Comparing the outcomes of MR-based versus CT-based tumor delineation in locally advanced non-small cell lung cancer treated with hypo-fractionated radiotherapy and concurrent chemotherapy.

Background: Delineating gross tumor volume (GTV) using computed tomography (CT) imaging is the standard for lung cancer contouring, but discrepancies among observers compromise accuracy and reliability. Magnetic resonance imaging (MRI) provides superior soft-tissue resolution compared to CT, thus, we design this retrospective study to compare the treatment outcomes of magnetic resonance-based (MR-based) and CT-based tumor delineation in locally advanced non-small cell lung cancer (LA-NSCLC) patients treated with hypo-fractionated concurrent chemoradiotherapy (hypo-CCRT).

Methods: A total of 293 LA-NSCLC patients treated with hypo-CCRT from three trials between October 2015 and October 2020 were screened. Ninety patients with each MR-based delineation and CT-based delineation of the primary tumor were selected for analysis. In the MR-based delineation group, T1-enhanced MR images was rigidly registered with 10 respiratory phases of planning CT images, respectively. The primary tumors were contoured on each respiratory phase based on co-registered MRI. The locoregional progression-free survival (LPFS), progression-free survival (PFS), overall survival (OS) and toxicities in both groups were analyzed.

Results: The 2-year LPFS rate was 69.2% [95% confidence interval (CI): 59.6-80.2%] in the MR-based delineation group and 61.0% (95% CI: 50.9-73.0%) in the CT-based delineation group (P=0.37). There was no significant difference in median PFS (P=0.45) or OS (P=0.69) between the two groups. The MR-based delineation group had smaller planning target volume (186.1 vs. 315.3 cm³, P<0.001), lower incidences of ≥G2 pneumonitis (10% vs. 24.4%, P=0.001) and ≥G3 esophagitis (2.2% vs. 15.6%, P<0.001). In evaluating the patterns of recurrence, in-field recurrences were the dominant type in both groups (21 out of 27 patients in MR-based delineation group, 24 out of 32 patients in CT-based delineation group).

Conclusions: MR-based delineation in hypo-CCRT was feasible and achieved similar treatment efficacy to CT-based delineation. The use of MR imaging to reduce the target volume resulted in promising local control and lower incidence of radiation-induced toxicities.