A 16-year-old boy with high-grade osteosarcoma of the proximal tibia is staged before surgery. MRI reveals a second discontinuous nodule of tumour in the medullary canal of the same tibia, 6 cm proximal to the main mass. No other lesion or metastasis is present. According to the Enneking system, this is staged as:

• A Stage IIA
• B Stage IIB
• C Stage IIIA
• D Stage IIIB ✓

Explanation

A skip metastasis is a discontinuous tumour focus in the same bone as the primary lesion. In the Enneking system this upgrades the local extent to T3, and a high-grade tumour (G2) with a skip lesion but no distant metastasis is Stage III. Because the tumour is high grade and extracompartmental behaviour is typical, it is Stage IIIB. Stage IIA/IIB require a single continuous lesion, and IIIA implies an intracompartmental high-grade tumour with skip lesions.

Reference: Campbell's Operative Orthopaedics, 14th ed.

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