Journal Article Correction factors to convert microdosimetry measurements in silicon to tissue in 12C ion therapy

David, BOLST  ,  guatelli, susanna  ,  Thuy Linh, Tran  ,  Lachlan, Chartier  ,  Lerch, Michael  ,  松藤, 成弘  ,  Rosenfeld, Anatoly

62 ( 6 )  , pp.2055 - 2069 , 2017-03 , IOP Publishing
Silicon microdosimetry is a promising technology for heavy ion therapy (HIT)quality assurance, because of its sub-mm spatial resolution and capability todetermine radiation effects at a cellular level in a mixed radiation field. Adrawback of silicon is not being tissue-equivalent, thus the need to convert thedetector response obtained in silicon to tissue. This paper presents a methodfor converting silicon microdosimetric spectra to tissue for a therapeutic 12Cbeam, based on Monte Carlo simulations. The energy deposition spectra ina 10 μm sized silicon cylindrical sensitive volume (SV) were found to beequivalent to those measured in a tissue SV, with the same shape, but withdimensions scaled by a factor κ equal to 0.57 and 0.54 for muscle and water,respectively. A low energy correction factor was determined to account forthe enhanced response in silicon at low energy depositions, produced byelectrons. The concept of the mean path length lPath to calculate the linealenergy was introduced as an alternative to the mean chord length l becauseit was found that adopting Cauchy’s formula for the l was not appropriatefor the radiation field typical of HIT as it is very directional. lPath can bedetermined based on the peak of the lineal energy distribution produced bythe incident carbon beam. Furthermore it was demonstrated that the thicknessof the SV along the direction of the incident 12C ion beam can be adoptedas lPath . The tissue equivalence conversion method and lPath were adoptedto determine the RBE10, calculated using a modified microdosimetric kineticmodel, applied to the microdosimetric spectra resulting from the simulationstudy. Comparison of the RBE10 along the Bragg peak to experimental TEPC measurements at HIMAC, NIRS, showed good agreement. Such agreement demonstrates the validity of the developed tissue equivalence correctionfactors and of the determination of lPath .

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