Focal nodular hyperplasia is the second most common benign liver tumor. Benign liver tumors are much more common than malignant ones, and up to 25% of all humans can have them without any symptoms. The most common benign tumor is hemangioma, and it is usually accidentally discovered on imaging done for another cause. Like hemangioma, focal nodular hyperplasia is usually discovered accidentally and tends to occur in females more commonly than males. The reason behind that was postulated to be hormonal.
Focal nodular hyperplasia constitutes about 8% of all liver tumors and 25% of benign tumors. It is usually discovered in the 3rd and 4th decades of life. However, it can occur much later in males and is diagnosed in up to the 8th decade. In the past, focal nodular hyperplasia was thought to be a type of hepatocellular adenoma as it was discovered with a similar incidence. Modern imaging techniques have, however, shown that it occurs more commonly and has a different appearance and behavior. Biopsy also helped differentiate between the two different tumors.
What causes focal nodular hyperplasia?
Understanding why a tumor occurs helps doctors treat or prevent it. Tumors are complex and rarely do they have a single cause, making their prevention challenging. In the past, it was thought that the only cause for focal nodular hyperplasia was hormonal, and that oral contraceptives are the main culprits. This was proven false, and the main cause was found to be related to the blood vessels of the liver. The liver has a vast network of blood vessels called sinusoids, which communicate with the portal blood coming from the gastrointestinal tract and the systemic blood going to the heart. When a vascular malformation occurs, liver tissue can respond erratically, causing increased division of cells. Although oral contraceptive pills were found to be innocent of the development of focal nodular hyperplasia, studies done on them showed that they are capable of enlarging an existing one.
The case of a liver lesion unrelated to hormones yet one that can increase in size under their influence is typical of most benign tumors of the liver. Focal nodular hyperplasia is considered benign because it contains normal liver cells arranged abnormally and dividing rapidly rather than deformed cells that act invasively. This can only be confirmed by a biopsy. Both hemangiomas and focal nodular hyperplasia tend to occur simultaneously.