Brain Tumors versus Brain Metastases Differing between these two types of brain lesions is a common source of confusion for many people. Primary brain tumors are tumors that start in the brain, and are actually quite rare, with an estimated 22,020 new cases in 2010. Brain metastases, commonly called "brain mets" are far more common, and are tumors that have traveled to the brain from another area of the body. It is estimated that between 100,000-170,000 patients develop brain metastases each year. Let's use an example to better understand this latter concept: a lung cancer is first formed in the lung tissue, but tumor cells can break off from the original mass and travel through the bloodstream or lymph system to other areas of the body, including the brain. This spreading of the tumor is known as "metastasis". When a lung cancer metastasizes to the brain, this "brain tumor" is ... read more on Brain Metastases and Available Treatments