Department of Radiation Oncology, Northwestern Memorial Hospital, Northwestern University Feinberg School of Medicine, Chicago, Illinois, USA.
Department of Radiation Oncology, Vanderbilt University School of Medicine, Nashville, Tennessee, USA.
Med Phys. 2022 Jun;49(6):4043-4055. doi: 10.1002/mp.15624. Epub 2022 Apr 7.
The equivalent square (ES) concept has been used for traditional radiation fields defined by the machine collimating system. For small fields, the concept S was introduced based on measuring dosimetric field width (full-width half maximum, FWHM) of the cardinal axis of the beam profiles. The pros and cons of this concept are evaluated in small fields and compared with the traditional ES using area and perimeter (4A/P) method based on geometric field size settings, for example, light field settings.
One hundred thirty-seven square and rectangular fields from 5-50 mm with every possible permutation (keeping one jaw fixed and varying other jaw from 5 to 50 mm) were utilized to measure FWHM for the validation of S . Using a microSilicon detector and a scanning water tank, measurements were performed on an Elekta (Versa) machine with Agility head and a Varian TrueBeam with different MLC/Jaw design to evaluate the S concept and to understand the effect of exchange factor in small fields. Field output factors were also measured for all 137 fields.
The data fitting for fields ranging from 5-50 mm between the traditional 4A/P method and S shows differences and indicates a linear relationship with distinct separation of slope for Elekta and Varian machines. For Elekta Agility machine ES based on 4A/P < S and for the VarianTrueBeam 4A/P > S for square fields. Our measured data show that both methods are equally valid but does vary by the machine design. The field output factor is dependent on the elongation factor as well as machine design. For fields with sides ≥10 mm, the exchange factor is nearly identical in both machines with magnitude up to 4%, which is close to measurement uncertainty (±3%), but for small fields (< 10 mm), the Elekta machine has higher exchange factors compared to the Varian machine.
The results demonstrate that the two concepts for defining equivalent field (S and 4A/P) are equivalent and can be directly related through an empirical equation. This study confirms that 4A/P is still valid for small fields except for very small fields (≤10 mm) where source occlusion is a dominating factor. The S method is potentially sensitive to measurement uncertainty due to measurement of FWHM which is machine-, detector- and user-dependent, while the 4A/P method relies mainly on geometry of the machine and has less dependency on type of machine, detector, and user. The exchange factors are comparable for both types of machines. The conclusion is based on data from an Elekta with Agility head and a Varian TrueBeam machine that may have potential for bias due to light field/collimator set up and alignment. Care should be taken in extrapolating these data to any other machine.
等效方（ES）概念已用于由机器准直系统定义的传统辐射场。对于小场，基于测量束轮廓的主轴线的剂量学场宽度（半峰全宽，FWHM），引入了 S 概念。该概念的优缺点在小场中进行了评估，并与基于几何场尺寸设置（例如光场设置）的传统 ES 相比，使用面积和周长（4A/P）方法进行了比较。
利用 5-50mm 的 137 个正方形和矩形场，以及每种可能的排列（固定一个遮线器并将另一个遮线器从 5 到 50mm 变化），用于测量 FWHM 以验证 S。使用微硅探测器和扫描水箱，在配备有 Agility 头的 Elekta（Versa）机器和具有不同 MLC/遮线器设计的 Varian TrueBeam 上进行测量，以评估 S 概念并了解小场中的交换因子的影响。还测量了所有 137 个场的射野输出因子。
5-50mm 之间的传统 4A/P 方法和 S 之间的数据拟合表明存在差异，并表明 Elekta 和 Varian 机器之间存在明显的斜率分离的线性关系。对于 Elekta Agility 机器，基于 4A/P 的 ES< S，而对于 VarianTrueBeam，4A/P> S，适用于正方形场。我们的测量数据表明，两种方法都是等效的，但因机器设计而异。射野输出因子取决于伸长因子以及机器设计。对于边长≥10mm 的场，在两台机器中，交换因子几乎相同，幅度高达 4%，接近测量不确定度（±3%），但对于小场（<10mm），Elekta 机器的交换因子高于 Varian 机器。
结果表明，定义等效场的两个概念（S 和 4A/P）是等效的，可以通过经验方程直接相关。这项研究证实，4A/P 对于小场仍然有效，除了非常小的场（≤10mm），其中源遮挡是一个主导因素。S 方法由于测量 FWHM 而可能对测量不确定性敏感，因为 FWHM 测量依赖于机器、探测器和用户，而 4A/P 方法主要依赖于机器的几何形状，并且对机器、探测器和用户的类型的依赖性较小。两种类型的机器的交换因子是可比的。该结论基于 Elekta 配备有 Agility 头和 Varian TrueBeam 机器的数据，由于光场/准直器设置和对准，可能存在偏差。在将这些数据外推到任何其他机器时应谨慎。
