When sending patients for follow up chest CT's after SBRT or chemoRT, how do you determine whether to send for scans with or without IV contrast?

My rule for lung SBRT has been to minimize the use of contrast given the patient population we are treating (elderly, frail, and a desire to minimize kidney stressors), and the fact that we are following lung parenchymal lesions which are generally well visualized without contrast. I also have never really found the absence of contrast to impede me from detecting suspected mediastinal recurrence when patients are on surveillance.

For patients treated with definitive chemoradiotherapy, I am dealing with a different patient population and disease presentation; both reasons in my view warrant the use of contrast to optimize interpretation of the mediastinum.  

If the patient received SBRT or chemo/RT within 5 years, my preference is still chest CT with IV contrast, if the patient doesn’t have a significant allergy for IV contrast. Chest CT with IV contrast has much better image quality to evaluate lymph node status. However, if the patient has no evidence of disease for 5 years, I feel comfortable having chest CT without IV contrast as part of annual surveillance imaging.