OSLD nanoDot characterization for carbon radiotherapy dosimetry

Abstract

Objective: This study characterized OSLD nanoDots for use in a therapeutic carbon beam using the IROC framework for remote output verification. 
Approach: The absorbed dose correction factors for OSLD (fading, linearity, beam quality, angularity, and depletion), as defined by AAPM TG 191, were characterized for carbon beams. For the various correction factors, the effect of carbon LET was examined by characterizing in both a low and high LET setting. 
Main Results: Fading was not statistically different between reference photons and carbon, nor between low and high LET carbon; thus, the standard IROC-defined exponential function could be used to characterize fading. Dose linearity was characterized with a linear fit; while low and high LET carbon linearity was different, these differences were small and could be rolled into the uncertainty budget if using a single linearity correction. A linear fit between beam quality and dose-averaged LET was determined. The OSLD response at various angles of incidence was not statistically different, thus a correction factor need not be applied. There was a difference in depletion between low and high LET carbon irradiations, but this difference was small over the standard five readings. The largest uncertainty associated with the use of OSLDs in carbon was because of the kQ, with an uncertainty of 6.0%. The overall uncertainty budget was 6.3% for standard irradiation conditions. 
Significance: OSLD nanoDot response was characterized in a therapeutic carbon beam. The uncertainty was larger than for traditional photon applications. These findings enable the use of OSLDs for carbon absorbed dose measurements, but with less accuracy than conventional OSLD audit programs.