What a Mixed Response to Treatment Means

A mixed response — often written as a dissociated response — means that different parts of the same cancer are doing different things at the same time. Two liver deposits have shrunk; a lung nodule has grown. A lymph node has almost disappeared; a bone lesion looks slightly larger.

It is a real and recognized pattern, not an ambiguity in the reading. And it is one of the harder results to be handed, because it refuses to be either good news or bad news.

Cancer is not a single uniform tissue. As it grows and spreads, different deposits accumulate different genetic changes, so what began as one disease becomes a collection of related but distinguishable populations. A drug that a mutation in one deposit is vulnerable to may leave another untouched.

The surroundings matter as well. Blood supply differs between sites, which affects how much drug arrives. Immune cell access differs. Some locations — the brain in particular — sit behind barriers that many drugs cross poorly. A site that was previously irradiated behaves differently from one that was not.

Formal response criteria were built for clinical trials, where consistency matters more than nuance. Under RECIST 1.1, up to five lesions are selected as targets, their longest diameters are added together, and the sum is tracked. Partial response requires that sum to fall by at least 30 percent; progressive disease requires it to rise by at least 20 percent with an absolute increase of at least 5 millimetres, or a new lesion to appear.

A genuinely mixed picture gets absorbed into whichever category the arithmetic produces. That is why what your radiologist describes in the body of the report may feel different from the single word in the conclusion, and why the conversation with your oncologist matters more than the label.

If you are on a checkpoint inhibitor, apparent growth carries an extra possibility. Immune cells flooding into a tumor make it physically larger on a scan while the treatment is working. Previously invisible deposits can become visible for the same reason. This is pseudoprogression, and it resolves on subsequent imaging.

It is uncommon — reported in roughly 1 to 9 percent of people on checkpoint inhibitors in most series — but the consequence of missing it is serious, because it means stopping a treatment that was succeeding.

This is why iRECIST exists. Under those rules, imaging that meets progression criteria is recorded as unconfirmed progressive disease, treatment continues if you are clinically well, and a confirming scan is performed at four to eight weeks. Only if the disease has grown further at that point is progression confirmed.

Dissociated responses on immunotherapy are similarly uncommon, reported in around 3 to 9 percent of patients — and, importantly, associated with better survival than genuine progression.

Beyond the images, several things go into the decision.

How you feel is central. New pain, weight loss, breathlessness or a decline in day-to-day function pushes toward true progression. Feeling well pushes the other way. Tumor markers and, increasingly, ctDNA add information — falling levels during apparent growth argue strongly for pseudoprogression. The pattern matters too: one growing site among many responding ones reads very differently from several growing at once.

Continue and reassess. Most common when you are well, most disease is responding, and the growth is small or possibly pseudoprogression.

Treat the growing site locally. When one or a few sites progress while the rest stays controlled — oligoprogression — radiation, surgery or ablation to those sites while continuing the same systemic therapy is a well-established approach that can extend the useful life of a treatment considerably.

Biopsy the discordant site. Useful when it might not be cancer at all, or when a resistance mutation would open a different drug.

Change systemic treatment. Reserved for when progression is confirmed, widespread, or accompanied by symptoms.

A mixed response is one of the situations where the scan alone cannot decide. It is worth asking your team directly which of these paths they are weighing, and what would tip them one way or the other.

• Frontiers in Oncology — Tumor response assessment on imaging following immunotherapy
• NCI Dictionary — pseudoprogression
• NCI Dictionary — mixed response
• NCI — Immune Checkpoint Inhibitors