Musculoskeletal tumours

Overview

Primary bone tumours are uncommon, but they are favourite MCQ material because each has a typical age, site in the bone and X-ray sign. Metastases are far commoner than primary bone sarcomas in older adults.

The exam tests: matching tumour to age and site, classic X-ray signs, which lesions are benign, how to biopsy, and when to fix a bone with a metastasis.

Classification (by the tissue the tumour makes)

Age and site: the key to diagnosis

Age

Site within a long bone

Benign tumours and tumour-like lesions

Osteochondroma (cartilage-capped exostosis)

• The commonest benign bone tumour; really a developmental lesion.
• A bony outgrowth from the metaphysis, pointing away from the joint, covered by a cartilage cap.
• Common sites: around the knee (lower femur, upper tibia), upper humerus, iliac crest.
• Teenager with a painless hard lump; may cause bursa, nerve irritation or tendon catching.
• Stops growing when the growth plate closes.
• Hereditary multiple exostoses (autosomal dominant) carries a higher risk of malignant change than a solitary lesion.
• Warning signs of change to chondrosarcoma: growth or pain after skeletal maturity, a thick cartilage cap (Apley uses more than 1 cm; many texts use 2 cm in adults), scattered calcification in the cap, soft tissue spread. Pelvic lesions are the ones that often go unnoticed.
• Treatment: excise if symptomatic or suspicious.

Enchondroma

• Benign cartilage within the medulla; commonest bone tumour of the hand (phalanges).
• Often found after a pathological fracture of a finger.
• Multiple enchondromas: Ollier disease; with soft tissue haemangiomas: Maffucci syndrome (high malignant risk).

Osteoid osteoma

• Young males (usually under 30); femur and tibia in over half.
• Tiny lesion (nidus under 1 cm) with severe night pain, typically relieved by aspirin/NSAIDs (prostaglandin-driven).
• X-ray: small lucent nidus with surrounding dense sclerosis; CT shows the nidus best; bone scan intensely hot.
• In the spine (posterior elements) it can cause painful scoliosis.
• No malignant potential. Treatment: CT-guided radiofrequency ablation or excision of the nidus.
• Osteoblastoma is a larger (over about 2 cm) cousin, mostly in the vertebral posterior elements, not relieved so well by aspirin.

Chondroblastoma

• Rare; epiphysis in an adolescent before the growth plate closes (upper humerus, lower femur, upper tibia).
• Histology: chondroblasts with grooved nuclei and "chicken-wire" calcification.

Simple (unicameral) bone cyst

• Child; metaphysis of the upper humerus (commonest) or upper femur, next to the growth plate.
• Usually found after a pathological fracture.
• Central, lucent, thins the cortex. Fallen fragment sign (a fragment of cortex lying in the dependent part of the fluid-filled cyst) is typical.
• Contains clear straw-coloured fluid.
• Treatment: often heals after fracture; steroid (methylprednisolone) injection, or curettage and grafting.

Aneurysmal bone cyst

• Teenagers and young adults; metaphysis, also spine.
• Eccentric, expansile, trabeculated lucency; MRI shows fluid-fluid levels.
• Blood-filled spaces separated by septa with giant cells.
• Treatment: curettage and bone grafting (can recur).

Non-ossifying fibroma: harmless eccentric metaphyseal lucency with a scalloped sclerotic edge in children; leave alone.

Fibrous dysplasia: ground-glass bone, "shepherd's crook" femur; polyostotic form with café-au-lait spots and precocious puberty is McCune-Albright syndrome.

Giant cell tumour (osteoclastoma)

• About 5% of primary bone tumours.
• Young adult (20-40), slightly more in women, after the physis has closed.
• Sites: lower femur, upper tibia (around the knee), upper humerus, lower radius.
• Arises in the epiphysis/metaphysis and extends right up to the subchondral bone plate.
• X-ray: eccentric lytic lesion at the bone end, thin expanded cortex, soap-bubble appearance, no sclerotic rim, no matrix calcification.
• Histology: many osteoclast-like giant cells among mononuclear stromal cells (the stromal cells are the true tumour cells).
• Behaviour: benign but locally aggressive; recurs after curettage; rare lung metastases; can undergo malignant change (risk raised by radiotherapy).
• Check calcium, phosphate and alkaline phosphatase to exclude a brown tumour of hyperparathyroidism.
• Treatment: extended curettage with burr, adjuvant (phenol, liquid nitrogen or hydrogen peroxide) and filling with bone cement or graft; wide excision and reconstruction for aggressive or recurrent lesions; denosumab for unresectable sites.

Malignant primary bone tumours

Osteosarcoma

• Commonest primary malignant bone tumour (excluding myeloma).
• Age 10-20 (adolescent growth spurt); a second peak in the elderly from Paget disease or previous radiotherapy.
• Associations: retinoblastoma (RB1), Li-Fraumeni (TP53), Paget disease, radiation.
• Site: metaphysis of long bones, especially around the knee (lower femur, upper tibia) and upper humerus.
• Symptoms: constant pain worse at night, then a swelling; pathological fracture is uncommon.
• Blood: raised alkaline phosphatase and ESR.

X-ray signs:

• Mixed lytic and sclerotic destruction with a poorly defined edge.
• Sunburst (sunray) spicules: streaks of new bone radiating into the soft tissue.
• Codman triangle: new bone where the periosteum is lifted at the tumour edge.
• Neither sign is specific; both can occur with Ewing sarcoma and other aggressive lesions.

Spread: by blood, mostly to the lungs (about 1 in 10 have lung metastases at presentation); skip lesions in the same bone. Lung deposits are hard because they make tumour osteoid.

Histology: malignant spindle cells producing osteoid (the key to diagnosis).

