Case #4

• Pain is the usual presenting symptom in this tumor.

• Plain film shows a well-defined, expansile lytic lesion, which is centered at the distal fibular metaphysis and is bordered by a sclerotic rim.

• Biopsy specimen consisted of fragments of white-gray rubbery tissue. Low power view shows a moderately cellular chondromyxoid tissue with the following two characteristic features:
  1. Vague lobularity caused by alternating highly cellular and less cellular areas;
  2. Increased cellularity at the periphery of the lobules.

• Higher magnification view of the lobule shows mildly pleomorphic, angular and stellate cells set in bluish-pink chondromyxoid stroma. Note that the tumor lacks true hyaline cartilage matrix seen in enchondromas and chondrosarcomas. Another important feature is lack of mitotic activity.

Diagnosis: Chondromyxoid Fibroma

• Chondromyxoid fibroma (CMF) is a rare benign tumor with predominant occurrence in patients younger than 40 years. The peak incidence is between ages 10 and 20. It is one of two neoplasms of incompletely differentiated cartilage. The other neoplasm is chondroblastoma.
• Location. The most frequent skeletal sites are the knee area (30%), pelvis and the small bones of the feet. In the long bone, the tumor characteristically involves the metaphysis or metadiaphysis and is often seen in an eccentric position.
• Secondary cystic change in the form of aneurysmal bone cyst is not unusual.
• Differential Diagnosis of CMF includes chondroblastoma and, more importantly, chondrosarcoma. Distinguishing features of chondroblastoma are epiphyseal location and "chiken-wire" calcifications. The diagnostic criteria for chondrosarcoma include hyaline cartilage matrix, tumor permeation of the surrounding bone and mitotic activity. Keep in mind that CMFs, even those showing marked cellular pleomorphism, have extremely low number of mitoses never exceeding 1 per 50 hpf.
• Clinical Behavior. Recurrence rate averages 15%-20%. Large or recurrent lesions may be locally aggressive.