Dosimetric characterization of a novel intracavitary mold applicator for high dose rate endorectal brachytherapy treatment
(a) An intracavitary mold applicator developed by Nucletron B. V. for preoperative treatment of endorectal cancer. It is made of flexible silicone rubber, long and in diameter. It houses eight catheters within its circumference. (b) An diameter rod made of tungsten or lead can be inserted into the central cavity of the applicator for shielding the surrounding normal tissues.
(a) A photograph of the applicator inserted into an endocavitary balloon for additional radiation protection. (b) A CT slice showing the balloon filled with contrast medium. Note that the normal tissues are pushed away from the target.
A cross-sectional view of the applicator from the proximal side. The catheter numbering convention used for this work is shown.
GEANT4 simulation setup for calculating dose reduction due to a balloon injected with iodine solution. The tungsten-shielded applicator is immersed in a water phantom. In separate simulations, the source is positioned in catheters 3, 4, and 5. The scoring voxel of diameter and depth is in contact with the balloon at the extreme left of the figure.
An Exradin A14P ionization chamber is placed in the middle layer of a Lucite phantom for measuring dose reduction at various points around the applicator due to tungsten shielding. The chamber has a sensitive volume at the distal end. Slabs of various thicknesses can be inserted between the chamber and the applicator, and the source is sent to catheters 3–7 in separate measurements.
(a) GEANT4 calculations of the anisotropy function of the Nucletron HDR classic model of the source for six radial distances between and . (b) Comparison of GEANT4 with results.
Absolute isodose distributions around the applicator with the source loaded in (a) catheter 1, without shielding, (b) catheter 1, with tungsten shielding, (c) catheters 1, 2, and 8, without shielding, and (d) catheters 1, 2, and 8, with tungsten shielding. EBT film measurements are represented by solid lines and GEANT4 results are represented by dotted lines. The contours are labeled in units of .
Ratios of dose with (a) lead and (b) tungsten shielding to dose without shielding as calculated using GEANT4. The source is positioned in catheter 7. The abscissa represents the distance between the applicator center and a point on a line through a given catheter.
Distributions of the scatter to total dose ratios along the axial plane of (a) an unshielded and (b) a tungsten-shielded applicator with the source loaded at catheter 5. The voxel dimensions are .
Ratios of dose around the applicator with tungsten shielding to dose without shielding as (a) measured using ion chamber, and (b) calculated using GEANT4. The source is positioned in catheter 7. Each number in brackets represents the uncertainty of the last digit.
Dose along the direction as indicated by the line in the insert with the applicator rotated (a) clockwise and (b) counterclockwise by , and relative to the case without rotation. The source is in catheter 6 in both cases.
Radial dose function of the HDR classic model compared to . The standard deviations of the GEANT4 results are less than 0.1%. The PHSP model without the source present is in agreement with the full simulation.
Uncertainty analysis for EBT film measurements and GEANT4 calculations along the transverse plane of the source placed inside an unshielded intracavitary mold applicator in water. The distance is along the transverse axis originating from the source center. Types A and B uncertainties represent the statistical and systematic uncertainties, respectively.
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