Osteosarcoma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina MD FRCPC
September 11, 2026

Osteosarcoma is a cancer that starts in bone and is the most common type of bone cancer. Under the microscope, it’s defined by tumor cells that produce osteoid, the soft material that normally hardens into bone. No matter how the cells look otherwise, finding tumor cells producing osteoid is what makes the diagnosis.

This article explains what appears on an osteosarcoma pathology report. That includes the type and grade, the tests used to confirm difficult cases, and the margins. It also covers the measurement of how much tumor was killed by chemotherapy, which is one of the most important findings on a report after surgery.

Who gets osteosarcoma?

Osteosarcoma occurs at two age peaks. The larger peak is in teenagers and young adults, during the years of fastest bone growth. This is why the tumor most often arises near the knee, in the lower end of the thigh bone or the upper end of the shin bone. The second, smaller peak is in older adults, where it more often develops in a bone already affected by another condition.

It can develop in any bone. Tumors of the arms and legs are the most common and generally the most treatable. Spinal and pelvic tumors are less common and harder to remove completely.

What are the symptoms of osteosarcoma?

Bone pain. Often worse at night or with activity, and frequently present for weeks or months before diagnosis.
A lump or swelling over a bone. Sometimes growing quickly, and sometimes warm or tender.
A fracture. The bone breaks because the tumor has weakened it, sometimes after a minor injury that would not normally cause a break.
Reduced movement in a nearby joint.

Pain in a growing teenager is common and usually has an innocent cause. Pain that wakes a person at night, that is getting worse rather than better, or that comes with a lump is the pattern that prompts an X-ray.

What causes osteosarcoma, and can it be inherited?

In most people, the cause of osteosarcoma is unknown. The tumor cells typically have a highly disordered genome, with many chromosome changes at once rather than a single defining alteration, which differs from several other childhood cancers.

A minority of osteosarcomas arise in a recognized setting.

An inherited condition. Li-Fraumeni syndrome, caused by changes in the TP53 gene, and hereditary retinoblastoma, caused by changes in RB1, both carry a substantially increased risk. Bloom syndrome, Rothmund-Thomson syndrome, and Werner syndrome do as well.
Previous radiation. Osteosarcoma can develop in a bone that received radiation for another cancer, usually many years later.
Paget disease of bone. A condition of abnormal bone turnover in older adults.

Because this inherited condition affects a meaningful minority, genetic assessment is often discussed. It is raised particularly for a young patient, for someone who has had another cancer, or where there is a pattern of cancer in the family. A survivor of hereditary retinoblastoma who later develops osteosarcoma is a recognized situation rather than a coincidence.

How is the diagnosis made?

Osteosarcoma is diagnosed from a combination of imaging and tissue, and the two are always read together. X-rays and MRI show a destructive bone lesion with new bone formation and often a raised edge of periosteum. A biopsy then provides tissue, and a pathologist looks for tumor cells producing osteoid.

Where the biopsy is taken from matters more here than in most cancers. The track the needle travels through has to be removed with the tumor at the definitive operation. The biopsy is therefore usually performed at the center that will do the surgery, by or in consultation with the surgeon who will do it.

Most people then receive chemotherapy before surgery. The tissue removed at the operation is examined again, both to check the margins and to measure how much of the tumor the chemotherapy killed.

What does the grade mean?

The grade describes how abnormal the tumor cells look and how quickly they are expected to grow. Reports describe it as grade 1 (low), grade 2 (intermediate), or grade 3 (high).

Osteosarcoma differs from many cancers in that the grade usually follows automatically from the type. Once the pathologist identifies the osteosarcoma type, the grade is usually known, and many reports do not assess it separately. Most osteosarcomas are high grade.

Grade matters because it predicts behavior. Low-grade tumors rarely spread but can grow back where they started if not completely removed. High-grade tumors are more likely to spread, most often to the lungs, and are treated with chemotherapy as well as surgery.

