Obesity and Risk of Biochemical Failure for Patients Receiving Salvage Radiotherapy after Prostatectomy

Abstract

Obesity has been proposed as an independent risk factor for patients undergoing surgery or radiotherapy (RT) for prostate cancer. Whether prostate cancer in obese patients is inherently more aggressive or whether treatment failure is related to greater technical challenges is still a subject of controversy. Using body mass index (BMI) as a measure of obesity, we test its role as a risk factor for patients receiving salvage RT after prostatectomy. There is no study of obesity in this clinical setting for comparison. The rates of subsequent biochemical relapse were examined among 90 patients who underwent salvage RT between 1984 and 2004 for biochemical failure after radical prostatectomy. Median follow-up was 3.7 years. Four BMI groups were established according to the World Health Organization: (normal) <25, (overweight) 25-<30, (grade I obesity) 30-<35 and (grade II-III obesity) 35 kg/m2 or more. Univariate and multivariate proportional hazards regression analyses were performed for clinical, pathologic and treatment factors associated with time to relapse after salvage RT. There were 40 biochemical failures after salvage RT with a median time to failure of 1.2 years. BMI was not associated with any adverse clinical or pathologic factors, or with any advantageous treatment factors. On univariate analysis only the severely obese (BMI > = 35 kg/m2) showed higher relapse rates (HR 5.5, 95% CI 1.6-19.5, p = 0.008) while on multivariate analysis obesity was significant as either a continuous variable (HR 1.2, 95% CI 1.03-1.4, p = 0.017) or as a categorical variable, BMI > = 35 kg/m2 (HR 34.8, 95% CI 2.9-418.7, p = 0.005) and BMI 25-30 kg/m2 (HR 3.2, 95% CI 1.02-10.1, p = 0.045). Also significant were extra-capsular extension and pre-RT PSA level. The evidence suggests that obesity is a risk factor for salvage RT patients. Whether this results from greater biologic aggressiveness or technical inadequacies cannot be answered by this study. However, the very high failure rate observed for the severely obese patients hints at technical difficulties with RT. Our conclusion is supported by studies of primary radiotherapy for prostate cancer, where BMI has only been found to be a risk factor for patients treated with external beam but not for patients treated with brachytherapy. Techniques that optimize targeting, especially in obese patients, seem warranted at this time.