Hilal Acar

Surface dose measurements with GafChromic EBT film for 6 and 18 MV photon beams

The aim of this study was to determine the surface doses using GafChromic EBT films and compare them with plane-parallel ionization chamber measurements for 6 and 18 MV high energy photon beams. The measurements were made in a water equivalent solid phantom in the build-up region of the 6 and 18MV photon beams at 100 cm SSD for various field sizes. Markus type plane-parallel ion chamber with fixed-separation between collecting electrodes was used to measure the percent depth doses. GafChromic EBT film measurements were performed both on the phantom surface and maximum dose depth at the same geometry with ion chamber measurements. The surface doses found using GafChromic EBT film were 15%, 20%, 29%and 39%+/-2% (1SD) for 6 MV photons, 6%, 11%, 23% and 32%+/-2% (1SD) for 18 MV photons at 5, 10, 20 and 30 cm(2) field sizes, respectively. GafChromic EBT film provides precise measurements for surface dose in the high energy photons. Agreement between film and plane-parallel chamber measurements was found to be within +/-3% for 18 MV photon beams. There was 5% overestimate on the surface doses when compared with the plane-parallel chamber measurements for all field sizes in the 6 MV photon beams.

Surface dose and build-up region measurements with wedge filters for 6 and 18 MV photon beams

PurposeHigh-energy photons are most commonly used in radiotherapy to treat cancer. Wedge filters are required to obtain homogeneous dose distribution in the patient. Different wedge filter types create different surface doses. In this study, the effect of the virtual and physical wedge filters on the surface and build-up region doses was examined for 6- and 18-MV high-energy photon beams.Materials and methodsThe measurements were made in a water equivalent phantom in the build-up region at a 100-cm source-to-surface distance for various field sizes using virtual and physical wedge filters having different angles. A parallel-plate ion chamber was used to measure the percent depth doses.ResultsThe percentage dose at the surface increased as the field size increased for open, virtual, and physical wedged beams. For open, physical, and virtual wedged beams, the surface doses were found to be 15.4%, 9.9%, and 15.9% with 6-MV photons and 10.6%, 8.8%, 11.9% with 18-MV photons, respectively, at 10 × 10 cm2 field size.ConclusionBuild-up doses of virtual wedged beams were similar to those of open beams. Surface and buildup doses of physical wedged beams were lower than those of open and virtual wedged beams.

Evaluation of material heterogeneity dosimetric effects using radiochromic film for COMS eye plaques loaded with 125I seeds (model I25.S16)

PURPOSE (1) To measure absolute dose distributions in eye phantom for COMS eye plaques with (125)I seeds (model I25.S16) using radiochromic EBT film dosimetry. (2) To determine the dose correction function for calculations involving the TG-43 formalism to account for the presence of the COMS eye plaque using Monte Carlo (MC) method specific to this seed model. (3) To test the heterogeneous dose calculation accuracy of the new version of Plaque Simulator (v5.3.9) against the EBT film data for this seed model. METHODS Using EBT film, absolute doses were measured for (125)I seeds (model I25.S16) in COMS eye plaques (1) along the plaques central axis for (a) uniformly loaded plaques (14-20 mm in diameter) and (b) a 20 mm plaque with single seed, and (2) in off-axis direction at depths of 5 and 12 mm for all four plaque sizes. The EBT film calibration was performed at (125)I photon energy. MC calculations using MCNP5 code for a single seed at the center of a 20 mm plaque in homogeneous water and polystyrene medium were performed. The heterogeneity dose correction function was determined from the MC calculations. These function values at various depths were entered into PS software (v5.3.9) to calculate the heterogeneous dose distributions for the uniformly loaded plaques (of all four sizes). The dose distributions with homogeneous water assumptions were also calculated using PS for comparison. The EBT film measured absolute dose rate values (film) were compared with those calculated using PS with homogeneous assumption (PS Homo) and heterogeneity correction (PS Hetero). The values of dose ratio (film∕PS Homo) and (film∕PS Hetero) were obtained. RESULTS The central axis depth dose rate values for a single seed in 20 mm plaque measured using EBT film and calculated with MCNP5 code (both in ploystyrene phantom) were compared, and agreement within 9% was found. The dose ratio (film∕PS Homo) values were substantially lower than unity (mostly between 0.8 and 0.9) for all four plaque sizes, indicating dose reduction by COMS plaque compared with homogeneous assumption. The dose ratio (film∕PS Hetero) values were close to unity, indicating the PS Hetero calculations agree with those from the film study. CONCLUSIONS Substantial heterogeneity effect on the (125)I dose distributions in an eye phantom for COMS plaques was verified using radiochromic EBT film dosimetry. The calculated doses for uniformly loaded plaques using PS with heterogeneity correction option enabled were corroborated by the EBT film measurement data. Radiochromic EBT film dosimetry is feasible in measuring absolute dose distributions in eye phantom for COMS eye plaques loaded with single or multiple (125)I seeds. Plaque Simulator is a viable tool for the calculation of dose distributions if one understands its limitations and uses the proper heterogeneity correction feature.

SU‐E‐T‐396: Heterogeneity Effect on Dose Distributions for COMS Eye Plaques with 125I Seeds (Model I25.S16): A Radiochromic EBT Film Dosimetry Study

Purpose: To study the heterogeneity effect on dose distributions in eye phantom for COMS eye plaques with 125I seeds using radiochromic EBT film dosimetry. Methods: COMS eye plaques (14,16,18 and 20mm in size) uniformly loaded with 125I seeds (model I25.S16) were studied. EBT film (lot #36076‐003AL) was positioned in a polystyrene eye phantom above the eye plaque, in two configurations: (1) in the plaques central plane, (2) perpendicular to the central axis at depth=5 or 12mm (from inner sclera) in the eye phantom. With the seed air‐kerma‐strengths of 2.4–4U, the exposure times (3.5–8.7h) were adjusted to deliver ∼400cGy at 5mm depth. Calibration films were irradiated by a single 125I seed at ∼5mm distance, one at a time, for doses up to 1200cGy. All films were scanned using Epson 10000XL scanner and data were analyzed using Mephysto FilmCal Mc2 software. With the established calibration curve, dose conversion was performed. Central axis depth doses and off‐axis profiles were determined. Results: The film dose data were compared with the calculated doses using Plaque Simulator v5.3.9 with homogeneous assumption (Homo) and heterogeneity correction (Hetero). The dose ratio (film/Homo) and (film/Hetero) values were obtained. The dose ratio (film/Homo) values are substantially lower than unity (mostly between 0.8 and 0.9) for all plaque sizes studied, indicating dose reduction by COMS plaque compared with homogeneous assumption. The film dose data agree with the Hetero values within the uncertainty of measurement data. Conclusions: We found significant heterogeneity effect on the 125I (model I25.S16) dose distributions in an eye for COMS plaques using radiochromic EBT film dosimetry. The film data agree with the calculated doses using Plaque Simulator with heterogeneity correction. The dose reduction effect for COMS plaques uniformly loaded with 125I seed (model I25.S16) found in this study is similar to that for other 125I seed models.