Comparative study of the simulated and calculation gantry angle methods in tangential breast irradiation

Breast irradiation involves a complex geometric and field-matching technique. Simulators are used to obtain the best and accurate patient treatment positioning as well as irradiation geometry for radiation portals. However many centers in developing countries lack this important equipment. The study was designed to determine the accuracy and reliability of the gantry angle calculation method for tangential breast irradiation in comparison with the simulated method. This prospective study was conducted at the National Center for Radiotherapy and Nuclear Medicine, Accra, Ghana between June and October, 2012 with a sample size of 50 breast cancer patients. The simulator method was compared with the calculated method to assess if the two methods can be used interchangeably using Bland-Altman analysis. The sensitivity and specificity values of the calculated formula in estimating accurate gantry angle as well as beta gantry angle were also calculated using Receiver Operator Characteristic. The coefficient of variation (CV) of the results generated by the simulation method (65.18%) was similar to the coefficient of variation of the results generated by the proposed formula (65.30%). The CV of the beta angles results from the breast bridge (13.50%) was also consistent with the calculated formula (14.04%). The sensitivity and specificity of the calculated formula for gantry angle are 100% and 100% respectively. The sensitivity and specificity of the calculated formula for beta angle are 100% and 98% respectively. None of the gantry angle values generated by calculated formula was different from the corresponding value from the simulator by 3 degree or more. Within the limit of this study, the two methods can be used interchangeably without significant variation in treatment plan and outcome. (au)