A hypoechoic mass is a lump which appears relatively darker on an ultrasound scan, because it reflects fewer ultrasound waves. The significance of this finding varies depending on the context. Some tissues normally reflect more or less ultrasound waves than others. When part of an organ changes to reflect more or less ultrasound waves than usual, with the result that it appears brighter or darker than the surrounding tissue, this could indicate an area of disease. Sometimes an entire organ could appear more or less bright than it would normally in comparison with other organs, and this could also be a sign of illness.
Ultrasound works by sending out high frequency sound waves which bounce off tissues. A probe sends out the sound signals, and the reflected echoes are captured and transformed into a black and white image displayed on a screen. What are called hyperechoic areas tend to return more waves, while hypoechoic regions return relatively fewer waves. Where sound waves pass through water, there are usually no reflected waves and the area appears black, or anechoic.
A hypoechoic breast mass may be benign, as in the case of a non-cancerous tumor called a fibroadenoma. This appears on an ultrasound scan as a hypoechoic mass with smooth edges. Fibroadenomas are common in young women and may sometimes disappear by themselves, so they are usually only removed if they are large or increasing in size. A breast lump could also be a simple cyst which, being fluid-filled, has an anechoic, rather than a hypoechoic, center surrounded by a well-defined wall. Again, this is a benign, or non-cancerous, mass and can be treated by draining the contents using a needle.
An ovarian mass which appears hypoechoic on ultrasound could be a tumor known as a fibroma. This is a benign, solid growth which can grow quite large, sometimes becoming bigger than a grapefruit. An ovarian fibroma is usually removed surgically and the ovary may be preserved if possible.
One example of a hypoechoic mass, seen in the thyroid gland, is a benign tumor called a follicular adenoma. Confusingly, follicular adenomas may also appear hyperechoic, where more sound waves are reflected back, giving a brighter appearance. In either case, follicular adenomas seen on ultrasound scans tend to be surrounded by a ring which is hypoechoic. They are the most common kind of thyroid tumor, and they are often removed as a precaution because it can sometimes be difficult to distinguish them from thyroid cancer.
In the liver, a hypoechoic mass may represent a growth of cancer cells which have traveled from an original tumor elsewhere in the body. This kind of malignant mass, which originates elsewhere, is known as a metastasis, plural metastases. In most cases where cancer has spread to the liver, the disease is not curable, but treatments such as chemotherapy may shrink tumors and increase life expectancy.