Abstract P1-07-06: Weight gain during pre- and postmenopausal years results in earlier onset of breast cancer. The Tromsø cohort study
Authors: Lofterød T , Frydenberg H , Flote VG , Risberg T , Eggen AE , McTiernan A , Mortensen E , Wist EA , Akslen LA , Reitan JB , Wilsgaard T , Thune I 
Published: May 2016

Abstract
Background: Obesity is both an independent risk factor, and a prognostic factor of postmenopausal breast cancer. In contrast, the association between premenopausal obesity/leanness, and subsequent weight gain and breast cancer outcomes, is still unclear. Furthermore, the association between adult weight gain, weight and age at diagnosis, and tumor characteristics is less studied.
Methods: During 1979-2007, a total of 18 990 women, aged 18-87 years, answered questionnaires and underwent clinical examination at a total of five repeated health surveys (attendance rate 68-82%). Height and weight were measured at each survey, and before surgery, among those women diagnosed with breast cancer during follow-up. Careful review of the respective medical records, including histopathological workup, was performed. Multivariate Cox Proportional Hazard models were used to study the importance of Body Mass Index (BMI kg/m2) and weight change on breast cancer risk, and to evaluate variation in breast tumor characteristics.
Results: During a median follow-up of 23.3 years, 579 women with invasive breast cancer were identified, and the cases were histologically verified. These breast cancer cases had a mean age at diagnosis of 56.3 years, and mean BMI at diagnosis of 25.3 kg/m2. Most (67 %) of the breast cancer patients had estrogen receptor (ER) positive tumors, 48 % had progesterone receptor (PgR) positive tumors, and 41 % had lymph node positive disease. We divided all participating women in three groups of weight change (&lt; 5kg, 5-15 kg, &gt;15 kg). When we compared women with less than 5 kg weight gain, to women with weight gain 5-15 kg, and to women with weight gain above &gt;15kg, we observed a RR of 1.43 (95% CI 1.07-1.90) and a RR of 1.86 (95% CI 1.30-2.68), respectively, for postmenopausal breast cancer. We divided women by quartiles of BMI (kg/m2) at entry, and observed that women in the lowest quartile of BMI (≤ 21.45 kg/m2), who had a subsequent weight gain &gt;15 kg, had a RR of 2.40 (95% CI 1.07-5.38) for postmenopausal breast cancer compared to women with the same BMI at entry, but who remained stabile in weight. We observed a 6 year difference in age at diagnosis for women diagnosed with breast cancer, who at study entry were in the same BMI group (&lt; 25kg/m2), but subsequently either experienced a large weight gain (&gt;15 kg), or remained stabile in weight (59.5 years vs. 64.4 years, p=0.007). Furthermore, we observed a 15 year difference in age at diagnosis for women diagnosed with breast cancer, who at study entry were in the same BMI group (≥ 25kg m2), but subsequently either experienced a large weight gain (&gt; 15 kg), or remained stabile in weight (60.3 years vs. 74.9 years, p=0.007).
Conclusion: Avoiding large weight gain during pre- and postmenopausal years may both protect against, and delay onset of postmenopausal breast cancer. Our findings support the importance of weight gain as a modifiable lifestyle factor for early onset of breast cancer.
Citation Format: Lofterød T, Frydenberg H, Flote VG, Risberg T, Eggen AE, McTiernan A, Mortensen E, Wist EA, Akslen LA, Reitan JB, Wilsgaard T, Thune I. Weight gain during pre- and postmenopausal years results in earlier onset of breast cancer. The Tromsø cohort study. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P1-07-06.