Optical‐CT gel‐dosimetry I: Basic investigations

Abstract

Comprehensive verification of the intricate dose distributions associated with advanced radiation treatments is now an immediate and substantial problem. The task is challenging using traditional dosimeters because of restrictions to point measurements (ion chambers, diodes, TLD, etc.) or planar measurements (film). In essence, rapid advances in the technology to deliver radiation treatments have not been paralleled by corresponding advances in the ability to verify these treatments. A potential solution has emerged in the form of water equivalent three dimensional (3D) gel‐dosimetry. In this paper we present basic characterization and performance studies of a prototype optical‐CT scanning system developed in our laboratory. An analysis of the potential role or scope of gel dosimetry, in relation to other dosimeters, and to verification across the spectrum of therapeutic techniques is also given. The characterization studies enabled the determination of nominal operating conditions for optical‐CT scanning. “Finger” phantoms are introduced as a powerful and flexible tool for the investigation of optical‐CT performance. The modulation‐transfer function (MTF) of the system is determined to be better than 10% out to 1 mm−1, confirming sub‐mm imaging ability. System performance is demonstrated by the acquisition of a 1×1×1 mm3 dataset through the dose distribution delivered by an x‐ray lens that focuses x rays in the energy range 40–80 KeV. This 3D measurement would be extremely difficult to achieve with other dosimetry techniques and highlights some of the strengths of gel dosimetry. Finally, an optical Monte Carlo model is introduced and shown to have potential to model light transport through gel‐dosimetry systems, and to provide a tool for the study and optimization of optical‐CT gel dosimetry. The model utilizes Mie scattering theory and requires knowledge of the variation of the particle size distribution with dose. The latter was determined here using the technique of dynamic‐light‐scattering.

Fast, high‐resolution 3D dosimetry utilizing a novel optical‐CT scanner incorporating tertiary telecentric collimation

“…Optical-CT was proposed as a cost effective alternative to MRI. 4,16,17,19 Detailed characterization studies 16,17 have revealed that optical-CT dose-readout in gel based 3D dosimeters can be hampered by scatter artifacts (caused by high scatter fraction of visible light), edge artifacts (caused by refractive index mismatch between the gel, container, and matching fluid) and other artifacts, including ring artifacts, digitization artifacts, and artifacts due to low transmission of visible light.…”

“…Broadly, 3D dosimetry systems consist of two essential components of a 3D dosimeter [e.g., polymer gels, 3-7 Fricke gels, 8,9 or PRESAGE™ (Ref. 10)] and a dose-readout modality [e.g., magnetic resonance imaging (MRI), 4,11,12 ultrasound, 13 x-ray computed tomography [x-ray CT] 14 and optical-CT. 15 [15][16][17][18] While several dose-readout modalities have been proposed for gel based dosimeters (polymer and Fricke gels), MRI and optical-CT (an optical equivalent of x-ray CT) remain predominant. Optical-CT was proposed as a cost effective alternative to MRI.…”

Temperature and hydration effects on absorbance spectra and radiation sensitivity of a radiochromic medium

“…[1][2][3][4][5][6][7] While many of the current polymer systems are utilized for two-or three-dimensional dose representation, high spatial resolution offers potential for successful implementation in point-based measurements. Radiochromic materials commercially available in GAFCHROMIC ® films ͑International Specialty Products, Wayne, NJ͒ have the desired high spatial resolution [8][9][10] and are able to provide sufficient signal from a subcubic millimeter volume.…”

Dosimetry tools and techniques for IMRT

“…228 For MR measurements requiring high accuracy and highspatial resolution, long imaging times ͑several hours͒ may be required to achieve the low uncertainty and high spatial resolution associated with standard dosimeters. An alternative readout technique, optical-CT, 166,203,204,209,227,[231][232][233][234][235] has been proposed for imaging PAG gels; an approach analogous to first-generation x-ray CT. In optical-CT the x-ray source is replaced by a visible laser and the x-ray detector replaced with a light-sensitive photodiode.…”