Pneumonia and Chest Infection — on a PET Scan

PET uses a radioactive glucose called FDG. Cancer cells consume glucose rapidly because they are dividing fast. Immune and inflammatory cells also consume it at high rates when they are actively fighting infection. The scan detects high consumption but cannot tell which type of cell is driving it. That is why a PET result always needs to be read alongside the CT images, your clinical history, and — when there is genuine uncertainty — a tissue biopsy.

It can be, if the urgency of staging outweighs the risk of a misleading result. Your oncologist will interpret the scan more cautiously in that context and may plan to biopsy any areas that remain ambiguous rather than acting on uncertain findings alone. If you are unsure why that decision was made, ask your oncologist to explain the trade-off. It is a reasonable question and you deserve a clear answer.

Mainly the lungs and the chest lymph nodes, which react to the same infection. If you also have a significant fever on the day of the scan, there can be more widespread increased uptake — in the bone marrow and spleen particularly — which makes the whole scan harder to interpret. A fever on scan day is something to tell the nuclear medicine team before the injection, not afterwards.

A tissue biopsy — a small sample taken from the area and examined under a microscope. This can be done by CT-guided needle biopsy through the chest wall, or by bronchoscopy if the area is accessible via the airway. A PET scan, CT, or chest X-ray can raise or lower suspicion, but none of them can confirm a cancer diagnosis. The pathology report from the biopsy is what decides.

Call your team before the appointment rather than deciding alone. A persistent cough may be from an active chest infection that would affect the scan, or it may be related to the cancer itself — in which case waiting could delay your diagnosis. Your team needs to know about the cough to decide whether to investigate it first or proceed and factor it into the interpretation. Do not wait until the morning of the scan to mention it.

Being on long-term antibiotics does not automatically rule out a PET scan, but your team needs to know. Chronic lung conditions — bronchiectasis, non-tuberculous mycobacterial infection, chronic aspiration — cause ongoing inflammation that affects how the scan is read. Tell your oncologist and the nuclear medicine team about any regular medications and underlying lung conditions before your appointment, so the result can be interpreted with that full context in mind.