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