Wu A, Krasin F
Department of Radiation Oncology, University of Pittsburgh, School of Medicine, Pennsylvania.
Med Phys. 1990 Sep-Oct;17(5):843-6. doi: 10.1118/1.596556.
One of the methods currently being used to treat choroidal melanoma employs an episcleral plaque containing I-125 radioactive seeds. However, comprehensive dosimetry studies on the plaque are scarce and controversial. For this work, we use film to study the dosimetry outside the lip of the gold shield of the eye plaque. This lip around the gold shield was made to protect the critical structures behind and adjacent to the lesion. Since the changes of energy spectrum of I-125 in tissue are negligible, film dosimetry seems to be a logical choice because of high spatial resolution required around the lip of the gold plaque. For this study, we first established an H and D curve with dose expressed in a unit of specific dose rate constant. This avoids absolute dose measurements. All film density measurements are made with a 1-mm aperture scan, normalized to the density at the prescription point for tumor of 3-5-mm apical height, i.e., 5 mm from the interior surface of sclera, and converted to percentage isodose curves. With a gold shield, it is found that when the plaque is placed against the optical nerve, the optical disk and macula, located at 2 mm outside the lip, on the exterior surface of sclera, may receive 85% of the prescription dose for a 12-mm plaque and 58% for a 16-mm plaque. For tumors of 8-mm apical height, the optical nerve would receive more than the prescription dose.
目前用于治疗脉络膜黑色素瘤的方法之一是使用含有碘-125放射性籽源的巩膜外斑块。然而，关于该斑块的全面剂量测定研究稀缺且存在争议。在本研究中，我们使用胶片来研究眼斑块金屏蔽边缘外的剂量分布。金屏蔽周围的这个边缘是为了保护病变后方和相邻的关键结构而设置的。由于碘-125在组织中的能谱变化可忽略不计，胶片剂量测定法似乎是一个合理的选择，因为在金斑块边缘周围需要高空间分辨率。在本研究中，我们首先建立了一条H-D曲线，剂量以特定剂量率常数为单位表示。这避免了绝对剂量测量。所有胶片密度测量均采用1毫米孔径扫描进行，以距巩膜内表面5毫米、顶端高度为3 - 5毫米的肿瘤处方点处的密度进行归一化，并转换为百分比等剂量曲线。有金屏蔽时，发现当斑块贴靠视神经放置时，位于巩膜外表面、距边缘2毫米处的视盘和黄斑，对于12毫米的斑块可能会接受85%的处方剂量，对于16毫米的斑块则为58%。对于顶端高度为8毫米的肿瘤，视神经所接受的剂量会超过处方剂量。
