  Purpose: This is a theoretical simulation study for proof of concept of
radiochromic film dosimetry to measure physical and biological doses without
plan-based quenching correction for patient-specific quality assurance of
carbon-ion radiotherapy. Methods: We took a layer-stacking carbon-ion beam
comprised of range-shifted beamlets. The dosimeter response was simulated
according to an experimental quenching model. The beam model followed a
treatment planning system. The beam was decomposed into finely arranged
beamlets with weights estimated by deconvolution of longitudinal dosimeter
responses. The distributions of physical and biological doses were
reconstructed from the estimated weights, and were compared with the plan. We
also evaluated the sensitivity to measurement errors and to erratic delivery
with an undelivered beamlet. Results: The reconstructed physical and biological
doses accurately reproduced the simulated delivery with errors approximately
corresponding to the measurement errors. The erratic beam delivery was easily
detectable by comparison of biological dose distribution to the plan.
Conclusions: We have developed a method to measure physical and biological
doses by longitudinal dosimetry of quenched response without using plan data.
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.
