Microdosimetric applications in proton and heavy ion therapy using silicon microdosimeters

Using the CMRP 'bridge' μ⁺ probe, microdosimetric measurements were undertaken out-of-field using a therapeutic scanning proton pencil beam and in-field using a ¹²C ion therapy field. These measurements were undertaken at Mayo Clinic, Rochester, USA and at HIMAC, Chiba, Japan, respectively. For a typical proton field used in the treatment of deep-seated tumors, we observed dose-equivalent values ranging from 0.62 to 0.99 mSv/Gy at locations downstream of the distal edge. Lateral measurements at depths close to the entrance and along the SOBP plateau were found to reach maximum values of 3.1 mSv/Gy and 5.3 mSv/Gy at 10 mm from the field edge, respectively, and decreased to ∼0.04 mSv/Gy 120 mm from the field edge. The ability to measure the dose-equivalent with high spatial resolution is particularly relevant to healthy tissue dose calculations in hadron therapy treatments. We have also shown qualitatively and quantitatively the effects critical organ motion would have in treatment using microdosimetric spectra. Large differences in spectra and RBE₁₀ were observed for treatments where miscalculations of ¹²C ion range would result in critical structures being irradiated, showing the importance of motion management. (authors)