Malignant chondroblastoma of the rib with scalp metastases in an adult patient: case report with molecular characterization.

Chondroblastomas represent less than 1% of all bone tumors. They are currently classified as benign neoplasms and "benign lung implants" have been described. Exceedingly rare case of malignant chondroblastomas have been published, and only three of them have scalp metastases. We present the case of a 57-year-old healthy man who noticed a rapidly growing mass on his left chest. Radiological imaging revealed an 8 cm expansile and lytic lesion located in the anterior segment of the sixth costal arch with a thinned cortex without periosteal reaction. The tumor was removed with wide margins. Seven years later, he presented with multiple superficial movable lesions on the scalp that were surgically removed. The costal mass was histologically composed of fused nests of discohesive polygonal cells with grooved nuclei, and scattered osteoclast-like giant cells embedded in an eosinophilic chondroid matrix. It presented a permeative growth pattern with entrapped pre-existing bone trabeculae and focal soft tissue extension. Scanty "chicken-wire" calcifications were detected. Moderate atypia and necrotic foci were observed. Occasional non-atypical mitotic figures were also observed. The cutaneous lesions demonstrated the same histopathological findings. Both the metastases and the primary tumor showed diffuse immunoreactivity for anti-histone H3K36M. Molecular study of the H3 histone family member 3B gene demonstrated a p.K37M mutation in exon 2 in the original mass and in the metastasis. Next-generation sequencing did not detect any other molecular alterations in the metastases. Malignant chondroblastomas are extremely rare tumors that most commonly arise in unusual locations, such as the rib or scapula, and in older adults. Permeative growth pattern, soft tissue extension, greater atypia, and higher mitotic rates are histopathological features of malignancy. H3K36M immunoreactivity and H3F3B gene mutations are key to achieving correct diagnosis. Wide resection and close follow-up of patients should be recommended. There is currently no consensus regarding the administration of adjuvant chemotherapy.