Notake Ryoichi, Kyuuma Yukishige, Sasamori Kouji, Sagawa Makoto, Annaka Yasushi, Saitoh Hidetoshi, Shibuya Hitoshi
Head and Neck Reconstruction Division, Department of Diagnostic Radiology and Oncology, Graduate School, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyoku, Tokyo, 113-8519, Japan,
Radiol Phys Technol. 2014 Jan;7(1):114-23. doi: 10.1007/s12194-013-0251-0. Epub 2013 Dec 10.
In external beam radiation therapy, it is important to calculate the output of the linear accelerator. The head scatter factor, S h, is one of the important factors for calculation of Monitor Unit, which changes with the size of the irradiation field. In irregular fields shaped by multileaf collimators (MLCs), it is difficult to calculate S h precisely. S h comprises backscatter from the upper and lower secondary collimators and scatter from the flattening filter. We measured the effect of backscatter on a monitor chamber (S b), and we modeled the scatter from a flattening filter using a Gaussian distribution. The modeled parameters used in this method are derived from measurements of square field sizes on the central axis. Furthermore, we divided an MLC irregular field in the shape of fans and integrated the scatter from a flattening filter by a method similar to Clarkson's sector integration. We were able to calculate S h with <1% error in comparison with measurements, even with a field setting with an error of >3% by the conventional method. This method requires no special measuring tools or software.
