Vaccine uptake on PET-CT
Inflammation After Injections, — Vaccines and Cannulas
A recent vaccination can make your shoulder and the lymph nodes in your armpit appear active on PET-CT. Seeing that on a report is alarming. It is also a documented pattern that radiologists are trained to look for — and one your team needs your vaccination history to interpret.
Medically reviewed by Dr. T. Raghavender Reddy, Medical Oncologist, MBBS · DM (Medical Oncology) · MD (Radiation Oncology) · Last reviewed September 2026
- Vaccines cause real inflammation — An injection triggers an immune response at the site and in nearby lymph nodes, which a PET scanner is designed to detect.
- The armpit nodes are the usual finding — Vaccines into the shoulder muscle drain to the axillary nodes on the same side — those are the nodes that light up.
- It can persist for several weeks — Vaccine-related uptake fades as the immune response settles, but that can take longer than people expect.
- Tell your team before the scan — The date, site, and type of vaccine help the radiologist interpret the finding correctly rather than treating it as unexplained.
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Yes. A recent vaccination can cause the injection site and nearby lymph nodes — most often the armpit on the same side — to show increased activity on PET-CT. This is a recognised pattern. Radiologists are trained to look for it, but they need to know your vaccination history to interpret the scan correctly.
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Why does a recent vaccine show up on a PET scan?
PET scans detect metabolic activity — cells working hard. When a vaccine enters a muscle, your immune system mounts a local response. That response involves increased cellular activity, and the scanner picks it up as FDG uptake.
Most vaccines in India are given into the deltoid, the muscle at the top of your shoulder. The lymph nodes that drain that muscle sit in the armpit on the same side. Both the injection site and those axillary nodes can show uptake.
The concern is that this pattern can look like disease on the scan. Without knowing about the vaccination, the resemblance is difficult to resolve from the images alone.
What happens when your team sees vaccine uptake on your scan?
Your vaccination history is documented
If you have told your team about a recent injection — ideally with the date, site, and vaccine type — the radiologist includes that information when writing the report.
The radiologist describes the pattern
Vaccine-related uptake tends to appear at the deltoid and in the ipsilateral axillary nodes. A radiologist familiar with this pattern will include vaccination as one possible explanation in the differential.
Your oncologist correlates imaging with your clinical picture
The scan finding is read alongside your examination findings, your disease history, and any other investigations. That combination guides what happens next.
If doubt remains, tissue is the answer
When the finding cannot be attributed to vaccination with confidence, a biopsy of the relevant node is how your team establishes what is there. Imaging cannot confirm or exclude disease — only tissue can.
How long does vaccine uptake last on PET?
Vaccine-related uptake fades as the local immune response settles, but it does not disappear in a few days. It can persist for several weeks, and in some patients for longer.
COVID-19 vaccines were studied extensively during the vaccination period because so many cancer patients had scans at the same time. Published radiology literature and guidance from ASCO noted uptake persisting well into the weeks after immunisation. If you have had a vaccine recently, tell your oncologist before the scan so the timing can be considered.
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What do the words in your PET report mean?
- FDG uptake
- FDG is a sugar the scanner tracks. Inflamed tissue, immune cells, and some tumour cells all absorb more of it. Uptake means that increased absorption was detected in a particular place.
- Axillary lymph nodes
- The lymph nodes in the armpit. They drain the arm, chest wall, and shoulder — and are the most common site of vaccine-related uptake after a deltoid injection.
- Ipsilateral
- On the same side. Ipsilateral axillary uptake after a right-arm injection means the nodes in the right armpit, not the left.
- Reactive lymphadenopathy
- Lymph node activity in response to a trigger — an infection, an injection, or a vaccine. Reactive describes the pattern, not the cause; it still needs clinical correlation.
- Differential diagnosis
- The list of possible explanations for a finding. Vaccine reaction on that list is not a verdict — it is one possibility your team weighs against the others.
Questions about vaccine uptake your team is asked most
I had my COVID vaccine months ago. Can it still be causing the uptake?
It is possible but less likely as more time passes. Most vaccine-related uptake settles within a few weeks. If a significant gap separates your last vaccination from the scan date, your oncologist and radiologist will weigh that timeline carefully. A longer gap makes vaccination a less complete explanation, and makes tissue confirmation more important — not less. The duration alone does not rule anything out; it is one factor in a clinical judgement that your team is best placed to make.
The uptake is on the same side as my breast cancer. Does that mean spread?
