Astrahan M A, Luxton G, Pu Q, Petrovich Z
Department of Radiation Oncology, University of Southern California School of Medicine, Los Angeles 90033, USA.
Int J Radiat Oncol Biol Phys. 1997 Sep 1;39(2):505-19. doi: 10.1016/s0360-3016(97)00118-1.
Episcleral plaque therapy (EPT) with sealed 125I sources is widely used in the treatment of choroidal melanoma. In EPT, as elsewhere in radiotherapy, concern for normal tissue tolerance has frequently been a dose-limiting factor. The concept of conformal therapy, which seeks to improve dose homogeneity within the tumor and greatly reduce the dose to uninvolved structures may provide a solution to this problem. Radioactive sources are typically distributed uniformly over the surface of an episcleral plaque and are sometimes offset slightly from the scleral surface to reduce the dose to the sclera relative to the apex and prescribed therapeutic margin at the tumor base. Nevertheless, it is not uncommon for scleral dose to exceed the dose to the apex of intermediate to tall tumors by a factor of 4 or more. The availability of low-energy sealed sources such as 125I prompted the development of gold-backed plaques to shield noninvolved periocular tissues. The concept of shielding can be extended to include collimation of individual sources. The potential advantages of individual source collimation include reduced scleral dose, more homogeneous tumor dose, and superior shielding of adjacent normal structures such as the fovea as compared to previous plaque designs.
A three-dimensional treatment-planning system has been extended to design a plaque that incorporates individually collimated 125I sources. Thermoluminescent dosimetry (TLD) and radiochromic film were used to compare calculated dose-rate distributions with measured dose rates in an acrylic phantom.
Calculations predict that source collimation in the form of a "slotted" gold plaque will achieve the purposes of the study. The collimating effect of the slots is demonstrated qualitatively using radiochromic film, and the accuracy of the calculation is demonstrated quantitatively with TLD.
The episcleral plaque described in this report is simpler to assemble than previous plaque designs. It produces a more homogeneous dose distribution in the tumor, reduces scleral dose by up to 50% as compared to conventional designs, and significantly reduces radiation dose to uninvolved structures adjacent to the plaque.
使用密封的¹²⁵I源进行巩膜外斑块治疗（EPT）广泛应用于脉络膜黑色素瘤的治疗。在EPT中，如同放疗的其他领域一样，对正常组织耐受性的关注常常是剂量限制因素。适形治疗的概念旨在改善肿瘤内的剂量均匀性并大幅降低对未受累结构的剂量，这可能为解决此问题提供一种方法。放射源通常均匀分布在巩膜外斑块表面，有时会从巩膜表面略微偏移，以相对于肿瘤基底的顶点和规定治疗边界降低对巩膜的剂量。然而，对于中等高度至高大肿瘤，巩膜剂量超过顶点剂量4倍或更多的情况并不少见。¹²⁵I等低能密封源的可用性促使了金背斑块的开发，以保护未受累的眼周组织。屏蔽的概念可以扩展到包括单个源的准直。与以前的斑块设计相比，单个源准直的潜在优势包括降低巩膜剂量、肿瘤剂量更均匀以及更好地屏蔽相邻正常结构，如黄斑。
一个三维治疗计划系统已得到扩展，用于设计一种包含单个准直¹²⁵I源的斑块。使用热释光剂量测定法（TLD）和放射变色膜在丙烯酸体模中比较计算的剂量率分布与测量的剂量率。
计算预测，呈“开槽”金斑块形式的源准直将实现研究目的。使用放射变色膜定性地证明了狭槽的准直效果，并使用TLD定量地证明了计算的准确性。
本报告中描述的巩膜外斑块比以前的斑块设计更易于组装。它在肿瘤中产生更均匀的剂量分布，与传统设计相比，巩膜剂量降低多达50%，并显著降低对斑块相邻未受累结构的辐射剂量。
