To see the explosive rise of proton-beam therapy, an expensive and controversial cancer treatment, look to the billboards of Belgium.

Ion Beam Applications SA, the Belgian company that leads the global market for huge proton-beam machines, is selling so many systems lately that it needs to boost its 1,200-strong workforce by 400 workers. It launched a big recruitment drive across the country this year, featuring radio and newspaper spots along with dozens of billboards and posters.

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