How Doctors Know If — Your Treatment Is Working

Targeted therapies work differently from chemotherapy. Chemotherapy is designed to shrink tumours, so success is measured in how much they reduce. Many targeted therapies are designed to stop cancer from growing rather than eliminate it immediately. A sustained stable disease result means the drug is preventing progression, which is often the goal. Your oncologist will explain what they expect from your specific treatment so you know what a good outcome looks like for you.

Scan timing is usually every two to three months, though your team may vary this based on your cancer type, treatment, and clinical picture. Scanning more frequently does not produce earlier decisions — tumours need time to show measurable change. Radiation exposure and the anxiety of more frequent waiting are also real factors your oncologist weighs. If you feel uncertain between scans, your symptoms and any blood markers your team follows are usually discussed at every visit, so contact your team if something concerns you.

This is called a mixed response, and it is more common than people realise. One tumour may shrink while another stays stable or even grows. It happens because tumours in different locations can have slightly different molecular characteristics, even within the same person. Your oncologist will assess the overall picture and may consider whether a biopsy of a non-responding lesion would change the plan. A mixed response does not automatically mean treatment has failed — ask your team what it means for your specific situation.

A rising marker between scans is worth telling your team about, but it does not automatically confirm progression. Markers can fluctuate for reasons unrelated to cancer growth, including infection or inflammation. Your oncologist will decide whether the rise is enough to bring your scan forward or whether to monitor it to the scheduled date. They will not change treatment on a marker result alone — a scan is needed to confirm whether there has been measurable change in the tumour itself.

All imaging has limits. CT and MRI detect tumours above a certain size; very small clusters of cells cannot be seen on a scan. This is why treatment often continues even after a complete response is recorded on imaging — undetectable disease may still be present. Your team uses the full clinical picture alongside the scan: how you feel, your blood markers, and your pattern of response over time. A complete response on imaging is a meaningful result; it is also the beginning of careful monitoring, not the end of it.

When progressive disease is confirmed on imaging, your oncologist will review the options. This may mean switching to a different targeted therapy if your tumour has a different actionable alteration, moving to chemotherapy, adding a second drug, or exploring a clinical trial. In some cancers, a repeat biopsy is recommended at progression to see whether the molecular profile has changed — this can open options that were not available at diagnosis. Progression on one treatment does not mean there are no further options, and that conversation with your oncologist is the right next step.