What are the types of osteosarcoma?

Osteosarcoma is divided into types by how the tumor cells look and where the tumor sits in the bone. The type appears in the diagnosis line of the report, and because it usually determines the grade, the types below are grouped by grade rather than listed flat.

High grade. These make up the great majority.

Conventional osteosarcoma. By far the commonest, and reported as osteoblastic, chondroblastic, or fibroblastic depending on what the tumor produces alongside osteoid. The three behave similarly and are treated the same way.
Telangiectatic osteosarcoma. Contains large blood-filled spaces and can resemble a noncancerous bone cyst on imaging.
Small cell osteosarcoma. Made of small round cells that resemble other small round cell tumors, so additional tests are needed to confirm it.
Giant cell-rich osteosarcoma. Contains many giant cells and can resemble a giant cell tumor of bone.
High-grade surface osteosarcoma. Arises on the outside of the bone rather than within it.
Epithelioid and anaplastic osteosarcoma. Uncommon types with markedly abnormal cells.
Osteosarcoma arising in Paget disease or after radiation. Defined by the setting rather than the appearance.

Intermediate grade.

Periosteal osteosarcoma. Grows from the outer covering of the bone and contains a prominent cartilage component.

Low grade. These are uncommon, grow slowly, and are generally treated with surgery alone.

Parosteal osteosarcoma. Grows on the surface of the bone, most often on the back of the thigh bone.
Low-grade central osteosarcoma. Arises within the middle of the bone and can look deceptively close to normal bone.

Low-grade tumors can occasionally change over time into a high-grade tumor, described as dedifferentiation. When that happens, the tumor is treated as a high-grade osteosarcoma.

Immunohistochemistry and molecular tests

Most osteosarcomas are diagnosed based on their appearance under the microscope, and no additional tests are needed. They become important in two situations.

Confirming that a tumor is bone-forming. A stain called SATB2 marks cells that are making bone, and it is used when the osteoid is scanty or the cells look unusual. It supports the diagnosis but is not specific to osteosarcoma on its own.

Separating a low-grade osteosarcoma from a noncancerous lesion. This is the more consequential use. Parosteal osteosarcoma and low-grade central osteosarcoma can look very similar under the microscope to noncancerous conditions such as fibrous dysplasia and desmoplastic fibroma. Getting this wrong in either direction has real consequences. Both of these low-grade osteosarcomas carry extra copies of a region of chromosome 12 containing the MDM2 and CDK4 genes. Testing for that amplification, either with immunohistochemistry or with FISH, separates them reliably. If your report describes a low-grade bone-forming tumor, it is reasonable to ask whether MDM2 and CDK4 testing was performed.

High-grade conventional osteosarcoma does not carry a single defining genetic change, so molecular testing is not used to make that diagnosis.

Treatment effect after chemotherapy

If chemotherapy was given before surgery, the pathologist examines the removed tumor to measure how much of it is dead, or non-viable. This is called the treatment effect or the percent necrosis, and it is one of the most important numbers on a resection report.

The tumor is sampled in a systematic grid across its largest cut surface, and the proportion of dead tumor is estimated. A result of 90 percent or more is considered a good response and is associated with a better outcome. Less than 90 percent is considered a poor response.

A poor response does not mean the chemotherapy was wasted or that treatment has failed. It is information used in planning what follows, and many people with a poor response are still cured.

Surgical margins

A margin is the edge of tissue the surgeon cuts to remove the tumor. The pathologist inks these edges and examines them under the microscope. A negative margin means no tumor cells were seen at the inked edge. A positive margin means the tumor reaches the edge and some may have been left behind.

Several margins are reported for a bone tumor. These include the bone cut above and below the tumor, the surrounding soft tissue, and any large vessels or nerves removed with the specimen. A positive margin is associated with a higher chance of the tumor coming back in the same place and usually leads to a discussion of further surgery or radiation.

