The cost of treating cancer in the developed world is spiralling and is "heading towards a crisis", an international team of researchers says.
Their Lancet Oncology report says there is a "culture of excess" with insufficient evidence about the "value" of new treatments and technologies.
It says the number of cancer patients and the cost of treating each one is increasing.
It argues for reducing the use and analysing the cost of cancer services.
About 12 million people worldwide are diagnosed with cancer each year. That figure is expected to reach 27 million by 2030.
The cost of new cancer cases is already estimated to be about £185bn ($286bn) a year.
A group of 37 leading experts from around the world say the burden of cancer is growing and becoming a major financial issue.
Their report says most developed countries dedicate between 4% and 7% of their healthcare budgets to dealing with cancer.
"The issue that concerns economists and policymakers is not just the amount of money spent on healthcare, but also the rate of increase in healthcare spending or what has become known as the cost curve."
It says the UK's total spend on breast cancer has increased by about 10% in each of the past four years.
"In general, increases in the cost of healthcare are driven by innovation. We spend more because we can do more to help patients."
For example, the number of cancer drugs available in the UK has risen from 35 in the 1970s to nearly 100, but the report warns they can be "exceedingly expensive".
It adds: "Few treatments or tests are clear clinical winners, with many falling into the category of substantial cost for limited benefit."
The cost of drugs is not the only target for criticism.
Lead author Prof Richard Sullivan told the BBC: "It's not just pharmaceuticals. Biomarkers, imaging and surgery are all getting through with very low levels of evidence - the hurdles are set too low."
The report calls for a proper evaluation of the relative merits of conventional surgery and less invasive robotic surgery.
Another criticism is "overusing" treatments and technologies.
"It is often easier to order a scan than to reassure the patient or physician on the basis of a careful history and a physical examination," the report claims.
There is also criticism of "futile care" - providing expensive chemotherapy which gives no medical benefit in the last few weeks of a patient's life.
Prof Sullivan said: "We're on an unaffordable trajectory. We either need to manage and reduce the costs or the cost will increase and then inequality rises between rich and poor."
He said failure to manage costs could result in a "train crash".
The report says solutions fall into two categories: reducing the cost of services or reducing the number of people using them.