Calcitriol and Dexamethasone in Treating Patients With Prostate Cancer Who Have Undergone Surgery or Radiation Therapy

DISEASE CHARACTERISTICS: * Histologically confirmed adenocarcinoma of the prostate meeting all of the following criteria: * Curatively treated with radical prostatectomy OR definitive radiotherapy * No signs of clinical recurrence or dissemination of prostate cancer by digital rectal examination without tumor * No local recurrence by CT scan or MRI of the pelvis * No metastases by bone scan and chest x-ray NOTE: Prior Prostascint scans allowed regardless of results * At least 3 prostate-specific antigen (PSA) measurements obtained over at least 90 days after radical prostatectomy on post-radiotherapy PSA nadir must be available * PSA at least 2 ng/mL after prostatectomy OR at least 4 ng/mL after prior radiotherapy * Patients previously treated with radiotherapy must have a PSA clearly rising from lowest value within 6 months after completion of therapy PATIENT CHARACTERISTICS: Age * 18 and over Performance status * ECOG 0-1 Life expectancy * At least 12 months Hematopoietic * Platelet count at least 100,000/mm\^3 * Hemoglobin at least 10 g/dL Hepatic * Bilirubin no greater than upper limit of normal (ULN) * Alkaline phosphatase no greater than 2 times ULN Renal * Creatinine less than 1.8 mg/dL * Phosphorus normal * No hypercalcemia (albumin-corrected calcium greater than ULN) * No nephrolithiasis * Single episode of renal lithiasis allowed provided episode occurred more than 5 years prior to study Other * Fertile patients must use effective double-barrier contraception for at least 1 week before, during, and for at least 2 weeks after study * No symptomatic pancreatitis * No uncontrolled diabetes * No known or suspected inability to comply with study requirements (e.g., abuse of alcohol/drugs or psychotic states) * Curatively treated condition associated with renal stones (e.g., hyperparathyroidism, bladder dysfunction, or obstructive uropathy) allowed provided patient has been free of stone formation for more than 5 years PRIOR CONCURRENT THERAPY: Biologic therapy * Not specified Chemotherapy * No prior chemotherapy for prostate cancer * At least 24 months since prior chemotherapy for other diseases Endocrine therapy * More than 6 months since prior hormonal therapy, including neoadjuvant or adjuvant androgen deprivation therapy (e.g., luteinizing hormone-releasing hormone analogue or antiandrogen) * No prior androgen deprivation therapy of more than 8 months duration * No prior hormonal therapy for prostate cancer more than 3 months after definitive local therapy * No concurrent androgen therapy Radiotherapy * See Disease Characteristics * More than 3 months since prior radiotherapy for locally recurrent prostate cancer * No concurrent radiotherapy, including for pain control Surgery * See Disease Characteristics Other * More than 4 weeks since prior investigational drugs * No concurrent medication known to affect systemic calcium metabolism, including any of the following: * More than 400 IU of cholecalciferol supplements * More than 500 IU of vitamin A supplements * Calcium supplements * Fluoride * Antiepileptics