Most bone tumours encountered in clinical practice are secondary forms (i.e. spread – metastases – from a separate primary cancer that started elsewhere), and it is incorrect to refer to these as ‘bone cancer’.
Doctors use the term bone tumours to refer to tumours which arise in the bone from the start (primary bone tumours), rather than those which are spread from a cancer that has originated elsewhere.
This point is of importance as the methods and data outlined in this section do not apply to secondary bone cancers/metastases.
The causes of most malignant bone tumours are not known. However, there are some osteogenic sarcomas that occur in patients who have the inherited form of retinoblastoma, for whom the possession of the Rb gene (in the genotype) predisposes to this condition; indeed, for those patients who have received radiotherapy to treat the retinoblastoma, the risk is even greater.
Patients with longstanding Paget’s disease of bone are also predisposed to develop osteogenic sarcoma, as are those who have received exposure to ionising radiation.
Most bone tumours encountered in clinical practice are secondary (i.e. metastases from primary cancer that started elsewhere), and it is a layman’s misnomer to refer to these as ‘bone cancer’. The doctor reserves this term to those tumours of bone which arise in the bone ab initio. The point is of importance as the methods, and data outlined below do not apply to bone tumours as secondary/metastases.
Primary bone tumours include osteogenic sarcoma (arising from the true bone forming cell) Ewing’s sarcoma (arising from a primitive pluripotential cell), chondrosarcoma (arising from the cartilage forming cell line) and other rare forms of sarcoma (e.g. malignant fibrous histiocytoma).
Primary bone lymphoma will not be considered here, nor the sarcomas which are more commonly associated with soft tissues such as bone and connective tissue.
Osteogenic sarcoma occurs with two peak incidences in life. The first is in adolescence (when the bones are actively growing) and the second is in late life when it is not infrequently associated with Paget’s disease of bone. Exposure to radiation predisposes to the later development of osteogenic sarcoma, and some patients with familial syndromes (par excellence those with hereditary retinoblastoma – but see introductory remarks concerning soft tissue sarcomas for others) are predisposed.
Ewing’s sarcoma tends to present in youth and is very unusual over the age of 40 years.
Both osteogenic sarcoma and Ewing’s sarcoma are less common in people of African extraction.
Chondrosarcomas, whilst certainly afflicting the young, are overall more common in later life again two peaks of age incidence.
Symptoms & diagnosis: bone tumours
The classical presentation to the doctor is with a painful swelling arising from the affected bone. Occasionally the patient may present with a fracture through the involved bone.
Whereas osteogenic sarcoma tends to involve the ends of long bones, Ewing’s tends to arise in the mid portions of long bones (the term long bones refer to limb bones such as the femur or humerus etc…).
X-rays are often highly suggestive of a bone cancer but the critical test is the biopsy (where the doctor obtains a piece of the tumour to examine down the microscope).
The x-ray picture of an osteogenic sarcoma is often highly typical with excess bone tissue laid down within and around the tumour; similarly, there are features on plain x-ray that guide the radiologist to be highly suspicious that a bone tumour is a Ewing’s sarcoma – but no feature on plain x-ray is diagnostic by itself.
Biopsy of the tumour is required to give the certain diagnosis. The biopsy specimen is then sent to the laboratory for microscopic examination/histopathology.
The doctor will then request staging scans: MR imaging is the best scan for delineating the extent of the tumour in the involved bone (see figure where the tumour in the tibia – the shin bone- is seen in side view, just below the knee joint) and also for defining the extent of extraosseous spread (spread outside the bone).
CT scanning of the chest is then performed for determining whether there are lung metastases and isotope bone scanning is best for determining whether there are bone metastases – the two commonest sites for distant relapse.
The oncologist will want to know if the tumour is confined to bone, whether it has spread outside the bone into the surrounding muscles (best assessed on MRI of the area) or whether it has spread further afield. With respect to distant spread, most sarcomas spread to the lungs first, which is best assessed by CT scanning of the chest. Thereafter, it spreads to other bones and other sites.
Treatment and outcomes: Bone tumours
Treatment of early bone tumours depends heavily on the expert input of several groups of experts.
Where the tumours can be surgically removed, then expert orthopaedic input is required and any operation may be attended by the immediate placement of a prosthetic implant (see figure).
However, it may not be best to go straight to operation. Often and for both Ewing’s sarcoma and osteogenic sarcoma – for which there is no evidence of spread, primary therapy is with chemotherapy. The drugs are given intravenously, which implies that they circulate around the body and not only shrink down the original/primary tumour but also knock out early spread of microscopic cells that could otherwise later clone in other body areas to become established metastases.
For both osteogenic sarcoma and Ewings sarcoma, a several month programme of chemotherapy precedes any surgery. In many cases, cancers that had grown beyond the confines of the bone can be shrunk back into the bone by such chemotherapy and the subsequent operation then has a better chance of obtaining clear margins (i.e getting that highly desirable buffer of uninvolved tissue between the cancer and the cut margin). Any cancer cells (that might have travelled away from the primary bone cancer) are also attacked at an early stage of their development (and hopefully annihilated).
The operation is often able (as shown in the figure) to place a prosthesis where the bone tumour was, so that the patient has a return to function of the limb. However, sometimes, the tumour is just too extensive and large to allow the approaches just outlined, and the patient may have to undergo a limb amputation.
Radiotherapy has a selected place when it is not possible to resect (cut out) the tumour completely, for example in the vertebral column (where it is less easy to fully resect the tumour).
Plain X-ray (left panel) and MRI scans (right panels) demonstrating an osteogenic sarcoma of the left femur.
The treatment of the patient with a localised primary bone tumour, particularly in youth, is aimed at cure. This is usually affected by primary chemotherapy, where the patient undergoes several cycles of a multiple chemotherapy drug regime and then (after shrinkage of the tumour by this means) surgery follows. If the chemotherapy has been useful in causing tumour shrinkage, then more chemotherapy may well be advised after the operation is over with cure as the target.
Radiotherapy may have a role in tumours that are not straightforward to operate e.g. vertebral/spinal tumours.
Occasionally, where there is early metastatic disease e.g. one or a few lung metastases from osteogenic sarcoma, surgical excision of these metastases may still allow cure and the timing of such surgery is carefully integrated into the chemotherapy regime.
Similarly, when an osteogenic sarcoma patient relapses some time after the whole therapy regime is completed, with lung metastatic nodules (but once again only with a single or a few) , then surgical excision or cyberknife to these nodules may be part of a ‘salvage’ regime that will include further (but different) chemotherapy, all with cure as the target again.
For relapsed Ewing’s sarcoma, the ‘salvage’ chemotherapy regime may be followed by a ‘high dose’ chemotherapy treatment and then autologous (nowadays a peripheral blood stem cell) transplant. (In this technique, a very high dose of chemotherapy is given that is sufficient to ‘knock out’ the tumour cells that survive but also, as a side effect, most vulnerable dividing cell population in the body (the bone marrow) and the patient is then rescued from bone marrow failure by the re-infusion of their own bone marrow progenitor cells that were harvested earlier).
Screening for bone tumours
Except for patients with the inherited form of retinoblastoma, there is no screening programme for this rare disease.