Two findings reported in many other cancers, perineural invasion and lymphovascular invasion, are uncommon in osteosarcoma. They are described when present but are not usually part of the report.

Extraosseous extension and pathologic stage

Extraosseous extension means the tumor has grown out of the bone into the surrounding soft tissue, such as muscle. Where a bone has several parts, as in the spine or pelvis, the report may also describe growth from one part into another. Both findings raise the stage and are associated with a higher risk of the tumor returning.

Osteosarcoma is staged using the TNM system, which combines the tumor itself, the lymph nodes, and spread to distant sites. Here are three things to know about how it works.

The T category depends on the bone’s location. Separate rules exist for the arms and legs, the spine, and the pelvis. A pT2 in a leg does not mean the same thing as a pT2 in the pelvis.
Size matters in the limbs. For tumors of the arms, legs, trunk, skull, and face, 8 centimeters is the dividing line between pT1 and pT2. Separate tumor deposits within the same bone raise it to pT3.
Lymph nodes are rarely involved. Osteosarcoma spreads through the bloodstream rather than the lymphatic system, so nodes are often not removed and the nodal category is frequently not assessed.

Spread to distant sites matters most, and the lungs are by far the most common place for osteosarcoma to travel. That is why a chest CT is part of the assessment at diagnosis.

What is the prognosis?

The outlook in osteosarcoma depends most on whether the tumor has spread, how completely it can be removed, and how well it responds to chemotherapy.

High-grade osteosarcoma confined to a limb. More than 70 percent of people are long-term survivors with modern treatment.
A good response to chemotherapy. When 90 percent or more of the tumor is dead at surgery, five-year survival can exceed 80 percent.
Low-grade osteosarcoma. Five-year survival is around 90 percent, and surgery alone is usually enough.
Disease that has already spread at diagnosis, or that comes back. Considerably harder to treat, with fewer than 30 percent surviving long term.

Other features linked to a worse outcome include tumors of the spine and pelvis, large or bulky tumors, and positive surgical margins.

The scale of the change in this disease is worth stating. Before chemotherapy was introduced, more than 8 in 10 people treated with surgery alone died of osteosarcoma. Today most people with localized disease are cured, and limb-sparing surgery has largely replaced amputation. These figures come from groups treated over many years and cannot predict what will happen to one person.

What happens after the diagnosis?

A team that includes an orthopedic oncologist, a medical or pediatric oncologist, a radiologist, and a pathologist manages osteosarcoma at a center that regularly treats bone sarcomas. Referral before the biopsy is preferred because biopsy placement strongly affects the later operation.

For high-grade disease, treatment usually begins with several cycles of chemotherapy, followed by surgery to remove the tumor with a margin of normal tissue, then further chemotherapy. Limb-sparing surgery, in which doctors replace the affected bone with a metal implant or bone graft, is possible for most people. Radiation is used mainly where surgery is not possible, because osteosarcoma responds to it poorly.

For low-grade disease, complete surgical removal is usually the only treatment needed.

Follow-up continues for years and includes imaging of the original site and of the chest. It also covers the long-term effects of treatment, particularly on the heart, hearing, kidneys, and fertility, as well as the function and durability of a limb-sparing reconstruction.

Questions to ask your doctor

What type of osteosarcoma is described on my report, and what grade is it?
Is this a high-grade or a low-grade tumor, and what does that mean for treatment?
If my tumor is low grade, was MDM2 and CDK4 testing done to confirm the diagnosis?
Has the tumor grown outside the bone?
What stage was assigned, and how is that measured for a tumor in this location?
Has the cancer spread anywhere else, and was a CT of my chest done?
After chemotherapy, what percentage of the tumor was dead?
If the response was less than 90 percent, does that change the plan?
Were the surgical margins negative?
Is limb-sparing surgery possible in my case?
Should I be assessed for an inherited condition, and should my relatives be?
Am I being treated in a clinical trial or under a standard protocol?
What follow-up will I need, and for how long?
What long-term treatment effects should I watch for?