Three sensitivity analyses were performed. First, a second matching step was performed in which two controls were selected for each case, where possible using a nearest neighbor and caliper metric. Controls needed to have propensity scores within 0.1 of the case to be selected. Thirty-eight of the 39 cases had at least one control using this method and for 36 cases two controls could be selected. The average difference between case and control propensity adjuvant RT was 0.008 (range 0.00003-0.095).
A second sensitivity analysis was performed to guard against immortal time bias. In order to mitigate the possibility of this effect, cases known not to have undergone adjuvant RT have been screened for suitable follow-up without a recurrence (local or regional recurrence, metastatic failure, and/or death) to ensure that if adjuvant RT had been prescribed as part of the multi-modality treatment regimen, that it would have been initiated. Three months was selected as the mandatory follow-up time. One to one matching was carried out and all 39 cases were matched to a control. A third sensitivity analysis was performed to account for stage migration seen in control patients that presented to the University of Michigan with more advanced disease. Patients that underwent adjuvant radiation were matched one to one with control group patients who did not receive adjuvant radiation, and who had the same stage at diagnosis as compared to stage at University of Michigan presentation.