Department of Radiation Oncology, Indiana University School of Medicine, Indianapolis, IN, USA.
Med Phys. 2013 Jan;40(1):011714. doi: 10.1118/1.4770285.
Dosimetric accuracy in radiation treatment of breast cancer is critical for the evaluation of cosmetic outcomes and survival. It is often considered that treatment planning systems (TPS) may not be able to provide accurate dosimetry in the buildup region. This was investigated in various treatment techniques such as tangential wedges, field-in-field (FF), electronic compensator (eComp), and intensity-modulated radiotherapy (IMRT).
Under Institutional Review Board (IRB) exemption, radiotherapy treatment plans of 111 cases were retrospectively analyzed. The distance between skin surface and 95% isodose line was measured. For measurements, Gafchromic EBT2 films were used on a humanoid unsliced phantom. Multiple layers of variable thickness of superflab bolus were placed on the breast phantom and CT scanned for planning. Treatment plans were generated using four techniques with two different grid sizes (1 × 1 and 2.5 × 2.5 mm(2)) to provide optimum dose distribution. Films were placed at different depths and exposed with the selected techniques. A calibration curve for dose versus pixel values was also generated on the same day as the phantom measurement was conducted. The DICOM RT image, dose, and plan data were imported to the in-house software. On axial plane of CT slices, curves were drawn at the position where EBT2 films were placed, and the dose profiles on the lines were acquired. The calculated and measured dose profiles were separated by check points which were marked on the films before irradiation. The segments of calculated profiles were stretched to match their resolutions to that of film dosimetry.
On review of treatment plans, the distance between skin and 95% prescribed dose was up to 8 mm for plans of 27 patients. The film measurement revealed that the medial region of phantom surface received a mere 45%-50% of prescribed dose. For wedges, FF, and eComp techniques, region around the nipple received approximately 80% of prescribed dose, although only IMRT showed inhomogeneous dose profile. At deeper depths mainly (6-11 mm depths), film dosimetry showed good agreement with the TPS calculation. In contrast, the measured dose at a 3-mm depth was higher than TPS calculation by 15%-30% for all techniques. For the tangential and IMRT techniques, 1 × 1 mm(2) grid size showed a smaller difference than that with a 2.5 × 2.5 mm(2) grid size compared to the measurements.
In general, TPS even with advanced algorithms do not provide accurate dosimetry in the buildup region, as verified by EBT2 film for all treatment techniques. For all cases, TPS and measured doses were in agreement from 6 mm in depth but differed at shallower depths. Grid size plays an important role in dose calculation. For accurate dosimetry small grid size should be used where differences are lower between TPS and measurements.
乳腺癌放射治疗中的剂量学准确性对于评估美容效果和生存至关重要。人们通常认为，治疗计划系统（TPS）可能无法在累积区域提供准确的剂量。本研究调查了各种治疗技术，如切线楔形野、内靶区（ITV）-外放边界（FF）、电子补偿器（eComp）和调强放疗（IMRT）。
在机构审查委员会（IRB）豁免的情况下，回顾性分析了 111 例患者的放射治疗计划。测量皮肤表面与 95%等剂量线之间的距离。为此，在人体无切片体模上使用 Gafchromic EBT2 胶片进行测量。在乳房体模上放置多层不同厚度的超级敷贴，并对其进行 CT 扫描以进行计划。使用两种不同网格尺寸（1×1 和 2.5×2.5mm2）的四种技术生成治疗计划，以提供最佳的剂量分布。将胶片放置在不同深度并使用选定的技术进行曝光。还在与体模测量同一天生成了剂量与像素值的校准曲线。将 DICOM RT 图像、剂量和计划数据导入内部软件。在 CT 切片的轴平面上，在放置 EBT2 胶片的位置绘制曲线，并获取线上的剂量分布。在照射前，将胶片上标记检查点以将计算和测量的剂量分布分开。将计算出的曲线的段拉伸，以使其分辨率与胶片剂量学匹配。
在回顾治疗计划时，有 27 名患者的计划中皮肤与 95%规定剂量之间的距离高达 8mm。胶片测量显示，体模表面的内侧区域仅接受了规定剂量的 45%-50%。对于楔形野、FF 和 eComp 技术，尽管只有 IMRT 显示不均匀的剂量分布，但乳头周围区域接受了大约 80%的规定剂量。在较深的深度（6-11mm 深度），胶片剂量学与 TPS 计算具有良好的一致性。相比之下，对于所有技术，3mm 深度处的测量剂量比 TPS 计算值高 15%-30%。对于切线和调强放疗技术，与 2.5×2.5mm2 网格尺寸相比，1×1mm2 网格尺寸的差异较小。
总的来说，正如 EBT2 胶片对所有治疗技术所验证的那样，即使是具有先进算法的 TPS，在累积区域也无法提供准确的剂量。对于所有病例，TPS 和测量剂量从 6mm 深度开始一致，但在较浅的深度上有所不同。网格尺寸在剂量计算中起着重要作用。为了进行准确的剂量学测量，应使用较小的网格尺寸，因为 TPS 和测量值之间的差异较小。
