Abstract
The INTRABEAM Carl Zeiss Surgical system (Oberkochen, Germany) is a miniature accelerator producing low energy photons (50 keV maximum). The published dosimetric characterization of the INTRABEAM was based on detectors (radiochromic films or ionization chambers) not allowing measuring the absorbed dose in the first millimeters of the irradiated medium, where the dose is actually prescribed. This study aims at determining with Magnetic Resonance Imaging (MRI) the sensitivity of a paramagnetic gel in order to measure the dose deposit produced with the INTRABEAM from 0 to 20 mm. Although spherical applicators are mostly used with the INTRABEAM system for breast applications, this study focuses on surface applicators that are of interest for cutaneous carcinomas. The irradiations at 12 different dose levels (between 2 Gy and 50 Gy at the gel surface) were performed with the INTRABEAM and a 4 cm surface applicator. The gel used in this study is a new « sensitive » material. In order to compare gel sensitivity at low energy with high energy, gels were irradiated by an 18 MV photon beam produced by a Varian Clinac 2100 CD. T2 weighted multi echo MRI sequences were performed with 16 echo times. The T2 signal versus echo times was fitted with a mono-exponential function with 95% confidence interval. The calibration curve determined at low energy is a linear function (r2 = 0.9893) with a sensitivity of 0.0381 s-1.Gy-1, a similar linear function was obtained at high energy (0.0372 s-1.Gy-1 with r2 = 0.9662). The calibration curve at low energy was used to draw isodose maps from the MR images. The PDD (Percent Depth Dose) determined in the gel is within 5%-1mm of the ionization chamber PDD except for one point. The dosimetric sensitivity of this new paramagnetic ferrous gel was determined with MRI measurements. It allowed measuring the dose distribution specifically in the first millimeters for an irradiation with the INTRABEAM miniature accelerator equipped with a surface applicator.
Highlights
Among detectors available to measure dose distribution, very few are capable of 3D dose measurements
Determination of paramagnetic ferrous gel sensitivity in low energy x-ray beam an irradiation in such gels can be measured by optical Computed Tomography (OCT), where the quantification is based on intensity difference of laser beams through the gels [6,7,8] or by magnetic resonance (MR) imaging [9,10]
This study focuses on surface applicators as they are easier to handle with gels and because the dose gradient with depth is the steepest among available applicators, making dose measurements more challenging
Summary
Among detectors available to measure dose distribution, very few are capable of 3D dose measurements. Paramagnetic gels have proven their relevance in the determination of the 3D dose distribution and their use has spread since several years [1,2,3,4,5]. Gels are used as chemical dosimeters and phantom at the same time and, in our study, the dose is determined from a measurable change on the MR signal impacted by the chemical state of the gel. The characterization of these chemical reactions enables to define gels as dosimeters. Gels record and preserve the spatial distribution information of the absorbed dose. One of the benefits of the gel relative to other dosimeters is its tissue equivalent density and the possibility to shape the gel as an anthropomorphic phantom for example
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