This article was last reviewed and updated on November 20, 2019.
by Jason Wasserman, MD FRCPC
Muscle is a special type of tissue that can quickly change in size and shape. For example, a muscle can become shorter (contract) or longer (extend). Muscles are found all over our body. Some muscles, like the ones in our legs that allow us to walk and run, are very large. Other muscles, like the ones inside that move our eyes, are very small. Most of the muscles in our body come from cells called immature muscle cells which over time turn into mature adult muscle cells. Pathologists call these immature muscle cells rhabdomyoblasts.
What is rhabdomyosarcoma?
Rhabdomyosarcoma is a type of cancer made up of immature muscle cells. A rhabdomyosarcoma is a type of cancer called a sarcoma. A sarcoma is a type of cancer that develops from mesenchymal tissues such as fat, bone and muscle.
Rhabdomyosarcoma can start anywhere in the body where there are skeletal muscle cells. The most common locations for rhabdomyosarcoma include the head and neck, arms and legs, bladder, trunk (chest and abdomen), and the around the eye. Most rhabdomyosarcomas occur in children although they can also occur in adults.
There are four different kinds of rhabdomyosarcoma based on how the cells look when examined under the microscope. Pathologists call these histologic types. The histologic type can only be determined after the tumour has been examined under the microscope by a pathologist.
The four different histologic types of rhabdomyosarcoma are:
How do pathologists make this diagnosis?
The first diagnosis of rhabdomyosarcomas is usually made after a small sample of the tumour is removed in a procedure called a biopsy. The biopsy tissue is then sent to a pathologist who examines it under a microscope. Additional tests such as immunohistochemistry and fluorescence in situ hybridization (FISH) may also be performed to confirm the diagnosis.
After a pathologist makes a diagnosis of rhabdomyosarcoma, patients are often treated first with chemotherapy and/or radiation therapy followed by surgery. The tumour is then removed completely as a resection specimen and sent to pathology for examination.
After surgical resection, the pathologist examines the tumour microscopically and provides your surgeon and oncologist with critical information required for your subsequent treatment.
This is the size of the tumour measured in centimeters. Tumour size will only be described in your report after the entire tumour has been removed. The tumour is usually measured in three dimensions but only the largest dimension is described in your report. For example, if the tumour measures 4.0 cm by 2.0 cm by 1.5 cm, your report will describe the tumour as being 4.0 cm.
Why is this important? Tumour size is used to determine the tumour stage (see Pathologic stage below). Tumours less than 5 cm are associated with better prognosis.
Grade (French Federation of Cancer Centers Sarcoma Group/FNCLCC)
Grade is a word pathologists use to describe how different the cancer cells in rhabdomyosarcoma look and behave compared to normal muscle cells. The grade can only be determined after the tumour is examined under the microscope by your pathologist.
The grade based on an internationally recognized system created by the French Federation of Cancer Centers Sarcoma Group. According to this system, a tumour can receive a grade of 1 through 3.
Only pleomorphic and spindle cell/sclerosing type rhabdomyosarcomas are given a grade. Embryonal and alveolar type rhabdomyosarcomas are not given a grade because for these types of rhabdomyosarcomas, grade does not help predict how the tumour will behave or how it will respond to treatment.
How do pathologists determine the tumour grade?
Your pathologist will determine the tumour grade by looking for three microscopic features which are described below. Points are given to each feature (from 0 to 3) and the total number of points determines the final grade of the tumour.
The final grade is based on the total number of points given to the tumour:
According to this system, pleomorphic or spindle cell/sclerosing type rhabdomyosarcomas can receive a grade of 1 through 3. Grade 1 tumours are also called ‘low grade’ while grade 2 and 3 tumours are grouped together into a category called ‘high grade’.
Why is this important? High grade tumours (grades 2 and 3) look and behave the least like normal muscle cells. High grade tumours are more likely to spread to other parts of the body or to re-grow after treatment.
Necrosis is a type of cell death and it commonly occurs in cancers. Your pathologist will closely examine the tumour for evidence of necrosis.
Most rhabdomyosarcomas start inside of a muscle, but the tumour can grow into other organs or tissues outside of the muscle. Your pathologist will examine samples of the surrounding organs and tissues under the microscope to look for cancer cells. Any surrounding organs or tissue that contain cancer cells will be described in your report.
Nerves are like long wires made up of groups of cells called neurons. Nerves send information (such as temperature, pressure, and pain) between your brain and your body. Perineural invasion means that cancer cells were seen attached to a nerve.
Why is this important? Cancer cells that have attached to a nerve can use the nerve to travel into tissue outside of the original tumour. This increases the risk that the tumour will come back in the same area of the body (recurrence) after treatment.
Blood moves around the body through long thin tubes called blood vessels. Another type of fluid called lymph which contains waste and immune cells moves around the body through lymphatic channels.
Cancer cells can use blood vessels and lymphatics to travel away from the tumour to other parts of the body. The movement of cancer cells from the tumour to another part of the body is called metastasis.
Before cancer cells can metastasize, they need to enter a blood vessel or lymphatic. This is called lymphovascular invasion.
