Alexander A, Seuntjens J
Medical Physics Unit, McGill University, Montreal, QC.
Medical Physics Department, The Ottawa Hospital, Ottawa, ON.
Med Phys. 2012 Jul;39(7Part2):4622. doi: 10.1118/1.4740098.
Modulated electron radiotherapy (MERT) takes advantage of the low distal dose of electrons to reduce dose to healthy tissue. The dosimetric advantage of MERT is clear when compared against single-field electron irradiation where MERT demonstrates superior target homogeneity and sparing; however the dosimetric advantage is unclear when comparing MERT with photon intensity-modulated radiotherapy (IMRT) where MERT techniques struggle to match the IMRT target homogeneity but with less total energy delivered to healthy tissues. In an effort to improve dosimetric benefits of MERT, this study investigated an inverse planning technique for the creation of hybrid MERT-IMRT mixed beam radiotherapy (MBRT) plans. The optimization process decouples the photon and electron beamlets for combined modality optimization. The input to the optimization algorithm was a series of patient-specific 3D dose distributions for the corresponding electron and photon beamlets, while the output was a list of weights that satisfied the optimization constraints. A photon IMRT Eclipse (Varian, Palo Alto, CA) plan and a MERT plan were created for a patient-specific sarcoma irradiation. The MERT plan was competitive in its ability to reduce dose to organs at risk and total-body dose; however, the plan suffered from poorer target conformity compared with the IMRT plan. The MBRT plan was created by adding two photon fields, divided into beamlets, to the electron beamlets of the MERT plan for reoptimization. The MBRT plan improved MERT target coverage with only minimal cost to healthy tissue dose. The MBRT plan provided clear dosimetric advantages over the IMRT and MERT plan.
调制电子放疗（MERT）利用电子束远端剂量较低的特点来减少对健康组织的剂量。与单野电子照射相比，MERT的剂量学优势明显，MERT在靶区均匀性和正常组织保护方面表现更优；然而，将MERT与光子调强放疗（IMRT）比较时，其剂量学优势并不明确，MERT技术在靶区均匀性上难以与IMRT相匹配，但输送到健康组织的总能量较低。为了提高MERT的剂量学效益，本研究探索了一种逆向计划技术，用于创建混合MERT-IMRT混合束放疗（MBRT）计划。优化过程将光子和电子子束解耦，以进行联合模态优化。优化算法的输入是一系列针对相应电子和光子子束的患者特异性三维剂量分布，输出则是满足优化约束的权重列表。针对一名特定患者的肉瘤照射，创建了一个光子IMRT Eclipse（瓦里安公司，加利福尼亚州帕洛阿尔托）计划和一个MERT计划。MERT计划在降低危及器官剂量和全身剂量方面具有竞争力；然而，与IMRT计划相比，该计划的靶区适形性较差。MBRT计划是通过在MERT计划的电子子束中添加两个光子野（分为子束）进行重新优化而创建的。MBRT计划在仅略微增加健康组织剂量的情况下改善了MERT的靶区覆盖。MBRT计划相对于IMRT和MERT计划具有明显的剂量学优势。
