⁃ Histologically confirmed HER-2 -positive breast cancer with newly-diagnosed brain metastases.

⁃ ECOG Performance Status (PS) of 0, 1, 2

⁃ Patients with 1-10 brain metastases will be candidates for tucatinib, capecitabine, and trastuzumab with SRS at the discretion of the treating radiation oncologist. Intra-cranial brain metastasis must measure 3 cm or less in the greatest dimension

⁃ Age 18 years or greater and being willing and able to sign a written informed consent. A signed informed consent must be obtained prior to any study specific procedures

⁃ Life expectancy at least 12 weeks

⁃ Any number of prior systemic therapies will be allowed, except tucatinib and capecitabine.

⁃ Hemoglobin ≥ 9g/dL, White blood count ≥3.0 × 10\^9/ L , Absolute Granulocyte count ≥1.5x 10\^9/ L and platelet count ≥100 × 10\^9/ L.

⁃ Serum bilirubin ≤ 1.5 × ULN

⁃ AST and / or ALT \<= 2 × ULN (≤ 5 × ULN when clearly attributable to the presence of liver metastases)

‣ Serum creatinine ≤ 1.5 × ULN or calculated creatinine clearance \> 60mL/min

‣ Ability to comply with study procedures and monitoring

‣ For women of childbearing potential, a negative pregnancy test should be obtained within one week prior to the start of therapy

‣ Male or female patients of reproductive potential need to employ two highly effective and acceptable forms of contraception throughout their participation in the study and for 7 months after last dose of tucatinib, capecitabine and trastuzumab.

∙ Highly effective and acceptable forms of contraception are:

• Male condom plus spermicide

• Cap plus spermicide

• Diaphragm plus spermicide

• Copper T

• Progesterone T

• Levonorgestrel-releasing intrauterine system (e.g., Mirena®)

• Implants

• Hormone shot or injection

• Combined pill

• Mini-pill

• Patch

∙ Postmenopausal woman on the study (that will not need contraception) is defined as:

• Amenorrhoeic for 1 year or more following cessation of exogenous hormonal treatments

• LH and FSH levels in the postmenopausal range for women under 50

• Radiation-induced oophorectomy with last menses \> 1 year ago

• Chemotherapy-induced menopause with \>1 year interval since last menses

• Surgical sterilization (bilateral oophorectomy or hysterectomy).

∙ Men and women and members of all races and ethnic groups are eligible for this trial.