To evaluate the utility of multi-parametric magnetic resonance imaging (MP-MRI), including dynamic contrast-enhanced MRI and diffusion-weighted MRI, for monitoring tumor tissue changes after volumetric-modulated arc radiotherapy in localized prostate cancer (PCa), and to compare the radiotherapy induced tumor tissue changes between conventional, moderate and extreme hypofractionated groups. Furthermore, we aimed to evaluate if follow-up by MRI has an incremental value compared to the standard care by prostate-specific antigen (PSA) serum level measurement.

Fifty-five men (mean age: 70±5 [SD] years; range: 60–79 years) with biopsy-proven PCa underwent MRI examination before radiotherapy, and at 3 and 12 months after radiotherapy. Pharmacokinetic analysis post-processing platform with dedicated software (Tissue 4D) was used to generate colorized parametric maps of enhancing tumors. The volume transfer constant (K^trans), reflux constant (K_ep), and initial area under curve (iAUC) were calculated from the tumors. Tumor apparent diffusion coefficient (ADC) value was measured on the ADC map. The patients were allocated into three radiotherapy groups: 17 conventional (39×2Gy), 16 moderate (20×3Gy) and 22 extreme hypofractionated (5×7.25Gy) regimen.

Sixty lesions were detected in the prostates of the 55 patients. Follow-up MRI showed decreases in tumor size and degree of enhancement. K^trans, K_ep, and iAUC all decreased at 3 months (P<0.001, respectively) and decreased further at 12 months (P<0.001, respectively). ADC increased at 3 months (P<0.001) and increased further at 12 months (P<0.001). There were no significant differences in the percentage changes of the measured MP-MRI parameters of the tumors from baseline to 12 months between the conventional, moderate and extreme hypofractionated regimen groups.

MP-MRI is a reliable tool for lesion detection and follow-up, providing both qualitative and quantitative data.