Dedifferentiated Chondrosarcoma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina MD FRCPC
October 6, 2026

Dedifferentiated chondrosarcoma is a type of bone cancer made of two very different parts sitting side by side. One part is a slow-growing cartilage cancer called a chondrosarcoma. The other part is a fast-growing cancer that no longer makes cartilage at all.

Dedifferentiated means part of the tumor has lost the features of the tissue it came from. Here, a cartilage-making cancer has changed in one area into a cancer that looks more primitive and grows more quickly.

About 1 in 10 chondrosarcomas are dedifferentiated. It usually affects older adults, with a median age of about 60, and most often develops in the pelvis, the thigh bone, or the upper arm bone.

This article explains how dedifferentiated chondrosarcoma is diagnosed and what your pathology report findings mean.

How is dedifferentiated chondrosarcoma different from other chondrosarcomas?

Most chondrosarcomas are made of cartilage throughout and are graded from 1 to 3. Grade 3 chondrosarcoma is the most abnormal, but it still makes cartilage.

Dedifferentiated chondrosarcoma is different. It contains a low-grade cartilage tumor, often grade 1 or grade 2, with an abrupt border next to a high-grade cancer that makes no cartilage. It is not given a grade number because it is a high-grade cancer by definition.

The distinction matters because dedifferentiated chondrosarcoma grows faster, spreads more often, and is sometimes treated with chemotherapy, unlike conventional chondrosarcoma.

What causes dedifferentiated chondrosarcoma?

Dedifferentiated chondrosarcoma develops when part of an existing chondrosarcoma acquires further genetic changes that make it grow faster. In many people, the original cartilage tumor had been present for years without causing symptoms.

About half of these tumors carry a change in a gene called IDH1 or IDH2. The same change is found in both parts of the tumor, suggesting that the fast-growing part developed from the adjacent cartilage tumor. This change develops in the tumor during a person’s lifetime and is not inherited.

For most people, no reason is found for why the change happened.

What are the symptoms of dedifferentiated chondrosarcoma?

The most common symptoms are pain and swelling over the affected bone. Some people have had mild pain for a long time, which then becomes worse over a few weeks or months as the fast-growing part enlarges.

Because the tumor weakens the bone, it can break with little or no force. This is called a pathologic fracture, and it causes a sudden increase in pain and difficulty using the affected limb.

How is the diagnosis made?

X-rays, CT, and MRI often show the two parts of the tumor. The cartilage part usually contains small flecks of calcium, while the fast-growing part destroys the surrounding bone and may extend into the nearby soft tissue.

The diagnosis is confirmed with a biopsy, which should be done at the center performing the final surgery. Ideally, the biopsy is taken from the area that looks most destructive on imaging, since that is where the high-grade part is usually found.

A biopsy may sample only one of the two parts. If it contains only cartilage, the tumor can be underestimated as a lower-grade chondrosarcoma. If it contains only the high-grade part, it can look like another type of sarcoma. For this reason, the diagnosis is sometimes made only after the whole tumor has been removed.

What does dedifferentiated chondrosarcoma look like under the microscope?

Under the microscope, the tumor has two parts with a sharp border between them:

The cartilage part. This looks like a low-grade chondrosarcoma, made of cartilage cells that are only mildly abnormal.
The high-grade part. This is made of very abnormal cells that do not make cartilage. Many of them are dividing, and dead tumor tissue is common.

The high-grade part can resemble other cancers. Most often it looks like undifferentiated pleomorphic sarcoma or osteosarcoma. Less often it resembles leiomyosarcoma, rhabdomyosarcoma, or angiosarcoma. Finding a cartilage tumor beside it is what tells the pathologist the diagnosis is dedifferentiated chondrosarcoma.

Molecular testing

Testing for an IDH1 or IDH2 change can help confirm the diagnosis when only the high-grade part was sampled. Finding the change supports a dedifferentiated chondrosarcoma rather than another type of sarcoma, which would not normally carry it.

The result may also make a person eligible for clinical trials of targeted drugs. Your oncologist can tell you whether one is available.

Tumor extension

Your report may describe whether the tumor has broken through the bone’s outer shell into surrounding muscle or other soft tissue. This is called extraosseous extension, and it is common in dedifferentiated chondrosarcoma.

The report may also note whether the tumor has grown into a nearby joint or an adjacent bone. In the spine and pelvis, which are made of several connected bones, spread from one bone to another raises the stage.

Margins

A margin is the edge of tissue cut by the surgeon. Margins are especially important for this tumor, because complete removal is the treatment most closely linked to a better outcome.

Negative margin. No cancer cells are seen at the cut edge. The report may give the distance between the tumor and the closest margin.
Positive margin. Cancer cells reach the cut edge, and some tumor may remain. This raises the chance the cancer will come back in the same place, and further treatment may be considered.

Your report will usually list several margins, including the cut ends of the bone, the surrounding soft tissue, and any large blood vessels or nerves that were removed.

Stage

Bone cancers are staged using the TNM system from the American Joint Committee on Cancer (AJCC), 8th edition. The tumor stage, called pT, depends on which bone is involved:

Arms, legs, trunk, skull, and face. The stage depends mainly on size. A tumor 8 cm or smaller is pT1, and a tumor larger than 8 cm is pT2. Separate tumor deposits within the same bone are pT3.
Spine and pelvis. These bones have their own staging rules, based on how many bone segments are involved and whether the tumor extends beyond the bone.
Lymph nodes. This tumor rarely spreads to lymph nodes, so surgeons usually do not remove them. When no nodes are examined, the report may say the nodal stage was not assigned.

When dedifferentiated chondrosarcoma spreads, it most often travels through the bloodstream to the lungs, which is why a chest CT scan is part of the assessment at diagnosis.

What is the prognosis?

Dedifferentiated chondrosarcoma has a less favorable outlook than other chondrosarcomas. In published series, about 10% to 25% of people are alive five years after diagnosis.

The outlook is better when the tumor is in a limb, has not spread, and is removed completely with negative margins. Tumors in the pelvis have a less favorable outlook, mainly because complete removal is harder. Some people already have spread to the lungs when the diagnosis is made.

These figures come from groups of people treated over many years and cannot predict what will happen to one person. Your care team can explain how they apply to you.

What happens after the diagnosis?

Dedifferentiated chondrosarcoma is managed by a team at a center that treats bone sarcomas regularly.

Surgery. The main treatment is removing the whole tumor with a rim of normal tissue. Part of the bone may be replaced with an implant or graft, and amputation is occasionally needed when the tumor surrounds major blood vessels or nerves.
Chemotherapy. Chemotherapy similar to that used for osteosarcoma is sometimes given, particularly to younger and fitter people. Its benefit is uncertain, and your oncologist will discuss whether it is recommended for you.
Radiation therapy. Radiation may be used when the tumor cannot be removed completely, or to relieve symptoms.
Clinical trials. Because standard treatments have limited benefit, your oncologist may discuss a clinical trial.
Follow-up. Regular imaging of the original site and of the chest continues for several years.

Questions to ask your doctor

What did the high-grade part of my tumor look like?
Was the whole tumor removed, and were all the margins negative?
Did the tumor grow outside the bone?
Has the cancer spread to my lungs or elsewhere?
Would chemotherapy be recommended for me, and why?
Was the tumor tested for an IDH1 or IDH2 change, and is a clinical trial an option?
How often will I need follow-up imaging?