Question

A 11-year-old boy presented with complaints of pain in the right arm near the shoulder. X-ray examination revealed an expansile lytic lesion in the upper third of the humerus. What is the most likely diagnosis?

• A. Giant cell tumor
• B. Unicameral bone cyst
• C. Osteochondroma
• D. Parosteal osteosarcoma

Hint:

Expansile lytic lesion in the metaphysis near an open growth plate in a child points to a simple (unicameral) bone cyst.

Answer 1

The Correct Option is B

Step 1: Note the key clues.
The patient is 11 years old, so the growth plates (physes) are still open. The pain sits near the shoulder, and the X-ray shows an expansile lytic lesion in the upper third (proximal metaphysis) of the humerus.

Step 2: Recall where a unicameral bone cyst forms.
A unicameral (simple) bone cyst is a fluid filled benign cavity that grows in the metaphysis of a long bone, right next to the growth plate. The single most common site in the whole skeleton is the proximal humerus, followed by the proximal femur. It shows up as a well defined, central, lytic area that can expand the bone cortex, exactly what is seen here. Most children have no symptoms until the thinned bone breaks after a minor fall, which is called a pathological fracture.

Step 3: Rule out giant cell tumor.
A giant cell tumor is an epiphyseal lesion that needs a closed growth plate to reach the end of the bone. It is almost always seen after skeletal maturity, usually between 20 and 40 years of age. An 11 year old still has open physes, so this diagnosis does not fit the age.

Step 4: Rule out osteochondroma.
Osteochondroma is the most common benign bone tumor overall, but it grows outward from the metaphysis as a bony cap with a cartilage cover. On X-ray it looks like a stalk or a broad based bump pointing away from the joint, not an expansile lytic cavity inside the bone.

Step 5: Rule out parosteal osteosarcoma.
Parosteal osteosarcoma is a low grade malignant tumor that forms on the outer surface of the bone, most often at the back of the distal femur, and it looks like a dense, sclerotic (bright white) mass, not a lytic lesion. It also favors an older age group, usually 20 to 40 years.

Step 6: Final answer.
An expansile lytic lesion in the metaphysis of the proximal humerus in a child points to a unicameral bone cyst.
\[ \boxed{\text{Unicameral bone cyst}} \]