Kurz Christopher, Landry Guillaume, Resch Andreas F, Dedes George, Kamp Florian, Ganswindt Ute, Belka Claus, Raaymakers Bas W, Parodi Katia
Department of Radiotherapy, University Medical Center Utrecht, Utrecht, Netherlands.
Phys Med Biol. 2017 Oct 19;62(21):8470-8482. doi: 10.1088/1361-6560/aa8de9.
Combining magnetic-resonance imaging (MRI) and proton therapy (PT) using pencil-beam scanning (PBS) may improve image-guided radiotherapy. We aimed at assessing the impact of a magnetic field on PBS-PT plan quality and robustness. Specifically, the robustness against anatomical changes and positioning errors in an MRI-guided scenario with a 30 cm radius 1.5 T magnetic field was studied for prostate PT. Five prostate cancer patients with three consecutive CT images (CT1-3) were considered. Single-field uniform dose PBS-PT plans were generated on the segmented CT1 with Monte-Carlo-based treatment planning software for inverse optimization. Plans were optimized at 90° gantry angle without B-field (no B), with ±1.5 T B-field (B and minus B), as well as at 81° gantry angle and +1.5 T (B G81). Plans were re-calculated on aligned CT2 and CT3 to study the impact of anatomical changes. Dose distributions were compared in terms of changes in DVH parameters, proton range and gamma-index pass-rates. To assess the impact of positioning errors, DVH parameters were compared for ±5 mm CT1 patient shifts in anterior-posterior (AP) and left-right (LR) direction. Proton beam deflection considerably reduced robustness against inter-fractional changes for the B scenario. Range agreement, gamma-index pass-rates and PTV V95% were significantly lower compared to no B. Improved robustness was obtained for minus B and B G81, the latter showing only minor differences to no B. The magnetic field introduced slight dosimetric changes under LR shifts. The impact of AP shifts was considerably larger, and equivalent for scenarios with and without B-field. Results suggest that robustness equivalent to PT without magnetic field can be achieved by adaptation of the treatment parameters, such as B-field orientation (minus B) with respect to the patient and/or gantry angle (B G81). MRI-guided PT for prostate cancer might thus be implemented without compromising robustness compared to state-of-the-art CT-guided PT.
将磁共振成像（MRI）与采用笔形束扫描（PBS）的质子治疗（PT）相结合，可能会改善图像引导放射治疗。我们旨在评估磁场对PBS-PT计划质量和稳健性的影响。具体而言，针对前列腺PT，研究了在半径为30 cm、磁场强度为1.5 T的MRI引导场景中，针对解剖结构变化和定位误差的稳健性。考虑了5例前列腺癌患者，他们有连续的三张CT图像（CT1-3）。使用基于蒙特卡洛的治疗计划软件进行逆向优化，在分割后的CT1上生成单野均匀剂量的PBS-PT计划。计划在90°机架角度且无磁场（无B）、±1.5 T磁场（B和负B）以及81°机架角度且+1.5 T磁场（B G81）的条件下进行优化。在对齐的CT2和CT3上重新计算计划，以研究解剖结构变化的影响。根据剂量体积直方图（DVH）参数变化、质子射程和伽马指数通过率比较剂量分布。为了评估定位误差的影响，比较了CT1患者在前后（AP）和左右（LR）方向上±5 mm移位时的DVH参数。对于B场景，质子束偏转大大降低了对分次间变化的稳健性。与无B相比，射程一致性、伽马指数通过率和计划靶体积（PTV）的V95%显著更低。对于负B和B G81，稳健性有所提高，后者与无B相比仅显示出微小差异。磁场在LR移位时会引起轻微的剂量学变化。AP移位的影响要大得多，并且在有磁场和无磁场的情况下相当。结果表明，通过调整治疗参数，如相对于患者的磁场方向（负B）和/或机架角度（B G81），可以实现与无磁场的PT相当的稳健性。因此，与目前先进的CT引导PT相比，在不影响稳健性的情况下，可能实施MRI引导的前列腺癌PT。
