When PET and CT Scans — Give Different Answers

Yes. Discordant findings are a recognised part of oncology imaging, not an unusual event. PET and CT are not designed to agree on every point — they are designed to be read together, each adding information the other cannot provide. When they point in different directions, that is the imaging working as designed by revealing the picture is incomplete. How often it happens varies by cancer type and the structures being imaged, but your team will have seen this before and will have a plan for resolving it.

Because they answer different questions and together give more information than either gives alone. A PET can reveal metabolic activity in a node that appears normal-sized on CT. A CT provides anatomical detail in areas where PET alone is insufficient — around the brain, for example, where high baseline glucose uptake can obscure other findings. The combination is more informative precisely because the two modalities differ. The disagreement is a feature of using both, not a reason to use only one.

No. A bright area on FDG-PET means cells in that location are consuming glucose at a higher rate than surrounding tissue. Cancer does this, but so do active infections, inflammation, post-surgical healing and a range of other conditions. ASCO and ESMO guidance acknowledges this false-positive risk, which is why imaging alone is never sufficient to confirm a diagnosis. A PET hot spot is a finding that needs interpretation alongside your full clinical picture — not a confirmed result on its own.

Yes, and it is entirely reasonable to ask. A second radiologist reviewing the same images may clarify the interpretation or confirm it. However, a second reading of the same images is unlikely to resolve a genuine discordance between two modalities — it can clarify what each scan shows individually, but it cannot tell you which finding reflects what is actually happening in that site. That answer requires tissue. A second opinion is most useful for confirming whether biopsy is the right next step.

Possibly, once the discordance is resolved. If biopsy confirms the PET finding reflects disease at a new site, your team may reassess staging and adjust the plan. If biopsy shows the finding is not cancer, the plan may continue unchanged. That decision cannot be made while the two scans remain in disagreement — which is why resolving the discordance is the priority before committing to a treatment direction. Your team will not change your plan on the basis of conflicting imaging alone.

Ask three things: what the discordant finding could represent, what investigation they recommend to resolve it, and when that investigation will happen. If biopsy is recommended, ask which site is being targeted and what the procedure involves. If further imaging is proposed instead, ask what it is expected to add and why biopsy has been deferred. You are entitled to a clear answer to each. Writing the questions down beforehand helps — these conversations are difficult to retain when you are anxious and waiting.