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Purpose Sentinel lymph node biopsy (SLNB) is currently the standard of care in clinically node-negative (cN0) breast cancer. The present study aimed to evaluate the negative predictive value (NPV) of 18F-FDG dedicated lymph node positron emission tomography (LymphPET) in cN0 patients. Methods This is a prospective phase II trial divided into two stages (NCT04072653). In the first stage, cN0 patients underwent axillary LymphPET, followed by SLNB. In the second stage, SLNB was omitted in patients with a negative preoperative axillary assessment after integrating LymphPET. Here, we report the results of the first stage; the primary outcome was the NPV of LymphPET to detect macrometastases of lymph nodes. Results 189 invasive breast cancer patients underwent LymphPET, followed by surgery, with definitive pathological reports. Forty patients had lymph node macrometastasis (LN-macro), and 16 patients had only lymph node micrometastasis (LN-micro). Of the 131 patients who had a negative LymphPET, 16 patients had LN-macro, the NPV was 87.8%. After combining axillary imaging evaluation with ultrasound and LymphPET, 100 patients were found to be both LymphPET and ultrasound negative, 9 patients had LN-macro, the NPV was 91%. Conclusions LymphPET can be used to screen potential patients to avoid SLNB with an NPV >90%. The second stage of the SOAPET trial is ongoing to confirm the safe omission of SLNB according to preoperational axillary evaluation when integrating LymphPET.