A boy with knee pain and an osteolytic mass is biopsied, showing pleomorphic cells. What is the most common bone malignancy in children?

The most common bone malignancy in children is osteosarcoma. This type of cancer primarily affects the long bones, particularly around the knee region, which aligns with the scenario presented where the boy has knee pain and an osteolytic mass. Osteosarcoma typically occurs during periods of rapid growth, commonly seen in adolescents, making it a frequent diagnosis in this age group.

Histologically, osteosarcoma is characterized by the presence of pleomorphic osteoblasts that produce osteoid and is typically aggressive. The biopsy findings of pleomorphic cells support the diagnosis of this malignancy. This type of cancer not only invades the bone but can also metastasize to distant sites, primarily the lungs, making early diagnosis and treatment crucial.

In contrast, Ewing sarcoma, while also a primary bone malignancy that commonly occurs in children, usually presents with different characteristics and often exhibits small, round blue cells on histological examination. Rhabdomyosarcoma is a soft tissue sarcoma that can occur in children but is not classified as a bone malignancy. Chondrosarcoma tends to occur in older individuals compared to the typical age range for osteosarcoma.

Thus, given the age of the patient, location of the