Dark spots on lungs are caused by a number of different things, including parasitic infections, bacterial infections and fungal infections, according to Dr. Lynne Eldridge writing for About Health. A dark spot on a lung CT does not always mean it's a cancerous growth. Dark spots can be cysts, pulmonary fibroids or blood vessel abnormalities.
A dark spot that shows up on a lung CT is called a lung nodule, states Dr. Eldridge. Nodules are usually less than 3 centimeters in diameter. Anything larger than a 3 centimeter spot is called a lung mass. There is a 40 percent chance that a lung nodule is cancer, stresses Eldridge, but there are other risk factors taken into consideration, such as the age of the individual, if he smoked or what the nodule actually looks like on the X-ray.
When looking at a lung nodule on a CT scan, different factors contribute to a lung nodule being malignant. The age of the individual is important, as more than half of dark spots found in people over the age of 50 are cancerous, states Dr. Eldridge. The individual may have a history of smoking or worked in an occupation where he was exposed to cancer-causing substances. Doctors look for whether or not the nodule continues to grow on repeated X-rays.