Comparisons of a proposed five-seed assay method with the single-seed and batch assay methods for I-125 seeds in ultrasound-guided prostate implants.

P C Lee, S J Starr, K Zuhlke, B J Moran
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引用次数: 9

Abstract

A simple five-seed assay method was proposed and investigated. A commercial well ion chamber system with an NIST-traceable single-seed calibration constant was used for the single-seed assays. A batch seed holder was used for batch assays. For the five-seed assays, a second single-seed holder was modified such that all five seeds were loaded in a central region of the well ion chamber. Compared with the same seed in the standard single-seed holder, the relative chamber responses for the five seed positions were 0.993, 0.993, 1.000, 1.001, and 0.977, respectively, indicating little or no position-dependent chamber response and no self-attenuation among seeds. Subsequent comparison of assays with the single-seed and five-seed methods indicated only 0.4% difference in charge collection. The five-seed calibration constant was therefore taken to be the same as the single-seed calibration constant. The reproducibility of the five-seed assay method was found to be better than 0.8%. When a dummy seed replaced an active seed, a nearly 20% reduction in charge was found, indicating that the proposed five-seed assay method can detect a dead seed. Clinical comparison of all three assay methods showed that they produced qualitatively the same assay results when the batch assay method was performed with extra care. Compared with the single-seed assay method, the five-seed method is equally simple, rigid, and reproducible, but it demands much less assay time. Compared with the batch assay method, the five-seed method is much more reproducible and reliable because of its rigid assay geometry; it only demands a moderate amount of assay time and can detect dead seeds. The American Association of Physicists in Medicine Task Group 40 (AAPM TG40) states that, for brachytherapy, ideally every (i.e., 100%) loose seed should be calibrated. For procedures involving large number of loose seeds, it then recommends that 10% of seeds be calibrated. The proposed five-seed assay is very simple to implement. It will facilitate the compliance of the "10%" recommendation from the AAPM TG40; it will make the "ideally 100%" statement from AAPM TG40 a more realistic and practical QA procedure in seed assaying.

超声引导前列腺植入物中I-125种子的五种检测方法与单种和批量检测方法的比较
提出并研究了一种简便的五种种子测定方法。采用具有nist可追溯单种子校准常数的商业井离子室系统进行单种子测定。分批种子固定器用于分批测定。对于五粒种子的测定,第二个单粒种子固定器进行了修改,使所有五粒种子都装载在井离子室的中心区域。与同一种子在标准单种器中相比,5个种子位置的相对室响应分别为0.993、0.993、1.000、1.001和0.977,表明种子间的室响应不依赖于位置,且不存在自衰减。随后与单种子法和五种子法的比较表明,电荷收集只有0.4%的差异。因此,五种种子的校准常数与单种种子的校准常数相同。结果表明,五种种子法的重复性优于0.8%。当假种子取代活性种子时,电荷减少了近20%,表明所提出的五种种子测定方法可以检测到死种子。所有三种分析方法的临床比较表明,当分批分析方法进行时,它们在质量上产生相同的分析结果。与单种子法相比,五种子法同样简单、严格、重复性好,但所需的分析时间短得多。与批量分析方法相比,五种种子法由于其严格的分析几何结构,具有更高的重复性和可靠性;它只需要适量的分析时间,可以检测到死亡的种子。美国医学物理学家协会第40任务小组(AAPM TG40)指出,对于近距离治疗,理想情况下,每个(即100%)松散的种子都应该进行校准。对于涉及大量松散种子的程序,建议对10%的种子进行校准。所提出的五种种子测定法实施起来非常简单。它将有助于遵守AAPM TG40的“10%”建议;它将使AAPM TG40的“理想100%”声明成为种子分析中更加现实和实用的质量保证程序。
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