Cortical scalloping and cortical penetration by small eccentric chondroid lesions in the long tubular bones: not a sign of malignancy?

The objective of this study was to evaluate by cross-sectional imaging the prevalence and degree of cortical scalloping by small eccentric chondromas correlated with histologic diagnosis and patient history. From 122 patients with histologically proven enchondromas and two patients without histology but with radiologic and clinical follow-up, 11 patients with small, eccentrically located chondromas in the long bones had cross-sectional imaging available. The lesions were evaluated for location, size, presence, and degree of cortical scalloping. The patient's medical charts and microscope slides were reviewed for relevant clinical history, clinical management, and histology. The chondromas ranged in size from 1.6 to 3.8 cm (mean 2.3 cm). Two lesions were located in the proximal femoral diaphysis, two in the distal femoral diaphysis, six in the distal femoral metaphysis, and one in the proximal tibial epimetaphysis. The lesions were curetted due to diagnostic uncertainty, continued pain, marked radiologic cortical penetration, or due to patient insistence on biopsy. All 11 lesions were benign, nine histologically, and two by stability over 4 and 7 years. The prevalence of cortical scalloping among eccentric chondromas was 100%. Cortical scalloping or occupancy ranged from 50 to 100% (mean 75%). All small eccentric chondromas in this study were associated with an appearance of cortical scalloping of varying degree. All curetted lesions were histologically bland without nuclear atypia. Based on the benign histology of nine lesions and lack of growth of two lesions over several years, the degree of cortical scalloping is felt to be a result of lesion location within the endosteum rather than biological activity or malignancy.