The 62 MeV proton beam for the treatment of ocular melanoma at Clatterbridge

A second treatment room and beam line has been constructed at the Cyclotron Unit at Clatterbridge for the purpose of using 62 MeV protons for the treatment of ocular melanoma. A uniform beam is produced by a double foil scattering system. The initial Bragg peak is spread across the target volume by the use of beam modulators. These are rotating four-vaned stepped absorbers made from Perspex. Two beam lines can be configured with different positions of modulators and range limiters. The first has a maximum penetration of 31.9 +/- 0.2 mm in water and the second a penetration of 31.2 +/- 0.2 mm. The second configuration has the advantage of less variation in beam penumbra, with a typical value of 1.7 +/- 0.1 mm for the 90% to 10% decrement lines. The patients are treated with individually shaped collimators. Beam output varies by less than 2% over the range of collimator areas used. The resulting whole-body dose equivalent to patient has also been assessed. In the first three years of operation over 250 patients have been treated.