The number of new cancer patients in the developing world reach 10 million per year by 2015 - and most will have no access to lifesaving treatment, say experts.
Radiation machines are scarce
The International Atomic Energy Agency (IAEA) says many people in poorer countries are denied access to treatment which could save or prolong their lives, and decrease their pain and suffering.
About 85% of the world's people live in developing countries - but these countries house only about one third of the world's radiotherapy facilities.
Mohamed ElBaradei, IAEA director general, said: "A silent crisis in cancer treatment exists in developing countries and is intensifying every year.
"At least 50% to 60% of cancer victims in the developing world can benefit from radiotherapy that destroys cancerous tumours, but most developing countries do not have enough radiotherapy machines or sufficient numbers of specialized doctors and other health professionals."
Some 15 African nations and several countries in Asia lack even one radiation therapy machine.
Ethiopia, which has 60 million people, possesses just one such machine, while other developing countries have one machine for several million people.
In contrast, the most developed countries have one machine for every 250,000 people.
'Crisis can be managed'
The IAEA estimates it would cost $2.4bn over the next 15 years to purchase sufficient machines, physicians and physicists to run them, and pay for the upkeep of the machines.
Dr Bhadrasain Vikram, an expert in radiation therapy working for the IAEA, said: "The growing cancer crisis in the developing world can be traced to people living longer, changing lifestyles, unhygienic living conditions and other important factors.
"This crisis is predictable and, to some degree preventable, depending on how well we start to manage it now."
The IAEA says that because of the types of cancer more prevalent in the developing world - such as cervical cancer - and because the cancers are often diagnosed at later stages, the vast majority should be treated by radiation.
Even for advanced cases where curative therapy may not always work, radiation is able to provide substantial pain relief.