Reddy Nandanuri M S, Mazur Andrzej K, Sampath Seshadri, Osian Adrian, Sood Brij M, Ravi Akkamma, Nori Dattatreyudu
Department of Radiation Oncology, The New York Hospital Queens, Flushing, NY, USA.
Med Dosim. 2008 Spring;33(1):55-61. doi: 10.1016/j.meddos.2007.05.001.
The purpose of this study is to understand the potential for dose dumping in normal tissues (>85% of prescription dose) and to analyze effectiveness of techniques used in reducing dose dumping during IMRT. Two hundred sixty-five intensity modulated radiation therapy (IMRT) plans for 55 patients with prostate, head-and-neck (H&N), and cervix cancers with 6-MV photon beams and >5 fields were reviewed to analyze why dose dumping occurred, and the techniques used to reduce dose dumping. Various factors including gantry angles, depth of beams (100-SSD), duration of optimization, severity of dose-volume constraints (DVC) on normal structures, and spatial location of planning treatment volumes (PTV) were reviewed in relation to dose dumping. In addition, the effect of partial contouring of rectum in beam's path on dose dumping in rectum was studied. Dose dumping occurred at d(max) in 17 pelvic cases (85% to 129%). This was related to (1) depth of beams (100 SSD [source-to-skin distance]), (2) PTV located between normal structures with DVC, and (3) relative lack of rectum and bladder in beam's path. Dose dumping could be reduced to 85% by changing beam angles and/or DVC for normal structures in 5 cases and by creating "phantom structures" in 12 cases. Decreasing the iterations (duration of optimization) also reduced dose dumping and monitor units (MUs). Part of uncontoured rectum, if present in the field, received a higher dose than the contoured rectum with DVC, indicating that complete delineation of normal structures and DVC is necessary to prevent dose dumping. In H&N, when PTV extends inadvertently into air beyond the body even by a few millimeters, dose dumping occurred in beam's path (220% for 5-field and 170%, 7-field plans). Keeping PTV margins within body contour reduced this type of dose dumping. Beamlet optimization, duration of optimization, spatial location of PTV, and DVC on PTV and normal structures has the potential to cause dose dumping. However, these factors are an integral part of IMRT inverse planning. Therefore, understanding these aspects and use of appropriate technique/s would reduce or eliminate the dose dumping and minimize time to obtain optimum plan.
本研究的目的是了解正常组织中剂量倾泻（超过处方剂量的85%）的可能性，并分析在调强放射治疗（IMRT）期间用于减少剂量倾泻的技术的有效性。回顾了55例前列腺癌、头颈癌（H&N）和宫颈癌患者的265个采用6-MV光子束且射野数大于5个的调强放射治疗（IMRT）计划，以分析剂量倾泻发生的原因以及用于减少剂量倾泻的技术。研究了包括机架角度、射束深度（100-源皮距）、优化持续时间、正常结构上剂量体积约束（DVC）的严格程度以及计划治疗体积（PTV）的空间位置等各种因素与剂量倾泻的关系。此外，还研究了射野路径中直肠部分勾画对直肠剂量倾泻的影响。17例盆腔病例在最大剂量（d(max)）处发生了剂量倾泻（85%至129%）。这与以下因素有关：（1）射束深度（100源皮距[源到皮肤的距离]），（2）PTV位于有DVC的正常结构之间，以及（3）射野路径中直肠和膀胱相对较少。通过改变射束角度和/或正常结构的DVC，5例患者的剂量倾泻可降至85%，12例患者通过创建“虚拟结构”实现了这一点。减少迭代次数（优化持续时间）也可减少剂量倾泻和监测单位（MU）。如果射野中存在未勾画的直肠部分，其接受的剂量高于有DVC的勾画直肠，这表明完整勾画正常结构和DVC对于防止剂量倾泻是必要的。在头颈癌中，当PTV无意中延伸到身体外的空气中哪怕几毫米时，射野路径中就会发生剂量倾泻（5野计划为220%，7野计划为170%）。将PTV边界保持在身体轮廓内可减少此类剂量倾泻。子野优化、优化持续时间、PTV的空间位置以及PTV和正常结构上的DVC有可能导致剂量倾泻。然而，这些因素是IMRT逆向计划不可或缺的一部分。因此，了解这些方面并使用适当的技术将减少或消除剂量倾泻，并将获得最佳计划的时间减至最短。
