MRI diagnosis of non-ossifying fibroma

Non-ossifying fibroma is a benign tumor that arises from the connective tissue of the bone marrow cavity. It has no tendency to form bones, so it is also called "non-ossifying". When the bones mature, they may disappear on their own. They are usually single, but occasionally multiple. This disease is common in adolescents aged 8 to 20 years old, with the highest incidence in the distal femoral epiphysis (29%), followed by the proximal tibia and both ends of the fibula. There are generally no symptoms in the early stages, and they are often discovered after trauma. A few present with local pain. Pathologically, the tumor is composed of tough and dense fibrous tissue and is surrounded by a bone shell. Microscopically, the tumor is composed of connective tissue cells, multinucleated giant cells, and foam cells. There is no bone formation in the lesion, and the surrounding bone tissue has reactive hyperplasia.

Imaging findings

Radiologically, it can be divided into cortical and medullary types according to its X-ray manifestations. The cortical type is mainly characterized by the lesion being biased to one side of the bone shaft, located within or below the cortex, and presenting as a single-chamber or multi-chamber, round, oval or lobed translucent area with thick bone ridges. The long axis of the lesion is mostly parallel to the bone shaft, and it can protrude into the medullary cavity as it develops into the bone, surrounded by dense capsules or sclerotic bands. There is no ossification. The medullary cavity type is characterized by the lesion being located in the center of the long bone, presenting as a single cystic or multi-cystic translucent area, with a relatively uniform density and a sclerotic edge. Multicystic patients have irregular bony gaps or bone ridges, and local cortical bone thinning.

The tumor components of MRI are collagen fibers, hemosiderin, etc., with low signal on T2; if it contains a large number of tissue cells, it will show high signal on T2; and the enhancement is obvious after enhancement.

Non-ossifying fibroma and fibrous cortical bone defect are closely related. Most scholars believe that they are the same disease at different stages, that is, before the age of 14, there is a fibrous cortical bone defect, and after the age of 14, it does not disappear on its own and develops into a non-ossifying fibroma or an ossifying fibroma. The two have the same predilection site and similar histological manifestations. Therefore, we also believe that fibrous cortical defects often occur bilaterally and symmetrically, and generally disappear on their own before the age of 14; if they do not disappear, they may develop into non-ossifying fibroma. At the same time, fibrous cortical defects are more common in children aged 4 to 8 years old, have a familial tendency, and disappear on their own within 2 to 4 years. They often appear multiple and symmetrical, presenting as cystic or lamellar bone defect areas, without expansion and invasion of the medullary cavity.

Key points for reading the film:

1. It is common in adolescents aged 8 to 20 years old

2. The epiphysis is biased towards the diaphysis

3. Mild expansive multilocular lesions

4. The diameter is greater than 2 cm, and the long axis is consistent with the backbone

5. No ossification, thick bone ridges

MRI tumor components are collagen fibers, hemosiderin, etc., T2 shows low signal; if it contains a large number of tissue cells, T2 shows high signal; after enhancement, it is obvious. If both T1 and T2 are low signal, it has certain characteristics, reflecting the mature fibrous tissue inside the lesion.