LONDON (Reuters) - Proton-beam therapy may need the world's most expensive medical machines but barriers to its use are falling fast as doctors test the super precise form of radiation in the treatment of a widening range of cancers, according to the leader in the field.

Belgium's Ion Beam Applications (IBA), which has a market share of around 50 percent, has a growing pipeline of potential proton therapy business worth more than 1 billion euros ($1.1 billion), its finance chief Jean-Marc Bothy said.

"We don't see any exhaustion of the pipeline at all. It's very promising how new opportunities are developing, while existing ones continue," he told Reuters during a visit to London.

IBA, which raised its revenue forecast for 2015 last month, has been a star of the European life sciences sector recently, with its shares up 95 percent so far this year.

The strong outlook has been driven by growing interest among cancer doctors and the introduction of more compact machines with a single treatment room, which are cheaper than traditional very large multi-room installations.

Bothy said there were now 122 proton therapy clinical trials in progress targeting more than a dozen different cancers.

"Not only are the number of clinical studies into proton therapy booming but they are no longer limited to brain, ocular, pediatric and prostate cancers," he said.

These days the technology is also being tested in liver, lung, gastric and pancreatic cancers, for example. Specialists are also investigating its use to treat left breast cancer, in order to minimize damage to the heart.

The process requires a beam of protons accelerated to two-thirds the speed of light. Since protons cause little damage to cells they pass through but are very good at killing tumors at the end of their path, damage to surrounding tissue is limited.

That makes them well suited to treating cancers in parts of the body where there is little room for error. But there has been a debate as to whether proton therapy is worth using in more common cancers, without more evidence.

Proton therapy hit the headlines in Britain last year when five-year-old Ashya King was removed from hospital by his parents, against the advice of doctors, and flown to Prague for proton treatment using an IBA machine. The family say he is now free of cancer.

In the United States, proton therapy costs about $1,100 per treatment session, or fraction, nearly double regular intensity-modulated radiation. Patients may receive 30 or more fractions.

Although half of all cancer patients receive radiation as part of their treatment, only 1 percent get proton beams. In future, IBA believes that could rise to 15 to 20 percent.

IBA faces fierce competition in the growing market from rivals such as Varian Medical Systems and Hitachi.

Bothy said the Belgian group would fight to keep market share but wouldn't sell at any price, noting that the cost of its Proteus One single-room system had already come down to around $20 million from $25 million at launch.

Currently, proton therapy is most common in North America and Europe but Bothy said there were big untapped opportunities in China, Japan and Latin America.

($1 = 0.8924 euros)