General principle:
Begin endocrine therapy only when its biologic trigger appears — biochemical recurrence, measurable receptor-positive disease, or symptom-producing tumor burden. Overtreatment wastes QoL; late treatment costs control.
Breast cancer
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Adjuvant (after surgery): start within 12 weeks post-operation.
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Neoadjuvant: start after biopsy confirmation ER/PR +, for 3–6 months to shrink the tumor.
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Metastatic: start promptly at diagnosis unless there’s visceral crisis (then use chemotherapy first).
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Switch or combine (AI → SERD ± CDK4/6) on progression or ESR1 mutation.
Prostate cancer
| Setting | Trigger | Start rule |
|---|---|---|
| Localized high-risk + RT | Before RT | 2–3 months prior → continue during and after |
| Locally advanced/metastatic | At diagnosis | Immediate ADT ± AR antagonist |
| Biochemical recurrence | PSA > 0.4 after prostatectomy or doubling time < 6 mo | Early ADT improves MFS; may delay for slow PSA rise & asymptomatic |
Clinical caution: confirm true biochemical failure (exclude lab noise or infection).
Quality of life: weigh bone, libido, and metabolic impact before early initiation.
(NCCN Breast 2025; ESMO Prostate 2024)