Purpose. To assess the association between tumour volume and occurrence of lung metastasis in patients with osteosarcoma and to determine the cut-off value. Methods.Records of 70 patients with histopathologically confirmed primary osteosarcoma in the extremities who had magnetic resonance imaging and computed tomography of the thorax less than one month before treatment were reviewed, with reference to the official report of tumour dimensions and lung metastasis by radiologists. The status of lung metastasis was assessed. Tumour volume was measured using the formula for an ellipsoidal mass. Results. Of the 70 patients with osteosarcoma, 33 (47%) had evidence of lung metastasis. Tumour volume was directly associated with occurrence of lung metastasis (p=0.048). The proportion having lung metastasis when the primary tumour volume exceeded 371 cm 3 was 69%, compared to 34% in those with smaller tumours. Conclusion. Larger tumours are more likely to

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