by Jason Wasserman MD PhD FRCPC
August 25, 2023
Leiomyoma is a non-cancerous type of tumour made up of specialized smooth muscle cells. These types of cells are normally found in the walls of hollow organs such as blood vessels, lungs, bladder, digestive tract, and uterus. A leiomyoma can develop in any location where smooth muscle cells are normally found. Fibroid is a special term used to describe a leiomyoma in the uterus.
The diagnosis of leiomyoma is typically made after the entire tumour is removed in a procedure called an excision. The diagnosis can also be made after only part of the tumour is removed in a procedure called a biopsy. The tissue is then sent to a pathologist who examines it under a microscope.
When examined under the microscope the smooth muscle cells in a leiomyoma are long and thin. Pathologists describe cells with this appearance as spindle cells. The cytoplasm (body) of the cell will contain pink material which pathologists describe as eosinophilic. The nucleus of the cell (the part that holds the genetic material) is typically oval-shaped with blunt ends. The tumour cells are commonly arranged in a pattern that pathologists describe as intersecting fascicles.
It is important for your pathologist to distinguish a non-cancerous leiomyoma from a cancerous tumour called leiomyosarcoma. Both leiomyoma and leiomyosarcoma are made up of smooth muscle cells and they can look similar when examined under the microscope. In order to confirm that the tumour is a leiomyoma, pathologists carefully look for tumour cells that are in the process of dividing to create new tumour cells. This process is called mitosis and the cells are called mitotic figures. Your pathologist will count the number of mitotic figures over an area of tissue to determine the mitotic rate. This rate is usually described as the number of mitotic figures seen in 10 or 50 high-powered fields (the amount of tissue that can be seen when the magnification of the microscope is very high) or over a specific area of tissue such as 2 millimetres square. Leiomyomas typically have very few mitotic figures or a low mitotic rate although some types of leiomyoma, such as those in the uterus are allowed to have more mitotic figures without being called leiomyosarcoma.
Other microscopic features that pathologists look for to tell the difference between a leiomyoma and a leiomyosarcoma are a type of cell death called necrosis and abnormal-looking tumour cells which pathologists describe as atypical or cytologic atypia. Most leiomyomas will not show any necrosis unless the tumour is very large and the blood supply to the tumour was insufficient for the size of the tumour. Abnormal-looking tumour cells are more common and may be seen in leiomyomas from different parts of the body. However, the combination of abnormal-looking tumour cells and mitotic figures will prompt your pathologist to consider the diagnosis of leiomyosarcoma.
Pathologists often perform a test called immunohistochemistry (IHC) to confirm the diagnosis. This test allows pathologists to see the types of proteins being made by the tumour cells. By knowing more about the proteins being made by the tumour cells, your pathologist can confirm that the tumour is made up of smooth muscle cells. When immunohistochemistry is performed, the tumour cells in a leiomyoma are typically positive or reactive for proteins normally found in smooth muscle cells such as smooth muscle actin (SMA), muscle-specific actin (MSA), desmin, and caldesmon. The tumour cells are negative or non-reactive for proteins found in other types of cells including S100, SOX-10, and cytokeratin.