Starting cancer treatment is a big step. Whether it’s chemotherapy, radiation, immunotherapy, or surgery, you want to know that it’s doing what it’s supposed to do. But how do doctors know if the cancer is shrinking, responding, or staying the same?

That’s where PET-CT comes in. It doesn’t just give a snapshot of what cancer looks like; it shows how your body is responding to treatment, sometimes even before symptoms change or physical tumours shrink.

This kind of live diagnostic insight is one of the biggest reasons PET-CT has become a critical tool in cancer care. It allows your care team to monitor your progress early, adjust your treatment plan if needed, and avoid unnecessary therapies that may not be working.

Let’s unpack how PET-CT helps doctors and patients make more confident, informed decisions throughout treatment.

What is PET-CT again? A quick refresher

PET-CT stands for Positron Emission Tomography – Computed Tomography. It combines two powerful types of scans into one:

• CT shows detailed images of the structures inside your body, like organs and tissues.
• PET uses a small amount of radioactive glucose to highlight areas where cells are especially active, often where cancer is growing or spreading.

This combination allows doctors to see not just where a tumour is, but also how it’s behaving and whether it’s responding to treatment.

How does PET-CT show progress?

Cancer cells typically consume more energy (glucose) than normal cells. The PET part of the scan lights up these “hotspots”, allowing doctors to measure the cancer’s activity, not just its size.

As treatment starts to work, the cancer becomes less active. On the scan, this looks like less glucose uptake, meaning those glowing areas start to dim. In some cases, this happens even before the tumour begins to shrink physically.

Why real-time feedback is so important

• You can avoid treatments that aren’t helping: Sometimes, a patient may begin chemotherapy only to find that the cancer isn’t responding. PET-CT helps identify this early so that treatment can be changed before more time is lost or side effects accumulate.
  Example: A patient with lymphoma may have a PET-CT scan after two or three cycles of chemotherapy. If the scan shows little to no response, doctors can adjust the drug combination or try a different approach, all without waiting for physical symptoms to return.
• You may be able to shorten your treatment: On the flip side, if a scan shows excellent response, your doctor may be able to reduce the length of treatment, meaning fewer side effects and quicker recovery.
  Example: In breast cancer, if a PET-CT shows that the tumour is no longer metabolically active, it might mean you’ve already gotten the maximum benefit from your current therapy.
• It helps personalise your care: Every cancer – and every person – responds differently. PET-CT ensures that treatment decisions aren’t one-size-fits-all. Instead, they’re based on your unique response at present.

When is PET-CT used to monitor progress?

Not all patients need PET-CT during treatment, but it plays a vital role in guiding care in certain cancers. Some of the most common cancers where PET-CT is used for response assessment include:

• Lymphoma – especially useful after a few rounds of chemotherapy to evaluate early response.
• Lung cancer – helps determine if chemotherapy or radiation is shrinking the tumour.
• Breast cancer – used in advanced or metastatic cases to evaluate how the cancer is reacting.
• Colorectal cancer – monitors spread to liver or lungs and response to systemic therapies.
• Melanoma – assesses response to immunotherapy, which can behave differently from chemo.
• Cervical and gynaecological cancers – helps track how well radiation or chemotherapy is targeting tumour activity.

Beyond treatment: spotting recurrence early

Even after treatment ends, PET-CT continues to play a valuable role. If a patient starts showing symptoms or if blood markers rise, a PET-CT can help detect whether cancer has returned, often before it’s visible on other scans.

This is especially helpful in:

• Colorectal cancer – when tumour markers like CEA (Carcinoembryonic Antigen) rise.
• Lymphoma – when follow-up scans are unclear.
• Melanoma and lung cancer – which may spread silently before symptoms appear.

Early detection of recurrence means earlier intervention and, often, better outcomes.

What does the scan actually look for?

During treatment, your care team is looking for a few key changes in your PET-CT scan:

• Less glucose uptake – which means the cancer is less active.
• No new areas lighting up – a good sign that the cancer hasn’t spread.
• A complete metabolic response – when the scan shows no signs of active cancer.

If the tumour continues to “light up” or new areas become active, your doctors will talk you through what that means and what can be done next.

Does a good scan mean the cancer is gone?

Not always, but it’s a strong indicator. PET-CT scans are one part of a bigger picture. Your doctor will consider your scan results, along with blood tests, physical exams, and how you’re feeling overall.

Sometimes, a scan may show residual activity even after treatment, but it may be inflammation or scar tissue rather than active cancer. Your care team will guide you through what it means in your particular case.

What if the results are unclear or unexpected?

That can happen. PET-CT is very sensitive and sometimes picks up things that aren’t cancerous. Inflammation, infection, or healing tissue can all cause areas to light up. Nuclear technicians are highly trained to distinguish between what is and isn’t cancer.

If your scan is unclear:

• Your team may recommend another scan after some time.
• They might order a biopsy for confirmation.
• Or they may simply continue monitoring, depending on your overall health.

The key is that PET-CT gives your team more insight and options, helping avoid unnecessary or premature treatment changes.

Your care, in real-time

Cancer treatment can be overwhelming, and waiting to find out if it’s working can be one of the most complex parts. PET-CT gives you and your care team answers sooner. It brings peace of mind when things are going well, and a chance to pivot early if they’re not.

At Life Healthcare, we believe that access to this kind of insight is essential to delivering patient-led, precision and comprehensive cancer care. That’s why we continue to invest in advanced nuclear imaging like PET-CT, and work closely with your oncology team to ensure every decision is guided by the most accurate information possible.

To learn more about PET-CT and how it fits into your treatment journey, visit our Health Library or speak to your Life Healthcare team.