Low-grade central osteosarcoma is a rare, slow-growing type of bone cancer. It is a form of osteosarcoma, a cancer of the cells that produce bone. Your report may use the shorter name LGCOS, or an older name, well-differentiated intramedullary osteosarcoma.
The word central means the tumor starts inside the bone, in the hollow center called the medullary cavity, where the bone marrow is. This sets it apart from parosteal osteosarcoma, a similar low-grade cancer that grows on the outside of the bone.
Low-grade central osteosarcoma is a low-grade cancer. It grows slowly and rarely spreads, and most people are cured with surgery alone. It can grow into surrounding bone and soft tissue, and it can come back if it is not completely removed.
This article explains how low-grade central osteosarcoma is diagnosed and what the findings in your pathology report mean.
Low-grade central osteosarcoma is very rare, making up only about 1% to 2% of all osteosarcomas. It usually affects adults in their 20s and 30s, which is older than the usual age for osteosarcoma. It is slightly more common in women.
Most develop near the ends of the long bones of the leg, particularly the thigh bone and the shin bone, close to the knee. Rarely, the tumor develops in the jaw, the pelvis, or the small bones of the hands and feet.
The most common symptom is pain in the affected bone, sometimes with swelling or a lump near a joint. Because the tumor grows slowly, symptoms often build up gradually, and some people have them for months or years before the diagnosis is made.
Low-grade central osteosarcoma is caused by extra copies of a region of chromosome 12 that contains two genes called MDM2 and CDK4. Extra copies of a gene are called amplification. These genes help control cell growth, and too many copies allow cells to keep growing.
This change develops in the tumor cells during a person’s lifetime. It is not inherited and cannot be passed on to children. It is the same change found in parosteal osteosarcoma.
X-rays, CT, and MRI usually show a large area inside the bone that may contain both dense and thinned areas. The tumor often has a less sharp border than a noncancerous bone growth, and it may break through the outer shell of the bone.
A biopsy confirms the diagnosis. The biopsy should be done at the center that will perform the final surgery, because the track left by the needle has to be removed along with the tumor later.
Under the microscope, the tumor is made of spindle cells, which are long, thin cells, along with strands of bone made by the tumor. The spindle cells show only mild nuclear atypia, meaning their nuclei look only slightly abnormal. The tumor grows between the existing bone and may extend through the outer shell of the bone into the nearby soft tissue.
Because the cells look so close to normal, low-grade central osteosarcoma can be mistaken for a noncancerous bone condition. The most common look-alike is fibrous dysplasia, a condition in which fibrous tissue replaces normal bone. Another is desmoplastic fibroma, a rare noncancerous bone tumor.
Telling them apart matters. A cancer treated as a noncancerous condition may be removed incompletely, and a tumor that comes back is more likely to change into a high-grade cancer.
Two kinds of tests help separate them:
All low-grade central osteosarcomas are low grade by definition. The cells grow slowly, look only slightly abnormal, and have a low risk of spreading.
In a minority of tumors, part of the cancer changes into a high-grade osteosarcoma, which grows faster and is more likely to spread. This change is called dedifferentiation. It is more common in tumors that have come back after incomplete removal.
Your report will state whether any high-grade area was found. This is the single most important finding for this tumor, because it changes both the outlook and the treatment.
Your report may describe whether the tumor has broken through the outer shell of the bone into the surrounding muscle or other soft tissue. This is called extraosseous extension, and it is linked to a higher chance of the tumor coming back.
The report may also note whether the tumor has grown into an adjacent bone. In the spine and pelvis, which are made of several connected bones, this raises the stage.
A margin is the edge of tissue cut by the surgeon. For low-grade central osteosarcoma, margins are the most important part of the surgery, because tumor left behind usually comes back and may return as a high-grade cancer.
Your report will usually list several margins, including the cut ends of the bone and the surrounding soft tissue.
Your report may state whether cancer cells were seen inside small blood vessels or around nerves. Both findings are rare in low-grade central osteosarcoma, and the report will usually say neither was identified.
Bone cancers are staged using the TNM system from the American Joint Committee on Cancer (AJCC), 8th edition:
The outlook is very good when the whole tumor is removed with negative margins. About 9 in 10 people are alive five years after diagnosis, and more than 8 in 10 at 10 years. Fewer than 5 in 100 develop spread to other parts of the body.
These figures describe groups of people and cannot predict what will happen to one person.
A team at a center that regularly treats bone sarcomas manages low-grade central osteosarcoma.