by Jason Wasserman MD PhD FRCPC and Bibianna Purgina MD FRCPC
September 5, 2022
A central atypical cartilaginous tumour (ACT) is a type of bone cancer. The tumour is called “cartilaginous” because it is made up of cartilage-producing cells. Cartilage is a type of connective tissue normally found throughout the body. These tumours start on the inside of a bone in a space called the “medulla”. Another name for this tumour is low-grade central chondrosarcoma.
Yes, central ACT is a type of cancer called a sarcoma. The tumour is considered locally aggressive which means it can spread into surrounding organs and tissues. However, it is very rare for central ACT to spread to other parts of the body.
Central ACT and grade 1 central chondrosarcoma are very similar tumours. The main difference between these tumours is that central ACTs are found in the appendicular skeleton which includes the bones of the arms, legs, hands, and feet. In contrast, grade 1 central chondrosarcomas are found in the axial skeleton which includes the bones of the pelvis, scapula, and skull.
Symptoms of a central ACT include pain and swelling over the involved bone. However, many patients with this tumour will experience no symptoms and the tumour will be found incidentally when imaging is performed for another reason.
People with the genetic syndrome enchondromatosis who have a mutation in the gene IDH1 or IDH2 are at increased risk for developing central ACTs. For people, without enchondromatosis who develop central ACT, the cause remains unknown.
When examined under the microscope, central ACT is made up of cells producing a type of connective tissue called cartilage. The tumour cells often grow in round groups called lobules. Central ACT can look very similar to a type of non-cancerous tumour called an enchondroma (another tumour made up of cartilage-producing cells), however, unlike enchondroma, the tumour cells in central ACT can be seen growing into and surrounding bone.
Larger central ACTs can break through the bone and grow into the surrounding organs or tissue such as muscle, tendons, or the joint space. If this has occurred, it may be included in your report and is usually described as extraosseous extension. If the tumour has grown into another part of the bone, that will also be described in your report. Tumour extension is important because it is used to determine the pathologic tumour stage (pT).
A margin is any tissue that was cut by the surgeon to remove the bone (or part of the bone) and tumour from your body. Depending on the type of surgery you have had, the types of margins, which could include proximal (the part of the bone closest to the middle of your body) and distal (the part of the bone farthest from the middle of your body) bone margins, soft tissue margins, blood vessel margins, and nerve margins.
All margins will be very closely examined under the microscope by your pathologist to determine the margin status. A margin is considered negative when there are no cancer cells at the edge of the cut tissue. A margin is considered positive when there are cancer cells at the edge of the cut tissue. A positive margin is associated with a higher risk that the tumour will regrow in the same site after treatment (local recurrence).
The pathologic stage for central ACT is based on the TNM staging system, an internationally recognized system originally created by the American Joint Committee on Cancer. This system uses information about the primary tumour (T), lymph nodes (N), and distant metastatic disease (M) to determine the complete pathologic stage (pTNM). Your pathologist will examine the tissue submitted and give each part a number. In general, a higher number means more advanced disease and a worse prognosis. The pathologic stage will only be included in your report after the entire tumour has been removed. It will not be included after a biopsy.
Central ACT is given a pathologic tumour stage (pT) from T1 to T3 based on the tumour size and the number of tumours found in the bone.
Central ACT is given a pathologic nodal stage (pN) of N0 or N1 based on the examination of lymph nodes.
Central ACT is given a metastatic stage only if the presence of metastasis has been confirmed by a pathologist. There are two metastatic stages for central ACT, M1a and M1b. If there is confirmed lung metastasis, then the tumor metastatic stage is 1a.
The metastatic stage can only be given if tissue from a distant site is sent for pathological examination. Because this tissue is rarely present, the metastatic stage cannot be determined, and it is typically not included in your report.