Cancers should no longer be categorised by where they are first formed in the body, but instead by similarities in tumour types, researchers say.
A US-led study of 33 cancer types from more than 10,000 patients found they could be reclassified into 28 clusters that shared similar molecules.
Reclassification would ultimately lead to better, more targeted treatments, said the researchers.
"It's time to rewrite the textbooks on cancer," one of the authors said.
'Break down silos'
Cancers have traditionally been classed and treated according to the part of the body from which they originate, such as the breasts or lungs.
But for this new study, the Pan-Cancer Atlas, scientists carried out analysis of 23 of the most common tumour types, and also 10 rare ones, to see which similarities and differences they had on both a genetic and cellular level.
They found that nearly two-thirds of cancer clusters that shared key molecular similarities were found in more than one area of the body.
One tumour type was found in 25 parts of the body, meaning it would traditionally be treated differently depending on which area it was found in, the study found.
They also found that common cancers, such as in the lung and breast, can be broken down into more than one cancer.
Prof Christopher Benz, from the Buck Institute for Research on Aging in California, told the BBC this was significant because it meant patients would not always be receiving the most effective treatment.
For example, in some cases medications for one type of cancer - such as of the bladder - should be used to treat another, like lung cancer, he said.
"Patients will have the best shot at successful treatment if their tumours can first be classified according to their genomic and molecular makeup," said Prof Benz.
Immunotherapy - where the body's immune system is enlisted to help fight cancer - is one treatment that could potentially be rolled out more widely, Prof Benz said.
Drugs used for the treatment of other conditions, such as rheumatoid arthritis, could also ultimately be repurposed to fight cancer.
In other cases, cancers that are harder to treat might require a combination of medications.
Lung and colorectal cancers are among those that would often benefit from more targeted treatment, Prof Benz said.
But he explained that it could take up to a decade for new treatments to become available because of the difficulty in getting drugs approved.
He called for oncology departments, which tend to focus on specific areas of the body, and drugs companies to work together better to improve care for patients.
"It's time to rewrite the textbooks on cancer, and it's time to break down the silos in clinical oncology that make it difficult for patients to take advantage of this paradigm shift in cancer classification," Prof Benz said.
Dr Justine Alford, from Cancer Research UK, said: "By revealing the molecular groups that cancers tend to fall into, this research opens up new possibilities for patients who would traditionally be treated based on where in the body their cancer is.
"Identifying patients most likely to benefit from a particular treatment could also help improve clinical trials.
"The real test now will be to put this knowledge into practice and find out if this way of treating patients helps save more lives," she added.
The research was published in Cell.