Dose Distribution In Build-up Region Research Articles

P213] Influence of beam spoiler and air gap on dose distribution in build-up region for X6 MV static field

Purpose Basing on CBCT images we noticed, that there are several different problems with reproducibility of bolus positioning, especially air gap occurrence. The aim of this study was to determine the influence of air gaps between bolus and phantom on dose distribution in build-up region for X6 MV. The novelty of the work is using low-density boluses fabricated with 3D printer. Methods Depth dose measurements were performed for X6 MV 10×10 cm2 open field, generated with Varian CL 2300 C/D. The measurements were carried out with the Markus plane parallel ionization chamber (PTW, 0.055 cm3 volume), connected to Unidos (PTW) electrometer. As 1 cm thick beam spoilers we used 3D-printed thermoplastic polyurethane (TPU) cuboids filled with 5%, 10%, 15% and 20% honeycomb structure (thickness of 0.5 g/cm2; 0.17 g/cm2; 0.22 g/cm2). To generate air gap we used homemade styrofoam frames. Measurements were performed at a physical depth of 1, 5, 10 and 15 mm for all beam spoilers without air gaps and with 10, 20, 30 and 40 mm air gaps, at constant 90 cm SSD. Results Our results showed that beam spoilers without air gap increased the dose in build-up region. The bigger spoiler filling is, the larger is surface dose. The same dependence can be seen on Percentage Depth-Dose curves, irrespectively of air gap size. Regardless of spoiler percentage filling with honeycomb structure, the larger air gap size is, the smaller dose on 1 mm depth. Conclusions Influence of beam spoiler filling and air gap on dose distribution in build-up region for X6 MV was investigated. Beam spoiler increased the dose in build-up region. The larger thickness of beam spoiler (larger filling) - the larger influence on build-up dose. Occurrence of air gaps lowers the surface dose. However, the influence of air gap in the range of 10–40 mm is not very visible. For bolus with density of 0.22 g/cm2; regardless of air gap size, the dose on 1 mm depth was always larger than 95% of maximum dose. The issue will be investigated for wedged and dynamic fields.

EP-1846: Influence of beam spoiler and air gap on dose distribution in build-up region for X6 MV open field

EP-1846: Influence of beam spoiler and air gap on dose distribution in build-up region for X6 MV open field

ELECTRON BEAM DOSE DISTRIBUTION IN THE PRESENCE OF NON-UNIFORM MAGNETIC FIELD

Introduction Magnetic fields are capable of altering the trajectory of electron beams andcan be used in radiation therapy.Theaim of this study was to produce regions with dose enhancement and reduction in the medium. Materials and Methods The NdFeB permanent magnets were arranged on the electron applicator in several configurations. Then, after the passage of the electron beams (9 and 15 MeV Varian 2100C/D) through the non-uniform magnetic field, the Percentage Depth Dose(PDDs) on central axis and dose profiles in three depths for each energy were measured in a 3D water phantom. Results For all magnet arrangements and for two different energies, the surface dose increment and shift in depth of maximum dose (dmax) were observed. In addition, the pattern of dose distribution in buildup region was changed. Measurement of dose profile showed dose localization and spreading in some other regions. Conclusion The results of this study confirms that using magnetic field can alter the dose deposition patterns and as a result can produce dose enhancement as well as dose reduction in the medium using high-energy electron beams. These effects provide dose distribution with arbitrary shapes for use in radiation therapy.

SU‐FF‐T‐77: Analysis of Dose Distributions in Buildup Region for IMRT Beams

Purpose: To measure the dose distribution in buildup region with a 2D ion chamber array. Measurements are compared with calculation from Eclipse and analyzed using Gamma Index method. Method and Materials: Dose distributions from 6 MV photon IMRT beams in buildup region are calculated with Eclipse TPS (Varian Medical Systems) in a phantom using sliding window technique for a head and neck case. Those beams are delivered by a Varian Clinac iX linear accelerator with 120‐leaf Millennium MLC and are measured with PTW729 ion chamber array (PTW, Inc.). All the measurements are performed at 0.5 cm depth with 96.5 cm SSD. The calculated and measured dose distributions are compared with Gamma index method using Verisoft software (PTW, Inc.). By changing the acceptance criteria in Gamma Index method from 5% dose difference, and 5 mm DTA (distance‐to‐agreement) to 20%, and 20 mm respectively, the comparison between measured and calculated dose distribution are analyzed. Results: Typical preliminary results are listed below: It can be seen for highly modulated beams, dose difference of 20% and positional difference of 20 mm can result between measurement and calculation. Conclusion: The dose distributions in the build up region measured by PTW 2D ion chamber array are compared with those calculated by Eclipse. The comparison is analyzed by Gamma Index method. Detailed measurement validation is underway with different intensity modulation levels and positional accuracy. This type of study can help us to better understand of the accuracy of dose calculations and measurement in buildup region.