Visualization of an Intra-Operative X-Ray Source With Liquid Scintillation Imaging

Peter A. Sandwall, PhD

Department of Radiation Oncology, OhioHealth - Mansfield , OH, USA

*Correspondence to: Dr. Peter A. Sandwall,
Department of Radiation Oncology, OhioHealth - Mansfield, Ohio, USA.

Copyright © 2020 Peter A. Sandwall. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Introduction
Intraoperative radiotherapy (IORT) for breast cancer has been actively studied for over 20 years [1-3]. The largest trials have explored low energy x-rays (30-50 kV), targeted intraoperative radiotherapy (TARGIT-IORT) and high-energy electrons (3-12 MeV), electron intraoperative radiotherapy (ELIOT) [4,5]. Designed to investigate non-inferiority and equivalence, results have been controversial but indicate acceptability for select patients [6]. Long-term TARGIT-IORT data has demonstrated comparable local control and overall survival to external beam radiotherapy [7].

The current protocol for commissioning low energy x-ray sources uses a parallel-plate ionization chamber [8]. Several investigators have also used radiochromic film for dosimetric analysis [9-11]. Film measurements are valuable for their spatial resolution which allows visualization of dose distributions. However, care must be taken when collecting measurements with film [12]. Film from different manufacturers have varying sensitivities and dependencies, including non-linear energy response [13]. While the use of ionization chambers is well established and reliable, film remains a tedious art form, with both methods consuming valuable clinical time.

Scintillation imaging is an active area of study with potential for rapid readout [14-16]. Coupling liquid scintillation with optical imaging, scintillation imaging allows near real-time visualization of volumetric dose distributions by collecting light intensity proportional to energy deposition. The current work demonstrates proof-of-principle for rapid characterization of an IORT x-ray source using liquid scintillation imaging.

Methods and Materials
Experiments were performed with a 50kVp x-ray source, IntraBeam (Ziess, Germany); bremsstrahlung photons are produced by a dithered electron beam impinging upon an Au target on the inner probe tip. The manufacturer (Ziess, Germany) provided leaded glass phantom was filled with commercial liquid scintillation cocktail (LSC), Ultima Gold (PerkinElmer, USA). LSC emits blue light proportional to imparted energy. Chemical compositions of commercial LSCs are proprietary; however, a generalized formula consists of organic solvent, surfactant, primary scintillator (emitting UV light), and secondary scintillator (emitting blue light).

Visualizations were collected with a 13 MP CMOS detector, 1080p resolution, with 30 fps video. Image processing was performed with ImageJ (National Institute of Health, USA) [17]. RGB channels separated and blue, color of LSC emissions, analyzed quantitatively. For visualization, smoothing was applied and intensity contour lines overlaid. Intensity with distance from surface of probe tip was determined by collecting a line profile and normalizing to maximum intensity at the surface of the x-ray source probe tip.

Results

Visualization of X-Ray Source
Acquired videos yielded visualization of the swept, dithered, nature of beam generation. Images provided visualization of emission intensity around the x-ray source probe tip and quantification of intensity fall-off with distance, See Figure 1.

Intensity Variation With Distance, Comparison With Published Values
Intensity variation with distance for RGB and blue channel, were plotted with Monte Carlo derived depth dose values in water. Scintillation images were observed to under-respond; nearly 10% at 5mm with blue channel, See Figure 2, Table 1.

Water Equivalence of Medium
The elemental composition of LSC fluid, provided by the manufacturer (Perkins-Elmer, USA), was compared to water and soft tissue, as defined by ICRU 44 [16]. LSC fluid was determined to have a lower effective Z (5.9) than water (7.2), See Table 2.

Discussion
Difference between LSC emission intensity and published depth dose data are believed to be due to the volumetric nature of dose deposition coupled with integrated collection of light emissions. Integration of light incident the detector leads to a higher intensity near the center of sphere (probe tip). Normalizing by this value results in points further from the tip appearing to under-respond, as observed.

Low energy ionizing radiation interactions are predominately photo-electric and highly dependent on atomic number, Z. Kirov, et al. studied a silicon doped LSC, others continue to explore this method to obtain a water equivalent LSC [19,20]. LSC fluid composition, optimization of image exposure, and derivation of a correction factor for integration along the ray path are areas for further study [21].

Conclusion
The results of this study confirm the feasibility of visualizing emissions of an IORT x-ray source with liquid scintillation imaging. This work demonstrates a low-cost method of real-time visualization of ionizing radiation. Further study of this dosimetry technique may yield a method for rapid commissioning and clinical deployment of IORT technology.

Author Contributions
Sole author performed experiments, analyzed data, and wrote communication.

Funding
This research received no external funding.

Acknowledgments
Work was performed at TriHealth Cancer Institute, Bethesda North Hospital. Clinical IORT program directed by Ching Ho, MD (surgeon) in collaboration with Jessica Guarnaschelli, MD (radiation oncologist) and Cameron Ditty (chief medical physicist). The author is grateful for the experience working alongside this team of professionals, applying technology to serve patients.

Conflicts of Interest
The author declares no conflict of interest.

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