RATIONALE: Estrogen can stimulate the growth of breast cancer cells. Hormone therapy using tamoxifen with or without octreotide may fight breast cancer by blocking the uptake of estrogen. It is not yet known which treatment regimen is more effective for breast cancer.

PURPOSE: Randomized phase III trial to compare the effectiveness of tamoxifen with or without octreotide in treating postmenopausal women who have stage I, stage II, or stage III breast cancer.

OBJECTIVES: I. Compare event-free, recurrence-free, and overall survival following adjuvant therapy with tamoxifen alone vs. tamoxifen plus octreotide long-acting release formulation in postmenopausal women with stage I/II/III breast cancer. II. Compare the toxicity and quality of life associated with each treatment regimen. III. Compare the effects of each treatment regimen on insulin-like growth factor-I (IGF-I) physiology, and study the relationship between IGF-I physiology and outcome.

OUTLINE: This is a randomized study. Patients are stratified by participating institution, when and whether they receive adjuvant chemotherapy, axillary lymph node status, and hormone receptor status. All patients are randomized within 12 weeks of definitive surgery. Patients receiving adjuvant chemotherapy prior to protocol treatment are randomized within 6 weeks after the last dose of chemotherapy. One group of patients receives daily oral tamoxifen, while a second group receives daily oral tamoxifen plus octreotide (long-acting release formulation) by monthly depot injection. Treatment in both groups continues for 5 years or until disease recurrence or development of a second malignancy. Patients are followed monthly for 4 months, every 4 months for 3 years, and every 6 months thereafter.

PROJECTED ACCRUAL: A total of 850 patients will be entered over 4.2 years in this multicenter study.

DISEASE CHARACTERISTICS: Histologically proven adenocarcinoma of the breast that is potentially curable Prior treatment with one of the following therapies required: Segmental mastectomy (lumpectomy) followed by radiotherapy Chest wall irradiation allowed only in patients with T4 dermal involvement on pathologic diagnosis Further excision or boost radiotherapy to the tumor bed recommended if microscopic disease found at mastectomy margins Total mastectomy Chest wall irradiation required if microscopic disease found at mastectomy margins Clinical stage T1-3a N0-2 M0 disease prior to surgery The following T4 features exclude: Chest wall extension Edema (including peau d'orange) Skin ulceration Satellite skin nodules confined to same breast Inflammatory carcinoma Pathologic stage T1-4 NX-2 M0 disease following surgery Eligible T4 tumors are those with dermal involvement on pathology assessment only Pathologic assessment of axillary lymph nodes required May be omitted in patients with clinical N0 status provided other entry criteria are met No bilateral breast cancer without complete resection of both sides Hormone receptor status: Estrogen and progesterone receptor status determined from primary tumor when possible by quantitative biochemical methods or immunohistochemistry Results recorded as positive or negative if immunohistochemistry used Unknown status does not exclude provided other entry criteria are met

PATIENT CHARACTERISTICS: Age: Postmenopausal Sex: Women only Menopausal status: Postmenopausal by one or more of the following: Amenorrhea lasting more than 1 year in women under 50 years of age with no prior hysterectomy No menses for 6 months prior to breast surgery in women 50 years of age and over with no prior hysterectomy Documented oophorectomy prior to breast cancer diagnosis Luteinizing hormone and follicle-stimulating hormone values diagnostic of postmenopausal status by local laboratory criteria Women 50 years of age and over with prior hysterectomy Performance status: ECOG 0-2 Life expectancy: At least 5 years Hematopoietic: WBC at least 3,000/mm3 Platelet count at least 100,000/mm3 Hepatic: (unless metastatic disease ruled out by radiologic exam) AST or ALT less than twice normal Alkaline phosphatase less than twice normal Renal: Not specified Other: No symptomatic gallbladder disease or cholecystitis No intercurrent illness that reduces life expectancy to less than 5 years No other major medical or psychiatric illness that precludes study treatment or follow-up No second malignancy within 5 years except: Adequately treated basal cell skin carcinoma Adequately treated cancer of the cervix, endometrium, colon, or thyroid Able and willing to complete quality-of-life questionnaires in English or French Illiteracy, loss of sight, or other inability to complete questionnaires does not exclude Accessible for treatment and follow-up

PRIOR CONCURRENT THERAPY: Biologic therapy: No prior biologic therapy Chemotherapy: Prior or concurrent adjuvant chemotherapy allowed at investigator's discretion Recommended regimens: CMF (cyclophosphamide/methotrexate/fluorouracil) CEF (cyclophosphamide/etoposide/fluorouracil) AC (doxorubicin/cyclophosphamide) Choice of adjuvant chemotherapy regimen defined prior to randomization if given concurrently with protocol therapy Endocrine therapy: No estrogen, progestins, or androgen therapy for a period of more than 30 days following pathologic diagnosis of breast cancer Prior tamoxifen allowed All hormonal therapy discontinued prior to randomization Radiotherapy: See Disease Characteristics Surgery: See Disease Characteristics