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Few studies have demonstrated changes in apparent diffusion coefficient (ADC) values during or after definitive radiotherapy (RT) in prostate cancer (PC) patients; however, its role in evaluating treatment response and prognostic significance on biochemical control in patients with PC treated with definitive RT has not been adequately investigated. We assessed early changes in ADC and serum prostate specific antigen (PSA) values after definitive RT in low- and intermediate-risk PC patients.

The clinical data and ADC parameters of 229 PC patients were retrospectively evaluated. All patients underwent DW-MRI for initial staging and RT planning, and after the completion of RT. All patients were treated with 78 Gy to the prostate ± seminal vesicles, 78 Gy to the prostate ± seminal vesicles, and 86 Gy to the intraprostatic lesion. No patient received pelvic nodal irradiation and hormonoterapy. Receiver operating characteristic curves were generated for primary tumor post-treatment ADC response to determine the cutoff for predicting recurrence. The prognostic factors predicting freedom from biochemical failure (FFBF) and progression-free survival (PFS) were analyzed using univariable and multivariable analyses.

Most patients (75.1%) had Gleason 6 tumor, had low-risk disease (64.4%), and were treated with SIB technique (85.6%). The median pretreatment serum PSA level was 8.3 ng/mL (range: 1.6–20.0 ng/mL). The primary tumor was found in the peripheral zone in 137 patients (59.8%) and in the central zone in 92 patients (40.2%). The mean pretreatment ADC value was 0.76 ± 0.16 × 10˗³ mm²/sec, which is significantly lower than the post-treatment ADC value (1.09 ± 0.20 × 10˗³ mm²/sec; p < 0.001) With a median follow-up time of 80.8 months, the 5-year FFBF and PFS rates were 95.9% and 89.3%, respectively. Eleven patients (4.8%) had PSA relapse, with a median of 34.4 months after the completion of RT. A statistically significant difference in post-treatment ADC values was noted between patients with and without recurrence (0.94 ± 0.07 vs. 1.10 ± 0.20 × 10˗³ mm²/sec; p < 0.001). Patients with a Gleason score of 6 and low-risk disease had significantly higher post-treatment ADC and PSA levels than their counterparts. The progression rate was 90.9% for patients with ADC ≤ 0.96 × 10˗³ mm²/sec and 25.2% for patients with ADC > 0.96 × 10˗³ mm²/sec (p < 0.001). In the multivariable analysis, a lower ADC value and intermediate-risk disease were independent predictors of worse FFBF. However, there were no significant prognostic factors for predicting PFS.

To the best of our knowledge, this is the first study to evaluate the feasibility of ADC values measured with DW-MRI for treatment response evaluations in PC patients undergoing definitive RT. Our findings suggest that the tumor ADC value measured four months after definitive RT could be used as a quantitative imaging modality for therapy monitoring.