Fibroid of the uterus

What is a fibroid?

A fibroid is a non-cancerous tumour that is made out of a specialized type of tissue called smooth muscle. The tumour develops from a layer of tissue called the myometrium in the wall of the uterus. Fibroids are very common tumours and they usually occur in women between 20 and 50 years old. Fibroids can start anywhere in the myometrium. Those that grow near the serosa are called subserosal. Those that grow near the endometrium are called submucosal. Another name for fibroid is leiomyoma.

The 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:

  1. Endometrium – The endometrium forms the inner lining of the uterus. The endometrium is composed of endometrial glands lined by one layer of columnar epithelium and surrounded by endometrial stroma.
  2. Myometrium – The myometrium is the middle layer and is made up of smooth muscle which allows the uterus to change size and contract.
  3. Perimetrium – The perimetrium is a thin layer of tissue that surrounds the outside of the uterus.

Gynecological tract


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, fibroids can grow to be very large which can cause pain and pressure on surrounding organs such as the bladder.

How do pathologists make this diagnosis?

Fibroids 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 fibroid is identified in the uterus, the report will simply include fibroid in the diagnosis section without providing additional information about the fibroid.

If the uterus is removed for the purpose of removing a fibroid (or multiple fibroids), the diagnosis may include additional information although the amount and type of information provided vary between pathologists and hospitals.


Tumour size

This is the size of the tumour measured in centimetres (cm). 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.

Tumour infarct or degeneration

Some fibroids 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. Infarcts are very common in fibroids that have been treated with hormone therapy or embolization.

In other cases, the cells do not die but the tumour shows signs of ageing that pathologists describe as degeneration. Some reports may use words like hydropic or hyaline to describe the degenerative changes seen in the tumour. Both infarcts and degeneration are common in leiomyomas and are not associated with a worse prognosis.

Mitotic activity

Cells divide in order to create new cells. This process is called mitosis and a cell that is dividing is called a mitotic figure. Dividing cells are seen in some fibroids and if present, the number of mitotic figures may be included in your report.


The tumour cells in a fibroid usually look very similar to normal smooth 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 fibroids. Although they look abnormal, atypical tumour cells are not cancer cells and are not associated with a worse prognosis.

Cellular leiomyomas

A cellular fibroid is a tumour that has more cells than the average fibroid. Despite the increased number of cells, the tumour is still non-cancerous.

by Jason Wasserman, MD PhD FRCPCn(updated July 27, 2021)
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