PII S0360-3016(99)00437-X CLINICAL INVESTIGATION Breast A POPULATION-BASED STUDY OF THE EFFECTIVENESS OF BREAST CONSERVATION FOR NEWLY DIAGNOSED BREAST CANCER LAWRENCE F. PASZAT, M.D., M.S., F.R.C.P.C.,* ² PATTI A. GROOME,PH.D.,* ² KARLEEN SCHULZE, M.MATH.,* ERIC J. HOLOWATY, M.D., M.SC., F.R.C.P.C., ‡ AND WILLIAM J. MACKILLOP, M.B., CH.B., F.R.C.P.C.* ² *Radiation Oncology Research Unit, Department of Oncology, Queen’s University, Kingston Regional Cancer Centre, and Kingston General Hospital, Kingston, Ontario, Canada; ² Department of Community Health and Epidemiology, Queen’s University, Kingston, Ontario, Canada; ‡ Ontario Cancer Registry, and the University of Toronto, Toronto, Ontario, Canada Purpose: Our objective was to evaluate the effectiveness of breast conservation for newly diagnosed breast cancer. Effectiveness was operationalized as two outcomes within 5 years of the diagnosis of breast cancer: the probability of mastectomy-free survival (either death or mastectomy count as event, whichever comes first), and the probability of mastectomy conditional on survival (mastectomy counts as event, observations censored at death). Methods and Materials: We linked records of 46,687 new cases of breast cancer from 1982 to 1991 in the Ontario Cancer Registry to records of surgery from 1982 to 1995, radiotherapy (RT) from 1982 to 1992, and median household income from the 1986 census. We labeled breast surgery within 4 months and postoperative RT within 12 months of diagnosis as treatment for newly diagnosed breast cancer. Surgery was categorized as mastectomy, lumpectomy plus RT, lumpectomy alone, or no surgical procedure. Among cases that did not undergo mastec- tomy within 4 months of diagnosis, we labeled mastectomy subsequent to 4 months after diagnosis as treatment failure. We performed life-table analysis and Cox proportional hazards regression, to describe the probability of mastectomy conditional on survival and the probability of mastectomy-free survival. Results: A total of 16,279 cases underwent lumpectomy as the maximum procedure on the breast within 4 months of diagnosis, and 49.7% of these received postoperative RT. Compared to the provincial mean, regions with higher rates of lumpectomy plus RT have higher probability of mastectomy-free survival and lower probability of mastectomy conditional upon survival 5 years after diagnosis of breast cancer. Conclusions: These findings are consistent with a hypothesis that breast conservation is effective in the overall breast cancer population of Ontario within the first 5 years after diagnosis. © 2000 Elsevier Science Inc. Breast cancer, Breast conservation, Radiotherapy, Effectiveness, Outcomes research. INTRODUCTION In the treatment of early-stage breast cancer, breast conser- vation is a strategy to treat the breast without mastectomy, commonly by means of lumpectomy plus adjuvant breast radiotherapy (RT). Randomized clinical trials have shown that lumpectomy plus RT in early breast cancer is effica- cious in avoiding recurrence of breast cancer in the ipsilat- eral breast (1, 2). Patients enrolled within clinical trials for their treatment often have better outcomes than patients treated outside of clinical trials. They are rigorously selected to be comparable, and their physicians, treating institutions, and treatment processes are subject to stringent guidelines (3). The treatment benefits seen in randomized clinical trials may be diminished when the treatment is implemented in the general population, because the spectrum of patients treated will be broader, and there will be a broader range of treatment processes and physician and institutional expertise outside of the context of a randomized clinical trial (4, 5). In contrast to the efficacy of interventions demonstrated in controlled clinical experiments, the effectiveness of an intervention is the outcome of care provided under ordinary conditions by average practitioners for typical patients (6). Evidence of the effectiveness of an intervention is the extent to which the benefits are observed in the general population (7). The use of lumpectomy has been described in Ontario (8 –10) as has the use of RT among women with newly diagnosed breast cancer (11). Regional variations in the use of each of these procedures have been present within On- tario. The effectiveness of breast conservation, and the Corresponding author: Dr. Lawrence Paszat, Radiation Oncol- ogy Research Unit, Kingston General Hospital, Apps 4, Kingston, Ontario, Canada K7L 2V7. E-mail: larry.paszat@cancercare.on.ca Acknowledgments—Dr. Paszat and Dr. Groome are Career Scien- tists of the Ministry of Health of Ontario. The Radiation Oncology Research Unit is supported by a grant from Cancer Care Ontario. We thank the Departments of Radiation Oncology of Ontario which provided access to their records. Accepted for publication 4 October 1999. Int. J. Radiation Oncology Biol. Phys., Vol. 46, No. 2, pp. 345–353, 2000 Copyright © 2000 Elsevier Science Inc. Printed in the USA. All rights reserved 0360-3016/00/$–see front matter 345