PET-CT When You Are — Taking Long-Term Steroids

PET-CT relies on FDG being absorbed by metabolically active cells. Corticosteroids reduce glucose metabolism in lymphoid tissue and can cross the blood-brain barrier to affect uptake in brain lesions. The tumour is still there, but it absorbs less FDG and appears less bright. In some cases an active disease site can look completely normal on the scan.

It means the result needs careful interpretation rather than a simple positive or negative read. The nuclear medicine physician writes the report in the context of your clinical history, including your steroid use. For most solid tumours the effect is modest. For lymphoma and brain lesions it can be significant, and a good report will say so explicitly rather than presenting the result as though steroids were not a factor.

Only if your oncologist decides that is safe and appropriate for your situation. Stopping corticosteroids suddenly — even briefly before a scan — can cause adrenal insufficiency or trigger a flare of the condition they are controlling. Whether a pause is worth the risk depends on your dose, how long you have been taking steroids, and what the scan is trying to answer. This is your referring doctor's decision, not the scanning centre's and not yours to make alone.

Lymphoma is the most well-documented. Steroids are directly cytotoxic to lymphoma cells and also independently suppress FDG uptake, so a staging or response-assessment scan done on steroids may underestimate disease extent. Brain tumours and brain metastases are also significantly affected because steroids reduce blood-brain barrier oedema, altering FDG distribution in brain tissue. For other solid tumours the effect is generally smaller, but still worth disclosing.

They note the steroid use in the report and adjust their interpretation accordingly. They may compare the current scan with previous imaging rather than reading it in isolation. Where uncertainty remains — for example, an area that looks quiet but could be suppressed rather than inactive — they will say so explicitly. This is why disclosing at booking matters: it gives the reporting team time to review prior scans and set the right interpretive frame before the images are produced.

In most cases, yes. Imaging services are familiar with reading PET-CT in patients on long-term steroids. The referring doctor may note that a repeat scan after steroid reduction would provide cleaner information, or may decide the current result gives enough to guide the next step. Whether to scan now or wait is a clinical judgement that depends on how urgently the result is needed and how much the steroid effect is expected to affect the specific question being asked.