What an interim scan is and why it matters

An interim scan is a scan done partway through a planned course of chemotherapy, rather than only at the start and the end. Its purpose is to check how the cancer is responding while there is still time to adjust the plan. The term is used most often for lymphoma, where an interim PET-CT scan has become an important part of treatment. PET-CT combines a CT scan, which shows the size and position of tissues, with a PET scan, which uses a small amount of radioactive sugar to show how active cells are. Active lymphoma cells take up more sugar and light up on the scan. After a few cycles, a scan showing that activity has settled suggests good early response; one showing persistent activity may lead the team to consider changes.

In Hodgkin lymphoma, the interim PET-CT is used in several treatment approaches to guide decisions. For some people with good early response, treatment may be de-escalated, for example by removing a medicine that has more long-term side effects or avoiding radiotherapy. For others, treatment may be intensified. In some other lymphomas, interim scans are also used, although their role varies. In solid cancers such as breast, lung, bowel or ovarian cancer, the equivalent is usually a CT scan after a set number of cycles, which checks whether tumours are shrinking, stable or growing, and guides whether to continue, change or move on to surgery or radiotherapy.

An interim scan is not a final verdict. Its findings are one part of the picture, alongside examination, blood tests and how you are coping. Some changes on interim scans are caused by inflammation, infection or healing rather than cancer, and may need careful review or further tests. Your oncologist interprets the scan in the context of your cancer and plan, and explains what, if anything, it changes. This page explains the purpose of interim scans; it cannot interpret your own scan.

Timing is set by the plan

Interim scans are scheduled at specific points so results can be compared with standard approaches.

Preparation matters for PET-CT

Fasting and sugar control affect PET-CT accuracy. Follow instructions carefully.

Results are discussed with your oncologist

The scan centre sends the report to your team, who explain it at your appointment.

It cannot interpret your interim scan or tell you what your result means. Your oncologist explains findings in the context of your cancer, treatment and other tests.

It also cannot tell you whether an interim scan is part of your plan. Not every cancer or treatment uses one.

Why PET-CT findings can be complicated

PET-CT shows activity, and not all activity is cancer. Inflammation, infection, recent injections that support blood counts, healing bone and even normal organs such as the bowel can light up. Radiologists and oncologists use standard scoring systems and clinical judgement to decide what a finding means. Sometimes further tests are needed before any change is made.

Preparing for a PET-CT

You will usually need to fast for some hours, avoid strenuous exercise the day before, and keep warm. People with diabetes need specific advice about medicines and food, because high sugar levels can affect the scan. Tell the scan team if you are pregnant or breastfeeding.

Radiation from interim scans

PET-CT and CT involve radiation. For people having cancer treatment, the benefit of guiding decisions usually outweighs the small added risk. Ask your team if you have concerns.

Cost of interim scans

PET-CT is more expensive than CT. Ask whether insurance or schemes cover it before booking.

Waiting for results

The wait after an interim scan can be stressful. Ask when results will be discussed so you know what to expect.

The Deauville score

PET-CT reports in lymphoma often include a score from a standard five-point scale, known as the Deauville score, which compares activity in lymphoma areas with activity in normal tissues such as the liver. Lower scores generally suggest less active disease. The score is one part of your oncologist's assessment and should not be read on its own.

How long results take

PET-CT reports are usually ready within a few days, and interim results are often discussed before the next planned cycle, so decisions about continuing or changing treatment can be made on time.

If the interim scan is unclear

Sometimes an area remains active but may be inflammation. Your team may recommend continuing treatment and repeating the scan later, doing another type of scan, or taking a small tissue sample to check before changing plans.

Interim scans in clinical trials

Many advances in interim scanning come from clinical trials. If you are in a trial, the scan schedule and the decisions based on it follow the trial plan, which your team will explain.

Your part in accurate scans

Arriving on time, following fasting and exercise instructions, keeping warm and lying still help produce clear, reliable images. Tell staff if you are in pain or anxious, so they can help you stay comfortable.

Talking to family about interim results

An interim result is a progress check, not the final outcome. Explaining this to family can help avoid over-reacting to either good or disappointing news.

What to do next

Ask whether an interim scan is planned and when, follow preparation instructions carefully, tell the scan team about diabetes or recent injections, attend your results appointment, and ask what the result means for your plan.