This ebook consists of a Thesis describing the research performed on the possible role of peripheral blood and tissue macrophages in diagnostic application in the detection colorectal cancer. The Thesis consists of multiple chapters covering the current possibilities in detection of colorectal cancer, the role of the immune system in this process, the discovery of tumormarker containing macrophages in tissue utilizing immunohistochemistry, (immuno) electron microscopy and in peripheral blood utilizing multiparameters flow cytometry. Furthermore the Thesis elaborates on the early application of a diagnostic tool and its validation when applying intracellular detection of tumormarkers (epitopes) in macrophages in peripheral blood (EDIM) . And finally the Thesis describes the application of the experimental test in comparison to PET-CT and serum inflammatory parameters and tumor markers when investigating the response to neoadjuvant chemoradiotherapy in rectal cancer patients. The Thesis also comprises of a english and dutch summary and a valorization paragraph including the abbreviations used in the Thesis as a whole.