)
Ewing’s sarcoma
group of primitive neuroectodermal tumours (PNET)
peak 5-20 years, males
site: most often long bones and pelvis, mandible (body)
• lytic destruction of the bone
• Gross
• grayish white, focal necrosis and haemorrhage
• Micro
• small uniform round cells with glycogen (PAS +)
• clinical features: pain and swelling
• very aggressive tumor with poor prognosis
• metastatic spread to lungs and other bones
Fibrous dysplasia
• non-neoplastic condition
• children and adolescents
• site: ribs, femur, tibia, maxilla, mandibula, humerus
• monostotic x polyostotic form
• micro
• well-demarcated localized area of bone replaced by fibrous tissue
with spicules of woven bone of fish-hook or Chinese letters shape
• McCune-Albright´s syndrome
= polyostotic FD + skin pigmentation + precocious puberty
Bone cysts
• Solitary bone cyst
• unicameral cavity lined by smooth fibrous membrane
• mandible
• Aneurysmal bone cyst
• multiple spaces filled with red blood cells separated by thin
fibrous septa with numerous osteoclasts
• rare in jaws
Joint diseases
1. Congenital defects
• Dysplasia coxae congenita
2. Trauma
3. Degenerative diseases
• Osteoarthritis
4. Inflammation
• Infective arthritis
• Lyme disease
• Tuberculous arthritis
• Rheumatoid arthritis
• Gout
5. Tumours
• Pigmented villonodular
Dysplasia coxae congenita
• autosomal recessive disorder
• hypoplasia of hip joint with congenital subluxation or
• untreated may lead to early secondary osteoarthritis
• disruption of synovial membrane – bleeding into joint
cavity (hemarthros)
• organization of hematoma  fibrous adhesions limiting
joint movements or ankylosis (join space replaced by
metaplastic bone)
• hemarthros often complicates disorders of coagulation
• most common of joint diseases (14% of adults)
• breakdown of the articular cartilage
• weight-bearing joints
• hip, knee, intervertebral joints, TMJ (asymptomatic)
• clinical features
• pain, joint deformity, limitation of movement, crepitus
(creaking sound heard on movement of the joint)
• narrowing of joint space
• Two main groups:
• primary OA: no known associated condition
• secondary OA: known associated event or disease causally
related to OA (congenital disorders, inflammation, trauma)
• Morphology:
• erosive changes of joint cartilage (“fibrillation“) +
flaking off of small portions of cartilage
• complete loss of cartilage – polishing of the denuded
bone (smooth ivory-like surface)
• thickening of subchondral bone plate, synovial fluid
under pressure may enter into small defects in the
bone – forming of subchondral pseudocysts
• bony outgrowths at the margins of the articular
cartilage (osteophytes)
Infective arthritis
• bacteria (St. aureus, N. gonorrhoeae)
• routes of infection:
• blood spread (most common)
• direct penetration (trauma)
• direct spread (osteomyelitis)
• morphology
• hyperemia + acute inflammatory infiltrate in synovial membrane,
exudate accumulation in the joint cavity (pyarthros)
• lysosomal enzymes released from neutrophils may cause
severe damage of the articular cartilage – secondary
Lyme disease
• spirochaete Borrelia burgdorferi transmitted to
humans via tick bites
• skin + nervous system + heart + joints
• 3 clinical stages:
• stage I:
• macular skin lesion (erythema migrans) + low-grade fever,
headache, arthralgia, muscle pain, LNpathy
• stage II: bloodstream dissemination of borrelia
• nervous system (meningitis, meningoencephalitis, neuritis)
• heart (atrioventricular block, pericarditis)
• stage III:
chronic disease (months to years after initial infection)
arthritis (large joints, similar to rheumatoid arthritis)
nervous system (encephalopathy)
skin (acrodermatitis chronica atrophicans Herxheimer)
Tuberculous arthritis
• children
• associated with infection (haematogeneous dissemination)
• site: knee, hip, elbow and ankle
• clinical features
• insidious development of pain, swelling and limitation of movement,
other signs of inflammation mild or absent
• Micro
• synovial hyperplasia, tuberculous granulomas in 90% of cases
Rheumatoid arthritis
• common multisystem autoimmune disease
• 3x commonly in premenopausal women as in males
• site: small joints of the hands and feet, knees, hips,
involvement frequently symmetrical, TMJ - minor symptoms
• rheumatoid factors (IgM directed against Fc´ portion of IgG)
positive in 95% pts.
