High-Grade Surface Osteosarcoma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina, MD FRCPC
October 12, 2026

High-grade surface osteosarcoma is a very rare type of bone cancer that grows on the bone’s surface. It is a type of osteosarcoma, a group of cancers in which the tumor cells make new bone. It accounts for fewer than 1% of all osteosarcomas.

“High grade” means the tumor cells look very abnormal under the microscope and the tumor can grow and spread quickly. “Surface” means the tumor starts on the outside of the bone, not in its hollow center.

Apart from where it starts, high-grade surface osteosarcoma looks and behaves like the common type of osteosarcoma that starts inside the bone. It is treated in the same way, with chemotherapy and surgery.

This article explains how high-grade surface osteosarcoma is diagnosed and what the findings in your pathology report mean.

How is it different from other osteosarcomas on the bone surface?

Most osteosarcomas start inside the bone. Three types start on the surface, and the names are easy to confuse:

Parosteal osteosarcoma. This is a low-grade tumor that grows slowly, most often on the back of the thigh bone just above the knee. Our article on parosteal osteosarcoma describes it in detail.
Periosteal osteosarcoma. This is an intermediate-grade tumor made mostly of cartilage. It usually grows on the shaft of the thigh bone or shin bone. Our article on periosteal osteosarcoma describes it in detail.
High-grade surface osteosarcoma. This is a high-grade tumor from the start. It is the least common and the most aggressive of the three.

The type matters because each one is treated differently and has a different outlook.

Who gets high-grade surface osteosarcoma?

High-grade surface osteosarcoma most often affects teenagers and young adults, although it can develop at any age. It is slightly more common in men than in women.

The tumor most often develops on the thigh bone (femur). The shin bone (tibia) and the upper arm bone (humerus) are the next most common sites. It usually grows on the shaft, the long middle part of the bone, or toward one end.

What causes high-grade surface osteosarcoma?

The cause is not known. Because the tumor is so rare, researchers haven’t studied it as closely as other osteosarcomas. Nothing you did or did not do caused it.

What are the symptoms?

The most common symptoms are a lump on the arm or leg, pain, and swelling. Symptoms usually develop over a few weeks to months, faster than with slower-growing surface tumors.

How is the diagnosis made?

The diagnosis is made by combining the imaging findings with the examination of tumor tissue under the microscope. Both are needed because the microscope shows the tumor is high grade, and the imaging shows it started on the surface.

An X-ray usually shows a mass on the surface of the bone that contains cloudy areas of new bone. The outer layer of the bone beneath the tumor may be partly worn away. An MRI or CT scan shows whether the tumor has grown into the bone’s hollow center, called the medullary cavity. In this tumor, there is little or no growth into the medullary cavity.

A biopsy removes a small sample of the tumor for examination. It should be planned at the hospital where the tumor will be treated. The path of the biopsy needle is removed along with the tumor during surgery, so its position must match the surgical plan.

Under the microscope, the tumor cells look very abnormal. They vary in size and shape, and many are dividing. The key finding is osteoid, the new, unhardened bone made directly by the tumor cells. The tumor may also make cartilage or fibrous tissue, and the amounts vary from one tumor to the next.

What else can look like high-grade surface osteosarcoma?

Pathologists and radiologists work together to separate this tumor from others that look similar:

Conventional osteosarcoma. This common type starts inside the bone and often grows out into the soft tissue. Under the microscope, the two look the same. Imaging separates them by showing where most of the tumor is located.
Dedifferentiated parosteal osteosarcoma. This is a parosteal osteosarcoma in which part of the tumor has changed to high grade, a process called dedifferentiation. It contains low-grade areas as well. High-grade surface osteosarcoma is high grade throughout.
Periosteal osteosarcoma. This tumor is mostly cartilage, and its cells look less abnormal.

Your report may mention a test for the MDM2 gene. Extra copies of MDM2 are found in most parosteal osteosarcomas, including those that have become high grade. A negative result supports the diagnosis of high-grade surface osteosarcoma.

Grade

Grade describes how abnormal the tumor cells look and predicts how the tumor is likely to behave. This tumor is high grade by definition. Reports may list this as “high grade,” grade 3, or grade 4, depending on the hospital’s system.

Treatment effect

Most people receive chemotherapy before surgery. After the tumor is removed, the pathologist examines it to see how much of it is no longer alive. This is called treatment effect or tumor necrosis, and it is reported as a percentage.

Good response. 90% or more of the tumor is no longer alive. This is linked to a better outlook.
Poor response. Less than 90% of the tumor is no longer alive. Your team may consider this when planning treatment after surgery.

Tumor extension

Your report will describe how far the tumor has grown. Most of the tumor sits on the bone surface and grows outward into the surrounding soft tissue. The report will also say whether the tumor has grown into the outer layer of the bone or the medullary cavity. Growth into the medullary cavity, if present, is limited.

Margins

A margin is the edge of tissue the surgeon cuts when removing the tumor. The pathologist examines the bone and soft-tissue margins to see whether any tumor cells are present at the cut edge.

Negative margin. No tumor cells are seen at the cut edge. This suggests the tumor was completely removed. The report may also give the distance from the tumor to the closest margin.
Positive margin. Tumor cells are seen at the cut edge. This suggests some tumor may remain, and more surgery or other treatment may be recommended.

Pathologic stage

The pathologic stage describes the tumor size and whether it has spread. It is assigned after the tumor is removed. For tumors in the arms, legs, trunk, skull, and face, the tumor stage (pT) is:

pT1. The tumor is 8 cm or smaller.
pT2. The tumor is larger than 8 cm.
pT3. There is more than one separate tumor in the same bone.

The spine and pelvis are staged using different rules.

Osteosarcoma rarely spreads to lymph nodes, so surgeons usually do not remove them. Your report may say that the nodal stage was not assigned. When this tumor spreads, it most often goes to the lungs.

What is the prognosis?

The outlook for high-grade surface osteosarcoma is similar to that of conventional osteosarcoma. Because the tumor is so rare, the available studies are small. The features that matter most are the same ones that matter for conventional osteosarcoma:

Spread at diagnosis. A tumor that has not spread to the lungs or other bones is more likely to be cured.
Response to chemotherapy. A good response is linked to a better outlook.
Margins. Complete removal with negative margins lowers the chance that the tumor will come back.
Tumor size and location. Smaller tumors and tumors in the arms and legs are easier to remove completely.

Your team can explain how these features apply to you.

What happens after the diagnosis?

A team at a center specializing in bone tumors treats high-grade surface osteosarcoma.

Chemotherapy. Chemotherapy is usually given both before and after surgery, using the same drugs that are used for conventional osteosarcoma.
Surgery. The tumor is removed in one piece with a rim of normal tissue around it, a procedure called a wide resection. In most cases, surgeons can save the arm or leg.
Radiation. Radiation is not standard treatment. Doctors may consider it if the tumor cannot be removed completely.
Follow-up. Follow-up includes regular examinations and imaging of the surgical site and the chest for many years.

Questions to ask your doctor

How was my tumor separated from the other types of surface osteosarcoma?
Did the tumor grow into the medullary cavity?
What percentage of the tumor was no longer alive after chemotherapy?
Were the margins negative, and how close was the nearest one?
What is the stage of my tumor?
Has the tumor spread to my lungs or anywhere else?
Has a pathologist who specializes in bone tumors reviewed my case?
What treatment is planned after surgery?
How often will I need follow-up visits and imaging?
What symptoms should I report between visits?