Stereotactic Body Radiotherapy (SBRT) is a form of high-precision radiotherapy playing a major role in patients diagnosed with early-stage non-small cell lung cancer (NSLCL), especially when surgery cannot be performed. It is a non-invasive, well-tolerated treatment, with an excellent ability to control disease recurrence. However, in some patients, disease response is suboptimal: understanding why this happens may open doors to more aggressive approaches, such as the combination with systemic therapies. Hence, the goal of this observational trial is to understand which clinical, imaging, and biological factors are associated with response to SBRT through the development of complex models. In other words, the main question it aims to answer is: Will this patient respond to radiation treatment based on the characteristics of their disease?. Participants will be treated according to the best clinical practice standards, in agreement with international, national, and internal guidelines. Researchers will compare data collected from patients treated with SBRT with those collected from a similar group of patients, who will be treated with surgery, to see which factors are actual predictors of response to SBRT, or rather are indicators of more or less aggressive disease behavior.