Mortality from myocardial infarction following postlumpectomy radiotherapy for breast cancer: a population-based study in Ontario, Canada.
OBJECTIVE
To compare the risk of mortality from myocardial infarction (MI) after left-sided postlumpectomy radiotherapy (RT) to the risk after right-sided postlumpectomy RT.


METHODS
We conducted a population-based cohort study of cases of invasive female breast cancer in Ontario, diagnosed between January 1, 1982 and December 31, 1987 (n = 25,570). Records of the Ontario Cancer Registry (OCR) were linked to hospital procedure and discharge abstracts and to RT records from Ontario cancer centers. A case was labelled as lumpectomy if this was the maximum breast surgery within 4 months of diagnosis. Postlumpectomy RT occurred up to 1 year postdiagnosis. Laterality was assigned from the laterality descriptor of the RT records. A case was labelled as having had a fatal MI if ICD code 410 (myocardial infarction) was recorded as the cause of death in the OCR. We used logistic regression to compare the likelihood of utilization of: 1. Dose per fraction > 2.00 Gy; 2. cobalt vs. linac; and 3. boost RT. We used life table analysis and the log rank test comparing the time to fatal MI from diagnosis of breast cancer between women who received left-sided postlumpectomy RT and women who received right-sided. We used Cox proportional hazards models to study the relative risk for left-sided cases overall, and stratified by age, RT characteristics, and among conditional survival cohorts.


RESULTS
Postlumpectomy RT was received by 1,555 left-sided and 1,451 right-sided cases. With follow-up to December 31, 1995, 2% of women with left-sided RT had a fatal MI compared to 1% of women with right-sided RT. Comparison of the time to failure between women who had left-sided RT and women who had right-sided RT showed the left-sided RT group to be associated with a higher risk of fatal MI (p = 0.02). Adjusting for age at diagnosis, the relative risk for fatal MI with left-sided postlumpectomy RT was 2.10 (1.11, 3.95).


CONCLUSIONS
Among women who received postlumpectomy RT for breast cancer in Ontario between 1982-1987, left-sided postlumpectomy RT was associated with a higher risk of fatal MI compared to right-sided.
Paszat
L F
LF
Department of Oncology, Queen's University and Kingston Regional Cancer Centre, Ontario, Canada.
Mackillop
W J
WJ
Groome
P A
PA
Schulze
K
K
Holowaty
E
E
D000328:Adult; D001943:Breast Neoplasms / Q000473:pathology / Q000532:radiotherapy* / Q000601:surgery; D015331:Cohort Studies; D003131:Combined Modality Therapy; D005260:Female; D006801:Humans; D015412:Mastectomy, Segmental; D008875:Middle Aged; D009203:Myocardial Infarction / Q000401:mortality*; D009864:Ontario / Q000453:epidemiology; D016016:Proportional Hazards Models; D011879:Radiotherapy Dosage; D012044:Regression Analysis; D012306:Risk