MR-Guided Online Adaptive Stereotactic Body Radiotherapy (SBRT) of Primary and Metastatic Lung Tumors

Abstract

<h3>Purpose/Objective(s)</h3> Radiotherapy has an important role in management of lung tumors. Online adaptive radiotherapy is a method that provides dosimetric accuracy by considering anatomical and functional changes with real-time images taken during treatment, and MR-guided radiotherapy is the latest technological development in this field. In our study, we aimed to present preliminary results and toxicity in patients who underwent magnetic resonance (MR) guided online adaptive radiotherapy (MRgRT) for primary or metastatic lung tumors. <h3>Materials/Methods</h3> Fifty-one patients with a diagnosis of primary lung cancer or lung metastases treated with MRgRT and a minimum follow-up period of 3 months were included. The treatment was carried out with the "step-and-shoot" IMRT technique and daily online adaptive planning. All patients were treated with breath-hold with audio-visual coaching. Patients were evaluated for treatment response, acute and late toxicity. <h3>Results</h3> The median age was 64 (range, 22-83 years) and 68.6% of the patients were male. MRgRT was performed in 21.6% of the patients for primary lung cancer and 78.4% for lung metastases from different primaries. MRgRT was applied to a single lesion in 38 patients and more than one lesion in 13 patients (a total of 69 lesions). Tumors were peripherally located in 78.3% of the patients and 21.7% of lesions were in the "no-fly zone". Median GTV volume was 1.8 cc (range, 0.2-172.1 cc). Target volume and doses of organs at risk met the criteria in all initial plans. Online adaptive treatment approach was applied in all patients. The median total dose was 50 Gy (range, 24-70), the median fraction number was 5 (range, 1-15 fractions), and the median fraction dose was 10 Gy (range, 4-34). The patients were treated every other day, except for two patients treated with 10 and 15 fractions. Median follow-up was 18.7 months. One- and 3-year overall survival (OS) rates were 92.4% and 68.0%, respectively. During the follow-up period, complete response was obtained in the MRgRT applied lesions in 37 patients, partial response was observed in 8 patients and local recurrence or progression was detected in 6 patients. One- and 3- year local progression free survival (LPFS) and distant progression free survival (DPFS) rates were 97.3%, 67.1% and 36.4%, 13.0%, respectively. There were no statistically significant differences in OS, LPFS and DPFS between primary and metastatic tumors. No Grade 3+ acute or late toxicity was observed. <h3>Conclusion</h3> In conclusion, MRgRT is a new method for the application of SBRT with better soft tissue contrast, no need for invasive marker placement, online adaptive radiotherapy in every fraction, and continuous real-time cine-MR tumor tracking. Our study shows that MRgRT is an acceptable method for the treatment of lung tumors. Randomized prospective studies with longer follow-up are needed comparing this technique with other modalities.