Uterine cancer, most frequently endometrial cancer, is a malignancy arising from the inner lining of the uterus (the endometrium). When patients experience abnormal uterine bleeding, especially after menopause, an ultrasound is often the first non-invasive imaging test used for evaluation. The ultrasound provides a clear image of the uterus and its lining, helping physicians determine if the internal structure shows characteristics that warrant further investigation. While the scan cannot definitively diagnose cancer, it identifies suspicious changes and guides the next diagnostic steps.
The Critical Measurement: Endometrial Thickness
The primary indicator of potential endometrial cancer on an ultrasound is the measurement of endometrial thickness. This measurement is highly dependent on a patient’s menopausal status. In postmenopausal women, who are at a higher risk for this cancer, the endometrium is expected to be thin due to hormone withdrawal.
For postmenopausal women experiencing abnormal bleeding, an endometrial thickness of 4 millimeters (mm) or less is considered low risk, often indicating benign atrophy. Thicknesses greater than 5 mm are a red flag and increase the suspicion of cancer or precancerous changes, prompting the need for a tissue biopsy. This cutoff is important for risk stratification and decision-making.
The risk assessment changes for postmenopausal women who are asymptomatic or taking hormone replacement therapy (HRT) or Tamoxifen. For asymptomatic postmenopausal patients, a thickness up to 8 to 11 mm may be acceptable, though elevated thickness still increases cancer risk. In premenopausal women, the measurement is less clear-cut because the endometrium naturally thickens and sheds during the menstrual cycle, reaching up to 16 mm. For these patients, a thickened endometrium is concerning, but the exact visual features are often more important than the measurement alone.
Secondary Visual Features Indicating Malignancy
Beyond the thickness measurement, several other visual characteristics on the ultrasound may suggest a malignant condition. A cancerous endometrium often appears heterogeneous, meaning its internal texture is non-uniform and irregular, unlike the smooth appearance of a normal lining. The borders of the endometrium, where it meets the muscular wall of the uterus (the myometrium), may also look irregular or poorly defined.
Malignancy can manifest as a distinct, focal mass or a polypoid lesion that bulges into the uterine cavity. Unlike a benign polyp, a cancerous mass may have a chaotic, lobulated, or irregular shape. Another concerning sign is myometrial invasion, where the tumor appears to spread beyond the endometrium into the uterine muscle layer. This invasion may be visualized as a disruption in the distinct dark line (the subendometrial halo) that normally separates the lining from the muscle. Color Doppler studies may also reveal increased, irregular, or chaotic blood flow within the thickened area, often associated with tumor neovascularization.
Techniques Used to Visualize the Uterus
To obtain detailed images of the endometrium, physicians primarily rely on Transvaginal Ultrasound (TVUS). This technique involves inserting a specialized transducer probe into the vagina, positioning it close to the uterus. The proximity of the probe allows for the use of higher-frequency sound waves, which produces a clearer, higher-resolution image of the endometrium than other methods.
Transabdominal Ultrasound (TAS), where the probe is placed on the abdomen, is sometimes performed first to gain a wider view of the entire pelvic area. However, TVUS is superior for assessing the fine details of the endometrial lining, which is essential for cancer screening. If the initial TVUS is inconclusive or the nature of the thickening is unclear, Saline Infusion Sonography (SIS), also known as hysterosonography, may be performed.
During an SIS procedure, a sterile saline solution is instilled into the uterine cavity through a thin catheter. The fluid distends the uterus, acting as an acoustic contrast agent that separates the walls. This makes it easier to distinguish between diffuse thickening and a focal lesion like a polyp or mass. This enhanced visualization helps determine the precise location and extent of any abnormality before proceeding to more invasive steps.
Confirming the Diagnosis After the Scan
An ultrasound is an imaging tool used for detection and risk assessment, but it cannot definitively diagnose cancer. The scan highlights an area of concern, such as a thickened or irregular lining, which necessitates a tissue sample for confirmation. The only way to confirm or rule out the presence of malignant cells is through a pathological examination of the endometrial tissue.
The standard procedure following a suspicious ultrasound is an endometrial biopsy, which can often be performed in a doctor’s office. A thin, flexible tube is inserted into the uterus to collect a sample of the lining for analysis. If the initial biopsy is not possible or the sample is inadequate for a clear diagnosis, dilation and curettage (D&C) is recommended.
A D&C, a minor surgical procedure, involves widening the cervix and using a specialized instrument to scrape or remove tissue from the entire uterine lining. The collected tissue is sent to a pathologist who examines the cells under a microscope to determine if cancer is present, including its type and grade. If ultrasound findings are borderline or the patient is asymptomatic, a follow-up scan may be recommended rather than an immediate biopsy.