Bone tumors

Bone tumors form when a bone cell divides uncontrollably and forms a mass or tumor.

If the tumor remains confined and doesn’t spread into surrounding tissues, then it’s considered benign.

But if the tumor invades into surrounding tissues and metastasizes or spreads through blood or lymph, then it’s considered malignant.

Malignant tumors can either be primary which is when they arise from the bone cells, or secondary, which is when a tumor developed somewhere else in the body, metastasized, and spread to the bones.

The most common sources of tumor cells that affect the bones but start somewhere else in the body, are the breast, prostate, the lungs, the thyroid, and the kidneys.

Now, even though the bones vary in size and shape, they are all made of the same types of cells, and chief among them are osteoblasts which build up new bone, and osteoclasts which help with bone breakdown or resorption.

Now in addition to these, there are some more primitive cells in the bone marrow called human mesenchymal stem cells and neuroectodermal cells, which have the ability to differentiate into many cell types including nerve, fat, bone, and cartilage cells.

Now, in terms of anatomy, looking at a long- bone like the femur - it has two epiphyses, which are the ends that contribute to joints with other bones.

Between the two epiphyses, is the diaphysis, also called the bone shaft.

In children and adolescents, there is an additional narrow portion between the epiphysis and the diaphysis called the metaphysis.

The metaphysis contains the growth plate, the part of the bone that grows during childhood.

In adults the growth plate has ossified and fused with the diaphysis and the epiphysis.

Now, there are genes that promote normal cell growth called proto-oncogenes.

With mutations, proto-oncogenes become oncogenes, and these overstimulate the cell growth.

To balance out cell growth, there are other genes called tumor suppressor genes which promote apoptosis or cell death of mutated cells.

So oncogenes or mutated tumor suppressor genes allow cells to keep growing uncontrollably resulting into tumors.

Alright, primary bone tumors can be divided up into ones that are most often benign, and those that are more often malignant.

Let’s start with the most common benign tumor, which is an osteochondroma and commonly affects males under 25 years.

Osteochondroma is thought to arise from mutations in the Exostosin 1, or EXT1 and Exostosin 2 or EXT2 genes, which encode the Exostosin 1 and 2 proteins.

These proteins help to synthesize heparin sulfate, which is a protein that regulates the growth of the growth plate.

So in osteochondroma, a tumor arises from the growth plate, and it typically results in a lateral bony projection.

This bony stalk is called an exostosis, and it has a cap, made mostly of hyaline cartilage.

Now, osteochondromas typically develop in the metaphysis of long bones, most usually the distal femur and the proximal tibia, around the knee joint, but they can also occur in flat bones like the ilium, one of the hip bones, as well as the scapula.

Another benign tumor is the giant- cell tumor of the bone - quite a name!

A risk factor for giant cell tumors is having a bony trauma like a fracture or radiation exposure.

The tumor cells arises from osteoclasts cells, but then develop into cells that have numerous nuclei - typically over 50 nuclei - so you can think of them as destructive little giants.

The giant cell tumor typically arises in the epiphysis of the long bones, like the distal femur and the proximal tibia.

On rare occasions, this tumor can become malignant, especially in elderly individuals.

Other benign tumors are osteoblastomas and osteoid osteomas, both of which arise from osteoblasts.

Both of these tumors classically form a nidus – which is a disorganized mix of small blood vessels, tiny rods of bone called trabeculae, and unmineralized bony tissue called osteoid.

In osteoblastomas, the nidi - which is plural for nidus - are large with a diameter of over 1.5 cm.

In osteoid osteomas, the nidi are smaller than 1.5cm in diameter and are often surrounded by a ring of sclerotic bone tissue.

That ring of sclerotic tissue often produces prostaglandins, which are small molecules that trigger the sensation of pain.

Osteoblastomas also tend to affect the bones of the axial skeleton, usually the mandible, and erode the surrounding bone.