Background: Multi-parameter tumor gene expression assays (MPAs) are validated tools to assist adjuvant chemotherapy decisions for post-menopausal women with luminal-type node-negative breast cancer. Currently there is less certainty for women with 1-3 involved axillary lymph nodes and no information on MPA use for patients with higher level nodal involvement. Three RCTs with available data report chemotherapy benefit for premenopausal women; with limited use of ovarian function suppression (OFS) for non-chemotherapy treated participants, chemotherapy-induced menopause may explain these results. Methods: OPTIMA is an international academic, partially-blinded RCT of test-directed chemotherapy treatment with an adaptive design. Women and men aged 40 or older with resected luminal-type breast cancer may participate if they fulfil one of the following stage criteria: pN1-2; pN1mi with pT ≥20mm; pN0 with pT ≥30mm. Consenting patients are randomized between standard treatment with chemotherapy followed by endocrine therapy or to undergo Prosigna testing; those with high-Prosigna Score ( > 60) tumors receive standard treatment whilst those with low-score tumors are treated with endocrine therapy alone. Patients are informed only of their treatment; test details, and randomization for chemotherapy-treated patients are masked. Clinical choice of chemotherapy is declared at randomization from a menu of standard regimens. Endocrine therapy must be for at least 5 years. Women postmenopausal at trial entry should receive an AI; men, tamoxifen; and premenopausal women, either an AI or tamoxifen, and OFS for 3 or more years; OFS initiation may be deferred because of post-chemotherapy amenorrhea. OPTIMA aims to randomize 2250 patients in each arm to demonstrate non-inferiority of test directed treatment, defined as not more than 3% below the estimated 85% 5-year IDFS for the control arm with a one sided 5% significance level. Power is 81% assuming recruitment over 96-months from January 2017 and 12 months minimum follow-up. OPTIMA also has at least 80% power to demonstrate 3.5% non-inferiority of IDFS for patients with low Prosigna Score tumors (estimated 65% of participants). Cox proportional hazards models will be used to explore important prognostic factors including menopausal status. Additional secondary endpoints include DRFI. A cost-effectiveness analysis of protocol specified MPA driven treatment against standard clinical practice will be conducted. At 31/01/2021, 2004 patients had been randomized. The DMC reviewed the trial in December 2020 with knowledge of related trial results and suggested that the trial continues as planned. OPTIMA is registered as ISRCTN42400492 and funded by the UK NIHR Health Technology Assessment Programme, award number 10/34/501. Clinical trial information: ISRCTN42400492.