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Bias in survival estimates created by a requirement for consent to enter a T clinical breast cancer registry
J. Mark Elwooda, , Roger J. Marshalla, Sandar Tin Tina, Mark E.P. Barriosb,d, Vernon J. Harveyb,c
a Department of Epidemiology and Biostatistics, School of Population Health, University of Auckland, Auckland, New Zealand b Auckland Breast Cancer Register, Greenlane Clinical Centre, Auckland District Health Board, Auckland, New Zealand c Regional Cancer and Blood Centre, Auckland District Health Board, Auckland, New Zealand d Department of Haematology, Royal North Shore Hospital, New South Wales, Australia
Background: A requirement for consent for inclusion may bias the results from a clinical registry. This study gives a direct measure of this bias, based on a population-based clinical breast cancer registry where the re-quirement for consent was removed after further ethical review and data could be re-analysed. Methods: In Auckland, New Zealand, the population-based clinical breast cancer registry required written pa-tient consent for inclusion from 2000-2012. A subsequent ethical review removed this requirement and allowed an analysis of consented and non-consented patients. Kaplan-Meier survival to 10 years (mean follow-up 5.1 years, maximum 13.9 years), demographic and clinical characteristics were compared. Of 9244 women with invasive cancer, 926 (10.4%) were not consented, and of 1642 women with ductal carcinoma in situ, 245 (14.9%) were not consented.
Results: Survival was much higher for consenting patients; invasive cancer, 5 year survival 83.2% (95% con-fidence limits 82.2–84.1%) for consenting patients, 57.1% (53.0–60.9%) for non-consenting, and 80.8% in all patients. Analyses based only on consenting patients overestimate survival in all patients by around 2% at 2, 5, and 10 years. Non-consented patients were older, more often of Pacific ethnicity, had fewer screen-detected cancers, and more often had metastatic disease; electron transport less frequently had primary surgery or systemic treatments. Conclusion: Data from a registry requiring active consent gives an upward bias in survival results, as non-con-senting patients have more extensive disease, less treatment, and lower survival. To give unbiased results active consent should be not required in a clinical cancer registry.
The Auckland Breast Cancer Registry was established in June 2000 by a voluntary group led by clinicians to collect demographic, clinical and pathological data on all newly diagnosed patients with breast cancer in the Auckland region (population around 1.4 million in 2016), and to document follow-up and outcomes [1,2]. The registry is popu-lation-based, including all newly diagnosed patients with primary breast cancer resident in the defined region, and is regularly linked to the statutory national cancer registry, and to national mortality data to ascertain deaths [1,2]. From 2000–2012, individual written consent was required for patients to be included in the registry. Clinicians were requested to present the project to potential participants when seen in a clinic, and patients were asked to sign the consent form with a sup-porting signature by a witness. The consent form assured patients that
their demographic information and data on their cancer data, treatment and follow-up would be collected using a key-coded technique suitable for potentially identifiable data, and only anonymized/unidentifiable data would be used for research and audit activities and for presenta-tion or publication. The consent form provided assurances that there was a strict protocol to ensure confidentiality, and that participation was voluntary and would not affect the patient’s care. Patients were usually approached for consent at their first visit to the specialist clinic. For some patients the process was delayed but the consent process was completed by first therapy, within 90 days of diagnosis.
By 2010, the registry management group was aware that consent was not being obtained from approximately 10% of patients. To achieve greater completeness, the group requested a review by the national statutory Health and Disability Ethics Committee, operated by the Ministry of Health, on whether the requirement for consent was