PET-CT scan timing
PET-CT After Steroids: — What They Do to Your Results
If you are on steroids and have a PET-CT coming up, you may have been told the scan could be affected. That concern is real. Steroids change how your body processes glucose, and that is exactly what a PET-CT measures.
Medically reviewed by Dr. C. Raghavendra Reddy, Medical Oncologist, MBBS (Gold Medal) · DNB · DM (Medical Oncology, Gold Medal) · Last reviewed September 2026
- The effect is real — Steroids reduce FDG uptake in tumour tissue, which can make active disease appear less active than it is.
- The scan is still useful — The nuclear medicine team can adjust their interpretation — but they need to know you are on steroids before the scan begins.
- Do not stop steroids on your own — Holding a dose before a scan without your oncologist's instruction can cause serious harm, especially in brain tumour patients.
- Your oncologist decides — Whether to pause steroids before a scan depends on your diagnosis, dose, and clinical situation — not general advice.
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Yes, steroids reduce FDG uptake in inflamed and cancerous tissue, which can make active lymphoma appear quieter than it is. The scan is not useless, but the nuclear medicine team must know you are on steroids. Whether to hold them before the scan is your oncologist's decision — never something to decide on your own.
At CION, a whole-body PET-CT starts from Rs 10,499 — among the lowest published prices in Hyderabad. Indicative price, as of September 2026.
How does steroid use change what the scan shows?
| What the scan measures | While on steroids | After steroids paused (where oncologist approves) |
|---|---|---|
| FDG uptake in tumour tissue | May be lower than true disease activity — active lymphoma can look quiet | More likely to reflect actual disease activity |
| Bone marrow signal | May appear suppressed, making spread into marrow harder to detect | Marrow pattern more likely to be readable |
| Effect of blood sugar | Steroids can raise blood sugar, which competes with the FDG tracer and reduces overall scan quality | Less interference from steroid-induced high blood sugar |
| Risk of a misleading result | Higher — tumour and inflammatory uptake can both be dampened | Lower, though no scan is perfect |
| When this applies | When stopping steroids is not clinically safe — most common in brain tumour patients | When a brief pause is safe — your oncologist decides |
What to do before your PET-CT if you are on steroids
- Tell the nuclear medicine team you are on steroids before the scan starts — not after.
- Bring your prescription or medicine strip to the appointment.
- Tell them the dose and how long you have been taking it.
- Tell them what the steroids are for — lymphoma treatment, brain swelling, or another reason all change how the result is read.
- Ask your oncologist before the appointment whether you should hold the dose. Never stop steroids on your own.
- Tell the team if your dose has changed recently, especially if it was reduced in the days before the scan.
Do steroids actually change what the scan picks up?
Yes. PET-CT works by detecting how much glucose cells are consuming. Cancerous tissue and inflamed tissue both use glucose faster than normal tissue — that is what makes them light up on the scan.
Corticosteroids — dexamethasone and prednisolone are the most common in cancer care — reduce this uptake by suppressing inflammation. Active lymphoma nodes can appear less bright than they really are.
Steroids also raise blood sugar in many patients. Higher blood sugar means glucose and the FDG tracer are competing for the same uptake, which reduces signal across the whole scan.
The scan is not useless. The nuclear medicine team can factor in your steroid use — but only if you tell them before the scan begins.
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Who decides whether to hold your steroids before the scan?
Your treating oncologist makes this decision — not the scan team, and not you.
For brain tumour patients on dexamethasone, stopping even briefly can cause rapid worsening of swelling and neurological symptoms. In most cases the steroids continue, and the scan is interpreted knowing that.
For lymphoma patients, the timing of the scan relative to your treatment cycle usually matters more than the steroid use on its own. Your oncologist sets that schedule deliberately.
If you are unsure, call your oncologist's team before the scan day. Arriving without having asked is more disruptive than calling early.
Did you know?
Corticosteroids can suppress FDG uptake in lymph nodes to a degree that residual active lymphoma no longer meets the standard threshold for a positive result on PET scoring — a recognised limitation in nuclear medicine guidance on lymphoma imaging.
This is why the scan result is always read alongside the full clinical picture, not in isolation.
Source: EANM/SNMMI guidelines on FDG-PET in lymphoma response assessment
What if your situation is more complicated?
I am on dexamethasone for a brain tumour. Can I have a PET-CT at all?