Rheumatoid arthritis
• inflammatory hyperplasia of synovial membrane with
increased vascularity, cellularity and synovial fluid
• inflammatory infiltrate (lymphocytes, plasma cells,
lymphoid follicles
• proliferation of inflammed hypervascular granulation
tissue termed pannus  degradation of underlying
articular cartilage  fibrous adhesions  limit joint
movement, formation of metaplastic bone (ankylosis)
• deformities of joints (ulnar deviation of fingers)
• periarticular rheumatoid nodules in 30%
• central area of fibrinoid necrosis + macrophages and fibroblasts
arranged in a palisaded fashion)
• complication: secondary amyloidosis
Variants of RA
• Juvenile rheumatoid arthritis (JRA): < 16 Y
• large joints predominantly involved, RFs often negative
• Still’s disease (systemic JRA)
• fever, leucocytosis, enlargement of liver, spleen and lymph nodes
• Felty’s syndrome: RA + splenomegaly + neutropenia
• disorder of purine metabolism leading to hyperuricaemia
• peak 30 – 60 Y, males
• deposition of monosodium urate crystals in articular
cartilage, synovial membrane and periarticular soft tissues
(tophi, tophaceous gout)
• clinical features:
• acute gouty arthritis: metatarsophalangeal joint of the big toe (70%),
ankle, knee, wrist, elbow
joint is red, hot, swollen, very painful and tender
• chronic gouty arthritis: follows recurrent episodes of acute gouty
arthritis, progressive erosion of cartilage and bone – limited joint
• Micro
• deposits of urate crystals + inflammatory infiltrate + foreign body-type
giant cells
Pigmented villonodular synovitis
• benign tumor rather than inflammatory condition
• site: knee joint (80%)
• clinical symptoms: mild pain, swelling, tenderness
• Gross
• brown-coloured thickened synovium thrown into small villous folds
• Micro
• diffuse proliferation of mononuclear cells with osteoclast-like
multinuclear giant cells and depositions of haemosiderin
Diseases of tendons, tendon sheaths
and bursae
• 1. Degenerative
• 2. Inflammatory
• area of myxoid degeneration of connective tissue of tendon
• on extensor surfaces of hand and feet
• Morphology: thin walled pseudocyst containing mucoid fluid
• = inflammation of tendon sheath
• purulent, rheumatoid (serofibrinous), tuberculous
• Tendovaginitis stenosans (deQuervain)
= stenosis of tendon sheath by accumulation of
fibrocartilaginous tissue – discontinual movement of affected
finger (digitus saltans)
• bursae around shoulder, elbow and knee
• acute bursitis (mechanical overload)
• serofibrinous exudate
• chronic bursitis (repeated traumatisation)
• fibrous thickening of wall, hyperplasia of synovial lining and fibrin
Diseases of skeletal muscle
• Muscle atrophy
• Muscle dystrophy
• Myasthenia gravis
• Inflammatory disorders (myositis)
Muscle atrophy
• Generalized: malnutrition, hypopituitarism, immobilisation
• Localized: immobilisation of one limb, denervation (trauma,
neuritis, poliomyelitis)
• Microscopic features: decrease in size of muscle fibers
Muscle dystrophy
• heterogeneous group of genetically determined disorders
• spontaneous progressive degeneration of muscle fibers
• Micro
• different size of muscle fibers (combination of atrophy and
• degenerative changes (fragmentation of the sarcoplasma,
• regeneration (cells with basophilic cytoplasm and more nuclei),
fibrosis, later lipomatosis
Muscle dystrophy
Duchenne-type dystrophy:
• X-linked recessive disorder – males affected only
• lack of dystrophin
• early childhood (5 years)
• pelvifemoral groups of muscles affected earliest (frequent falls,
gait disturbances, difficulty in rising)
• progress to other muscle groups (wheelchair bound at 10 to 12 Y)
• death at 20 Y (respiratory difficulties, pneumonia)
Becker-type dystrophy:
• much milder clinical picture
