Marek J. Maryanski

Dosimetry study of Re-188 liquid balloon for intravascular brachytherapy using polymer gel dosimeters and laser-beam optical CT scanner.

Angioplasty balloons inflated with a solution of the beta-emitter Re-188 have been used for intravascular brachytherapy to prevent restenosis. Coronary stents are in extensive clinical use for the treatment of de novo atherosclerotic stenoses. In this study, the effect of an interposed stent on the dose distribution has been measured for Re-188 balloon sources using the proprietary BANG polymer gel dosimeters and He-Ne laser-beam optical CT scanner. In polymer gels, after ionizing radiation is absorbed, free-radical chain-polymerization of soluble acrylic monomers occurs to form an insoluble polymer. The BANG polymer gel dosimeters used in these measurements allow high resolution, precise, and accurate three-dimensional determination of dosimetry from a given source. Re-188 liquid balloons, with or without an interposed metallic stent, were positioned inside thin walled tubes placed in such a polymer dosimeter to deliver a prescribed dose (e.g., 15 Gy at 0.5 mm). After removing the balloon source, each irradiated sample was mounted in the optical scanner for scanning, utilizing a single compressed He-Ne laser beam and a single photodiode. In the absence of a stent, doses at points along the balloon axis, at radial distance 0.5 mm from the balloon surface and at least 2.5 mm from the balloon ends, are within 90% of the maximum dose. This uniformity of axial dose is independent of the balloon diameter and length. Dose rate and dose uniformity for intravascular brachytherapy with Re-188 balloon are altered by the presence of stent. The dose reduction by the stent is rather constant (13%-15%) at different radial distances. However, dose inhomogeneity caused by the stent decreases rapidly with radial distance.

Determining optimal gel sensitivity in optical CT scanning of gel dosimeters.

A method for determining the gel sensitivity that is necessary for obtaining optimal image contrast in optical CT scanning of gel dosimeters is presented. The effective dynamic range of the OCTOPUS-ONE research scanner (MGS Research, Inc., Madison, CT) is analyzed. Optical density increments for selected straight-line paths across a gel cylinder to be scanned are calculated based on the optical properties of the polymer gel and the dose distribution from a commercial treatment planning system (Cadplan, Varian Corporation, Palo Alto, CA). Maximum optical density increment across the entire gel is obtained by searching the gel cylinder over a set of transverse planes at different rotational angles. The application of this quantity as a criterion for optimizing the quality of the optical CT scanning is demonstrated through dose verification of two representative treatment plans. When the MU dependence of the dose distribution for a treatment plan is linear, as is the case for static field irradiation, it is possible to scale the treatment plan such that the intensity variation of the signals received by the photodetector spans its entire dynamic range. For treatment plans that are possibly nonlinear, IMRT plans, for example, modification of the sensitivity of the gel material is necessary for the high-dose signals to be collected at a certain signal-to-noise ratio. Results obtained using the optimized CT scanning approach are compared with those from the treatment planning system and the film measurement.

Heterogeneity phantoms for visualization of 3D dose distributions by MRI-based polymer gel dosimetry

Heterogeneity corrections in dose calculations are necessary for radiation therapy treatment plans. Dosimetric measurements of the heterogeneity effects are hampered if the detectors are large and their radiological characteristics are not equivalent to water. Gel dosimetry can solve these problems. Furthermore, it provides three-dimensional (3D) dose distributions. We used a cylindrical phantom filled with BANG-3 polymer gel to measure 3D dose distributions in heterogeneous media. The phantom has a cavity, in which water-equivalent or bone-like solid blocks can be inserted. The irradiated phantom was scanned with an magnetic resonance imaging (MRI) scanner. Dose distributions were obtained by calibrating the polymer gel for a relationship between the absorbed dose and the spin-spin relaxation rate of the magnetic resistance (MR) signal. To study dose distributions we had to analyze MR imaging artifacts. This was done in three ways: comparison of a measured dose distribution in a simulated homogeneous phantom with a reference dose distribution, comparison of a sagittally scanned image with a sagittal image reconstructed from axially scanned data, and coregistration of MR and computed-tomography images. We found that the MRI artifacts cause a geometrical distortion of less than 2 mm and less than 10% change in the dose around solid inserts. With these limitations in mind we could make some qualitative measurements. Particularly we observed clear differences between the measured dose distributions around an air-gap and around bone-like material for a 6 MV photon beam. In conclusion, the gel dosimetry has the potential to qualitatively characterize the dose distributions near heterogeneities in 3D.

