Cheng C-W, Mitra R, Allen Li X, Das Indra J
Arizona Oncology Associates, 2625 N. Craycroft Road, Suite 100, Tucson, Arizona 85712.
Ochsner Clinic Foundation, 1516 Jefferson Highway, New Orleans, Louisiana 70121.
Med Phys. 2005 Jul;32(7Part1):2279-2287. doi: 10.1118/1.1943827.
In the management of early breast cancer, a partial breast irradiation technique called MammoSite (Proxima Therapeutic Inc., Alpharetta, GA) has been advocated in recent years. In MammoSite, a balloon implanted at the surgical cavity during tumor excision is filled with a radio-opaque solution, and radiation is delivered via a high dose rate brachytherapy source situated at the center of the balloon. Frequently air may be introduced during placement of the balloon and/or injection of the contrast solution into the balloon. The purpose of this work is to quantify as well as to understand dose perturbations due to the presence of a high-Z contrast medium and/or an air bubble with measurements and Monte Carlo calculations. In addition, the measured dose distribution is compared with that obtained from a commercial treatment planning system (Nucletron PLATO system). For a balloon diameter of 42 mm, the dose variation as a function of distance from the balloon surface is measured for various concentrations of a radio-opaque solution (in the range 5%-25% by volume) with a small volume parallel plate ion chamber and a micro-diode detector placed perpendicular to the balloon axis. Monte Carlo simulations are performed to provide a basic understanding of the interaction mechanism and the magnitude of dose perturbation at the interface near balloon surface. Our results show that the radio-opaque concentration produces dose perturbation up to 6%. The dose perturbation occurs mostly within the distances <1mm from the balloon surface. The Plato system that does not include heterogeneity correction may be sufficient for dose planning at distances ⩾10mm from the balloon surface for the iodine concentrations used in the MammoSite procedures. The dose enhancement effect near the balloon surface (<1mm) due to the higher iodine concentration is not correctly predicted by the Plato system. The dose near the balloon surface may be increased by 0.5%percm3 of air. Monte Carlo simulation suggests that the interface effect (enhanced dose near surface) is primarily due to Compton electrons of short range (<0.5mm). For more accurate dosimetry in MammoSite delivery, the dose perturbation due to the presence of a radio-opaque contrast medium and air bubbles should be considered in a brachytherapy planning system.
在早期乳腺癌的治疗中，近年来一种名为MammoSite（Proxima Therapeutic Inc.，佐治亚州阿尔法利塔）的部分乳腺照射技术得到了提倡。在MammoSite技术中，肿瘤切除时植入手术腔的球囊被一种不透射线的溶液充满，并且通过位于球囊中心的高剂量率近距离放射治疗源进行辐射。在放置球囊和/或将造影剂注入球囊的过程中，经常会引入空气。这项工作的目的是通过测量和蒙特卡罗计算来量化并了解由于高原子序数造影剂和/或气泡的存在而引起的剂量扰动。此外，将测量得到的剂量分布与从商业治疗计划系统（Nucletron PLATO系统）获得的剂量分布进行比较。对于直径为42 mm的球囊，使用小体积平行板电离室和垂直于球囊轴放置的微二极管探测器，测量不同浓度（体积分数范围为5% - 25%）的不透射线溶液在距球囊表面不同距离处的剂量变化。进行蒙特卡罗模拟以基本了解相互作用机制以及球囊表面附近界面处剂量扰动的大小。我们的结果表明，不透射线浓度产生的剂量扰动高达6%。剂量扰动主要发生在距球囊表面<1mm的距离内。对于MammoSite程序中使用的碘浓度，不包括不均匀性校正的Plato系统对于距球囊表面⩾10mm处的剂量规划可能就足够了。Plato系统不能正确预测由于较高碘浓度导致的球囊表面附近（<1mm）的剂量增强效应。每立方厘米空气可使球囊表面附近的剂量增加0.5%。蒙特卡罗模拟表明，界面效应（表面附近剂量增强）主要是由于短程（<0.5mm）的康普顿电子引起的。为了在MammoSite治疗中进行更准确的剂量测定，在近距离放射治疗计划系统中应考虑由于不透射线造影剂和气泡的存在而引起的剂量扰动。
