Targeting just one chemical inside cancerous cells could one day lead to a single test for a broad range of cancers, researchers say.
The same system could then be used to deliver precision radiotherapy.
Scientists told the National Cancer Research Institute conference they had been able to find breast cancer in mice weeks before a lump had been detected.
The same target chemical was also present in cancers of the lung, skin, kidney and bladder, they said.
The team, at the Gray Institute for Radiation Oncology and Biology at Oxford University, were looking for a protein, called gamma-H2AX, which is produced in response to damaged DNA. This tends to be one of the first steps on the road to a cell becoming cancerous.
The scientists used an antibody that is the perfect partner to gamma-H2AX and able to seek it out in the body.
This was turned into a cancer test by attaching small amounts of radioactive material to the antibody. If the radiation gathered in one place it would be a sign of a potential tumour.
The researchers trialled the test on genetically modified mice, which are highly susceptible to forming tumours.
Prof Katherine Vallis said lumps could be felt when the mice were about 120 days old, but "we detected changes prior to that at 90 to 100 days - before a tumour is clinically apparent".
She told the BBC that gamma-H2AX was a "fairly general phenomenon" and it "would be the dream" to develop a single test for a wide range of cancers.
However this is still at a very early stage.
Adding more radiation to the antibody could convert the test to a treatment.
Prof Vallis said "it is attracted to DNA damage", where it then delivers a dose of radiation, causing more damage and attracting even more antibodies - it is a "self-amplifying system".
Eventually the doses of radiation should do so much damage to the cancerous cells that they would be killed.
She said: "This early research reveals that tracking this important molecule could allow us to detect DNA damage throughout the body.
"If larger studies confirm this, the protein could provide a new route to detect cancer at its very earliest stage - when it is easier to treat successfully."
Dr Julie Sharp, from Cancer Research UK, said: "This important study reveals that targeting this key molecule could provide an exciting route for new ways to detect cancer at an earlier stage - and help to deliver radiotherapy and monitor its effect on tumours."