• dystrophin is abnormal
Myasthenia gravis
• acquired autoimmune disorder
• Ab bind to acetylcholine receptors on motor endplate –
defect of neuromuscular transmission
• association with hyperplasia of thymus in 60% and with
thymoma in 20%
• Clinical features
• abnormal muscle fatiguability, muscle weakness (eyelids)
• respiratory muscles may be severely affected
• Microscopic features: no light-microscopic abnormalities
= inflammation of muscles
• Myositis ossificans
• tumor-like lesion preceded by trauma
• central area of plump fibroblasts surrounded by zone of immature
woven and mature lamellar bone
• must be distinguished from extraskeletal osteosarcoma
Soft tissue tumors
• in non-skeletal mesodermal tissues such as adipose tissue,
fibrous tissue, muscle, blood vessels and peripheral nerves
• benign
• intermediate: locally aggressive x rarely metastasizing
• malignant
• from primitive mesenchymal stem cells
• classification according to their differentiation lines (e.g.
liposarcoma is not a tumor arising from lipoblast but
exhibiting lipoblastic differentiation)
Classification of soft tissue tumors
Lipomatous tumors
Smooth muscle tumors
Skeletal muscle tumors
Fibroblastic tumors
Nodular fasciitis
Fibrohistiocytic tumors
Benign fibrous histiocytoma
Malignant fibrous
Vascular tumors
Tumors of peripheral
Malignant peripheral nerve
sheath tumor
Tumors of uncertain origin
Synovial sarcoma
common benign tumor
subcutaneous tissue of the trunk and limbs, oral cavity
middle-aged and elderly
soft, slowly growing mass
• Micro: well-defined lobules of mature adipose tissue
• Lipoma variants:
• Angiolipoma
• Spindle cell lipoma
• most common malignant soft tissue tumor
• adults (peak 40-60 Y)
• site: lower limb and retroperitoneal space
• Key diagnostic feature: multivacuolated lipoblast
• subtypes:
• Well-differentiated liposarcoma (atypical lipomatous
tumor): closely resembles lipoma
• Myxoid liposarcoma
• Round cell liposarcoma
• Pleomorphic liposarcoma
• Dedifferentiated liposarcoma
• Mixed liposarcoma
Smooth muscle tumors
skin, subcutaneous tissue, uterus, gastrointestinal tract
bundles of well-differentiated smooth muscle cells with bright
eosinophilic cytoplasm and blunt-ended nuclei
mesentery, retroperitoneal space, wall of large veins, skin,
subcutaneous tissue, deep soft tissues of limbs
signs of malignancy
large size, high mitotic rate, necrosis, cellular pleomorphism
• adult type: head and neck of middle aged individuals
• round or polygonal cells with eosinophilic cytoplasm and abundant
cytoplasmatic glycogen
• genital type: polypoid growth of the cervix, vagina or
vulva in middle age
• large rhabdomyoblasts with cross-striations
• fetal type: boys < 3 Y, head and neck
• mixture of immature skeletal muscle and myxoid stroma
• cardiac rhabdomyoma: in infancy
• fatal by the age of 5 years, association with tuberous sclerosis in
about 50%
• multiple nodules in heart ventricles
• “spider cells“
• most common malignant soft tissue tumour in infants
and young children
• Dg.: demonstration of rhabdomyoblasts
• Embryonal rhabdomyosarcoma: most common
• early childhood, head and neck, genitourinary system
• Bothryoid rhabdomyosarcoma (grape-like)
• ER with polypoid configuration and myxoid consistency
• Alveolar rhabdomyosarcoma: 10 to 20 Y
• muscles of limbs and trunk
• Pleomorphic rhabdomyosarcoma: adults
• limbs of adults
Nodular fasciitis
• benign fibroblastic proliferation
• adolescents and young adults
• rapidly growing nodule within subcutaneous tissue, forearm
• Micro
• immature fibroblasts arranged in short bundles, numerous mitoses,
cellular pleomorphism not present
Superficial fibromatoses
Palmar fibromatosis (Dupuytren’s contracture)
• middle-aged men