Sensitivity calibration procedures in optical-CT scanning of BANG 3 polymer gel dosimeters

The dose response of the BANG 3 polymer gel dosimeter (MGS Research Inc., Madison, CT) was studied using the OCTOPUS laser CT scanner (MGS Research Inc., Madison, CT). Six 17 cm diameter and 12 cm high Barex cylinders, and 18 small glass vials were used to house the gel. The gel phantoms were irradiated with 6 and 10 MV photons, as well as 12 and 16 MeV electrons using a Varian Clinac 2100EX. Three calibration methods were used to obtain the dose response curves: (a) Optical density measurements on the 18 glass vials irradiated with graded doses from 0 to 4 Gy using 6 or 10 MV large field irradiations; (b) optical-CT scanning of Barex cylinders irradiated with graded doses (0.5, 1, 1.5, and 2 Gy) from four adjacent 4 x 4 cm2 photon fields or 6 x 6 cm2 electron fields; and (c) percent depth dose (PDD) comparison of optical-CT scans with ion chamber measurements for 6 x 6 cm2, 12 and 16 MeV electron fields. The dose response of the BANG3 gel was found to be linear and energy independent within the uncertainties of the experimental methods (about 3%). The slopes of the linearly fitted dose response curves (dose sensitivities) from the four field irradiations (0.0752 +/- 3%, 0.0756 +/- 3%, 0.0767 +/- 3%, and 0.0759 +/- 3% cm(-1) Gy(-1)) and the PDD matching methods (0.0768 +/- 3% and 0.0761 +/- 3% cm(-1) Gy(-1)) agree within 2.2%, indicating a good reproducibility of the gel dose response within phantoms of the same geometry. The dose sensitivities from the glass vial approach are different from those of the cylindrical Barex phantoms by more than 30%, owing probably to the difference in temperature inside the two types of phantoms during gel formation and irradiation, and possible oxygen contamination of the glass vial walls. The dose response curve obtained from the PDD matching approach with 16 MeV electron field was used to calibrate the gel phantom irradiated with the 12 MeV, 6 x 6 cm2 electron field. Three-dimensional dose distributions from the gel measurement and the Eclipse planning system (Varian Corporation, Palo Alto, CA) were compared and evaluated using 3% dose difference and 2 mm distance-to-agreement criteria.

SU-GG-T-357: Initial Evaluation of a Fast Optical CT Scanner for Gel Dosimetry

Purpose: To evaluate the performance of the OCTOPUS™‐10X optical CTscanner (MGS Research, Inc., Madison, CT) based on a single laser diode and a single photodiodedetector.Method and Materials: Optical CTscanner using a single laser beam and a single photodiodedetector does not suffer from the light scattering problem inherent in scanners using broad light sources. The OCTOPUS™‐10X optical CTscanner is an upgrade of the OCTOPUS™ research scanner with improved design for faster motion of the laser beam and faster data acquisition process. The motion of the laser beam in the new configuration is driven by the rotational motion of a scanning mirror. The center of the scanning mirror and the center of the photodiodedetector are adjusted to be on the focal spots of two coaxial Fresnel lenses. A glass water tank is placed between the two Fresnel lenses to house gel phantoms and matching liquids. The laser beam scans over the water tank in parallel beam geometry for projection data as the scanning mirror rotates at the speed of less than 0.1 second per circle. Signal sampling is performed independently of the motion of the scanning mirror. Technical issues with regard to the new design of the scanner are addressed, including projection data extraction from raw samples, non‐uniform pixel averaging and reference image normalization. Results: The mechanical stability of the scanner, the accuracy of the non‐uniform pixel averaging algorithm and the effectiveness of the reference image normalization method are analyzed. Gamma analysis are performed between the 3D dose distributions from the gel measurement and the Eclipse planning system for a 12MeV, 6cm × 6cm single electron field irradiation and a 5‐field IMRT plan. Conclusion: The OCTOPUS™‐10X optical CTscanner can reconstruct 3D dose distribution from gel dosimeters with reasonable accuracy in a timely fashion.

