Purpose:With the establishment of the IGRT as a standard technique, the extra dose that is given to the patients should be taken into account. Furthermore, it has been a recent decrease of the dose threshold in the lens, reduced to 0.5 Gy (ICRP ref 4825‐3093‐1464 on 21st April, 2011). The purpose of this work was to evaluate the extra dose that the lens is receive due to the Cone‐Beam (CBCT) location systems in Head‐and‐Neck treatments.Methods:The On‐Board Imaging (OBI) v 1.5 of the two Varian accelerators, one Clinac iX and one True Beam, were used to obtain the dose that this OBI version give to the lens in the Head‐and‐Neck location treatments. All CBCT scans were acquired with the Standard Dose Head protocol (100 kVp, 80 mA, 8 ms and 200 degree of rotation). The measurements were taken with thermoluminescence (TLD) EXTRAD (Harshaw) dosimeters placed in an anthropomorphic phantom over the eye and under 3 mm of bolus material to mimic the lens position. The center of the head was placed at the isocenter. To reduce TLD energy dependence, they were calibrated at the used beam quality.Results:The average lens dose at the lens in the OBI v 1.5 systems of the Clinac iX and the True Beam is 0.071 and 0.076 cGy/CBCT, respectively.Conclusions:The extra absorbed doses that receive the eye lenses due to one CBCT acquisition with the studied protocol is far below the new ICRP recommended threshold for the lens. However, the addition effect of several CBCT acquisition during the whole treatment should be taken into account.
Purpose: The Valencia HDR skin applicators are accessories of the microSelectron HDR afterloading system (Nucletron) cup shaped to limit the dose to the irradiation area. Applicators sizes are: diameter 2 and 3 cm. The typical prescription depth is 3 mm. This work aims to evaluate the backscatter and electron contamination of an internal shielding used to reduce the dose to the ocular globe when the lesion is located at the eyelid. Monte Carlo (MC) and experimental methods have been used. Method and Materials: The geometry has been: the Valencia applicator (2 cm diameter) on the surface of a solid polystyrene slab phantom; inside, a 2 mm lead slab located at 3 mm depth. MC simulation characteristics: GEANT4 code (Version 9.0); Scored absorbed dose; Cell size: 0.1 mm in the depth axis; Photon histories: 3×108. Radichromic EBT film dosimetry setup and processing: Films depths: 1,5, 3, 5 and 7 mm, inside the phantom; Irradiations, with and without the lead shielding slab; Flatbed scanner; Matlab software to obtain the red channel and to filter noise; Verisoft (PTW) software utilities to calibrate the optical density against dose and to analyse the results. Results: MC simulation gives an overdose close the lead (backscatter component) of about 190% (range: 0.5 mm). The electron contamination under the lead is about 130% (range: 1 mm). These results agree with measurements with radichromic film within experimental uncertainties. The dose reduction behind the shield is about 60%. Conclusion: The use of an internal 2 mm lead shielding, in eyelid skin treatments with the Valencia applicators has been evaluated. The reduction in dose to the ocular globe with a 2 mm lead shielding is about 60%. A minimum bolus of 0.5 mm and 1 mm thickness must be added above and below the shielding, respectively to remove the dose contamination.
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