This article was reviewed and updated on September 1, 2019
by Jason Wasserman, MD PhD FRCPC
A leiomyoma is a non-cancerous tumour that develops from the muscle inside the uterus.
Leiomyomas are very common among women 20 to 50 years of age.
Although they are non-cancerous, the tumour can grow to become very large which can cause symptoms such as pain and bleeding.
Another name for leiomyoma is a fibroid.
The normal uterus and endometrium
The uterus is a pear-shaped hollow organ that is in the female pelvis between the rectum (distal segment of the large bowel) and the urinary bladder. The upper part of the uterus (fundus) is attached to the fallopian tubes while the lower part is connected to the vagina through the uterine cervix.
The walls of the uterus are made up of three layers:
Myometrium – The myometrium is the middle layer and is made up of smooth muscle which allows the uterus to change size and contract.
Serosa – The serosa is a thin layer of tissue that surrounds the outside of the uterus.
What is a leiomyoma?
A leiomyoma is a non-cancerous tumour that is made out of muscle and develops within the myometrium of the uterus. Another name for leiomyoma is a fibroid.
Leiomyomas are very common tumours and they usually occur in women between 20 and 50 years old. Leiomyomas can start anywhere in the myometrium. Those that grow near the serosa are called subserosal. Those that grow near the endometrium are called submucosal.
These tumours can bulge into the inner cavity of the uterus where they can cause bleeding or even interfere with pregnancy. Although they are benign, leiomyomas can grow to be very large which can cause pain and pressure on surrounding organs such as the bladder.
Leiomyomas are often diagnosed after the uterus has been removed, often for other conditions such as cancer of the cervix, endometrium, or ovaries. In this situation, if a leiomyoma is identified in the uterus, the report will simply include ‘leiomyoma’ in the diagnosis section without providing additional information about the leiomyoma.
If the uterus is removed for the purpose of removing a leiomyoma (or multiple leiomyomata), the diagnosis may include additional information although the amount and type of information provided varies between pathologists and institutions.
This is the size of the tumour measured in centimeters (cm). 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.
Infarct or degeneration
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.
Why is this important? Infarcts are very common in leiomyomas that have been treated with hormone therapy or embolization.
In other cases the cells do not die but the tumour shows signs of aging called that pathologists describe as degeneration. Some reports may use words like hydropic or hyaline to describe the degenerative changes seen in the tumour.
Why is this important? Both infarcts and degeneration are common in leiomyomas and are not associated with worse prognosis.
Cells divide in order to create a new cells. This process is called mitosis and a cell that is dividing is called a mitotic figure. Dividing cells are seen in some leiomyomas and if present, the number of mitotic figures may be included in your report.
The tumour cells in a leiomyoma usually look very similar to normal muscle cells. However, when examined under the microscope, some tumour cells may look different.
Atypical tumour cells are larger and darker than normal muscle cells. Atypical cells are often seen in very large leiomyomas.
Why is this important? Although they look abnormal, atypical tumour cells are not cancer cells.
A cellular leiomyoma is a tumour that has more cells than the average leiomyoma. Despite the increased number of cells, the tumour is still non-cancerous (benign).