Your pathology report for parosteal osteosarcoma

by Jason Wasserman MD PhD FRCPC
June 28, 2025

Parosteal osteosarcoma is a rare type of low-grade bone cancer that forms on the outer surface of a bone, rather than inside the bone. The word “parosteal” means “beside the bone,” and this tumour starts in the thin layer of tissue that covers the bone, called the periosteum. It grows slowly and is considered a low-grade malignant tumour, which means it can invade surrounding tissues but usually spreads to other parts of the body only in rare cases. Parosteal osteosarcoma is the most common type of surface osteosarcoma.

Where in the body is parosteal osteosarcoma usually found?

Approximately 70% of parosteal osteosarcomas occur at the back of the lower thigh bone (distal femur), near the knee joint. Other common locations include the top of the tibia (the shin bone) and the humerus (the upper arm bone). Less frequently, the tumour may develop in the jaw, ribs, or the small bones of the hands and feet.

Who gets parosteal osteosarcoma?

Parosteal osteosarcoma usually affects young adults, most commonly in their 20s and 30s. It accounts for about 4% of all osteosarcomas and is slightly more common in females than in males.

What are the symptoms of parosteal osteosarcoma?

Symptoms of parosteal osteosarcoma typically include:

• A firm, painless lump near a joint, especially the knee.

• Pain in the affected area, which may occur if the tumour grows into nearby tissues.

• Reduced joint movement, especially if the tumour is near the knee.

Symptoms may be present for a long time, sometimes for over a year, before the tumour is diagnosed.

How is the diagnosis made?

The diagnosis is usually made after a biopsy, which is a procedure to remove a small sample of the tumour for examination under a microscope. On X-rays, parosteal osteosarcoma appears as a hard, lobulated mass growing on the bone surface, often with a dense center and a clear border separating it from the underlying bone. On MRI or CT scans, the tumour may appear to invade the outer layer of bone (cortex) or extend into nearby soft tissues.

Under the microscope, the tumour is made up of well-formed bone trabeculae (strands of immature bone) and spindle-shaped cells that show only mild atypia (abnormal features). In some cases, the tumour may contain cartilage, or areas where it has begun to transform into a more aggressive (high-grade) cancer.

What other tests may be performed to confirm the diagnosis?

To help confirm the diagnosis and distinguish parosteal osteosarcoma from other benign bone tumours, pathologists may perform additional tests such as immunohistochemistry (IHC) or fluorescence in situ hybridization (FISH). These tests are especially helpful when the diagnosis is unclear or when only a small biopsy sample is available.

Immunohistochemistry

Immunohistochemistry (IHC) is a test that uses special stains to detect the presence of proteins in tumour cells. When this test is performed, most parosteal osteosarcomas show strong expression of MDM2 and CDK4. These proteins are not usually found in benign bone conditions, making them useful for diagnosis.

Molecular testing

A test called FISH (fluorescence in situ hybridization) can be used to detect amplification of the MDM2 gene, a common genetic change seen in parosteal osteosarcoma. This test helps distinguish parosteal osteosarcoma from other bone tumours that may look similar under the microscope.

What does grade mean for parosteal osteosarcoma?

All parosteal osteosarcomas are considered low-grade tumours by definition. This means that the cancer cells grow slowly, look relatively similar to normal bone cells, and have a low risk of spreading to other parts of the body.

However, in 15–40% of cases, the tumour can develop areas of high-grade transformation, also called dedifferentiation. These areas look more aggressive under the microscope and behave more like high-grade osteosarcoma, which grows faster and has a higher risk of spreading. When dedifferentiation is present, the prognosis is worse, and chemotherapy may be recommended in addition to surgery.

What is a margin, and why is it important?

A margin is the edge of tissue removed during surgery. Pathologists examine the margins to see if any cancer cells are present at the cut edge:

• Negative margin – no cancer cells at the edge (preferred).

• Positive margin – cancer cells are seen at the edge, meaning some tumour may have been left behind.

