The spread of cancer and the effect of drugs to combat it could be closely monitored using tiny implants.
Getting drug doses right can be difficult
US researchers are perfecting a way to use microscopic particles which stick to chemicals in cancer cells and show up during scans.
Their latest advance is to find a way to keep a supply of the particles inside the body for longer periods.
However, the Massachusetts Institute of Technology (MIT) technique has not yet been tested in humans.
Working out the right dose of a cancer drug, or whether it is working properly, is difficult for doctors.
Conventional scans and blood tests often do not reveal how much of a drug is reaching the inside of a tumour.
The MIT project aims to provide doctors with more information about conditions inside clusters of cancer cells.
They are using microscopic nanoparticles made out of iron oxide and coated with a sugar called dextran.
The surface of these tiny particles is coated with antibodies, immune system components which can be designed to attach themselves to specific molecules found inside cancer cells.
When the target molecules are present, the particles clump together and can be spotted using conventional MRI scanning.
The novelty in this project is the tiny silicon container which holds the iron oxide nanoparticles - this allows the chemical reaction to take place without the nanoparticles being quickly dissipated out of the tumour.
The advantage of this is to allow doctors a longer-term and more detailed view of conditions within the cancer.
Grace Kim, who is involved in the project, said: "When you're cooking a turkey, you can take the temperature with a thermometer - but with something like this, you can not only find out the temperature, but the moisture, saltiness and whether there's enough rosemary."
The team is now preparing for further laboratory tests before the technique can be tried out on humans.
They are hopeful the particles can be made to clump together by human chorionic gonadotrophin, a hormone produced by some cancers but not normally by healthy people, unless they are pregnant.
Experts warned that the technology needs to develop further before it can be useful in patients.
Cancer Research UK medical director Professor John Toy said: "It would be extremely helpful to know at the start of treatment that anti-cancer drugs are indeed reaching their target or, if not, that other treatment is required.
"If it were possible to monitor intimately the response of cancer cells to treatment, this would offer valuable detailed information on the effectiveness of treatment.
"But, of course, there are millions of cancer cells in tumours and technology has a long, long way to travel yet before it can offer these desirable attributes."