Department of Medical Physics, University of Wisconsin-Madison, Madison, Wisconsin 53705, USA.
Med Phys. 2010 Jun;37(6):2693-702. doi: 10.1118/1.3429089.
Intracavitary accelerated partial breast irradiation (APBI) has become a popular treatment for early stage breast cancer in recent years due to its shortened course of treatment and simplified treatment planning compared to traditional external beam breast conservation therapy. However, the exit dose to the skin is a major concern and can be a limiting factor for these treatments. Most treatment planning systems (TPSs) currently used for high dose-rate (HDR) 192Ir brachytherapy overestimate the exit skin dose because they assume a homogeneous water medium and do not account for finite patient dimensions. The purpose of this work was to quantify the TPS overestimation of the exit skin dose for a group of patients and several phantom configurations.
The TPS calculated skin dose for 59 HDR 192Ir APBI patients was compared to the skin dose measured with LiF:Mg,Ti thermoluminescent dosimeters (TLDs). Additionally, the TPS calculated dose was compared to the TLD measured dose and the Monte Carlo (MC) calculated dose for eight phantom configurations. Four of the phantom configurations simulated treatment conditions with no scattering material beyond the point of measurement and the other four configurations simulated the homogeneous scattering conditions assumed by the TPS. Since the calibration TLDs for this work were irradiated with 137Cs and the experimental irradiations were performed with 192Ir, experiments were performed to determine the intrinsic energy dependence of the TLDs. Correction factors that relate the dose at the point of measurement (center of TLD) to the dose at the point of interest (basal skin layer) were also determined and applied for each irradiation geometry.
The TLD intrinsic energy dependence for 192Ir relative to 137Cs was 1.041 +/- 1.78%. The TPS overestimated the exit skin dose by an average of 16% for the group of 59 patients studied, and by 9%-15% for the four phantom setups simulating treatment conditions. For the four phantom setups simulating the conditions assumed by the TPS, the TPS calculated dose agreed well with the TLD and MC results (within 3% and 1%, respectively). The inverse square geometry correction factor ranged from 1.023 to 1.042, and an additional correction factor of 0.978 was applied to account for the lack of charged particle equilibrium in the TLD and basal skin layer.
TPS calculations that assume a homogeneous water medium overestimate the exit skin dose for intracavitary APBI treatments. It is important to determine the actual skin dose received during intracavitary APBI to determine the skin dose-response relationship and establish dose limits for optimal skin sparing. This study has demonstrated that TLDs can measure the skin dose with an expanded uncertainty (k = 2) of 5.6% when the proper corrections are applied.
与传统的外部束乳腺保留治疗相比，腔内加速部分乳房照射（APBI）由于其治疗疗程缩短和治疗计划简化，近年来已成为治疗早期乳腺癌的一种流行方法。然而，皮肤出口剂量是一个主要关注点，并且可能是这些治疗方法的限制因素。目前大多数用于高剂量率（HDR）192Ir 近距离治疗的治疗计划系统（TPS）由于假设均匀的水介质并且不考虑有限的患者尺寸而高估了出口皮肤剂量。这项工作的目的是量化一组患者和几种体模配置的 TPS 对出口皮肤剂量的高估。
将 59 例 HDR 192Ir APBI 患者的 TPS 计算的皮肤剂量与 LiF：Mg，Ti 热释光剂量计（TLD）测量的皮肤剂量进行比较。此外，还将 TPS 计算的剂量与 TLD 测量的剂量和 Monte Carlo（MC）计算的剂量进行了比较，共涉及 8 种体模配置。四个体模配置模拟了没有散射材料超出测量点的治疗条件，另外四个配置模拟了 TPS 假设的均匀散射条件。由于这项工作的校准 TLD 是用 137Cs 照射的，而实验照射是用 192Ir 进行的，因此进行了实验以确定 TLD 的固有能量依赖性。还确定了将测量点（TLD 中心）处的剂量与感兴趣点（基底皮肤层）处的剂量相关的校正因子，并将其应用于每种辐照几何形状。
192Ir 相对于 137Cs 的 TLD 固有能量依赖性为 1.041 +/- 1.78%。TPS 对研究的 59 例患者组的出口皮肤剂量平均高估了 16%，对四个模拟治疗条件的体模设置高估了 9%-15%。对于四个模拟 TPS 假设条件的体模设置，TPS 计算的剂量与 TLD 和 MC 结果非常吻合（分别在 3%和 1%以内）。平方反比几何校正因子的范围为 1.023 至 1.042，还应用了另外一个 0.978 的校正因子，以说明 TLD 和基底皮肤层中不存在带电粒子平衡。
假设均匀水介质的 TPS 计算高估了腔内 APBI 治疗的出口皮肤剂量。确定腔内 APBI 期间实际接受的皮肤剂量以确定皮肤剂量 - 反应关系并建立最佳皮肤保护的剂量限制非常重要。这项研究表明，当应用适当的校正时，TLD 可以以扩展不确定度（k = 2）为 5.6%测量皮肤剂量。
