Follow up is a very common recommendation by the radiologist on many types of imaging studies. To me, an abnormality that needs follow up is not clearly benign or malignant or dangerous to the health of the patient. It is an abnormality that is either probably benign, or too small to tell as is the case with a lung nodule under a centimeter.
The best way to tell if something is concerning is if it grows or changes over time. While benign abnormalities can also change, something that is changing and growing becomes more concerning. If a lung nodule or spot in the lung grows over 6 months, then it is more concerning then a nodule which stays the same.
Follow up is appropriate if the radiologist thinks there is little harm in waiting say 6 months and repeating the study. Finding out now what an abnormality is may not be possible or involve a risky invasive procedure like a biopsy or surgery. This may be overkill and pose risk for an abnormality which has a good likelihood of being harmless and unimportant.
If an abnormality does change or become more concerning in appearance, then more testing or invasive procedures can be considered. First do no harm is what many physicians learn in the course of their training. Recommending invasive procedures that pose risk to the patient for a probably benign abnormality goes against this.
Abnormalities like pulmonary nodules, or kidney cysts that are complex have established guidelines for follow up. Other abnormalities encountered on imaging studies may be at the discretion of the radiologist. Some radiologists may recommend follow up or testing based on their experience instead of published guidelines from experts and societies.
Many follow up imaging studies are still not conclusive and further follow up may be recommended. Some cancers grow very slowly, and we can only be sure after months to years of follow up. This will still pose little harm to most patients but potentially prevent the patient from getting a major surgery with all of its risks.
Recommending follow up studies in a certain amount of time is very common in radiology. This does not mean you have something dangerous. Instead, something has been found which is not clearly benign or poses a major threat to health. It’s something in between those two extremes, or something which we can’t tell because it’s too small. Finding out for sure may pose some risks or simply not be reasonably possible. For example, having surgery for a tiny lung spot- a bit of overkill!