Effect of multileaf collimator-defined segment size on S_c

Purpose: Tertiary multileaf collimators (MLCs) in certain linear accelerators have been found to influence the factor S_c. This effect on S_c is often ignored at the time of commissioning and beam data collection since the scatter/output factors are usually measured with the MLCs retracted. This may be a negligible effect for majority of conventional radiotherapy portals since the jaws are set to conform with MLCs as closely as possible. In the case of IMRT treatments, however, the jaw is often set at a fixed size, and the MLCs form the segments, or control points, of each beam, thus the difference between MLC and jaw settings may be significant. Methods: The authors quantified the magnitude of this scatter difference for various field sizes defined by MLCs and jaws and characterized the variation in this scatter with MLC and jaw settings. They measured S_c and S _c,p factors for square fields (apertures) of different sizes for jaw settings from 10×10 to 36×36 cm². Results: The data indicate that the larger the difference between MLC-defined aperture and jaw settings, the more significant the difference between the output factors. The authors fitted curves to the MLC output factors for any given jaw size in order to parametrize this aperture effect as a function of jaw and MLC setting. Conclusions: The quantification of the change in the output factor due to MLCs may be important for better modeling of dose in IMRT deliveries and other applications such as in vivo dosimetry.