Inclusion Criteria:

• Women ≥ 18 years of age
• Histological confirmation of ER+, HER2- breast cancer, newly diagnosed and tissue available for central confirmation of Ki-67 measurement. Ki-67 will be repeated for patients who have had this done externally.
• Scheduled for definitive breast surgery

• Detectable disease as defined by mammography, breast ultrasound of greater than 5mm in size.

  • NOTE: Tumor lesions in a previously irradiated area are not considered measurable disease; Disease that is measurable by physical examination only is not eligible

• Post menopausal as defined by:

  • Self-reported last menstrual period greater than 12 months, or
  • Bilateral oophorectomy, or
  • Follicle stimulating hormone (FSH) >20 mIU/mL, and estradiol level ≤ 20 pg/mL
• Not taking aromatase inhibitor or a selective estrogen receptor modifier
• Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0, 1 or 2
• Platelet count ≥ 100,000/mm^3 (obtained ≤ 90 days prior to registration)
• Absolute neutrophil count (ANC) ≥ 1,000/mm^3 (obtained ≤ 90 days prior to registration)
• Hemoglobin ≥ 11 g/dL (obtained ≤ 90 days prior to registration)
• Serum transaminase [alanine aminotransferase (ALT) or aspartate aminotransferase (AST)] ≤ 1.2 x upper limit of normal (ULN) (obtained ≤ 90 days prior to registration)
• Alkaline phosphatase ≤ 1.2 x ULN (obtained ≤ 90 days prior to registration)
• Serum creatinine ≤ 1.2 x ULN (obtained ≤ 90 days prior to registration)
• Provide written informed consent
• Ability to complete the Symptom Experience Diary by themselves or with assistance

Exclusion Criteria:

• Current use of any medicinal mushrooms
• Patient with locally advanced cancer who will require neoadjuvant therapy or metastatic cancer
• Currently on chemotherapy
• Concurrent endocrine therapy (selective estrogen receptor modifiers or aromatase inhibitors)
• Allergy to mushrooms
• Co-morbid systemic illnesses or other severe concurrent disease which, in the judgment of the investigator, would make the patient inappropriate for entry into this study or interfere significantly with the proper assessment of safety and toxicity of the prescribed regimens
• Immunocompromised patients

• Patients known to be HIV positive and currently receiving antiretroviral therapy.

  • NOTE: Patients known to be HIV positive, but without clinical evidence of an immunocompromised state, are eligible for this trial

• Uncontrolled intercurrent illness including, but not limited to:

  • Ongoing or active infection
  • Symptomatic congestive heart failure
  • Unstable angina pectoris
  • Cardiac arrhythmia
  • Or psychiatric illness/social situations that would limit compliance with study requirements
• Receiving any other investigational agent which would be considered as a treatment for the primary neoplasm

• Other active malignancy 3 years prior to registration

  • EXCEPTIONS: Non-melanotic skin cancer or carcinoma-in-situ of the cervix
  • NOTE: If there is a history of prior malignancy, they must not be receiving chemotherapy treatment for their cancer

Note: Other protocol defined Inclusion/Exclusion Criteria may apply.

Eligibility last updated 03/17/2025. Questions regarding updates should be directed to the study team contact.