Determination of paramagnetic ferrous gel sensitivity in low energy x-ray beam produced by a miniature accelerator

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.

Fig 1. Irradiation configuration with plastic containers.

Fig 2. Sagittal view of the plastic containers acquisition set-up inside the MR scan.

Fig 3. Gel dose response.

(A) Measurement at low energy for 12 irradiation dose levels (2, 4, 6, 8, 10, 15, 20, 25, 30, 35, 40 and 50 Gy). The equation of the linear regression (dotted line) is $\text{R}2-{\text{R}}_2{}_{{}_0}=0.0381*\text{Dose}+0.0836({\text{r}}^2=0.9893)$. (B) Measurement at high energy for 6 irradiation dose levels (2.5, 5, 10, 15, 20 and 25 Gy). The equation of the linear regression (dotted line) is $\text{R}2-{\text{R}}_2{}_{{}_0}=0.0372*\text{Dose}+0.0326({\text{r}}^2=0.9662)$.

Fig 4. Dose distribution in gel.

(A) Isodose map obtained for the irradiation dose level of 50 Gy. The black line defines the position of the Percent Depth Dose (PDD). (B) Normalized PDD measured from gel (blue) and from ion chamber (orange) with respective error bars. Indicative point of normalization at 0 mm for the ion chamber is obtained by extrapolation with an exponential fit.

Fig 5. Gel dose response measured with vegetable oil at low energy for 6 irradiation dose levels (15, 20, 25, 30, 35 and 40 Gy).

The equation of the linear regression (dotted line) is ${\text{R}}_2-{\text{R}}_2{}_{{}_0}=0.0811*\text{Dose}+0.1627({\text{r}}^2=0.9757)$.