by Jason Wasserman MD PhD FRCPC
September 12, 2022
A leiomyoma is a non-cancerous tumour that starts in the wall of the uterus. The tumour is made up of specialized smooth muscle cells that are normally found in a part of the uterus called the myometrium. Leiomyomas are very common tumours and they usually occur in women between 20 and 50 years old. Another name for leiomyoma of the uterus is fibroid.
Small leiomyomas typically do not cause any symptoms. Larger tumours can cause symptoms such as abdominal pain, pressure, and vaginal bleeding.
In the uterus, there is no difference between a leiomyoma and a fibroid. Both terms are used to describe the same type of non-cancerous tumour.
Leiomyomas are often diagnosed after all or a part of the uterus has been removed and tissue has been examined under the microscope by a pathologist. For many patients, the uterus is removed for other reasons and the leiomyoma is found incidentally.
When examined under the microscope, a leiomyoma is made up of long thin cells called spindle cells. These cells are often described as growing in fascicles which are long interconnecting chains of cells. Thick-walled blood vessels and open spaces called cysts are commonly seen. Rare mitotic figures (tumour cells dividing to create new tumour cells) may be also seen.
Pathologists describe a leiomyoma as subserosal if the tumour is found just under a thin layer of tissue on the outside surface of the uterus called the serosa.
Pathologists describe a leiomyoma as intramural if the tumour is located within the wall of the uterus in a layer of tissue called the myometrium.
Pathologists describe a leiomyoma as submucosal if the tumour is located just below a thin layer of tissue on the inside surface of the uterus called the endometrium.
Degenerative change is a term pathologists use to describe the breakdown of tissue over time. It is very common to see degenerative changes in a leiomyoma, especially larger ones that have been growing for a long time.
Some leiomyomas grow so large that blood cannot get to all of the cells in the tumour. The cells that do not receive blood undergo a type of cell death called necrosis and the area of the tumour that dies is described as an infarct. Infarct-like necrosis is very common in leiomyomas that have been treated with hormone therapy or embolization.
Mitotically active means that dividing tumour cells called mitotic figures were seen within the tumour. Mitotically active leiomyomas are more common in younger women, pregnant women, and in women previously treated with certain medications such as tamoxifen and progesterone. Despite the increased mitotic activity, these tumours are still non-cancerous.
The tumour cells in a leiomyoma usually look very similar to the smooth muscle cells normally found in the myometrium of the uterus. However, when examined under the microscope, some leiomyomas contain tumour cells that are larger and darker than normal smooth muscle cells. pathologists describe these cells as atypical. Although they look abnormal, atypical tumour cells are not cancer cells and are not associated with a worse prognosis.