A differential diagnosis is best formulated with the help of plain radiographs. The radiographical features of a lesion indicate if the tumour is benign or malignant, and the latter, whether it is a primary tumour of bone, a malignant change in a pre-existing condition or a metastasis (Fig. 22.50). It is important to perform radiographs in at least two planes so as to demonstrate all the radiological features (Fig. 22.51).

Further investigations suggested by the differential diagnosis are:

• Haematological: full blood count, ESR/CRP, renal function tests, liver function tests including serum calcium, thyroid function tests, acid phosphatase/prostatic specific antigen.

• Cytological: sputum cytology, urine analysis.

• Nuclear medicine: bone scan to look for other areas of bony involvement and to determine if the lesion is 'hot' (Fig. 22.52). However, note that some tumours can show cold spots on bone scan or be negative.

• Further plain X-rays: skeletal survey depending on results of bone scan, radiographs of other hot sports on the scan, chest X-ray to assess for pulmonary metastases.

• CTscan: to assess the bony architecture of the lesion in an axial plane, extent of the tumour and presence or absence of cortical penetration. CT scan of the chest for metastases, especially for soft-tissue tumours.

• MRI: to assess for extracompartmental spread of tumours and degree of soft-tissue involvement. Also important in preoperative planning for clearance biopsies, extent of marrow involvement, measurement of resection margins and planning surgical approach for prosthetic implantation.

If curative resection is to be attempted, then it is important to stage malignant tumours, especially those involving the soft tissues and the above investigations should be helpful in aiding staging and should preferably be undertaken prior to a biopsy, as open-open biopsies in particular can disrupt the

Figure 22.47. Solitary exostosis of the distal femur.

soft-tissue planes and therefore interfere with interpretation of the radiological and MRI characteristics of tumours.

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