Department of Therapeutic Radiology-Radiation Oncology, University of Minnesota, Minneapolis, Minnesota 55455, USA.
Med Phys. 2010 Jun;37(6):2731-7. doi: 10.1118/1.3431997.
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.
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 x 10 to 36 x 36 cm2.
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.
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.
在某些直线加速器中，三级多叶准直器（MLC）已被发现会影响因子 S(c)。在进行调试和采集光束数据时，通常会忽略这种对 S(c)的影响，因为散射/输出因子通常是在 MLC 缩回的情况下测量的。对于大多数常规放射治疗射野，这种影响可能是微不足道的，因为准直器的设置尽可能与 MLC 相吻合。然而，在调强放射治疗（IMRT）中，准直器通常设置为固定大小，而 MLC 则形成每个射束的段或控制点，因此 MLC 和准直器设置之间的差异可能很大。
作者量化了由 MLC 和准直器定义的不同射野大小的散射差异的大小，并描述了这种散射随 MLC 和准直器设置的变化。他们测量了不同大小的方形射野（孔径）的 S(c)和 S(c,p)因子，准直器设置从 10 x 10 到 36 x 36 cm2。
数据表明，MLC 定义的孔径和准直器设置之间的差异越大，输出因子之间的差异就越显著。作者为给定的准直器尺寸拟合了 MLC 输出因子曲线，以便将这种孔径效应作为准直器和 MLC 设置的函数进行参数化。
量化由于 MLC 引起的输出因子变化对于更好地模拟 IMRT 治疗和其他应用（如体内剂量测定）中的剂量可能很重要。
