U.O. di Fisica Sanitaria, Policlinico Gemelli UCSC, Roma, Italy.
J Appl Clin Med Phys. 2010 Sep 2;11(4):3275. doi: 10.1120/jacmp.v11i4.3275.
An electronic portal imaging device (EPID) is an effective detector for in vivo transit dosimetry. In fact, it supplies two-dimensional information, does not require special efforts to be used during patient treatment, and can supply data in real time. In the present paper, a new procedure has been proposed to improve the EPID in vivo dosimetry accuracy by taking into account the patient setup variations. The procedure was applied to the breast tangential irradiation for the reconstruction of the dose at the breast midpoint, Dm. In particular, the patient setup variations were accounted for by comparing EPID images versus digitally reconstructed radiographies. In this manner, EPID transit signals were obtained corresponding to the geometrical projections of the breast midpoint on the EPID for each therapy session. At the end, the ratios R between D(m) and the doses computed by the treatment planning system (TPS) at breast midpoints, D(m,TPS), were determined for 800 therapy sessions of 20 patients. Taking into account the method uncertainty, tolerance levels equal to ± 5% have been determined for the ratio R.The improvement of in vivo dosimetry results obtained (taking into account patient misalignment) has been pointed out comparing the R values obtained with and with-out considering patient setup variations. In particular, when patient misalignments were taken into account, the R values were within ± 5% for 93% of the checks; when patient setup variations were not taken into account, the R values were within ± 5% in 72% of the checks. This last result points out that the transit dosimetry method overestimates the dose discrepancies if patient setup variations are not taken into account for dose reconstruction. In this case, larger tolerance levels have to be adopted as a trade-off between workload and ability to detect errors, with the drawback being that some errors (such as the ones in TPS implementation or in beam calibration) cannot be detected, limiting the in vivo dosimetry efficacy.The paper also reports preliminary results about the possibility of reconstructing a dose profile perpendicular to the beam central axis reaching from the apex to the lung and passing through the middle point of the breast by an algorithm, similar to the one used for dose reconstruction at breast midpoint. In particular, the results have shown an accuracy within ± 3% for the dose profile reconstructed in the breast (excluding the interface regions) and an underestimation of the lung dose.
电子射野影像装置（EPID）是用于体内透射剂量测量的有效探测器。事实上，它提供二维信息，在患者治疗期间不需要特殊的努力来使用，并且可以实时提供数据。在本文中，提出了一种新的方法，通过考虑患者设置变化来提高 EPID 体内剂量测量的准确性。该方法应用于乳房切线照射，以重建乳房中点 Dm 的剂量。具体来说，通过比较 EPID 图像与数字重建射线照相来考虑患者设置变化。通过这种方式，对于每个治疗疗程，在 EPID 上获得了对应于乳房中点的几何投影的 EPID 传输信号。最后，对于 20 名患者的 800 个治疗疗程，确定了 D(m)与治疗计划系统（TPS）在乳房中点处计算的剂量 D(m,TPS)之间的比值 R。考虑到方法不确定性，已经确定了比值 R 的容限水平为±5%。通过比较考虑和不考虑患者设置变化时获得的体内剂量测量结果，指出了体内剂量测量结果的改善。特别是，当考虑患者未对准时，对于 93%的检查，R 值在±5%以内；当不考虑患者设置变化时，对于 72%的检查，R 值在±5%以内。最后一个结果表明，如果不考虑患者设置变化来重建剂量，则透射剂量测量方法会高估剂量差异。在这种情况下，必须采用更大的容限水平作为工作量和检测错误能力之间的权衡，缺点是一些错误（例如 TPS 实现或束校准中的错误）无法检测到，从而限制了体内剂量测量的效果。本文还报告了通过类似于用于乳房中点剂量重建的算法，重建从顶点到肺部并穿过乳房中点的垂直于束中心轴的剂量分布的初步结果。特别是，结果表明在乳房中重建的剂量分布的精度在±3%以内（不包括界面区域），并且低估了肺部剂量。
