BackgroundThe aim of this study was to analyze biopsychosocial factors affecting how patients cope with cancer and adjuvant treatment and to appraise psychological distress, coping, perceived social support, quality of life and SDM before and after adjuvant treatment in breast cancer patients compared to colon cancer patients.MethodsNEOcoping is a national, multicenter, cross-sectional, prospective study. The sample comprised 266 patients with colon cancer and 231 with breast cancer. The instruments used were the Brief Symptom Inventory (BSI), Mini-Mental Adjustment to Cancer (Mini-MAC), Multidimensional Scale of Perceived Social Support (MSPSS), Shared Decision-Making Questionnaire-Patient (SDM-Q-9) and Physician’s (SDM-Q-Doc), and the European Organization for Research and Treatment of Cancer Quality of Life Questionnaire (EORTC-QLQ).ResultsBreast cancer patients reacted worse to the diagnosis of cancer with more symptoms of anxiety, depression, and somatization, and were less satisfied with their involvement than those with colon cancer (p = 0.003). Participants with colon cancer were older and had more physical symptoms and functional limitations at the beginning of adjuvant treatment, while there were scarcely any differences between the two groups at the end of adjuvancy, at which time both groups suffered greater psychological and physical effects and scored lower on coping strategies, except for anxious preoccupation.ConclusionsBreast cancer patients need more information and involvement of the oncologist in shared decision-making, as well as and more medical and psychological support when beginning adjuvant treatment. Both breast and colon cancer patients may require additional psychological care at the end of adjuvancy.