Christie Medical Physics and Engineering (CMPE), The Christie NHS Foundation Trust, Wilmslow Road, Manchester, M20 4BX, United Kingdom. Division of Cancer Sciences, Faculty of Biology, Medicine and Health, University of Manchester, Manchester, M13 9PL, United Kingdom. Joint first author.
Phys Med Biol. 2018 Jun 20;63(12):125020. doi: 10.1088/1361-6560/aac749.
The MR-Linac will provide excellent soft tissue contrast for on-treatment imaging. It is well known that the electron return effect (ERE) results in areas of increased and decreased dose at air/tissue boundaries, which can be compensated for in plan optimisation. However, anatomical changes may affect the quality of this compensation. In this paper we aim to quantify the interaction of anatomical changes with ERE in head and neck (H&N) cancer patients. Twenty patients treated with either 66 Gy or 60 Gy in 30 fractions were selected. Ten had significant weight-loss during treatment requiring repeat CT (rCT) and ten had PTVs close to the sinus cavity. Plans were optimised using Monaco to meet the departmental dose constraints and copied to the rCT and re-calculated. For the sinus patients, we optimised plans with full and empty sinus at both 0 T and 1.5 T. The effect of the opposite filling state was next evaluated. No clinically relevant difference between the doses in the PTV and OARs were observed related to weight-loss in 0 T or 1.5 T fields. Variable sinus filling caused greater dosimetric differences near the walls of the sinus for plans optimised with a full cavity in 1.5 T, indicating that optimising with an empty sinus makes the plan more robust to changes in filling. These findings indicate that current off-line strategies for adaptive planning for H&N patients are also valid on an MR-linac, if care is taken with sinus filling.
MR-Linac 将为治疗中的成像提供出色的软组织对比度。众所周知，电子返回效应（ERE）会导致在空气/组织边界处增加和减少剂量的区域，这可以在计划优化中得到补偿。然而，解剖结构的变化可能会影响这种补偿的质量。在本文中，我们旨在量化解剖结构变化与头颈（H&N）癌症患者 ERE 的相互作用。选择了 20 名接受 66 Gy 或 60 Gy 分 30 次治疗的患者。其中 10 名患者在治疗过程中体重明显减轻，需要重复 CT（rCT），10 名患者的 PTV 接近窦腔。使用 Monaco 优化计划以满足部门剂量限制，并将其复制到 rCT 并重新计算。对于窦腔患者，我们在 0 T 和 1.5 T 下分别对充满和排空窦腔的计划进行了优化。接下来评估了相反填充状态的效果。在 0 T 或 1.5 T 场中，体重减轻与 PTV 和 OAR 中的剂量无明显差异。在 1.5 T 中对充满空腔的计划进行优化时，可变窦腔填充会导致窦腔壁附近的剂量差异更大，这表明优化时使用空窦腔会使计划对填充变化更具鲁棒性。这些发现表明，如果对窦腔填充加以注意，当前用于 H&N 患者自适应计划的离线策略也适用于 MR-Linac。
