Determination of water absorbed dose in a carbon ion beam using thimble ionization chambers

The method to measure absorbed dose to water in a field of carbon ions as a pplied for the heavy ion therapy project at the Heavy Ion Research Laborato ry in Darmstadt (GSI), Germany, is described in detail. Thimble ionization chambers with a water absorbed calibration factor are applied. The dose obt ained with this method was compared with that obtained at the heavy ion the rapy facility HIMAC at the National Institute of Radiological Sciences in C hiba, Japan, using the Japanese code of practice. The agreement found was b etter than 1%. The combined uncertainty of the determination of absorbed do se to water was estimated to amount to 5%.