Purpose: To quantify the correlation between gross tumor volume (GTV) margins and local control (LC) outcome in head and neck (H&N) and non‐small cell lung cancer (NSCLC) patients. Methods: Interfractional motion simulation was done using the robustness analysis tool in CERR. This tool applies shifts to individual patients fraction by fraction within an entire course of treatment obtaining a final integral dose distribution. The shifts are defined assuming a Gaussian distribution of possible positioning for every fraction. Because of their fractionation schemes, 35 shifts were applied to the H&N, and 30 to the NSCLC dataset. We applied isotropic random shifts sampled from three independent normal distributions all with standard deviations of 3mm. Systematic errors were not included in this study, nor roll rotations. We repeated the process 10 times and averaged the integral doses. Ring structures around the GTV extending isotropically were created. The ring's width ranged from 2 mm to 15mm in H&N, and from 2mm to 20mm in NSCLC. A cell‐kill based equivalent uniform dose (cEUD) metric was calculated for each structure. A logistic regression model was built using cEUD for the margins and for the GTV. LC prediction was evaluated using Spearman correlation coefficient (Rs) on the entire datasets and also on cross‐validation. Results: Adding a margin to GTV did not improve LC correlation in H&N after cross‐validation (Rs= 0.398 p=0.0003 for the GTV alone, and Rs= 0.371 p=0.0007 for the next best added margin). For NSCLC we improved from Rs=0.393 (p=0.003) to Rs=0.413 (p=0.0015) at 5mm margin. Conclusion: Our analysis suggests that in H&N the dose given to the tissue surrounding the GTV is not correlated with LC, but it is for NSCLC. This may be because immobilization devices and setup verification systems are more accurate in H&N and also because the differences in prescription doses.