Duan Jun, Shen Sui, Spencer Sharon A, Ahmed Raef S, Popple Richard A, Ye Sung-Joon, Brezovich Ivan A
Department of Radiation Oncology, University of Alabama at Birmingham, 619 South 19th Street, Birmingham, AL 35233, USA.
Int J Radiat Oncol Biol Phys. 2004 Nov 1;60(3):959-72. doi: 10.1016/j.ijrobp.2004.06.213.
The conventional single-isocenter and half-beam (SIHB) technique for matching supraclavicular fields with head-and-neck (HN) intensity-modulated radiotherapy (IMRT) fields is subject to substantial dose inhomogeneities from imperfect accelerator jaw/MLC calibration. It also limits the isocenter location and restricts the useful field size for IMRT. We propose a dynamic field-matching technique to overcome these limitations.
The proposed dynamic field-matching technique makes use of wedge junctions for the abutment of supraclavicular and HN IMRT fields. The supraclavicular field was shaped with a multileaf collimator (MLC), which was orientated such that the leaves traveled along the superoinferior direction. The leaves that defined the superior field border moved continuously during treatment from 1.5 cm below to 1.5 cm above the conventional match line to generate a 3-cm-wide wedge-shaped junction. The HN IMRT fields were optimized by taking into account the dose contribution from the supraclavicular field to the junction area, which generates a complementary wedge to produce a smooth junction in the abutment region. This technique was evaluated on a polystyrene phantom and 10 HN cancer patients. Treatment plans were generated for the phantom and the 10 patients. Dose profiles across the abutment region were measured in the phantom on films. For patient plans, dose profiles that passed through the center of the neck lymph nodes were calculated using the proposed technique and the SIHB technique, and dose uniformity in the abutment region was compared. Field mismatches of +/- 1 mm and +/- 2 mm because of imperfect jaw/MLC calibration were simulated, and the resulting dose inhomogeneities were studied for the two techniques with film measurements and patient plans. Three-dimensional volumetric doses were analyzed, and equivalent uniform doses (EUD) were computed. The effect of field mismatches on EUD was compared for the two match techniques.
For a perfect jaw/MLC calibration, dose profiles for the 10 patients in the 3-cm match zone had an average inhomogeneity range of -1.6% to +1.6% using the dynamic-matching technique and -3.7% to +3.8% according to the SIHB technique. Measurements showed that dose inhomogeneities that resulted from 1-mm and 2-mm jaw/MLC calibration errors were reduced from as large as 27% and 45% with the SIHB technique to less than 2% and 5.7% with the dynamic technique, respectively. For -1-mm, -2-mm, +1-mm, and +2-mm jaw/MLC calibration errors, respectively, treatment plans for the 10 patients yielded average dose inhomogeneities of -5.9%, -3.0%, +2.7%, and +5.8% with the dynamic technique as compared to -22.8%, -11.1%, +9.8%, and +22.1% with the SIHB technique. Calculation based on a dose-volume histogram (DVH) showed that the SIHB technique resulted in larger changes in EUD of the PTV in the junction area than did the dynamic technique.
Compared with the conventional SIHB technique, the dynamic field-matching technique provides superior dose homogeneity in the abutment region between the supraclavicular and HN IMRT fields. The dynamic feathering mechanism substantially reduces dose inhomogeneities that result from imperfect jaw/MLC calibration. In addition, isocenter location in the dynamic field-matching technique can be chosen for reproducible patient setup and for adequate IMRT field size rather than being dictated by the match position. It also allows angling of the supraclavicular field to reduce the volume of healthy lung irradiated, which is impractical with the SIHB technique. In principle, this technique should be applicable to any treatment site that requires the abutment of static and intensity-modulated fields.
用于锁骨上野与头颈部（HN）调强放疗（IMRT）野匹配的传统单等中心半束（SIHB）技术，因加速器准直器/多叶准直器（MLC）校准不完善而存在显著的剂量不均匀性。它还限制了等中心位置，并限制了IMRT的有效射野大小。我们提出一种动态野匹配技术以克服这些局限性。
所提出的动态野匹配技术利用楔形连接来衔接锁骨上野和HN IMRT野。锁骨上野由多叶准直器（MLC）塑形，其叶片沿上下方向移动。定义上野边界的叶片在治疗过程中从传统匹配线下方1.5 cm连续移动到上方1.5 cm，以产生一个3 cm宽的楔形连接。HN IMRT野通过考虑锁骨上野对连接区域的剂量贡献进行优化，从而产生一个互补楔形，以在衔接区域产生平滑连接。该技术在聚苯乙烯模体和10例HN癌症患者身上进行了评估。为模体和10例患者生成了治疗计划。在模体中使用胶片测量衔接区域的剂量分布。对于患者计划，使用所提出的技术和SIHB技术计算穿过颈部淋巴结中心的剂量分布，并比较衔接区域的剂量均匀性。模拟了由于准直器/MLC校准不完善导致的±1 mm和±2 mm的野匹配误差，并通过胶片测量和患者计划研究了两种技术产生的剂量不均匀性。分析三维体积剂量，并计算等效均匀剂量（EUD）。比较了两种匹配技术野匹配误差对EUD的影响。
对于完美的准直器/MLC校准，使用动态匹配技术时，10例患者在3 cm匹配区域的剂量分布平均不均匀性范围为 -1.6%至 +1.6%，而根据SIHB技术为 -3.7%至 +3.8%。测量表明，由1 mm和2 mm准直器/MLC校准误差导致的剂量不均匀性，使用SIHB技术时分别高达27%和45%，而使用动态技术时分别降至小于2%和5.7%。对于 -1 mm、-2 mm、+1 mm和 +2 mm的准直器/MLC校准误差，10例患者的治疗计划使用动态技术时平均剂量不均匀性分别为 -5.9%、-3.0%、+2.7%和 +5.8%，而使用SIHB技术时分别为 -22.8%、-11.1%、+9.8%和 +22.1%。基于剂量体积直方图（DVH）的计算表明，与动态技术相比，SIHB技术在连接区域导致PTV的EUD变化更大。
与传统的SIHB技术相比，动态野匹配技术在锁骨上野和HN IMRT野之间的衔接区域提供了更好的剂量均匀性。动态羽化机制显著降低了由于准直器/MLC校准不完善导致的剂量不均匀性。此外，动态野匹配技术中的等中心位置可以选择，以实现可重复的患者摆位和足够的IMRT射野大小，而不是由匹配位置决定。它还允许锁骨上野倾斜以减少受照射的健康肺体积，这对于SIHB技术是不切实际的。原则上，该技术应适用于任何需要衔接静态野和调强野的治疗部位。
