Investigation of the Field Size Effect on Wedge Field Isodose Curves Angle for Two Energies; 6 & 18 MV, produced by VARIAN 2100C Linac

Abstract

Introduction: Nowadays, considerable developments in the field of radiotherapy have been achieved. They include the advances made in the equipments and treatment planning techniques which require highly complex calculations. Such achievements have made it possible to treat cancer patients not only with higher radiation dose but also with higher precision and consequently increasing the chance of curing the cancer. However, the conventional techniques requiring physical wedge are still being used but with a lesser frequency. One of the wedge parameters needed to be measured is the wedge angle. It is the angle that the horizontal line creates with the tilted isodose curve at a specific depth and for a certain field size. In this study, the variation of wedge angle for different field sizes was evaluated using dosimetric and mathematical method. Material and Methods: For the wedge fields with a dimension of 6×6 to 20×20 cm 2 , the wedge angle for two photon energies of 6 and 18 MV was measured by the dosimetric method. For these measurements, the conventional wedges having the nominal wedge angle of 15, 30, 45 & 60 were used. The theoretical method suggested by Saw et al. is also used to indirectly calculate the slope of isodose curve by the dose profile and percent depth dose data. The dose profile, percentage depth dose and isodose curves were drawn for all the field sizes and the tilt of isodose curve at 10 cm depth, according to international definition, is considered as the wedge angle. The data were obtained using the theoretical equation of wedge angle and it was compared to the dosimetric data. Results: The result obtained in this work shows that the wedge angle increases with the field size. For a 6×6 cm 2 field size, the calculated wedge angle has the highest difference in comparison to the nominal wedge angle. The difference is equal to 14.7 degree for a 45° wedge and a 6 MV photon. The highest difference for a 45° wedge angle, a field size of 10×10 cm 2 and a 6 MV photon is 9.2 degree. Comparing the calculated and measured wedge angles shows a maximum difference of 4 degree for 6 and 18 MV photon beams. Discussion and Conclusion: The wedge angle varies with field size. In order to get a better dose distribution in the conventional radiotherapy, it is necessary to use the appropriate wedge angle which generates the desired slope for the isodose line and for the specific field size.

Keywords

• Profile Curve
• PDD Curve
• Isodose Curve
• Wedge Angle