Imaging Cherenkov photons emitted during radiation therapy can provide real-time information of the external beam field. It is well established that Cherenkov emission is correlative to dose deposited; however, differential photon energies and tissue attenuation properties, along with complicated camera geometries, entangle this relationship and introduce variability in the Cherenkov emission-to-dose ratio from patient-to-patient. This study aims to better understand the effects of optical properties, skin color, and patient-specific geometries (i.e. angle of camera incidence and curvature) on the Cherenkov emission-to-dose relationship. To do so, a series of phantom experiments were conducted with tissuesimulating optical phantoms and an andromorphic breast phantom in which optical properties, curvature, and camera angle of incidence were all examined as a function of normalized Cherenkov emission-to-dose. To acquire clinical Cherenkov data along with patient skin color, Cherenkov images and OSLD measurements for the ground-truth surface dose were collected weekly on 13 whole-breast radiotherapy patients, alongside high-resolution 3D color and texture scans. Phantom results suggest there to be a moderately strong correlation between dose percent error and patient curvature (R2 = 0.57), as well as angle of camera incidence (R2 = 0.56). Initial patient results suggest there to be a correlation between the redness of a patient’s skin, and the Cherenkov emission-to-dose ratio, with higher amounts of redness correlating to lower Cherenkov signal. By better characterizing these trends, we are potentially able to find generalizable optics-based corrections that improve the accuracy in mapping Cherenkov emission to real-time skin dose.
The emergence of the Halcyon linear accelerator has allowed for increased patient throughput and improved treatment times for common treatment sites in radiation oncology. However, it has been shown that this can lead to increased surface dose in sites like breast cancer compared with treatments on conventional machines with flattened radiation beams. Cherenkov imaging can be used to estimate surface dose by detection of Cherenkov photons emitted in proportion to energy deposition from high energy electrons in tissue. Phantom studies were performed with both square beams in reference conditions and with clinical treatments, and dosimeter readings and Cherenkov images report higher surface dose (25% for flat phantom entrance dose, 5.9% for breast phantom treatment) from Halcyon beam deliveries than for equivalent deliveries from a TrueBeam linac. Additionally, the first Cherenkov images of a patient treated with Halcyon were acquired, and superficial dose was estimated.
It has been reported and discussed that electrical current can be produced when an insulating material interacts with ionizing radiation. We have found that high-resolution images can be obtained from insulating materials if this current is guided by an electric field to the pixels of a TFT array. The charge production efficiency of insulators is much smaller than that of photoconductor materials such as selenium, silicon, or other conventional semiconductors. Nevertheless, when the intensity of the ionizing radiation is sufficiently high, a charge sensitive TFT imaging array with only dielectric material can produce high MTF images with contrast resolution proportional to the intensity of the radiation. The function of the dielectric in this new detector may be similar to that of an ionization chamber. Without the semiconductor charge generating material, the dielectric imaging detector does not exhibit charge generation fatigue or charge generation saturation. Prototype detectors have been tested using diagnostic x-ray beams with energy ranging from 25 kVp to 150 kVp, and therapeutic 2.5MV, 6MV, 10MV, and 15MV photon beams (with and without an electron built-up layer), electron beams, broad area proton beams, and proton pencil beams in the energy range of 150 MeV. High spatial resolution images up to the Nyquist frequency have been demonstrated. The physics, structure, and the imaging properties as well as the potential application of this detector will be presented and discussed.
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