Margins help determine whether additional treatment is needed. The most commonly described margins in osteosarcoma include:

• Proximal bone margin – the cut closest to the body.

• Distal bone margin – the cut farthest from the body.

• Soft tissue margins – any nearby muscle or fat removed.

• Blood vessel or nerve margins – if large vessels or nerves were removed with the tumour.

What is the pathologic stage for parosteal osteosarcoma?

Parosteal osteosarcoma is staged using the TNM staging system for bone cancers; however, some experts believe that surface tumours, such as parosteal osteosarcoma, may not fit this system perfectly. The TNM staging system is divided into three parts.

• T (Tumour) – Size of the tumour and extent of growth.

• N (Nodes) – Whether cancer cells are found in nearby lymph nodes.

• M (Metastasis) – Whether cancer has spread to other parts of the body.

The T stage depends on the location of the tumour in the body and helps doctors understand how advanced the disease is.

T stage for tumours in the appendicular skeleton, trunk, skull, or face:

These are bones of the arms, legs, shoulder, pelvis (excluding the spine), and head.

• pT1 – The tumour is 8 cm or smaller in greatest dimension.

• pT2 – The tumour is larger than 8 cm in greatest dimension.

• pT3 – The tumour is seen in multiple separate areas of the same bone (discontinuous tumour nodules), which indicates a more aggressive tumour.

T stage for tumours in the spine:

The spine is made up of a series of vertebral bones. Staging here depends on the number of vertebrae involved and whether critical structures are affected.

• pT1 – The tumour is limited to one or two adjacent vertebral bones.

• pT2 – The tumour involves three adjacent vertebral bones.

• pT3 – The tumour involves four or more adjacent vertebral bones, or any non-adjacent vertebral bones.

• pT4 – The tumour has grown into the spinal canal or invaded major blood vessels. This is a more advanced and potentially dangerous stage.

T stage for tumours in the pelvis:

The pelvis is a complex structure made up of several bones. The T stage is based on the size of the tumour and the number of bones involved.

• pT1 – Tumour is limited to one pelvic bone and does not extend outside the bone (no extraosseous extension).

  • pT1a – Tumour is 8 cm or smaller.

  • pT1b – Tumour is larger than 8 cm.

• pT2 – Tumour involves one pelvic bone with extraosseous extension, or two pelvic bones without extraosseous extension.

  • pT2a – Tumour is 8 cm or smaller.

  • pT2b – Tumour is larger than 8 cm.

• pT3 – Tumour involves two pelvic bones with extraosseous extension.

  • pT3a – Tumour is 8 cm or smaller.

  • pT3b – Tumour is larger than 8 cm.

• pT4 – Tumour involves three pelvic bones or is growing across the sacroiliac joint into the spine.

  • pT4a – Tumour invades the sacral nerve openings (neuroforamina).

  • pT4b – Tumour grows around or into major blood vessels such as the external iliac artery or vein.

Other tumour stages:

• pT0 – No tumour is found in the tissue that was examined.

• pTX – The tumour cannot be staged because there is not enough information, such as when the tumour is received in small fragments or incompletely.

What is the prognosis for parosteal osteosarcoma?

The prognosis for pure low-grade parosteal osteosarcoma is excellent. When the tumour is completely removed with wide surgical margins, the 5-year survival rate is around 90%. Most people are cured with surgery alone.

However, if the tumour is not fully removed or only partially scraped out (incomplete resection), it may come back (recur). If the tumour develops high-grade areas, it becomes more aggressive and may metastasize (spread) to other parts of the body, most often the lungs. These high-grade tumours have a worse prognosis and usually require chemotherapy.

Questions to ask your doctor

• Was the tumour completely removed with clear margins?

• Did the tumour show any high-grade or dedifferentiated areas?

• Will I need chemotherapy or other treatment?

• What is my risk of recurrence or metastasis?

• What follow-up tests will I need?