Beddar A S, Briere T M, Ouzidane M
Department of Radiation Physics, Division of Radiation Oncology, The University of Texas M D Anderson Cancer Center, 1515 Holcombe Blvd, Box 94, Houston, TX 77030, USA.
Phys Med Biol. 2006 Sep 21;51(18):N331-7. doi: 10.1088/0031-9155/51/18/N01. Epub 2006 Aug 30.
Intraoperative radiation therapy (IORT) consists of delivering a large, single-fraction dose of radiation to a surgically exposed tumour or tumour bed at the time of surgery. With the availability of a mobile linear accelerator in the OR, IORT procedures have become more feasible for medical centres and more accessible to cancer patients. Often the area requiring irradiation is larger than what the treatment applicators will allow, and therefore, two or more adjoining fields are used. Unfortunately, the divergence and scattering of the electron beams may cause significant dose variations in the region of the field junction. Furthermore, because IORT treatments are delivered in a large single fraction, the effects of underdosing or overdosing could be more critical when compared to fractionated external beam therapy. Proper matching of the fields is therefore an important technical aspect of treatment delivery. We have studied the matching region using the largest flat applicator available for three different possibilities: abutting the fields, leaving a small gap or creating an overlap. Measurements were done using film dosimetry for the available energies of 4, 6, 9 and 12 MeV. Our results show the presence of clinically significant cold spots for the low-energy beams when the fields are either gapped or abutted, suggesting that the fields should be overlapped. No fields should be gapped. The results suggest that an optimal dose distribution may be obtained by overlapping the fields at 4 and 6 MeV and simply abutting the fields at 9 and 12 MeV. However, due to uncertainties in the placement of lead shields during treatment delivery, one may wish to consider overlapping the higher energy fields as well.
术中放射治疗（IORT）是指在手术时对手术暴露的肿瘤或肿瘤床给予大剂量单次放射。随着手术室中移动直线加速器的出现，IORT程序对医疗中心来说变得更加可行，癌症患者也更容易接受。通常，需要照射的区域比治疗施源器所能允许的区域大，因此，会使用两个或更多相邻野。不幸的是，电子束的发散和散射可能会在野交界区域引起显著的剂量变化。此外，由于IORT治疗是大剂量单次给予的，与分次外照射治疗相比，剂量不足或过量的影响可能更为严重。因此，野的恰当匹配是治疗实施的一个重要技术方面。我们使用可获得的最大平板施源器，针对三种不同情况研究了匹配区域：野相邻、留小间隙或重叠。使用薄膜剂量测定法对4、6、9和12兆电子伏的可用能量进行了测量。我们的结果表明，当野有间隙或相邻时，低能束存在临床上显著的冷点，这表明野应该重叠。不应有间隙野。结果表明，通过在4和6兆电子伏时重叠野，在9和12兆电子伏时简单相邻野，可能获得最佳剂量分布。然而，由于治疗实施过程中铅屏蔽放置的不确定性，人们可能也希望考虑对高能野进行重叠。
