Longitudinal radiochromic-film dosimetry for carbon-ion radiotherapy

This is a feasibility study for radiochromic film (RCF) dosimetry to measure physical and biological doses for quality assurance of carbon-ion radiotherapy. We used a layer-stacking carbon-ion beam comprised of range-shifted beamlets from the Heavy Ion Medical Accelerator in Chiba (HIMAC) at the National Institute of Radiological Scienecs (NIRS). Using biophysical beam model in a treatment planning system with its measured RCF response, the layer-stacking beam was decomposed into finely arranged beamlets with weights estimated by deconvolution of longitudinal RCF responses. The distributions of physical and biological doses were reconstructed from the estimated weights and were compared with the plan. We have developed a method to measure physical and biological doses by longitudinal dosimetry of quenched response. The method only involves a general optimization algorithm, a radiobiology model, and experimental beamlet data, and requires no extra corrections. Theoretically, this approach is applicable to various dosimeters and to proton and ion beams of any delivery method, regardless of quenching or biological effectiveness.ESTRO Meets Asia 201