Differentiating clinical and radiographic features of enchondroma and secondary chondrosarcoma in the foot

Enchondroma is the most common benign tumor of the bones of the foot. Chondrosarcoma in this area is relatively rare with malignant transformation from enchondroma occurring rarely. In contrast to similar tumors in the appendicular skeleton, it is difficult to distinguish between these two tumors when they occur in the foot.

We reviewed the medical records and radiographs of all patients with enchondroma and chondrosarcoma arising from enchondroma (secondary chondrosarcoma) from the radiologic archives at the Armed Forces Institute of Pathology (AFIP) and identified those patients with tumors involving the bones of the foot. There were 755 patients with enchondroma of which 34 (4.8%) involved the foot; there were 340 patients with secondary chondrosarcoma and 14 (4.1%) involved the foot. We compared clinical and radiographic features of both these lesions. We also compared interobserver differences not only for diagnosis but also for the presence of scalloping, fracture, cortical destruction, and mineralized matrix.

Size and location were statistically significant variables differentiating the two tumors (p = 0.03). Enchondromas had a mean size of 2.7 cm(2). Lesions that occurred in the hindfoot and midfoot were more likely to be malignant compared to those in the forefoot. In comparing interobserver reliability, most disagreement occurred regarding the presence or absence of matrix with the examiners concurring only 51% of the time. With regard to diagnosis, the examiners' accuracy was 71% and 80%. Their accuracy increased only to 83% when they agreed.

Our findings suggest that it is difficult to differentiate enchondroma from secondary chondrosarcoma in the foot. Concern for malignant change is warranted for cartilage bone tumors of the foot if they exceed 5 cm(2), or if they arise in the midfoot or hindfoot. In these cases, we recommend either biopsy or close clinical followup.