Yes, PET-CT is used in brain tumour management alongside MRI. If you are on dexamethasone, the scan can go ahead, but the nuclear medicine team needs to know the dose before it starts. Stopping dexamethasone even briefly can worsen swelling and neurological symptoms — your oncologist will almost always advise against holding it. The result will be interpreted knowing steroids were on board.
My steroids are part of my lymphoma treatment, such as R-CHOP. Does the scan team still need to know?
Yes. Steroids are part of many lymphoma regimens, and the nuclear medicine team needs to know your treatment history and when your last cycle was. Scan timing relative to your cycle is usually set deliberately — scanning too close to a steroid-containing cycle makes results harder to interpret. Do not rearrange the timing yourself; call your oncology team if you are unsure when the scan should happen.
Can I skip today's steroid dose to make the scan more accurate?
No — not without your oncologist's explicit instruction. Missing one dose without guidance can cause rebound inflammation, adrenal suppression in long-term users, or rapid neurological worsening in brain tumour patients. The scan team cannot advise you to hold steroids. If you think skipping a dose might help, call your oncologist's team the day before the scan and let them decide.
Will the scan report say that steroids may have affected the result?
It should. A complete nuclear medicine report notes medications known to affect FDG uptake, including corticosteroids, and accounts for them in the interpretation. This is why giving an accurate medication history before the scan matters — without it, the report cannot account for the steroid effect. Ask your oncologist to walk you through the report if you want to understand how it was factored in.
My blood sugar was high on the scan day because of my steroids. Does that ruin the scan?
High blood sugar at the time of FDG injection reduces scan quality, because glucose and the tracer compete for the same uptake pathway. The team checks your blood sugar before injecting the tracer; if it is above the level they consider reliable for a good result, they may reschedule. Ask your oncologist before the scan day whether your blood sugar needs managing ahead of the appointment.
Book a PET-CT at CION from Rs 10,499 — among the lowest published prices in Hyderabad — with an oncologist-reviewed report and a free Rs 950 consultation, across 4 partner centres. Indicative price, as of September 2026.
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Common questions
Frequently asked questions
Does being on steroids mean my PET-CT result will be wrong?
Not necessarily wrong, but harder to interpret. Steroids reduce FDG uptake in tumour and inflamed tissue, which can make active disease appear less active than it is. The nuclear medicine team can account for this — but only if they know you are on steroids and at what dose. If your oncologist believes the scan will be uninterpretable, they may reschedule or add a different imaging tool rather than proceed.
How long before the scan do steroids need to be stopped to improve the result?
There is no single timing that applies to everyone, and this is your oncologist's clinical decision — not something to work out from a general page. The effect depends on the steroid type, dose, and how long you have been on it. If a pause is clinically safe for you, your oncologist will say how many days to hold before the scan. Never choose a timing on your own.
I take low-dose prednisolone for arthritis or asthma. Do I still need to mention it?
Yes. Low-dose steroids taken for non-cancer reasons can still affect FDG uptake, though the effect is usually smaller than with cancer-dose steroids. Tell the scan team about all steroids you are taking, including those prescribed for an unrelated condition. Do not stop them before the scan without asking the prescribing doctor.
Can the scan team tell from the images alone that I am on steroids?
Sometimes. Patterns such as suppressed bone marrow signal or less-active lymph nodes than expected can suggest steroid use. But the team cannot determine your dose or timing from the image alone. This is why giving an accurate medication history before the scan matters — a result read without that information may not fully account for the steroid effect.
Should I ask for my PET-CT to be postponed until I am off steroids?
That is your oncologist's decision. For some patients, a brief pause before the scan is safe and produces a more reliable result. For others — particularly those on steroids for brain swelling — pausing is not safe. And for some, the scan timing relative to treatment matters more than the steroid situation. Ask your oncologist what the scan is for and whether your steroid use is likely to affect how the result is used.
What is different about a PET-CT on steroids for lymphoma versus a brain tumour?
For lymphoma, the main concern is that steroids suppress activity in the lymph nodes, making residual disease harder to detect. Scan timing relative to treatment cycles is usually managed deliberately. For brain tumours, steroids cannot usually be held, and PET findings are read alongside MRI rather than alone. In both cases, the nuclear medicine team needs to know what you are on and why — the interpretation differs by diagnosis.