Assessment of response to radiotherapy for prostate cancer: Value of diffusion-weighted MRI at 3 T

OBJECTIVE. The objective of our study was to investigate the changes of apparent diffusion coefficient (ADC) values in prostate cancers before and after radiotherapy at 3 T using a phased-array coil. MATERIALS AND METHODS. Forty-nine patients with biopsy-proven prostate cancer who received radiotherapy underwent diffusion-weighted imaging (DWI) at 3 T and were included in the study. Biopsies in all patients were performed before the initial MRI examination (range, 15-35 days before MRI; mean, 23.4 days). All 49 patients underwent DWI (b values = 0 and 1,000 s/mm²) before and 1-5 months after the completion of radiotherapy. The changes in ADC values were measured for cancers and benign tissues before and after therapy. Additionally, the changes in serum prostate-specific antigen (PSA) levels were evaluated before and after therapy. RESULTS. A total of 57 cancers (peripheral zone, n = 45; transition zone, n = 12) were found in 46 patients. For the tumors, the mean ADC value after therapy (1.61 × 10^-3 mm²/s) was increased compared with the mean ADC value before therapy (1.0 × 10 ^-3 mm²/s) (p < 0.001). After radiotherapy, the mean ADC values of benign peripheral zones and of benign transition zones were statistically significantly decreased compared with those before radiotherapy (p < 0.05). Before treatment, a significant difference of ADC values between the tumors and benign tissues was found (p < 0.001), whereas there was no significant difference of ADC values between them after treatment (p > 0.1). The median PSA level after therapy (0.49 ng/mL) was decreased compared with the median PSA level before therapy (20.0 ng/mL). CONCLUSION. With the use of a 3-T MR scanner, our preliminary results suggest that ADC values may be useful as an imaging biomarker for monitoring therapeutic response of prostate cancer to radiotherapy.