Microdosimetric measurements in the high energy proton cancer therapy beams at Clatterbridge and Orsay

Microdosimetric measurements have been made in the 62MeV proton beam in Clatterbridge, UK, and in the 73MeV beam in Orsay, France. Both beams are currently used for the treatment of ocular melanoma (eye tumours). Measurements were made using a planar, wall-less proportional counter, designed specifically for use in proton beams. Measured lineal energy spectra indicated a shift to higher lineal energies with depth of beam penetration, with the most significant increases occurring near the end of each beams range. This trend was observed in both unmodulated and range modulated beams. The measured spectra were used to calculate mean quality factors, Q-bar, using the latest ICRP relationships between Q and LET∞. For the Clatterbridge and Orsay full-range modulated beams, Q-bar was 1.07±0.06 and 1.26±0.3, respectively, at beam entrance. These values increased to 5.33±0.4 and 4.42±0.4, respectively, at the end of the beams range, with the most significant increase in Q-bar occurring in the final 2-3mm of range. The effect the increases in Q-bar will have on dose distribution with depth of penetration of the proton beams will be discussed, as too the implications this will have regarding the planning of proton therapy