This prospective cohort study included patients drawn from 22 hospitals belonging to the Spanish National Health Service (SNS), which covers the majority (99.8%) of the population of Spain. All covered residents have free access to their primary care physician and to the emergency departments of the hospitals. All of the hospitals have similar technological and human resources.

Patients with a diagnosis of colon cancer attending the surgical services of any of these hospitals to undergo surgery between June 2010 and December 2012 were informed of the goals of the study and invited to voluntarily participate. In order to take part in the study, a patient had to provide informed consent. All information was kept confidential. The Institutional Review Boards of the participating hospitals approved this project.

Data collected Data collected upon hospital admission included information on sociodemographic data, clinical data (including information about onset of symptoms, habits, personal and family background, comorbidities, diagnostic tests, pre-intervention treatments), preoperative data (including analytical, tumor markers and diagnostic tests), data of the outpatient anesthesia data on the surgical intervention, pathology data, and data related to the remaining days of admission (including the presence of complications, need for reintervention or death). Subsequently, data were collected up to 30 days after surgery (analytical, diagnostic tests, presence of complications, readmissions, reintervention or death). Finally, information was collected through the year including need for radiation therapy, chemotherapy (treatment schedule, cycles, complications of treatment, supportive care), laboratory results and diagnostic tests performed, presence of complications, tumor recurrence, readmission or reintervention and death.

Patients completed the following questionnaires before surgery and one year after surgery: EuroQol-5D, EORTC QLQ-C30 (The European Organization for Research and Treatment of Cancer quality of life questionnaire) and QLQ-CR29 (European Organisation for Research and Treatment of Cancer questionnaire module for colorectal cancer), and HADS. The EuroQol-5D is a generic HRQoL questionnaire which has demonstrated good psychometric properties and allow for comparison with other populations. It has been translated and validated in Spanish. This instrument consists of two parts: the first is a description of the state of health in five dimensions: mobility, self-care, usual activities, pain / discomfort and anxiety / depression. Each of these dimensions is measured by three answer options that define different levels of severity. The second is a visual analogue scale in which patients rate their health on a scale displayed in the form of a thermometer (20 mm), whose ends are 0 (worst imaginable health state) to 100 (best state imaginable health). The EORTC QLQ-C30 is a questionnaire for evaluating the HRQoL for cancer patients undergoing treatment, which has shown good validity and reliability in its Spanish adaptation (Cronbach's alpha coefficient of 0.7). It is one of the instruments measuring quality of life in cancer patients most commonly used and allow the comparison of the results obtained in patients with colorectal cancer patients with other cancers. It consists of 30 items, 28 of them with four possible answers (in all, a little, a lot, a lot) and two items with seven Alternative (visual analogue scale where one is "very poor" and seven "excellent"). The timeframe should refer to the previous week. The scores on each scale are transformed so that the final values are between 0 and 100. In addition to this overall score, the instrument consists of three scales: Global health status: in which a high score represents a high performance in terms of overall health; Functional -scale: consists of five subscales of operation: physical, role, emotional, cognitive and social; Symptoms scale consisting of a number of symptoms: fatigue, nausea, vomiting, pain, dyspnea, insomnia, decreased appetite, constipation, diarrhea and financial difficulties. A high score on each of these symptoms represents a high level of symptoms or problems in this area. The QLQ-CR29 was then developed after revising the QLQ-CR38, and was demonstrated internationally to have both sufficient validity and reliability to support its use as a supplement to the EORTC QLQ-C30 to assess patient-reported outcomes during treatment for colorectal cancer in clinical trials and other settings. The QLQ-CR29 contains 29 items, and is described in detail elsewhere. Briefly, there are 18 items addressing gastrointestinal symptoms, pain and problems with micturition, and there are separate scales for the participants with or without a stoma and separate items addressing sexual function for men and women. The response categories for each item are the same as those used in the QLQ-C30.