Patterns of metastatic progression after definitive radiation therapy for early-stage and locally advanced non-small cell lung cancer

Current preclinical models of metastatic disease (particularly oligometastases) suggest that metastases appear in a hierarchical order. We attempted to identify systematic patterns of metastasis in non-small cell lung cancer (NSCLC) after radiation therapy (XRT). We analyzed 1074 patients treated from 12/21/1998 through 8/20/2012 with ≥60 Gy definitive radiation for initially non-metastatic NSCLC. Location and time of metastases were recorded. Regional nodal failure was noted, as was subsequent distal failure. For further analysis, we considered only the five most common sites of metastasis (bone, brain, liver, adrenal, and lung). Metastatic progression over time was defined and patterns elucidated with Chi square tests. Histologic findings were analyzed with Wilcoxon rank sum tests. A significant multistep linear progression was not apparent. Having a first metastasis in lung or bone was associated with respective 16% (median 2.4 months) and 15% likelihoods (median 7.9 months) of secondary brain metastasis. Initial metastasis in the brain led to metastasis in another organ 29.3% of the time, most often in the lung, bone, and liver (medians 3.6, 7.9, and 3.1 months). Adenocarcinoma was more likely than squamous to metastasize to the brain (18 vs. 9%) and any of the five major sites (41 vs. 27%). We did not appreciate dominant patterns suggesting a multi-step hierarchical order of metastasis. Rather, our findings suggest that certain subgroups may develop different patterns of spread depending on a variety of factors.