P Guo

Characterization of a new radiochromic three‐dimensional dosimeter

The development of intensity-modulated radiotherapy (IMRT) has created a clear need for a dosimeter that can accurately and conveniently measure dose distributions in three dimensions to assure treatment quality. PRESAGE is a new three dimensional (3D) dosimetry material consisting of an optically clear polyurethane matrix, containing a leuco dye that exhibits a radiochromic response when exposed to ionizing radiation. A number of potential advantages accrue over other gel dosimeters, including insensitivity to oxygen, radiation induced light absorption contrast rather than scattering contrast, and a solid texture amenable to machining to a variety of shapes and sizes without the requirement of an external container. In this paper, we introduce an efficient method to investigate the basic properties of a 3D dosimetry material that exhibits an optical dose response. The method is applied here to study the key aspects of the optical dose response of PRESAGE: linearity, dose rate dependency, reproducibility, stability, spectral changes in absorption, and temperature effects. PRESAGE was prepared in 1 x 1 x 4.5 cm3 optical cuvettes for convenience and was irradiated by both photon and electron beams to different doses, dose rates, and energies. Longer PRESAGE columns (2 x 2 x 13 cm3) were formed without an external container, for measurements of photon and high energy electron depth-dose curves. A linear optical scanning technique was used to detect the depth distribution of radiation induced optical density (OD) change along the PRESAGE columns and cuvettes. Measured depth-OD curves were compared with percent depth dose (PDD). Results indicate that PRESAGE has a linear optical response to radiation dose (with a root mean square error of -1%), little dependency on dose rate (-2%), high intrabatch reproducibility (< 2%), and can be stable (-2%) during 2 hours to 2 days post irradiation. Accurate PRESAGE dosimetry requires temperature control within 1 degrees C. Variations in the PRESAGE formulation yield corresponding variations in sensitivity, stability, and density. CT numbers in the range 100-470 were observed. In conclusion, the small volume studies presented here indicate PRESAGE to be a promising, versatile, and practical new dosimetry material with applicability for radiation therapy.

Three-dimensional imaging of whole rodent organs using optical computed and emission tomography

We explore the potential of optical computed tomography (optical-CT) and optical emission computed tomography (optical-ECT) in a new area-whole organ imaging. The techniques are implemented on an in-house prototype benchtop system with improved image quality and the capacity to image larger samples (up to 3 cm) than previous systems based on stereo microscopes. Imaging performance tests confirm high geometrical accuracy, accurate relative measurement of linear attenuation coefficients, and the ability to image features at the 50-microm level. Optical labeling of organ microvasculature was achieved using two stains deposited via natural in vivo circulatory processes: a passive absorbing ink-based stain and an active fluorescin FITC-lectin conjugate. The lectin protein binds to the endothelial lining, and FITC fluorescense enables optical-ECT imaging. Three-dimensional (3-D) optical-CT images have been acquired of a normal rat heart and left lung and a mouse right lung showing exquisite detail of the functional vasculature and relative perfusion distribution. Coregistered optical-ECT images were also acquired of the mouse lung and kidney. Histological sections confirmed effective labeling of microvasculature throughout the organs. The advantages of optical-CT and optical-ECT include the potential for a unique combination of high resolution and high contrast and compatibility with a wide variety of optical probes, including gene expression labeling fluorescent reporter proteins.

IMRT verification using a radiochromic/optical-CT dosimetry system

This work represents our first experiences relating to IMRT verification using a relatively new 3D dosimetry system consisting of a PRESAGETM dosimeter (Heuris Inc, Pharma LLC) and an optical-CT scanning system (OCTOPUSTM TM MGS Inc). This work builds in a step-wise manner on prior work in our lab.

Quality assurance in 3D dosimetry by optical-CT

In this paper we present a QA phantom and procedure designed for efficient evaluation of the basic imaging performance of any optical-CT scanning system. Example results are presented from two optical-CT systems, an in-house CCD based system and the OCTOPUSTM system from MGS Research.

Investigation of the dosimetric characteristics of PRESAGETM

In this work we perform a detailed investigation of the fundamental dosimetric characteristics of PRESAGETM, to determine its potential for dosimetry. A common problem encountered when attempting to evaluate a new material, is how to efficiently evaluate the large multitude of potential mechanisms and response-characteristics that may affect the practicality and accuracy of that material for dosimetry. We introduce a new method designed to enable rapid, accurate and convenient evaluation of any material which has an optical dose-response, and can be formed into columns of precise dimension (e.g. spectro-photometric cuvettes). The method is also optimised to enable evaluation of dosimetric characteristics with the minimum volume of material, to minimise cost, and problems associated to storage, transportation etc.

Simple 3D validation experiments for PRESAGETM/optical-CT dosimetry

Detailed studies of the basic dosimetric properties of small volumes of PRESAGETM have confirmed its promise as a new material with enhanced properties for 3D dosimetry. PRESAGETM is a transparent polyurethane material containing a radiochromic leucodye. Principle advantages include a relative insensitivity of the dose response to atmospheric exposure, and a radiochromic optical contrast which is light absorbing rather than light-scattering (peak optical density change is at ~633 nm). The absorptive nature of the contrast is more amenable to accurate dose read-out by optical-computed-tomography. This work represents our first feasibility tests of large volumes of PRESAGETM for dosimetric validation of radiation treatments.