SU‐GG‐T‐208: 3‐D Dosimetric Comparison of IMRT with 2.5 Mm HD120 MLC Using Optical CT Based Polymer Gel and PRESAGE Dosimeters

Purpose: To evaluate and compare the 3‐D dose distributions for IMRT with 2.5 mm HD120 MLC using optical CT based polymergel and PRESAGE dosimeters.Method and Materials: In this study, a polymer BANG geldosimeter and a PRESAGE phantom, together with an optical CT scanner, were employed to implement 3‐D dose distribution measurements. Both dosimeters, with 15 cm diameter and 14 cm height, were modified to optimal and linear dose‐response characteristics. A slice thickness of 2.5 mm without spacing was used for CT simulation on both the patient and cylindrical phantoms. The Varian Eclipse treatment planning system was used to design the IMRT radiosurgery plan for a patient with a 2.5 cc small brain tumor treated with 6 MV photon beams. To correlate the optical density response with radiation dose, the same batch of gel and PRESAGE phantoms were irradiated with a 16 MeV electron beam to a known dose at the depth of dmax. The optical density at a specific depth and the PDD table of the electron beam can be used to obtain the optical density dose response of the dosimeters. Both phantoms were scanned with 1 mm pixel resolution using a commercial optical CT scanner, OCTOPUS™ (MGS Research Inc., Madison, CT).Results: Both measured dose distributions from gel and PRESAGE and calculated results are in reasonable agreement. However, the isodose lines from the measurements show more variation than those from the calculation, and this trend is more significant for the 2.5 mm MLC. These discrepancies may be partly attributed to the fact that the calculation grid for the planning system is 2.5 mm yet the resolution of gel measurements is 1 mm, as well as the Trilogy TX having a smaller leaf width. Issues and difficulties on 3‐D dosimetric comparison will be presented.

SU‐GG‐T‐290: A Fast Optical CT Scanner for Gel Dosimetry

Purpose: To evaluate the performance of a fast optical CTscanner and discuss methods for improving scanning speed in 3D geldosimetry.Method and Materials: The performance of a fast optical laser CTscanner for 3D geldosimetry (OCTOPUS‐10X, MGS Research, Madison, CT) is evaluated. This scanner uses three stepper motors to guide a single laser beam to move in three dimensions relative to the gel to be scanned. Fresnel lenses were used to generate parallel‐ or fan‐beam geometry inside the scanning tank. Non‐uniform averaging was applied to the parallel‐beam data to compensate for the non‐uniform linear speed of the laser beam refracted by the first Fresenel lens. Image reconstruction was done using filtered back‐projection functions “iradon” and “ifanbeam” built in Matlab. The geometric accuracy of the scanner in two fast‐scanning modes was tested using test objects. Dosimetric accuracy of the scanner in conjunction with BANG polymergel dosimeter was tested by comparing dose distributions from the gel dosimeters with those from Eclipse treatment planning system and film dosimetry for several static and dynamic treatment plans. Results: The scanner can acquire a single slice image of 100 pixels ×100 pixels from an irradiated gel in 30 seconds. This is about 20 times faster than the previous model of optical CTscanner based on translational‐rotational motion. The reconstructed images of test objects show the geometric accuracy of the scanner is at sub‐pixel level. The reconstructed dose distributions from the two different scanning modes agree well between themselves and are also in good agreement with the planning system and film dosimetry.Conclusion: Optical CTscanner based on a single laser beam and rotational motion of 3 stepper motors can reproduce complete 3D dose distributions from regular external beam treatment plans in a matter of half an hour.

3-D dose verification for IMRT using optical CT based polymer gel dosimetry

In this study BANG® polymer gels in conjunction with OCTOPUS™ optical CT scanner (MGS Research Inc., Madison, CT) was employed to measure the relative 3D dose distribution of an IMRT treatment. Measured relative dose distributions from the gel measurement were compared with those from treatment planning system calculations and EDR2 film measurements with regard to planar dose distributions in axial, coronal, and sagittal orientations.