Inclusion Criteria

DISEASE CHARACTERISTICS:

- Histologically confirmed prostate cancer, meeting all of the following criteria:

- Prostate-specific antigen (PSA) < 10.0 ng/mL

- T1c-T2a disease

- Gleason sum of 6 or 7 (secondary pattern 4 only)

- Patients with low risk disease must meet the following criteria:

- Gleason pattern 3 + 3

- PSA < 10.0 ng/mL

- Clinical T2a disease

- Patients with Gleason secondary pattern 4 (i.e., Gleason Pattern 3 + 4) are eligible
but must not have a primary pattern 4, PSA > 10 ng/mL, or clinical T2b disease

- Measurable disease on MRI of at least 0.2 cc, based on planimetry volume

- Biopsy-proven disease within 2 years of screening visit

- No biopsy artifact on MRI scan (minimum 12-week interval between biopsy and
baseline MRI)

- Eligible for active surveillance according to the criteria set out by the National
Institute for Health and Clinical Excellence

PATIENT CHARACTERISTICS:

- ALT and AST ≤ 2 times the upper limit of normal (ULN)

- Alkaline phosphatase ≤ 2 times ULN

- Bilirubin ≤ 1.5 times ULN

- Estimated glomerular filtration rate (eGFR) ≥ 60 mL/min

- Able to swallow and retain oral medication

- Able and willing to participate in the study for its duration

- Able to read and write (health-outcomes questionnaires are written)

- Able to understand instructions related to study procedures and give written informed
consent

- No history of another malignancy within five years that could affect the diagnosis of
prostate cancer

- No history or current evidence of drug or alcohol abuse within the last 12 months
that might confound the results of the study or pose additional risk to the patient

- No known hypersensitivity to any 5α-reductase inhibitor or to any drug chemically
related to dutasteride

- No contraindication for undergoing gadolinium-enhanced MRI, including any of the
following:

- Inability to see tumor focus of ≥ 0.2 cc on T2 sequences

- Previous allergic reaction to gadolinium

- Serum creatinine > ULN

- Incompatible pacemaker

- Metal fragments in eyes

- Hip replacements that give artifact with prostate/pelvis views

- Any artifact or condition that reduces image quality of MRI (e.g., inability to
keep still)

- No unstable serious co-existing medical condition(s) including, but not limited, to
any of the following:

- Myocardial infarction, coronary bypass surgery, unstable angina, cardiac
arrhythmias, clinically evident congestive heart failure, or cerebrovascular
accident within 6 months prior to screening visit

- Uncontrolled diabetes

- Peptic ulcer disease uncontrolled by medical management

PRIOR CONCURRENT THERAPY:

- No prior radiotherapy (external-beam or brachytherapy), high-intensity focused
ultrasound (HIFU), or photodynamic therapy (PDT)

- No prior chemotherapy

- At least 3 months since prior and no concurrent prostatic surgery, including TUNA,
TURP, TUIP, laser treatment, thermotherapy, balloon dilatation, prosthesis, and
ultrasound ablation

- No prior oral glucocorticoids

- Glucocorticoids, except inhaled or topical, are not permitted within 3 months
prior to visit one

- No prior GnRH analogues (e.g., leuprolide, goserelin)

- No prior or concurrent hormonal treatment (e.g., megestrol, medroxyprogesterone,
cyproterone, DES) of prostate cancer

- No current and/or prior use of the following medications:

- Finasteride (Proscar, Propecia), or dutasteride (GI198745, AVODART) exposure
within 12 months prior to study entry

- Any other investigational 5α-reductase inhibitors within the past 12 months

- Anabolic steroids within the past 6 months

- Drugs with antiandrogenic properties within the past 6 months (e.g.,
spironolactone, flutamide, bicalutamide, cimetidine, ketoconazole,
progestational agents)

- The use of cimetidine is permitted prior to study entry

- The use of topical ketoconazole is permitted prior to and during the study

- No participation in another investigational or marketed drug trial within the 30 days
prior to the first dose of study drug or anytime during the study period