Background: The 8th edition of the American Joint Committee on Cancer (AJCC) staging has introduced\nprognostic stage based on anatomic stage combined with biologic factors. We aimed to validate the prognostic\nstage in HER2-positive breast cancer patients enrolled in the ShortHER trial.\nMethods: The ShortHER trial randomized 1253 HER2-positive patients to 9 weeks or 1 year of adjuvant trastuzumab\ncombined with chemotherapy. Patients were classified according to the anatomic and the prognostic stage. Distant\ndisease-free survival (DDFS) was calculated from randomization to distant relapse or death.\nResults: A total of 1244 patients were included. Compared to anatomic stage, the prognostic stage downstaged\n41.6% (n = 517) of patients to a more favorable stage category.\nFive-year DDFS based on anatomic stage was as follows: IA 96.6%, IB 94.1%, IIA 92.4%, IIB 87.3%, IIIA 81.3%, IIIC\n70.5% (P < 0.001). Five-year DDFS according to prognostic stage was as follows: IA 95.7%, IB 91.4%, IIA 86.9%, IIB\n85.0%, IIIA 77.6%, IIIC 67.7% (P < 0.001). The C index was similar (0.69209 and 0.69249, P = 0.975).\nWithin anatomic stage I, the outcome was similar for patients treated with 9 weeks or 1 year trastuzumab (5-year\nDDFS 96.2% and 96.6%, P = 0.856). Within prognostic stage I, the outcome was numerically worse for patients\ntreated with 9 weeks trastuzumab (5-year DDFS 93.7% and 96.3%, P = 0.080).\nConclusions: The prognostic stage downstaged 41.6% of patients, while maintaining a similar prognostic\nperformance as the anatomic stage. The prognostic stage is valuable in counseling patients and may serve as\nreference for a clinical trial design. Our data do not support prognostic stage as guidance to de-escalate treatment.
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