Why is this important? Lymphovascular invasion increases the risk that cancer cells will be found in a lymph node or a distant part of the body such as the lungs.
A margin is any tissue that was cut by the surgeon in order to remove the tumour from your body. Whenever possible, surgeons will try to cut tissue outside of the tumour to reduce the risk that any cancer cells will be left behind after the tumour is removed.
Your pathologist will carefully examine all the margins in your tissue sample to see how close the cancer cells are to the edge of the cut tissue. Margins will only be described in your report after the entire tumour has been removed.
A margin is considered positive when there are cancer cells at the very edge of the cut tissue.
A negative margin means there were no cancer cells at the very edge of the cut tissue. If all the margins are negative, most pathology reports will say how far the closest cancer cells were to a margin. The distance is usually described in millimeters.
Why is this important? A positive margin is associated with a higher risk that the tumour will grow back (recur) in the same site after treatment.
If you have been diagnosed with rhabdomyosarcoma on a biopsy, you will often receive chemotherapy and/or radiation therapy before the operation to remove your tumour. If you have received either of these treatments before your surgery, your pathologist will examine all the tissue sent to pathology to see how much of the tumour is still alive (viable).
There are different systems used to describe treatment effect for rhabdomyosarcoma. Most commonly, your pathologist will describe the percentage of tumour that is dead.
Why is this important? A tumour showing 90% or more therapy response (meaning 90% of the tumour is dead and 10% or less of the tumor is still alive) is considered a good response to therapy and is associated with better prognosis.
Lymph nodes are small immune organs located throughout the body. Cancer cells can travel from the tumour to a lymph node through lymphatic channels located in and around the tumour (see Lymphovascular invasion above). The movement of cancer cells from the tumour to a lymph node is called a metastasis.
Your pathologist will carefully examine each lymph node for cancer cells. Lymph nodes that contain cancer cells are often called positive while those that do not contain any cancer cells are called negative. Most reports include the total number of lymph nodes examined and the number, if any, that contain cancer cells.
Why is this important? Well differentiated liposarcoma rarely spreads to lymph nodes. However, if cancer cells are found in a lymph node this increases the risk that the cancer will spread to other parts of the body.
The pathologic stage for rhabdomyosarcoma is based on the TNM staging system, an internationally recognized system originally created by the American Joint Committee on Cancer.
Additional information including the location of the tumour and the surgery performed is used for children diagnosed with embryonal, alveolar, or spindle cell/sclerosing rhabdomyosarcoma. All of this information is then combined to determine the risk of the cancer coming back in the future.
The TNM 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 worse prognosis.
Tumour stage (pT) for rhabdomyosarcoma
The tumour stage for rhabdomyosarcoma varies based on the body part involved. For example, a 5 centimeter tumour that starts in the neck 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.
Tumour stage for tumours starting in the head and neck:
Tumour stage for tumours starting on the outside of the chest, back, or stomach and the arms or legs (trunk and extremities):
Tumour stage for tumours starting in the abdomen and organs inside the chest (thoracic visceral organs):
T-stage for tumours starting in the space at the very back of the abdominal cavity (retroperitoneum):
Tumour stage for tumours starting in the space around the eye (orbit):
Nodal stage (pN) for rhabdomyosarcoma
Rhabdomyosarcoma is given a nodal stage of 0 or 1 based on the presence of cancer cells in a lymph node. If no cancer cells are seen in any of the lymph nodes examined, the nodal stage is N0. If cancer cells are seen in any of the lymph nodes examined, the nodal stage becomes N1.
Metastatic stage (pM) for rhabdomyosarcoma
Rhabdomyosarcoma is given a metastatic stage of 0 or 1 based on the presence of cancer cells at a distant site in the body (for example the lungs). The metastatic stage can only be determined if tissue from a distant site is submitted for pathological examination. Because this tissue is rarely present, the metastatic stage cannot be determined and is listed as MX.
Each cell in your body contains a set of instructions that tell the cell how to behave. These instructions are written in a language called DNA and the instructions are stored on 46 chromosomes in each cell. Because the instructions are very long, they are broken up into sections called genes and each gene tells the cell how to produce piece of the machine called a protein.
Sometimes, a piece of DNA falls off one chromosome and becomes attached to a different chromosome. This is called a translocation and it can result in the cell making a new and abnormal protein. If the new protein allows the cell to live longer than other cells or spread to other parts of the body, the cell can become a cancer.
Why is this important? The alveolar type of rhabdomyosarcoma often contains a translocation that combines the genes for PAX3 or PAX7 with the genes for FOXOA1, NCOA1, or AFX which results in the production of an abnormal protein.
Pathologists test for these translocations by performing either fluorescence in situ hybridization (FISH) or next generation sequencing (NGS) on a piece of the tissue from the tumour. If you were diagnosed with the alveolar type of rhabdomyosarcoma, your report will say which test was performed and which translocation was found in the tumour.
Because the other types of rhabdomyosarcoma (embryonal, pleomorphic, and spindle/sclerosing) do not usually have translocations, no additional molecular tests may have been performed if you were diagnosed with one of these types of tumours.