In radiotherapy, field sizes are defined in terms of the dimensions of the irradiation area. However, geometric square fields result in irradiation areas with rounded corners, which become almost elliptical for small fields. Superellipses are a family of curves encompassing shapes lying between ellipses and rectangles. The purpose of this work was to analyze the advantages and disadvantages of a novel approach that describes small-field sizes with superellipses. Square fields with nominal side lengths ranging from 0.5 to 10 cm were irradiated with two different linacs using 6 and 10 MV photon beams with and without flattening filters. Field size dimensions and output factors were measured by employing radiochromic films and the Radiochromic.com software. An alternative definition of equivalent square small-field size based on the superellipse (S_se) was introduced. The degree n of the superellipse for 10 cm nominal fields measured between 14.8 ± 1.0 to 27.7 ± 1.9. However, it decreased with the field size, down to between 2.26 ± 0.10 and 2.64 ± 0.15 for 0.5 cm nominal side lengths. A relation between the degree n and the equivalent square small-field size (S_clin) as defined by Cranmer-Sargison et al. [“A methodological approach to reporting corrected small field relative outputs,” Radiotherapy and Oncology 109, 350–355 (2013)] was found. For nominal side lengths of 10 cm, S_se was between 0.34 ± 0.04% and 0.10 ± 0.01% smaller than S_clin, while for 0.5 cm nominal side length S_se was between 9.5 ± 0.6% and 7.4 ± 0.7% smaller than S_clin. There was no significant difference in the goodness of the regression between using S_se or S_clin to fit field output factors with the function proposed by Sauer and Wilbert. Small fields were found to be more accurately characterized with superellipses. The advantages and disadvantages of describing field sizes with superellipses were examined. Field output factors can be derived with equivalent square small-field sizes based on the superellipse approach.

在放射治疗中，根据照射区域的尺寸定义场大小。然而，几何方形场会导致带圆角的照射区域，对于小场来说几乎变成椭圆形。超级椭圆是一系列曲线，包含位于椭圆和矩形之间的形状。这项工作的目的是分析一种用超椭圆描述小视场尺寸的新方法的优缺点。标称边长范围为 0.5 到 10 cm 的方形场被两种不同的直线加速器使用 6 和 10 MV 光子束照射，有和没有平坦滤波器。通过使用放射变色薄膜和 Radiochromic.com 软件测量场尺寸尺寸和输出因子。基于超椭圆​​的等​​效正方形小视场尺寸的另一种定义（S _se ) 被引入。在 14.8 ± 1.0 到 27.7 ± 1.9 之间测得的 10 cm 标称视场的超椭圆度数n。然而，它随着视野大小而减小，对于 0.5 cm 标称边长，下降到 2.26 ± 0.10 和 2.64 ± 0.15 之间。Cranmer-Sargison 等人定义的度数n与等效方形小视野大小 ( S _clin )之间的关系。[“一种报告校正小场相对输出的方法，”放射治疗和肿瘤学 109, 350–355 (2013)] 被发现。对于 10 cm 的标称边长，S _se比S _clin小0.34 ± 0.04 %到 0.10 ± 0.01 %，而对于 0.5 厘米的标称边长，S _se比S _clin小9.5 ± 0.6 %到 7.4 ± 0.7 %。使用S _se或S _clin拟合场输出因子与 Sauer 和 Wilbert 提出的函数之间的回归优度没有显着差异。发现用超椭圆更准确地表征小场。研究了用超椭圆描述场大小的优点和缺点。场输出因子可以基于超椭圆​​方法用等效的方形小场尺寸导出。