Parham Melton D, Ahmad Salahuddin, Jin Hosang
Sun Nuclear Cooperation, Melbourne, FL, USA.
Department of Radiation Oncology, University of Oklahoma Health Sciences Center, Oklahoma City, OK, USA.
Int J Part Ther. 2021 Mar 8;7(4):19-28. doi: 10.14338/IJPT-20-00077.1. eCollection 2021 Spring.
To investigate dosimetric implications of biodegradable Biozorb (BZ) markers for proton accelerated partial breast irradiation (APBI) plans.
Six different BZs were placed within in-house breast phantoms to acquire computed tomography (CT) images. A contour correction method with proper mass density overriding for BZ titanium clip and surrounding tissue was applied to minimize inaccuracies found in the CT images in the RayStation planning system. Each breast phantom was irradiated by a monoenergetic proton beam (103.23 MeV and 8×8 cm) using a pencil-beam scanning proton therapy system. For a range perturbation study, doses were measured at 5 depths below the breast phantoms by using an ionization chamber and compared to the RayStation calculations with 3 scenarios for the clip density: the density correction method (S1: 1.6 g/cm), raw CT (S2), and titanium density (S3: 4.54 g/cm). For the local dose perturbation study, the radiographic EDR2 film was placed at 0 and 2 cm below the phantoms and compared to the RayStation calculations. Clinical effects of the perturbations were retrospectively examined with 10 APBI plans for the 3 scenarios (approved by our institutional review board).
In the range perturbation study, the S1 simulation showed a good agreement with the chamber measurements, while excess pullbacks of 1∼2 mm were found in the S2 and S3 simulations. The film study showed local dose shadowing and perturbation by the clips that RayStation could not predict. In the plan study, no significant differences in the plan quality were found among the 3 scenarios. However, substantial range pullbacks were observed for S3.
The density correction method could minimize the dose and range difference between measurement and RayStation prediction. It should be avoided to simply override the known physical density of the BZ clips for treatment planning owing to overestimation of the range pullback.
研究可生物降解的Biozorb（BZ）标记物对质子加速部分乳腺照射（APBI）计划的剂量学影响。
将六种不同的BZ放置在内部乳腺模型中以获取计算机断层扫描（CT）图像。应用一种轮廓校正方法，对BZ钛夹及其周围组织进行适当的质量密度覆盖，以尽量减少在RayStation计划系统的CT图像中发现的不准确之处。使用笔形束扫描质子治疗系统，用单能质子束（103.23 MeV和8×8 cm）照射每个乳腺模型。对于射程扰动研究，使用电离室在乳腺模型下方的5个深度处测量剂量，并与RayStation在3种夹子密度情况下的计算结果进行比较：密度校正方法（S1：1.6 g/cm³）、原始CT（S2）和钛密度（S3：4.54 g/cm³）。对于局部剂量扰动研究，将放射成像EDR2胶片放置在模型下方0和2 cm处，并与RayStation的计算结果进行比较。对3种情况的10个APBI计划进行回顾性研究，检查扰动的临床效果（经我们机构审查委员会批准）。
在射程扰动研究中，S1模拟结果与电离室测量结果吻合良好，而在S2和S3模拟中发现有1～2 mm的过度回拉。胶片研究显示夹子造成了局部剂量阴影和扰动，这是RayStation无法预测的。在计划研究中，3种情况之间在计划质量方面未发现显著差异。然而，在S3中观察到了明显的射程回拉。
密度校正方法可使测量值与RayStation预测值之间的剂量和射程差异最小化。由于高估了射程回拉，在治疗计划中应避免简单地覆盖BZ夹子已知的物理密度。
