Abstract P1-08-19: 10-year conditional recurrence risks, overall and relative survival for breast cancer patients in the Netherlands: Taking account of event-free years

Background Survival estimates valid at the time of diagnosis are of limited value for (ex-)breast cancer patients who survived several years, as it includes information on already deceased patients. This study analyzed the 10-year conditional risk of recurrent breast cancer in specific prognostic subgroups according to T and N stage and breast cancer subtypes. Secondly, we investigated 10-year conditional overall (OS) and relative survival (RS), adjusted for confounding. Patients and methods We selected all women diagnosed in 2005 with operated T1-2N0-1 breast cancer from the Netherlands Cancer Registry. Patients were classified into T1N0, T1N1, T2N0 and T2N1 stage. Ten-year conditional recurrence rates were calculated for every year from diagnosis for patients without an event (local (LR), regional recurrence (RR), distant metastasis (DM) or death). Ten-year conditional OS was calculated using multivariable Cox regression. RS was estimated by dividing patient survival rates by those of the general Dutch population. Results We included 7,969 patients: 52.3% had T1N0, 15.3% T1N1, 19.9% T2N0 and 12.5% T2N1 stage. For T1N0, 10-year LR rates changed from 4.6% at diagnosis to 0.5% in year 10. RR rates decreased from 2.3% to 0.2% and DM rates decreased from 7.8% to 0.6%. For T2N1 stage, the LR, RR and DM rates decreased from 6.2% to 0.8%, 5.2% to 0.4% and 19.6% to 1.5%, respectively. Of all patients, 1,702 patients (21.4%) had an unknown breast cancer subtype and were consequently excluded from the analyses according to subtype. Of the remaining 6,267 patients, 3,774 (60.2%) had luminal A, 1,465 (23.4%) had luminal B, 314 (5.0%) had HER2 positive and 714 (11.4%) had triple negative disease For the luminal A subtype, LR, RR and DM rates ranged from 3.9% to 0.4%, 1.7% to 0.5% and 7.3% to 1.1%, while for triple negative these rates ranged between 5.6% to 0.7%, 4.9% to 0.2% and 16.7% to 0%, respectively. Differences between subgroups attenuated over time and all recurrence rates became ≤1.5% in year 10. Ten-year OS and RS, adjusted for confounding, showed diminishing risk differences between subgroups over time. Conclusion Differences in recurrence rates, OS and RS between prognostic subgroups decreased as years passed by. These results highlight the importance of taking into account disease-free years to more accurately predict (ex-)breast cancer patients' prognosis over time. Citation Format: van Maaren MC, Strobbe LJ, Smidt ML, Moossdorff M, Poortmans PM, Siesling S. 10-year conditional recurrence risks, overall and relative survival for breast cancer patients in the Netherlands: Taking account of event-free years [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P1-08-19.