It may mean spread, or it may reflect vaccine-related uptake. This is one of the most significant differentials on oncology PET, and imaging alone cannot resolve it. Your oncologist will correlate the scan with your clinical examination, the node characteristics, and your vaccination history. If that correlation leaves doubt, a biopsy of the relevant node is the way to reach a definitive answer. Do not rely on the scan report alone — ask your oncologist what the finding means in your specific clinical context.
I had a cannula or chemotherapy injection in my arm. Can that cause uptake too?
Yes. Any local tissue injury — from a cannula, a previous biopsy, or an intramuscular injection — can cause localised FDG uptake as the tissue responds and repairs. This type of uptake tends to sit at the exact procedure site rather than spreading to the lymph nodes, and it usually settles more quickly than post-vaccination uptake. Tell your team about all recent injections and procedures before a PET scan, not only vaccines.
Should I delay my scan if I have just had a vaccine?
That depends on how urgent the scan is. If there is clinical flexibility, your oncologist may suggest waiting a few weeks for uptake to settle before scanning. If the scan is needed urgently — to assess treatment response or an acute finding — the benefit of scanning now outweighs the interpretive difficulty, provided the vaccination history is documented so the radiologist can account for it. Raise this with your oncologist before the scan rather than on the day.
Does the BCG vaccine I had as a child still show up on scans?
BCG given in early childhood is not usually an active source of current uptake. In rare cases, BCG can reactivate — particularly in people who are immunosuppressed — and that can appear on imaging. If your report mentions uptake near a childhood BCG scar site, mention it to your team. Whether it is relevant depends on the clinical context and your immune status, and is a question for your treating oncologist to assess alongside the other findings.
Did you know?
During the COVID-19 vaccination period, oncology radiology societies documented a significant rise in axillary lymph node uptake on PET-CT in vaccinated cancer patients. ASCO and international radiology bodies now recommend that vaccination history — including site and date — be recorded before every oncology PET scan.
The guidance exists because vaccine-related uptake and nodal metastasis can be indistinguishable on imaging alone. History is not a formality; it changes how the scan is reported.
Source: ASCO guidance on COVID-19 vaccination and oncology imaging; published radiology literature 2021–2023
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Common questions
Frequently asked questions
Does every vaccine cause uptake on PET, or only some?
Most intramuscular vaccines can cause some local uptake. COVID-19 vaccines were studied most extensively because of their timing with routine oncology imaging, but flu vaccines, hepatitis vaccines, and other intramuscular injections can produce the same effect. The consistent recommendation from ASCO and international radiology bodies is to document vaccine history — type, site, and date — before any oncology PET scan, regardless of which vaccine was given.
Can my radiologist tell the difference between a vaccine reaction and cancer in the nodes?
Sometimes, when the location is characteristic and the vaccination history is known. A finding confined to the deltoid and ipsilateral axillary nodes, in a patient known to have had a recent injection on that side, supports a benign explanation. But imaging cannot confirm what tissue contains. When the clinical stakes are high — as they are with axillary nodes in a breast cancer patient — a biopsy is the appropriate next step to give a definitive answer.
I forgot to mention my vaccine before the scan. What do I do now?
Tell your oncologist at your next appointment and ask whether it should be passed to the radiologist before the report is finalised, or noted as a formal addendum. If a treatment decision is being made based on that report, make sure the team knows about the vaccination before they act on the findings. It is never too late to add relevant history, and it may change the interpretation significantly.
My report says 'reactive axillary lymphadenopathy' — is that the vaccine?
Vaccination is one common cause, but 'reactive' is a description of the appearance, not a confirmed diagnosis. It means the radiologist considers a benign explanation plausible given the pattern — not that disease is excluded. Your oncologist will decide whether that explanation fits your full clinical picture or whether further investigation is needed. Ask them what the finding means specifically for you.
The uptake is on the opposite side from where I had the vaccine. What does that mean?
Vaccine-related uptake typically follows the lymph drainage from the injection site, which runs to the axillary nodes on the same side. Uptake on the opposite side would not be explained by vaccination alone and would need separate assessment. Tell your team exactly which arm received the injection so they can interpret the laterality correctly — left and right matter here.
We use traditional or herbal treatments alongside cancer treatment. Should we mention those?
Yes, mention everything. Some preparations applied to the skin or given at traditional medicine clinics can cause local tissue changes visible on imaging. Tell your treating team what you are taking or applying — including traditional and herbal treatments — so they have a complete picture when interpreting your results. Nothing in that history will be judged; it will be used to help you.