The IMPRINT-study is a prospective biomarker study assessing the impact of different treatment field sizes and associated radiation doses on the patient's microbiome and metabolome, whereby the link with radiation-induced GI toxicities will be emphasized. Blood, urine and fecal samples will be longitudinally collected at 4 different time points: (1) shortly before, (2) during and (3) shortly after RT treatment, as well as (4) one-month post-RT. To our knowledge, this is the first clinical research project relating the impact of multiple radiation parameters on fecal-, urine- and blood-based biomarkers to risk of GI toxicities in a homogeneously defined study population.