Brugmans M J, van der Horst A, Lebesque J V, Mijnheer B J
Radiotherapy Department, The Netherlands Cancer Institute/Antoni van Leeuwenhoek Huis, Amsterdam.
Int J Radiat Oncol Biol Phys. 1999 Mar 1;43(4):893-904. doi: 10.1016/s0360-3016(98)00489-1.
In conformal radiotherapy of lung tumors, penumbra broadening in lung tissue necessitates the use of larger field sizes to achieve the same target coverage as in a homogeneous environment. In an idealized model configuration, some fundamental aspects of field size reduction were investigated, both for the static situation and for a moving tumor, while maintaining the dose homogeneity in the target volume by employing a simple beam-intensity modulation technique.
An inhomogeneous phantom, consisting of polystyrene, cork, and polystyrene layers, with a 6 x 6 x 6 cm3 polystyrene cube inside the cork representing the tumor, was used to simulate a lung cancer treatment. Film dosimetry experiments were performed for an AP-PA irradiation technique with 8-MV or 18-MV beams. Dose distributions were compared for large square fields, small square fields, and intensity-modulated fields in which additional segments increase the dose at the edge of the field. The effect of target motion was studied by measuring the dose distribution for the solid cube, displaced with respect to the beams.
For the 18-MV beam, the field sizes required to establish a sufficient target coverage are larger than for the 8-MV beam. For each beam energy, the mean dose in cork can significantly be reduced (at least a factor of 1.6) by decreasing the field size with 2 cm, while keeping the mean target dose constant. Target dose inhomogeneity for these smaller fields is limited if the additional edge segments are applied for 8% of the number of monitor units given with the open fields. The target dose distribution averaged over a motion cycle is hardly affected if the target edge does not approach the field edge to within 3 mm.
For lung cancer treatment, a beam energy of 8 MV is more suitable than 18 MV. The mean lung dose can be significantly reduced by decreasing the field sizes of conformal fields. The smaller fields result in the same biological effect to the tumor if the mean target dose is kept constant. Intensity modulation can be employed to maintain the same target dose homogeneity for these smaller fields. As long as the target (with a 3 mm margin) stays within the field portal, application of a margin for target motion is not necessary.
在肺部肿瘤的适形放射治疗中，肺组织中的半影增宽使得需要使用更大的射野尺寸，以达到与均匀环境中相同的靶区覆盖。在一个理想化的模型配置中，研究了射野尺寸减小的一些基本方面，包括静态情况和移动肿瘤的情况，同时通过采用一种简单的射束强度调制技术来保持靶区内的剂量均匀性。
使用一个由聚苯乙烯、软木和聚苯乙烯层组成的非均匀体模，在软木内部有一个6×6×6 cm³ 的聚苯乙烯立方体代表肿瘤，用于模拟肺癌治疗。对采用8 MV或18 MV射束的前后位-后前位（AP-PA）照射技术进行了胶片剂量测定实验。比较了大方形射野、小方形射野以及强度调制射野（其中额外的子野增加了射野边缘的剂量）的剂量分布。通过测量相对于射束发生位移的实心立方体的剂量分布，研究了靶区运动的影响。
对于18 MV射束，建立足够靶区覆盖所需的射野尺寸比8 MV射束的大。对于每种射束能量，通过将射野尺寸减小2 cm，同时保持平均靶区剂量不变，软木中的平均剂量可显著降低（至少降低1.6倍）。如果将额外的边缘子野应用于开放射野所给监测单位数量的8%，则这些较小射野的靶区剂量不均匀性是有限的。如果靶区边缘与射野边缘的距离不接近到3 mm以内，则在一个运动周期内平均的靶区剂量分布几乎不受影响。
对于肺癌治疗，8 MV的射束能量比18 MV更合适。通过减小适形射野的尺寸，可以显著降低平均肺剂量。如果平均靶区剂量保持不变，较小的射野对肿瘤产生相同的生物学效应。可以采用强度调制来保持这些较小射野相同的靶区剂量均匀性。只要靶区（有3 mm的边界）保持在射野范围内，就无需为靶区运动设置边界。
