RESUMEN
Conservative surgery of primary bone tumors, which has become possible owing to the progress made by chemotherapy, must be carcinological, and it is therefore based on a thorough assessment of local tumor extension. CT and MRI have proven their great value for this assessment. While CT clearly shows the calcified tissue and the extension into soft tissue, MRI is more effective to delineate the tumoral and intraosseous margins and, more importantly, for the exploration within bone, for which it has the advantage of making a study in all planes possible. The anatomical substrate and exact meaning of the MR signal of pathological tissue still remains to be specified. However, MRI already appears to be the essential examination for the assessment of local bone tumor extension. If it did not have contraindications, it could replace CT within the next few years.