Benign tumors are masses of cells that can’t invade neighboring tissues or organs, and therefore are usually defined as non-cancerous.
Benign liver tumors are actually pretty common, and typically don't cause any serious complications, and there are three major types: cavernous hemangiomas, focal nodular hyperplasias, and hepatocellular adenomas.
Cavernous hemangiomas are the most common form and are these masses or swelling of the endothelial cells in blood vessels of the liver that, when we check it out on histology, they form these huge vascular spaces that sort of look like a system of caves.
In other words, instead of blood flowing through a tube, the blood goes into a giant cavern with endothelial cells randomly sprinkled throughout.
Although these vascular spaces look huge on histology, most patients have relatively small lesions, usually less than about 1.5 cm, and therefore don’t have any symptoms; in more rare cases, with larger lesions, patients may develop symptoms and in very rare cases, experience rupture and intraperitoneal bleeding.
Finding and diagnosing these hemangiomas can be done through several imaging techniques like ultrasound, CT scans, and MRI.
The second most common type of benign liver tumor is a focal nodular hyperplasia, or FNH.
These are like these localized (focal) aggregates (nodular) of rapidly reproducing liver cells (hyperplasia).
FNH is actually the most common non-blood vessel-related benign tumor in the liver and are seen slightly more in women than men, but can happen at any age.
Basically this is a loosely used term to describe when nodules or aggregations of seemingly benign hepatocytes are found in the liver.
Ultimately, we don’t really know why these form, but it’s thought that they could be a response to vascular injury of some kind that leads the hepatocytes to ramp up reproduction and form these aggregates of cells.
Another reason it’s thought to be a result of vascular injury is that there’s almost always an abnormally large blood vessel in the center, with smaller branches radiating out into the periphery.
And further supporting the injury hypothesis, is that there’s almost always a characteristic gross finding of centralized fibrous scar tissue produced by stellate cells.
On histology you’ll often see this fibrous tissue as well, sometimes called fibrous septae, which means wall or separation.