Theranostics, explained: seeing the cancer and then treating exactly what you saw

"Theranostics" is an ugly word for an elegant idea. It is a compression of therapy and diagnostics, and it describes treatment where the test and the treatment use the same molecule.

If you have been told about Lu-177 PSMA therapy or PRRT and are trying to work out what it is, start here.

The idea

Imagine a molecule that fits a particular protein on a cancer cell the way a key fits a lock, and ignores everything else in the body.

Attach a camera tag to that key, inject it, and the scanner shows you every place in the body where those cancer cells are. That is a diagnostic scan — PSMA PET for prostate cancer, DOTA-TATE PET for neuroendocrine tumours.

Now take the same key and attach a therapy tag instead — an isotope that emits short-range radiation as it decays. Inject it, and the key carries that radiation directly to the same cells it found before. The radiation travels only a fraction of a millimetre, so it damages the tumour cell and largely spares whatever is around it.

Same key. Different payload.

Why it is different from what came before

Conventional radiotherapy aims a beam from outside at a target you can point to. It is extremely effective for disease in one place, and it cannot help with disease scattered through the skeleton.

Chemotherapy circulates everywhere and affects every fast-dividing cell in the body, including hair follicles, bone marrow and gut lining. That is where the side effects come from.

Theranostics circulates everywhere but only accumulates where the target protein is. It reaches disease anywhere in the body, and it concentrates the dose at the cancer rather than distributing it evenly through healthy tissue.

And crucially, you can see in advance whether it will work. The diagnostic scan tells you whether that patient's tumour actually expresses the target. If the disease does not light up, the therapy will not reach it — and you know that before treatment starts rather than after three cycles.

Very little else in oncology offers that.

The three established treatments

Lu-177 PSMA for advanced prostate cancer. For men whose cancer has spread and has stopped responding to hormone therapy. A PSMA PET/CT confirms that the disease takes up the molecule; treatment is then given as an injection every six to eight weeks, usually for four to six cycles. Trial evidence has shown improved survival and, notably, better quality of life than the comparator treatments — largely because side effects are modest.

Lu-177 DOTA-TATE (PRRT) for neuroendocrine tumours. For NETs that express somatostatin receptors, confirmed on a DOTA-TATE PET/CT. Given as four cycles roughly two months apart. It has become a standard option for progressive, well-differentiated NETs, and often controls disease for years.

I-131 for thyroid cancer. The oldest of the three by decades — used since the 1940s, long before anyone coined the word theranostics. Thyroid cells absorb iodine; radioactive iodine given by mouth is taken up by remaining thyroid tissue and by metastases, and destroys them. It remains one of the most effective cancer treatments in existence.

What treatment is actually like

More ordinary than most people fear.

For Lu-177 therapies it is an intravenous infusion over about half an hour. You stay in a shielded room for a day or two — a legal and safety requirement, because you are temporarily radioactive — with your own bathroom, a phone and a television. Relatives can usually visit for limited periods at a distance.

You go home afterwards with instructions about distancing from children and pregnant women for a short period. Most people continue their normal routine.

Common side effects are mild nausea for a day or two, tiredness, and in Lu-177 PSMA a dry mouth, because salivary glands also express PSMA and take up some of the dose. Blood counts are monitored, since bone marrow receives some radiation. Serious complications are uncommon but real, and your treating team will go through them with you.

Who it is not for

Honesty matters more than enthusiasm here.

Theranostics is generally used when standard treatments have been tried, or where the disease type makes it the appropriate next step. It is not a first-line replacement for surgery or chemotherapy in early disease.

It requires the target to be present. Some prostate cancers, particularly those that have become neuroendocrine-differentiated, express little PSMA. Some neuroendocrine tumours lose their somatostatin receptors as they dedifferentiate. In both cases the scan will show it, and the honest conclusion is that this treatment is not the right one.

It also requires adequate kidney function and adequate bone marrow reserve.

What we are building

Prismaa currently provides the diagnostic half: gallium-68 PSMA, DOTA-TATE/TOC and FAPI PET/CT on a digital scanner. Patients who go on to therapy have been travelling elsewhere for it.

We are commissioning a dual-head gamma camera and a shielded radionuclide therapy suite so that both halves can happen in the same place — scan, confirm the target, and treat, without a journey to another city between each step.

We are not giving a date, because commissioning a therapy facility involves regulatory approvals that do not run to a marketing timetable. If you have a diagnosis for which this is relevant, tell us and we will contact you when the service opens.

Questions to ask your oncologist

1. Does my cancer type have a theranostic option?
2. Have I had the diagnostic scan that shows whether my disease expresses the target?
3. Where does this sit in the sequence relative to my other options?
4. What would we do if it does not work, and how soon would we know?

Call us on +91 93169 25516 if you would like the diagnostic side explained before you see your oncologist — it is much easier to have that conversation already knowing what the scan is for.