Accurate dose measurements using Cherenkov emission polarization imaging

Abstract

Purpose Cherenkov radiation carries the potential of direct in‐water dose measurements, but its precision is currently limited by a strong anisotropy. Taking advantage of polarization imaging, this work proposes a new approach for high‐accuracy Cherenkov emission dose measurements. Methods Cherenkov radiation produced in a 15 × 15 × 20‐cm3 water tank is imaged with a cooled charge‐coupled device (CCD) camera from four polarizer transmission axes [0, 45, 90, 135°]. The water tank is positioned at the isocenter of a 5 × 5‐cm2, 6‐, and 18‐MV photon beam. Using Malus’ law, the polarized portion of the signal is extracted. Corrections are applied to the polarized signal following azimuthal and polar Cherenkov emission angular distributions extracted from Monte Carlo simulations. Projected percent depth dose and beam profiles are measured and compared with the prediction from a treatment planning system (TPS). Results Corrected polarized signals on the central axis reduced deviations at depth (mean ± standard deviation) from 8% ± 5% to 0.8% ± 1% at 6 MV and 8% ± 7% to 1% ± 3% at 18 MV. For the profile measurement, differences between the corrected polarized signal and the TPS calculations are 1% ± 3% and 2% ± 3% on the central axis at 6 and 18 MV respectively. In these conditions, Cherenkov emission is shown to be partly polarized. Conclusions This work proposes a novel polarization imaging approach enabling high‐precision water‐based dose measurements using the Cherenkov radiation. The method allows a correction of the Cherenkov emission anisotropy within 4% on the beam central axis and in depth.

Direct in-water radiation dose measurements using Cherenkov emission corrected signals from polarization imaging for a clinical radiotherapy application

“…The resulting images correspond to the Cherenkov signal summed over the water tank thickness along the optical axis. As a result, projected dose distributions are measured 18 . For each set of measurements, projected percent depth dose (PPDD) and projected profiles at depth of maximum dose ( ) were extracted.…”

“…Given the intrinsic polarization of Cherenkov emission, polarization imaging has potential benefits for dose measurements in radiotherapy that have not yet been exploited. Recently, a letter was published that presented promising results when using polarization imaging to correct the Cherenkov emission anisotropy which enabled accurate dose measurements 18 . The method takes advantage of polarization imaging to isolate the polarized contribution of the Cherenkov signal.…”

Effects of signal source to camera distance in Cherenkov dosimetry using polarized imaging

“…Cherenkov light is often considered as a parasite signal in light-based dosimetry such as with scintillators [1] but it has been recently demonstrated that it can be used as the primary signal for dosimetry using polarized imaging [2,3]. Cherenkov dosimetry has several benefits, one of which making real-time radiation treatment dosimetry possible [4].…”

“…Cherenkov light is polarized by nature, however, the Cherenkov signal produced in a water phantom and captured by a camera is not entirely polarized due to scattering. To correct this, Cloutier et al developed a method based on the Malus law (𝐼 = 𝐼 𝑝𝑜𝑙 (cos(𝛼 0 − 𝛼)) 2 + 𝐼 𝑟𝑎𝑛𝑑.𝑝𝑜𝑙 ) [5] and Monte Carlo simulations to extract the polarized portion of the light (𝐼 𝑝𝑜𝑙 ) captured by a camera and calculate angular correction distributions to have a signal proportional to dose [2]. Such distributions account for Cherenkov photon loss due to photon interactions with the environment.…”

“…The objective of this work is to experimentally characterize and quantify the effects of the Cherenkov source to camera distance on 𝐼 𝑝𝑜𝑙 as to avoid calculating the angular correction distributions from Monte Carlo simulations. [2,3]. To do so, a 15×15×20 cm 3 water tank was irradiated by a 5×5 cm 2 , 6 MV photon beam (Truebeam, Varian Medical Systems, Palo Alto, USA) at a 600 MU/min dose rate for 2 minutes.…”