Exhibit Hall | Forum 2
Purpose: Lung MR-guided radiotherapy (MRgRT) with Elekta Unity MR-linac (MRL) offers potentially superior target visualization relative to other MRL options due to its higher magnetic field (B =1.5T vs. 0.35T). However, the high magnetic field also introduces unique planning challenges in low electron density (ED) regions like lung due to secondary electron return effect (ERE) related to the magnetic field. Here we describe and validate a strong tesla electron return effect reduction (STEER) planning solution that enables safe lung MRgRT.
Methods: Ten patients with lung cancer who were treated with hypofractionated radiation were selected for this dosimetry analysis; seven were treated on a standard linac and three on an MRL. The dose range was 35-50 Gy in 5 fractions. Average GTV or ITV size was 99.7±130.9cc and PTV was 164.2±195.9cc. Deviating from conventional planning techniques, cases were planned with two novel and practical
methods: (1) 18-25 parallel-opposed step-and-shoot IMRT fields were utilized to mitigate ERE; (2) PTV ED was set to 1.0 (relative to water) during optimization to reduce dose painting and removed for final dose calculation. STEER is simple and reproducible for other clinics to utilize. Plan quality was evaluated using department objectives.
Results: All plans met department constraints. GTV/ITV coverage on average was 98.8±1.7% and PTV was 94.5±1.5% Average conformity and heterogeneity index was 1.22±0.1 and 1.19±0.1, respectively. Higher intermediate dose spillage was observed with average gradient index of 6.41± 1.6. Dose-limiting organ criteria such as the heart and bronchus maximal dose on average were 1410.4 ± 1471cGy and 805.5 ± 675.7cGy lower than their constraints, respectively. Despite increase in total fields, average modulation factor was 2.5±0.4.
Conclusion: Deviating from conventional planning techniques enables safe delivery of adaptive stereotactic lung in high field MRgRT. STEER helps to limit lung MRgRT specific challenges. Further study is warranted.