Uterine leiomyosarcoma (uterus)

What is uterine leiomyosarcoma?

Uterine leiomyosarcoma is a type of cancer that starts from specialized smooth muscle cells found in the wall of the uterus. It is part of a group of cancers called sarcomas. Most patients are over the age of 50 years at the time of diagnosis.

The uterus

The uterus is a pear-shaped hollow organ found in the female pelvis between the rectum (the end of the large bowel) and the urinary bladder. The upper part of the uterus is called the fundus and it is attached to the fallopian tubes while the lower part is connected to the vagina through the uterine cervix. The middle of the uterus is called the body or corpus.

The walls of the uterus are made up of three layers:

  • Endometrium – The endometrium is the tissue that covers the inside surface of the uterus. The endometrium is made up of endometrial glands lined by one layer of cells that form a barrier called the epithelium. The epithelium is surrounded by supporting tissue called the endometrial stroma.
  • Myometrium – The myometrium is the middle layer and is made up of smooth muscle which allows the uterus to change size and contract. Leiomyosarcoma starts from the cells normally found in the myometrium.
  • Perimetrium – The perimetrium is a thin layer of tissue that surrounds the outside of the uterus.

Gynecological tract

How do pathologists make this diagnosis?

The diagnosis of uterine leiomyosarcoma is usually made after the entire uterus is removed in a procedure called a hysterectomy. The ovaries and cervix may be removed at the same time. The uterus is then sent to a pathologist who examined the tumour and other tissues under the microscope.

Most tumours are found in the body or corpus of the uterus. However, these tumours can also start in the wall of the cervix. When examined under the microscope, most uterine leiomyosarcomas are made up of long thin smooth muscle cells. Because of their shape, pathologists often describe these cells as spindle cells. These same smooth muscle cells are also found in a non-cancerous type of tumour called a leiomyoma. In order to tell the difference between leiomyosarcoma and leiomyoma, your pathologist will look for the following three histologic features:

  • Dividing tumour cells – Tumour cells divide in order to create new cells. This process is called mitosis. A cell that is in the process of dividing is called a mitotic figure. Dividing tumour cells are typically found in leiomyosarcoma. In contrast, very few dividing cells should be found in a non-cancerous leiomyoma.
  • Abnormal looking tumour cells – Pathologists use the terms atypical or atypia to describe cells that are abnormal-looking in shape, size, or colour. Atypical tumour cells are commonly found in leiomyosarcoma.
  • Dead or dying tumour cells – Cancers grow more quickly than non-cancerous tumours. As they grow, the tumour cells run out of energy and die. This kind of cell death is called necrosis. Necrosis may be seen in leiomyosarcoma. However, it is rare to see necrosis in a non-cancerous leiomyoma.

uterine leiomyosarcoma

Some uterine leiomyosarcomas are made up of cells that are more round. Pathologists call these cells epithelioid because they look similar to specialized epithelial cells found on the surface of many types of tissues. Finally, some uterine leiomyosarcomas are made up of a small number of tumour cells surrounded by a loose connective tissue that pathologists describe as myxoid. As with spindle type leiomyosarcoma, pathologists look for mitotic figures, atypical tumour cells, and necrosis when making the diagnosis of epithelioid or myxoid leiomyosarcoma.

uterine leiomyosarcoma epithelioid

Your pathologist may also perform a test called immunohistochemistry to confirm the diagnosis. Immunohistochemistry allows your pathologist to see different types of proteins inside the tumour cells. When this test is performed on a tissue sample from a uterine leiomyosarcoma the tumour cells will be positive or reactive for proteins normally found in smooth muscle in smooth muscle cells such as h-caldesmon, smooth muscle actin (SMA), and desmin. These immunohistochemical markers are also positive in leiomyoma so these results need to be considered in combination with the histologic features (see above) in order to make the correct diagnosis.

Other tissues or organs involved

Several other organs and tissues are directly attached or very close to the uterus including the ovaries, fallopian tubes, vagina, bladder, and rectum. Adnexa or adnexal are terms used to describe the fallopian tubes, ovaries, and ligaments connected directly to the uterus. The spread of cancer cells directly into any of these organs or tissue is important because it is used to determine the pathologic tumour stage (see Pathologic stage below) and because it is associated with a worse prognosis.

Lymphovascular invasion

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. 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.

lymphovascular invasion


A margin is the normal tissue that surrounds a tumour and is removed with the tumour at the time of surgery. The margins will only be described in cases where the tumour extends into the cervical stroma or other tissues surrounding the uterus and after the entire tumour has been removed.

A negative margin means that no tumour cells were seen at any of the cut edges of tissue. A margin is called positive when there are tumour cells at the very edge of the cut tissue. A positive margin is associated with a higher risk that the tumour will recur in the same site after treatment.


Lymph nodes

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 metastasis.

Your pathologist will carefully examine all lymph nodes 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.

Lymph nodes examined are usually divided into those found in the pelvis and those found around a large blood vessel in the abdomen called the aorta. The lymph nodes found around the aorta are called para-aortic. Lymph nodes on the same side as the tumour are called ipsilateral while those on the opposite side of the tumour are called contralateral.

If cancer cells are found in a lymph node, the size of the area involved by cancer will be measured and described in your report.​

  • Isolated tumour cells – The area inside the lymph node with cancer cells is less than 0.2 millimetres in size.
  • Micrometastases – The area inside the lymph node with cancer cells is more than 0.2 millimetres but less than 2 millimetres in size.
  • Macrometastases – The area inside the lymph node with cancer cells is more than 2 millimetres in size.

Cancer cells found in a lymph node is associated with a higher risk that the cancer cells will be found in other lymph nodes or in a distant organ such as the lungs. The number of lymph nodes with cancer cells and the size of the largest tumour deposit is also used to determine the nodal stage (see Pathologic stage below).

Lymph node

Pathologic stage​

​The pathologic stage for uterine leiomyosarcoma is based on the TNM staging system, an internationally recognized system originally created by the American Joint Committee on Cancer. This 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 a worse prognosis.

Tumour stage (pT)

The tumour stage for uterine leiomyosarcoma is based on the size of the tumour and the growth of the tumour outside of the uterus.

  • T1a – The tumour only involves the uterus and is 5 cm or less in size.
  • T1b – The tumour only involves the uterus and is more than 5 cm in size.
  • T2a – The tumour has spread into the adnexal structures surrounding the uterus.
  • T2b – The tumour has spread to other organs or tissue in the pelvis.
  • T3a – The tumour has spread to at least one site inside the abdomen.
  • T3b – The tumour has spread to more than one site in the abdomen.
  • T4 – The tumour has grown directly into the bladder or the rectum.
Nodal stage (pN)

The nodal stage for uterine leiomyosarcoma is based on the number of lymph nodes in the pelvis that contain cancer cells.

  • N0 – No cancer cells were found in any of the lymph nodes examined.
  • N0(i+) – Cancer cells were found in at least one lymph node from the pelvis but the area with cancer cells was not larger than 2 millimetres (only isolated cancer cells or micrometastasis).
  • N1 – Cancer cells were found in at least one lymph node from the pelvis and the area with cancer cells was greater than 2 millimetres (macrometastasis).
  • NX – No lymph nodes were sent for examination.
Metastatic stage (pM)

The metastatic stage for uterine leiomyosarcoma is based on the presence of cancer cells in a lymph node outside of the pelvis or 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.

by Jason Wasserman MD PhD FRCPC (updated September 27, 2021)
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