Do doctors know by looking at the pet and ct scans if the patient has cancer?

Yes: Yes they will especially pet scan , final sure answer is biopsy of the lesion.

Most of time: Pet ct scan involves injection of isotope f18 FDG which accumulates in areas of metabolism of glucose. In general the greater amount of uptake in lesion, the more likely the area is malignant. Sometimes infection can be active in uptake as well. By comparing uptake with ct lesion is localized. Often helps to guide biopsy looking for most active part for viable tumor.

Yes: Pet and pet/ct scans can tell physicians a lot about whether a lesion has metabolic activity and if they are potentially more concerning. To understand more about pet scans please read more here and check out it's related/linked articles and videos: http://www.Conciergeradiologist.Com/pet-scan.Html.

Usually: Pet scans show how much glucose a lesion is consuming for food. More glucose, more metabolism. Depending on the history, high metabolic lesions are much more likely to be cancer. However, other things (inflammation, infection) can also have increased metabolism. Also, very small cancers (less than 1 cm or 1/2 inch) can sometimes be falsely negative on pet.