Towards four dimensional (4D) dosimetry for radiation-therapy

The development of accurate and convenient dosimetry tools with the capacity to comprehensively verify advanced four-dimensional treatments is an important and urgent goal for radiation therapy physicists. At present, implementation into the clinic is being severely hampered and delayed by the difficulty in adequately verifying these techniques using traditional dosimetry methods. The work presented here represents an important step towards providing a solution.

A dual-purpose CCD based micro-optical-CT scanning system.

The concept of three dimensional (3D) dosimetry by optical-computed-tomography (optical-CT) of radiation induced optical contrast was first introduced in 1996 (Gore J C, Ranade M, Maryanski M J and Schulz R J 1996 Phys. Med. Biol. 41 2695-2704 and Maryanski M J, Zastavker Y Z and Gore J C 1996 Phys. Med. Biol. 41 2705-2717) and developed later by other groups. These works describe a first generation optical-CT system based on measurement of the transmission of single scanning laser beam that scanned the dosimeter in a rastering manner. Second generation systems can be categorized as macroscopic scanners developed for 3D dosimetry, and microscopic scanners developed for embryo imaging. Here we introduce a new system that bridges this divide. It is designed to be a dual purpose, micro-optical-CT system, with capability to perform both 3D micro-dosimetry on dosimeters up to 5 cm diameter, and to image structure and function of optically cleared tissue samples in transmission mode (optical-CT) and emission mode (optical-ECT).

SU‐FF‐T‐257: The Study of the Dosimetric Properties of ‘RadGel’, a New Dosimeter for Three‐Dimensional Gel Dosimetry

Purpose: The development of advanced radiation treatment techniques such as IMRT has prompted an immediate need for a dosimetrysystem that can provide accurate and convenient measurement of complex three‐dimensional (3D) dose distributions. Geldosimetry has proved a promising candidate, but present gel‐dosimeters are still not in widespread routine clinical use. In this work we investigate the dosimetricproperties of a new 3D dosimetrymaterial, RadGel™, which has potential for application in radiation therapy.Method and Materials: Samples of RadGel TM contained in optical cuvettes (1cm×1cm×5cm) were irradiated in a series of experiments to determine sensitivity to dose,dose rate, energy, and stability of response with time post‐irradiation. Pre and post irradiation measurements were made of the optical‐density profile along the long axis of the cuvettes using a custom laser scanning system at 632nm. The radiation induced spectral optical density (OD) change of RadGel™ was also measured by a conventional spectrophotometer. Results: The spectrophotometric measurements indicated that peak radiation induced OD change occurred at ∼600nm (FWHM ∼100nm). A linear relationship was observed between OD changes and dose, and negligible dependence on dose rate. OD measurements a week after irradiation revealed significant degradation of optical response compared with scans 24 h post irradiation, but the timescale of these changes is still much improved from existing non‐scattering 3D dosimetrymaterials.Conclusion: The RadGel™ material has several attractive qualities for 3D dosimetry. In particular RadGel™ is more robust than established non light‐scattering 3D dosimeters to exposure to air and water. Together with an improved stability of response with time RadGel™ appears a practical and convenient material. Further tests are required on larger volumes of RadGel™ to determine true potential.

WE-E-AUD-03: Characterization of a Novel, Fast, In-House, CCD-Based Optical-CT Scanner for 3D Dosimetry in Radiochromic Dosimeters

Purpose: Practical tools for comprehensive three‐dimensional (3D) dosimetry are urgently needed. Here we evaluate a novel, in‐house, second‐generation optical‐computed tomography (optical‐CT) scanner for 3D‐dosimetry in PRESAGE™ dosimeters. Primary advantages over laser‐based, first‐generation scanners include much faster scanning times and potential for high resolution. Methods: The optical‐CT scanner performance was evaluated by comparing dose readout with that from the ‘gold‐standard’ first‐generation (MGS) scanner for three irradiation schemes of progressive complexity (a single rectangular‐beam, a 3D‐conformal and an IMRT treatment). The scanner incorporates a red‐filtered (633 nm) uniform area‐backlight, but is distinguished by incorporation of a customized tertiary telecentric lens‐system, which enables image formation with parallel light‐ray geometry. 2D projection images were acquired on a CCDcamera of an irradiated PRESAGE™ cylinder (5 cm diameter). Multiple 2D projections over a 360°‐scan were used to reconstruct a 3D map of attenuation coefficients. Results: A complete set of high‐resolution (1392×1040 pixels, 50μm) projection images was acquired in ∼5 minutes with the new (CCD)scanner. This represents an improvement in speed of >2 orders of magnitude and potential for high‐resolution dosimetry compared to the first‐generation MGS scanner. The speed advantage for CCD‐scanner arises because, unlike the MGS scanner, an entire 2D‐projection is acquired in a single acquisition. Line profiles showed that 3D dose readout from the CCDscanner was in good agreement with independent readout from the MGS scanner as well as the calculated treatment plan for all three irradiation schemes. Gamma comparison indicated agreement with MGS readout within 4% dose‐difference and 4mm distance‐to‐agreement. While the signal‐to‐noise ratio for MGS scanner is better by a factor of 3, the attributes of faster speed and higher resolution are highly desirable for practical 3D‐dosimetry. Conclusion: This work demonstrates the feasibility of accurate 3D dosimetry with the new optical‐CT scanner used in combination with radiochromic (non‐scattering) dosimeters.