Radiograph

OSTEOSARCOMA

Osteosarcomas are malignant bone forming tumours and the second most common primary bone tumour after multiple myeloma . They account for ~20% of all primary bone tumours.

Introduction

Osteosarcomas can be either primary or secondary, and these have differing demographics. 

Primary osteosarcoma - typically occurs in young patients (10-20 years) with 75% occurring before the age of 20; which is logical because the growth centers of the bone are more active during the puberty to adolescence time period. Therefore the epidemiology can be easily understood. For less clear reasons, there is a slight male predominance.

Secondary osteosarcoma - occurs in the elderly, usually secondary to malignant degeneration of Paget's disease, extensive bone infarcts or post radiotherapy for other conditions.

Epidemiology

Patients usually present with bone pain, occasionally accompanied by a soft-tissue mass or swelling. At times, the first symptoms are related to pathologic fracture.

The distribution of primary and secondary osteosarcomas is also different. 

Primary tumours typically occur in the metaphyseal regions of long bones, and have a striking predilection for the knee, with up to 60% occurring there

Secondary tumours on the other hand, have a much wider distribution largely mirroring the combined incidence of their underlying condition, and thus much have a higher incidence in flat bones, especially the pelvis (a favourite site of Paget's disease).

Clinical presentation

Osteosarcomas can be divided into a number of sub types according to degree of differentiation, location within the bone, and histological variants 3. 

These sub types vary in imaging findings, demographics and biological behaviour, and include :

Intramedullary ~ 80%◦ conventional high-grade - most common and discussed in this

article◦ telangiectatic osteosarcoma◦ low-grade osteosarcoma

Surface or juxtacortical ~ 10-15%◦ intracortical osteosarcoma◦ parosteal osteosarcoma◦ periosteal osteosarcoma

Extra skeletal ~ 5%◦ extra skeletal osteosarcoma

Pathology

Macroscopically osteosarcomas are bulky tumours where a heterogeneous cut surface demonstrates areas of haemorrhage, fibrosis and cystic degeneration. Their extension within the medullary cavity is often much more extensive than the bulky part of the tumour would suggest. Areas of bone formation are characteristic of osteosarcomas, with the degree of bone formation varying widely.

Macroscopy

Microscopically poorly formed trabecular bone is seen with (in the typical high grade conventional sub type) cellular pleomorphism and mitoses. Variable amounts fibrocytic and chondroblastic appearing cells may also be encountered. 

Microscopy

They typically occur at the metadiaphysis of tubular bones in the appendicular skeleton. Common sites include 

femur: ~ 40% (especially distal femur) tibia: ~ 16% (especially proximal tibia) humerus: ~ 15%  Other less common sites include fibula innominate bone mandible (gnathic osteosarcoma) maxilla vertebrae

Location

Plain Film: Conventional radiography continues to play an

important role in diagnosis. Typical appearances of conventional high grade osteosarcoma include:

medullary and cortical bone destruction wide zone of transition, permeative or moth-eaten

appearance aggressive periosteal reaction

◦ sunburst type◦ Codman triangle◦ lamellated (onionskin) reaction - less frequently seen

soft-tissue mass tumour matrix ossification / calcification

◦ variable:  reflects a combination of the amount of tumour bone production, calcified matrix, and osteoid

Radiological Features

The role of CT is predominantly in assisting biopsy and staging, but adds little to plain radiography and MRI in direct assessment of the tumour. The exception to this rule is predominantly lytic lesions in which small amounts of mineralized material may be inapparent on both plain film and MRI

CT

MRI is proving essential in accurate local staging and assessment for limb sparing resection, particularly for evaluation of intraosseous tumour extension and soft-tissue involvement. Assessment of the growth plate is also essential as up to 75 - 88% of metaphyseal tumours do cross the growth plate into the epiphysis .

T1◦ soft tissue non-mineralized component : intermediate signal intensity◦ mineralised / ossified components : low signal intensity◦ peri-tumoural oedema : intermediate signal intensity◦ scattered regions of haemorrhage will have variable signal (see 

ageing blood on MRI)◦ enhancement : solid components enhance

T2◦ soft tissue non-mineralized component : high signal intensity◦ mineralised / ossified components : low signal intensity◦ peri-tumoural oedema : high signal intensity.

MRI

Work-up: local staging by MRI prior to biopsy and distant staging with bone scan and chest CT.

Cure, if achievable requires aggressive surgical resection often with amputation followed by chemotherapy. If a limb-salvage procedure is feasible, a course of multi-drug chemotherapy precedes surgery to downstage the tumour, followed by wide resection of the bone and insertion of an endoprosthesis. Currently, the 5-year survival rate after adequate therapy is approximately 60-80% 4.

The most frequent complications of conventional osteosarcoma are pathologic fracture and the development of metastatic disease, particularly to bone, lung and regional lymph nodes.

Treatment and prognosis

General differential considerations include

osteomyelitis Other tumours

◦metastatic lesion to bone◦Ewing sarcoma◦aneurysmal bone cyst

Differential diagnosis

Haemophilic pseudotumour is a rare complication that occurs in 1-2% ofhaemophiliacs. 

Pathology Most develop in the muscles of the pelvis

and lower extremity, where the large muscles have a rich blood supply, or in bone following intraosseous bleeding.

Hemophilic Pseudotumour

Plain film X ray is useful in diagnosing intraosseous

pseudotumours. Pseudotumours appear as well defined,unilobular or multiloculated, lytic, expensile lesions of variable size. It can involve metadiaphysis and epiphysis of long bones.

Other findings include endosteal scalloping, perilesional sclerosis, cortical thinning or thickening, trabeculations and septations. Pathological fractures can also be present.

Ultrasonography, CT, and MR imaging  have important  roles in detecting pseudotumours, especially  when  they are in the pelvis. 

Radiographic features

CT is useful in detecting both the extent of soft-tissue masses and the involvement of bone. Pseudotumours contain coagulated  blood and are surrounded by a thick  wall. Contrast-enhanced CT is useful in determining  the thickness of  the wall. In the acute stage, the center of the pseudotumour is hypodense on CT, but the periphery  is  isodense and  indistinguishable from surrounding muscle.

CT

In the acute and subacute stages, sonography shows a central  anechoic area with increased echoes behind the lesion, caused by fluid in the pseudotumour. A distinct plane separates the mass  from the surrounding muscles.

CT shows the thick wall more consistently than does ultrasonography. Multiple irregular echoes represent solid material that cannot be documented on CT. Differential diagnosis from abscess may be difficult.

Ultrasound

The hypointense rim of a pseudotumor on both  T1 and T2-weighted images consists of fibrous tissue that contains haemosiderin .

MRI