• nodular thickening of palmar aponeurosis - flexion
deformities of fingers
Plantar fibromatosis (Ledderhose’s disease)
• nodular thickening of plantar aponeurosis
Penile fibromatosis (Peyronie’s disease)
• abnormal curvature of penis
• nodules of well-differentiated fibroblasts arranged in long
sweeping bundles
Deep fibromatoses (desmoid
• abdominal wall, young adults, women who have borne children,
detected in peripartum or postpartum period, sometimes in
surgical scars
• young adults, mesentery, + Gardner’s syndrome
• the most aggressive, adults in third and fourth decades, pectoral
and pelvic girdles
General features
• deep intramuscular location, large size (up to 10-15cm),
infiltrative growth pattern, high risk of recurrence after excision
uncommon malignant neoplasm
middle aged adults
deep soft tissues of lower limbs and trunk
bundles of spindle shaped cells arranged at angles to one
another (“herring-bone pattern“), frequent mitoses
Infantile fibrosarcoma
• within first two years of life, better prognosis
Benign fibrous histiocytoma
• common lesion, most frequently on skin of lower leg
• papule or nodule, often deeply pigmented
• within the mid-dermis, spindle cells arranged in curious
whorled pattern (storiform pattern)
Malignant fibrous histiocytoma
• deep soft tissues of limbs, retroperitoneum
• irregularly arranged plump, eosinophilic, spindle-shaped
cells with darkly staining and often bizarre nuclei,
numerous mitoses, interspersed macrophage-like cells,
storiform pattern in some areas
• MFH represents a morphological pattern shared by wide
variety of poorly differentiated malignant neoplasms
• = heterogeneous group of unrelated lesions
(pleomorphic sarcomas, dedifferentiated carcinomas, highgrade lymphomas etc.)
• hamartomas rather than true neoplasms
Capillary haemangioma
• skin, mass of proliferated capillaries
Pyogenic granuloma
• rapidly developing polypoid nodule, many small capillaries
similar to those seen in granulation tissue, myxoid stroma,
surface often eroded, common in oral cavity
Cavernous haemangioma
• skin, liver, aggregates of large dilated thin-walled vessels
lined by flattened endothelium
Irregular anastomosing vascular spaces lined by endothelial
cells with large hyperchromatic nuclei, highly aggressive
tumor with poor prognosis
Kaposi’s sarcoma
HHV8, often patients with AIDS, multiple bluish plaques
and nodules on the skin of lower extremities, spindleshaped cells with the presence of red cells containing slits
and haemosiderin deposition
Schwannoma (neurilemmoma)
• smooth lobulated lesion attached to a nerve
Microscopic features: two patterns recognized:
• Antoni A – compact areas formed by regular interlacing
bundles of uniform spindle-shaped cells, often foci of
nuclear palisading, Verocay bodies
• Antoni B – looser open areas, small cells with rounded
• not infrequently multiple, part of neurofibromatosis
• infiltrates and expands the affected nerve
Microscopic features
• spindle-shaped cells with elongated wavy nuclei set in
myxoid stroma
Malignant peripheral nerve sheath
tumor (MPNST)
• adults
• most common locations: neck, forearm, lower leg, buttock
• large mass producing enlargement of a major nerve
• relatively uniform spindle-shaped cells with
hyperchromatic nuclei and high mitotic activity
Synovial sarcoma
• term “synovial sarcoma“ is a misnomer
• young adults (15-40 Y)
• from extraarticular soft tissues, often near joint capsules and
• Microscopic features: tumour consists of two cell types:
• Epithelial (forming nests and gland-like spaces)
• Spindle (arranged in sheets)
• classification
• Biphasic (both cell types)
• Monophasic (spindle or epithelial cells only)
• frequent recurrences, metastases in lymph nodes and lungs