Dosimetry intercomparison of four proton therapy institutions in Germany employing spot scanning

Abstract Aim To verify the consistency of dose and range measurement in an interinstitution comparison among proton therapy institutions in Germany which use the pencil-beam scanning technique. Methods Following a peer-to-peer approach absorbed dose and range have been intercompared in several missions at two hosting centers with two or three visiting physics teams of participating institutions using their own dosimetry equipment. A meta-analysis has been performed integrating the results of the individual missions. Dose has been determined with ionization chambers according to the dosimetry protocol IAEA TRS-398. For determination of the depth of the distal 80% dose the teams used either a scanning water phantom, a variable water column or a multi-layer ionization chamber. Results The systematic deviation between measured doses of the participating institutions is less than 1%. Ranges differ systematically less than 0.4 mm. Conclusions The match of measured dose and range is better than expected from the respective uncertainties. As all physics teams agree on the assessment of absorbed dose and range, an important prerequisite for a start of joint clinical studies is fulfilled.