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Comment & Response
September 5, 2019

Reanalysis of Data Comparing Prophylactic Cranial Irradiation vs Observation in Patients With Locally Advanced Non–Small Cell Lung Cancer

Author Affiliations
  • 1Department of Statistics and Actuarial Science, The University of Hong Kong, Hong Kong
  • 2Department of Biostatistics, The University of Texas, MD Anderson Cancer Center, Houston
JAMA Oncol. Published online September 5, 2019. doi:10.1001/jamaoncol.2019.3593

To the Editor In randomized clinical trials with time-to-event outcomes, hazard ratios (HRs) are often reported, and misinterpretation of the HR is common.1 In the long-term update of the NRG Oncology/RTOG 0214 phase 3 trial, Sun et al2 reported that prophylactic cranial irradiation (PCI) decreased the incidence of brain metastasis (BM) and prolonged disease-free survival but did not affect overall survival in patients with locally advanced non–small cell lung cancer. Based on an HR of 0.43 (95% CI, 0.24-0.77; P = .003) for BM, the authors posited that patients in the PCI arm were 57% less likely to develop BM than those in the observation arm. Such an interpretation is inappropriate because, unlike the relative risk, the HR is not a ratio of 2 probabilities and does not convey a message of likelihood of developing BM. Instead, we claim that the hazard of BM for patients in the PCI arm was reduced by 57% compared with patients in the observation arm. Using a stratified log-rank test, a significant difference in disease-free survival (HR, 0.76; 95% CI, 0.59-0.97; P = .03) was shown. However, the Kaplan-Meier survival curves in Figures 2A and 2B of the article by Sun et al2 are intertwined during the first year, and the spread between the 2 survival curves is quite irregular throughout the entire follow-up (sometimes the curves even cross), which suggests a possible violation of the proportional hazards (PH) assumption. The log-rank test is optimal under the PH assumption; it incurs power loss otherwise. Moreover, if the PH assumption does not hold, the HR from the Cox model is not a meaningful quantity and its interpretation is unclear. A more robust (assumption-free) approach to quantifying treatment effect uses the restricted mean survival time (RMST),3-5 which is defined as the area under the Kaplan-Meier curve. We reanalyzed the data constructed from Figure 2B,2 and the difference in 10.5-year RMSTs between the 2 arms was 0.46 (95% CI, −0.245 to 1.154; P = .203), favoring PCI. During the 10.5-year follow-up, patients receiving PCI enjoyed less than 6 months disease-free survival on average compared with those not receiving PCI, which was not statistically significant. The log-rank test without stratification yielded P = .16, which is consistent with the RMST result. We recommend that the authors reanalyze the data using the stratified RMST-based test (stratified by the AJCC stage, prior surgery, histologic characteristics, and Zubrod performance status) owing to a lack of power for the log-rank test under deviations from the PH assumption.

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