by Jason Wasserman MD PhD FRCPC
January 23, 2024
Alveolar soft part sarcoma is a rare type of cancer that typically starts in the body’s soft tissues, including muscles, fat, and connective tissues. Despite its name, alveolar soft part sarcoma does not start in the lung’s alveolar structures, as one might think from the word “alveolar.” Instead, it gets its name from the way the tumour cells are arranged in small nests that resemble the small air sacs in the lungs.
The symptoms of alveolar soft part sarcoma can vary depending on where the tumour is located. Commonly, it manifests as a painless, slow-growing lump or mass in the affected area. It most frequently occurs in the legs or arms but can also be found in the head, neck, chest, or abdomen. Symptoms might only become noticeable as the tumour grows and affects surrounding tissues or organs.
The exact cause of alveolar soft part sarcoma is not well understood. Like many cancers, it’s thought to result from genetic changes in the body’s cells that lead to uncontrolled growth. However, it’s important to note that these changes are usually not inherited but occur randomly.
Alveolar soft part sarcoma is associated with a specific genetic change known as a chromosomal rearrangement. This involves the fusion of part of chromosome X (Xp11) with chromosome 17 (17q25), resulting in the ASPSCR1-TFE3 gene fusion. This fusion is believed to play a key role in the development of the tumour.
Diagnosing alveolar soft part sarcoma typically involves a combination of medical imaging and a biopsy. Imaging tests like MRI or CT scans can detect the presence of a mass and its size and location. A biopsy, where a small tumour sample is removed and examined under a microscope, is important for confirming the diagnosis.
Under the microscope, alveolar soft part sarcoma cells have a distinctive appearance. They are often arranged in nests or clusters separated by thin walls of blood vessels, resembling the alveoli in the lungs. The cells are usually large, and the cytoplasm (body) of the cell can appear clear, eosinophilic (pink), or granular. The nucleus (the part of the cell that holds the genetic material) is typically round and prominent clumps of genetic material called nucleoli are often seen. Mitotic figures (dividing cells) may be seen, but the mitotic rate is usually low (the number of mitotic figures in a given tissue area).
Immunohistochemistry (IHC) involves using special stains (antibodies) that react with specific proteins in the tumour cells. In the case of alveolar soft part sarcoma, these tests can highlight certain proteins that are typically found in the tumour cells, helping to distinguish alveolar soft part sarcoma from other types of tumours that can look similar when examined under the microscope.
Molecular tests on alveolar soft part sarcoma tissue focus on identifying the specific chromosomal changes that are characteristic of this cancer. The most significant change is a translocation between chromosomes X and 17, which creates the ASPSCR1-TFE3 gene fusion. The details of these tests include:
All alveolar soft part sarcomas are considered high grade tumours. This means they tend to grow and spread more aggressively than low grade tumours.
If you received chemotherapy and/or radiation therapy before the operation to remove the tumour, your pathologist will examine all the tissue sent to pathology to see how much of the tumour was still alive at the time it was removed from the body. Pathologists use the term viable to describe tissue that was still alive when it was removed from the body. In contrast, pathologists use the term non-viable to describe tissue that was not alive when it was removed from the body. Most commonly, your pathologist will describe the percentage of tumours that is non-viable.
Pathologists use the term “perineural invasion” to describe a situation where cancer cells attach to or invade a nerve. “Intraneural invasion” is a related term that specifically refers to cancer cells found inside a nerve. Nerves, resembling long wires, consist of groups of cells known as neurons. These nerves, present throughout the body, transmit information such as temperature, pressure, and pain between the body and the brain. The presence of perineural invasion is important because it allows cancer cells to travel along the nerve into nearby organs and tissues, raising the risk of the tumour recurring after surgery.
Lymphovascular invasion occurs when cancer cells invade a blood vessel or lymphatic channel. Blood vessels, thin tubes that carry blood throughout the body, contrast with lymphatic channels, which carry a fluid called lymph instead of blood. These lymphatic channels connect to small immune organs known as lymph nodes scattered throughout the body. Lymphovascular invasion is important because it enables cancer cells to spread to other body parts, including lymph nodes or the lungs, via the blood or lymphatic vessels. As such, lymphovascular invasion is associated with an increased risk of developing metastatic disease.
In pathology, a margin is the edge of tissue removed during tumour surgery. The margin status in a pathology report is important as it indicates whether the entire tumour was removed or if some was left behind. This information helps determine the need for further treatment.
Pathologists typically assess margins following a surgical procedure, like an excision or resection, that removes the entire tumour. Margins aren’t usually evaluated after a biopsy, which removes only part of the tumour. The number of margins reported and their size—how much normal tissue is between the tumour and the cut edge—vary based on the tissue type and tumour location.
Pathologists examine margins to check if tumour cells are present at the tissue’s cut edge. A positive margin, where tumour cells are found, suggests that some cancer may remain in the body. In contrast, a negative margin, with no tumour cells at the edge, suggests the tumour was fully removed. Some reports also measure the distance between the nearest tumour cells and the margin, even if all margins are negative.
Lymph nodes are small immune organs found throughout the body. Cancer cells can spread from a tumour to lymph nodes through small lymphatic vessels. For this reason, lymph nodes are commonly removed and examined under a microscope to look for cancer cells. The movement of cancer cells from the tumour to another part of the body, such as a lymph node, is called a metastasis.
Cancer cells typically spread first to lymph nodes close to the tumour, although lymph nodes far away from the tumour can also be involved. For this reason, the first lymph nodes removed are usually close to the tumour. Lymph nodes further away from the tumour are only typically removed if they are enlarged and there is a high clinical suspicion that there may be cancer cells in the lymph node.
If any lymph nodes were removed from your body, they will be examined under the microscope by a pathologist, and the results of this examination will be described in your report. The examination of lymph nodes is important for two reasons. First, this information determines the pathologic nodal stage (pN). Second, finding cancer cells in a lymph node increases the risk that cancer cells will be found in other parts of the body in the future. As a result, your doctor will use this information when deciding if additional treatment, such as chemotherapy, radiation therapy, or immunotherapy, is required.
The pathologic stage for alveolar soft part sarcoma is based on the TNM staging system, an internationally recognized system 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 a more advanced disease and a worse prognosis.
The tumour stage for alveolar soft part sarcoma varies based on the body part involved. For example, a 5-centimetre tumour that starts in the head will be given a different tumour stage than a tumour that starts deep in the back of the abdomen (the retroperitoneum). However, in most body sites, the tumour stage includes the tumour size and whether the tumour has grown into surrounding body parts.
Alveolar soft part sarcoma is given a nodal stage of 0 or 1 based on the presence or absence of tumour cells in one or more lymph nodes. If no tumour cells are seen in any lymph nodes, the nodal stage is N0. If no lymph nodes are sent for pathological examination, the nodal stage cannot be determined, and the nodal stage is listed as NX. If tumour cells are found in any lymph nodes, then the nodal stage is listed as N1.
Doctors wrote this article to help you read and understand your pathology report. If you have additional questions, contact us.