• Breast Cancer Eligibility:

‣ Population 1: Pathologically confirmed ER and/or PR positive, locoregionally recurrent or metastatic breast cancer that is refractory to endocrine therapy (progression on at least one line of endocrine therapy and determined by the investigator that Study Participant would not benefit from additional endocrine therapy) who have received at least one line of chemotherapy or antibody drug conjugate in the metastatic setting (recurrence within 6 months of adjuvant chemotherapy counts as one line of therapy). There is no limit on prior number of lines of endocrine therapy. Prior treatment with CDK4/6, AKT, PI3K and/or mTOR inhibitors is permitted.

⁃ Population 2: Pathologically confirmed ER positive and HER2 negative locoregional or metastatic breast adenocarcinoma that is not amenable to resection, with progression on at least one line of prior endocrine therapy in the adjuvant or metastatic setting and starting next line of therapy that will include endocrine therapy, such as tamoxifen, fulvestrant, aromatase inhibitor, or elacestrant with or without ovarian suppression.

⁃ Population 3: Study Participants with pathologically confirmed, estrogen and/or progesterone receptor (ER and/or PR) positive and HER2 negative breast adenocarcinoma who are ineligible for Population 1 or 2.

• At least one target or non-target lesion per RECIST 1.1

• Male or non-pregnant, non-lactating female Study Participant age ≥18 years. Female Study Participant of child-bearing potential and male Study Participant (if sexually active) must agree to use adequate method(s) of effective contraception during their participation in the study.

∙ Medically acceptable adequate contraception for sexually active females with child-bearing potential include: 1) surgical sterilization (such as tubal ligation or hysterectomy), 2) approved hormonal contraceptives, 3) barrier method (such as condom or diaphragm) used with a spermicide, or 4) intrauterine device (IUD).

‣ Medically acceptable adequate contraception for sexually active males include: 1) surgical sterilization (such as vasectomy), 2) a condom used with a spermicide.

• Eastern Cooperative Oncology Group (ECOG) Performance Status ≤2.

• Adequate hepatic function as defined below (within 28 days of dosing with 68Ga R10602):

∙ Serum alanine aminotransaminase (ALT)/ aspartate aminotransaminase (AST) ≤3 × upper limit of normal (ULN) or ≤5 × ULN if liver metastases are present, and

‣ Serum bilirubin: total ≤1.5 × ULN (unless due to Gilbert's syndrome or hemolysis in which case total ≤3.0 × ULN).

• Adequate renal function as measured by creatinine clearance calculated by the Cockcroft-Gault formula (≥60 mL/minute), determined within 28 days of dosing with 68Ga-R10602.

• Able to understand and willing to sign an informed consent form (ICF).