Prognostic Factors as a Function of Disease-free Interval After Definitive (Chemo)radiation for Non–Small Cell Lung Cancer Using Conditional Survival Analysis

Purpose: We analyzed overall and disease-free survival (OS and DFS) after definitive (chemo)radiation for stage III non–small cell lung cancer with 2 statistical methods: Kaplan-Meier (KM) analysis, with diagnosis as index date, and conditional survival (CS) analysis, with a variety of disease-free index dates, and determined whether prognostic factors varied based on the reference date. Materials and Methods: All 651 patients analyzed received definitive (chemo)radiotherapy for stage III non–small cell lung cancer in November 1998 to December 2010 at a single institution; all had Karnofsky performance status scores ≥60 and received ≥60 Gy. OS and DFS were first calculated with the KM method, and then CS was used to calculate 2 outcomes: OS conditioned on DFS time (OS|DFS) and DFS conditioned on DFS time (DFS|DFS). Factors predicting OS and DFS conditioned on 1-, 2-, and 3-year DFS were sought in univariate and multivariate analyses. Results: KM analysis produced 1-, 2-, and 3-year DFS rates of 48%, 30%, and 26%; OS rates were 64%, 41%, and 29%. By CS analysis, both OS|DFS and DFS|DFS showed an increase in 5-year OS after 6 months, and CS after 30 months approached 100%. On multivariate analyses, age and concurrent chemoradiation predicted OS|DFS; age, smoking history, tumor histology, disease stage, and radiation dose predicted DFS|DFS. Conclusions: CS analysis showed that the probability of long-term survival increases sharply after 6 months with no evidence of disease; factors predicting survival differed based on the method and endpoint used.

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