Staging and treatment:

1. MRI of the whole bone (extent, skip lesions), CT chest, bone scan.
2. Biopsy in the specialist centre (see below).
3. Neoadjuvant multi-agent chemotherapy for about 8-12 weeks.
4. Wide resection with limb salvage (endoprosthesis or graft), or amputation if needed.
5. More chemotherapy; tumour necrosis over 90% in the specimen is a good prognostic sign.

Variants: parosteal osteosarcoma (low grade, on the back of the lower femur, older patients, better prognosis; wide excision alone) and periosteal osteosarcoma.

Ewing sarcoma

• Small round blue cell tumour of children and teenagers (about 5-20 years).
• Genetics: translocation t(11;22), EWS-FLI1 fusion; CD99 positive.
• Site: diaphysis of long bones (femur, tibia, fibula), also pelvis, ribs and clavicle.
• Clinical: pain, warm tender swelling, fever, raised ESR and WCC: mimics osteomyelitis.
• X-ray: permeative moth-eaten destruction in the shaft with a layered onion-peel periosteal reaction; a large soft tissue mass; sunray and Codman signs may also be seen.
• Cut surface may look like pus (necrotic tumour), another trap.
• Treatment: chemotherapy first, then surgery and/or radiotherapy (Ewing is radiosensitive), then more chemotherapy.

Chondrosarcoma

• Adults 40-60, men more often.
• Pelvis, upper femur, upper humerus, ribs, scapula.
• Primary (central) or secondary (from an osteochondroma or enchondroma).
• X-ray: lytic area with ring-and-arc or popcorn calcification, cortical scalloping.
• Slow growing; does not respond well to chemotherapy or radiotherapy, so wide surgical excision is the treatment.

Comparison of the big three

Myeloma

• Commonest primary malignancy of bone in adults (Bailey); suspect in anyone over about 65 with back pain.
• Punched-out lytic lesions (skull "pepper pot"), vertebral collapse, anaemia, hypercalcaemia, renal failure, very high ESR.
• Bone scan is often normal (lesions are purely lytic): use skeletal survey, MRI or PET.
• Diagnosis: serum/urine protein electrophoresis (monoclonal band, Bence Jones protein), bone marrow.

Metastatic bone disease

• Commonest malignant lesion in bone overall; over 50 years they outnumber all primary bone tumours.
• Primary sources: breast (commonest), prostate, kidney, lung, thyroid.
• Sites: vertebrae, pelvis, upper femur, upper humerus, ribs; spread is mainly by blood (vertebral venous plexus for the spine).
• Mostly osteolytic; osteoblastic (sclerotic) deposits suggest prostate (also breast sometimes).
• In children under about 6: neuroblastoma.
• Features: pain (often at night), pathological fracture, hypercalcaemia, spinal cord compression.
• Bone scan (99mTc) is the most sensitive test for silent deposits (but may miss myeloma and very aggressive lytic deposits).

Management of metastases

• Analgesia, bisphosphonates or denosumab, radiotherapy (pain relief).
• Prophylactic fixation of a long bone when fracture is likely. The Mirels score (site, pain, lytic or blastic, size relative to cortex) predicts fracture risk; Apley treats a score of 8 or more as high risk, needing fixation before radiotherapy.
• Rule of thumb: destruction of half of one cortex on any view, or avulsion of the lesser trochanter, means fracture is close.
• Spine: decompression and stabilisation for cord compression with a reasonable outlook.
• A solitary "metastasis" may be a primary bone tumour: biopsy before fixing it.

Soft tissue tumours

• Most soft tissue lumps are benign (lipoma is the commonest).
• Red flags for sarcoma (refer to a sarcoma centre): larger than 5 cm, deep to the fascia, increasing in size, painful, or recurrence after excision.
• Common sarcomas in adults: undifferentiated pleomorphic sarcoma, liposarcoma (often retroperitoneal or thigh), synovial sarcoma. In children: rhabdomyosarcoma.
• Investigations: ultrasound first, then MRI; core needle biopsy planned along the line of excision; CT chest (lungs are the main metastatic site).
• Treatment: wide excision with a cuff of normal tissue including the biopsy track; radiotherapy for high-grade tumours (before or after surgery); chemotherapy has a limited role except in some (e.g. rhabdomyosarcoma).

Principles: staging and biopsy

Enneking staging

Biopsy rules

• Stage first, biopsy last: imaging (X-ray, MRI of the whole bone, CT chest) before the biopsy, because biopsy changes the MRI picture.
• Biopsy should be done at the specialist centre, ideally by (or with) the surgeon who will do the resection.
• Place the track in line with the planned incision and through a single compartment, avoiding neurovascular structures, so the whole track can be excised with the tumour.
• Core needle biopsy is usually preferred; open (incisional) biopsy if needed. Excisional biopsy only for small, clearly benign lesions.
• Send tissue for histology and culture.

Quick revision

• Ewing: child, diaphysis, onion peel, fever, t(11;22), radiosensitive.
• Osteosarcoma: teenager, metaphysis around knee, sunburst, Codman triangle, lung metastases, chemo then resection.
• Giant cell tumour: young adult, epiphysis to subchondral plate, soap bubble, recurs; exclude brown tumour.
• Osteochondroma: commonest benign; suspect malignancy if it grows after maturity or has a thick cap.
• Osteoid osteoma: night pain relieved by aspirin, nidus under 1 cm; CT shows it; RF ablation.
• Simple bone cyst: proximal humerus of a child, fallen fragment sign, steroid injection.
• Over 50: metastasis (breast commonest; prostate sclerotic) or myeloma (bone scan may be cold).
• Soft tissue lump over 5 cm, deep, growing or painful: MRI and specialist biopsy.

Written by The House Job from standard orthopaedic references (Apley; Bailey & Love).

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