Chemoprevention With Tamoxifen in Pre-Invasive Pancreas Mucinous Cystic Neoplasms Not Undergoing Immediate Resection

Inclusion Criteria: * Age ≥ 19 years * Clinically diagnosed pre-invasive pancreatic mucinous cystic neoplasm (MCN), measurable by cross-sectional imaging * Surgical resection of the lesion is not planned due to cyst features, patient factors or patient preference * Females of reproductive potential and males with partners of reproductive potential must agree to employ two methods contraception throughout the study and for up to 3 months following treatment. Non-child-bearing potential is defined as age 45 years or older and no menses for greater than or equal to 12 months or any age with surgical removal of the uterus and/or both ovaries. * Estimated glomerular filtration rate (eGFR) \> 30mL/min/1.73m2 * Willing and able to provide informed consent to and abide by the protocol Exclusion Criteria: * Presence of invasive pancreatic adenocarcinoma or high-grade dysplasia * Presence of a solid component or mural nodule, main pancreatic duct dilation or abrupt caliber change, obstructive jaundice, lymphadenopathy * Current or prior use of tamoxifen or another estrogen antagonist including, but not limited to, clomifene, raloxifene, fulvestrant, anastrazole; subjects who have previously used an estrogen antagonist are eligible provided the last use was at least 5 years prior to enrollment. * Current or planned use of hormonal treatments including estrogen, progesterone, androgens, hormone replacement therapy or other types of hormonal contraceptives including implants and depot injections; levonorgestrel-releasing intrauterine device (IUD) is permitted. * Contraindications to tamoxifen include: * Pregnancy or nursing * Known allergy or hypersensitivity to tamoxifen * Cataracts which affect visual acuity (ie. symptomatic) * Retinopathy which affects visual acuity (ie. symptomatic) * Current warfarin use * History of deep vein thrombosis or pulmonary embolism or other condition which, in the opinion of the investigator, may significantly increase the individual's risk of venous thromboembolism * History of stroke * Known endometrial hyperplasia or personal history of endometrial carcinoma, uterine sarcoma and uterine carcinosarcoma * History of intestinal disease or major gastric surgery likely to alter absorption of tamoxifen or inability to swallow oral medications * Uncontrolled illness including but not limited to ongoing or active infection requiring IV antibiotics, symptomatic congestive heart failure, unstable angina or uncontrolled cardiac arrhythmias, or other conditions which might jeopardize or preclude the ability of the patient to take tamoxifen or the safety of follow-up visits, scans and procedures * Elective surgery planned for the study period * Participation in another clinical study with an investigational product during the last 28 days * Any participant, in the opinion of the investigator, who will not be able to tolerate treatment, or the participant is unsuitable to participate in the study and is unlikely to comply with study procedures, restrictions and requirements