A radiologist reading a chest X-ray done for respiratory symptoms may identify a nodule or abnormal spot and recommend a chest CT. A chest x ray is a good starting point but a chest CT is much better at looking at the lungs. A chest X-ray produces a picture based on how much radiation passes through the tissues of the chest. Tissues like bone will be white because they let less radiation pass than the lungs which are dark.
A chest X-ray is often done with one or two pictures. One shows the structures from the front to back and one from the side. There are many overlapping structures in the chest. There are bones, blood vessels, bronchial tubes, heart and mediastinum which is the middle part of the chest. These structures overlap in the chest since the X-ray is only done with one or two pictures. The X-ray beam passes through multiple structures as it travels through the chest.
Lung cancer is the main concern for a spot or nodule in the lungs. A lung cancer will let less radiation through than the lung around it. A lung cancer will look more white than the surrounding lung. Unfortunately, with X-ray, blood vessels can overlap ribs and produce shadows which looks like a small cancer. Parts of the ribs can look like white spots that mimic cancer. Shadows from nipples can look like a lung cancer. Areas of scar tissue or pneumonia can look like cancer as well.
A spot on chest X-ray therefore does not have to be an abnormality. A chest X-ray is a way to quickly look at the lungs for abnormalities. Unfortunately, normal structures can overlap and mimic abnormalities. Benign abnormalities can look like cancer. This is particularly true for nodules in the lungs. It is better to be safe than sorry. Lung cancer is lethal and detecting it early is important. Getting a negative chest CT after finding a spot on chest X-ray is great news!