BPOP versus differentials

This is a case of 17 year old female with pain and swelling over 5th metacarpal head dorsally.  Case submitted by: Dr. Wael Nemattalla, MD, Radiodiagnosis from  Egypt. 

Differential diagnosis:

1. Periosteal chondrosarcoma (juxta-cortical chondrosarcoma).

2. Parosteal osteosarcoma.

3. BPOP (bizarre parosteal osteochondromatous proliferation).

4. Reactive florid periostitis.

5. Rarely is cartilage cap chondrosarcoma with totally destroyed most of the osteochondroma but the presence of such large bony component in X-ray is mitigating against such assumption. 

The mass over dorsum of 5th metatarsal head shows malignant features in MR like cortical interruption, bone marrow involvement and avid heterogeneous soft tissue enhancement. Concerning internal bony structure in X-ray film; periosteal chondrosarcoma (juxta-cortical chondrosarcoma) is preferred over other diagnoses. STIR hyperintensity is in favor of chondromatous tumor. 

Parosteal osteosarcoma may also included in DD due to younger patient's age but is rare in hands. 

BPOP (bizarre parosteal osteochondromatous proliferation) is a benign lesion and is common in small bones of hands and feet in younger age groups but usually associated with T2 hyperintense cartilage cap and peripheral and/or partial enhancement of the bony mass; cortex is intact in BPOP; in our case cortex and bone marrow are involved. 

Reactive florid periostitis is also in DD but usually associated with intact cortex and laminated periosteal reaction. Sometimes florid reactive periostitis may have aggressive appearance similar to malignancy; it may have cortical and bone marrow involvement as well as marked soft tissue enhancements.