ABSTRACT
Objective To evaluate the effect of ion chamber sensitive volume on absolute dose verification in CyberKnife plan.Methods Solid water phantoms were scanned by a CT scanner, single-field plan, multi-field isocentric plan and sequential optimized plan were designed by the treatment planning system.Absolute doses were measured at the specified point in each plan using the ion chambers with sensitive volumes of 0.007 cm3(A16), 0.24 cm3(A12 s), and 0.6 cm3(PTW30013) and compared with calculated values.Results For the single-field plan, the relative error increased as the aperture size of collimator decreased;with relative errors within ±2%, the smallest aperture sizes of collimator were 12.5 mm (A16), 25 mm (A12 s), and 30 mm (PTW30013).For the multi-filed isocentric plan, the relative errors were 0.26%±3.90%(A16),-6.28%±14.33%(A12 s), and-9.41%±14.10%(PTW30013).For the sequential plan optimized with 15 mm cone, the relative error was 0.79%±1.43%;for the sequential plan optimized with 7.5 mm cone, the relative error was 2.01%±8.39%.In absolute dose verification for clinical plans, there was no significant difference between the results measured by these ion chambers (P=0.985).Conclusions There is no significant effect of ion chamber sensitive volume on absolute dose verification in CyberKnife plan under the following two situations:(1) the collimator with a relatively large aperture is used;(2) the sensitive volume of ion chamber is totally covered by the prescription isodose line.
ABSTRACT
Objective:To study the dose measurements methods of neutron andγ-ray for 252Cf neutron after-load radiotherapy machine. Methods:To measure the neutron-γmixed field with one ion chamber that have the similar sensitivities of neutron andγ-ray, another ion chamber that only have sensitivities for γ-ray, little sensitivity for neutron. Verification measurement results with calculated values. Results:Calculate key parameters, measure neutron andγ-ray dose rate at the position of 2.5cm, 5cm, 7.5cm and 10cm from the 252Cf, the Maximum deviation is-5.22%compared with calculated values. Conclusion:The method of twin ion chamber can measure the neutron andγ-ray mixed field dose.
ABSTRACT
In order to evaluate the radio-protective advantage of an enhanced dynamic wedge (EDW) over a physical wedge (PW), we measured peripheral doses scattered from both types of wedges using a 2D array of ion-chambers. A 2D array of ion-chambers was used for this purpose. In order to confirm the accuracy of the device, we first compared measured profiles of open fields with the profiles calculated by our commissioned treatment planning system. Then, we measured peripheral doses for the wedge angles of 15 degrees, 30 degrees, 45 degrees, and 60 degrees at source to surface distances (SSD) of 80 cm and 90 cm. The measured points were located at 0.5 cm depth from 1 cm to 5 cm outside of the field edge. In addition, the measurements were repeated by using thermoluminescence dosimeters (TLD). The peripheral doses of EDW were (1.4% to 11.9%) lower than those of PW (2.5% to 12.4%). At 15 MV energy, the average peripheral doses of both wedges were 2.9% higher than those at 6MV energy. At a small SSD (80 cm vs. 90 cm), peripheral dose differences were more recognizable. The average peripheral doses to the heel direction were 0.9% lower than those to the toe direction. The results from the TLD measurements confirmed these findings with similar tendency. Dynamic wedges can reduce unnecessary scattered doses to normal tissues outside of the field edge in many clinical situations. Such an advantage is more profound in the treatment of steeper wedge angles, and shorter SSD.