Lesion size threshold is the most common imaging feature used to assess response to therapy. Size as an imaging feature has its limitations. Quantitative imaging biomarkers (QIBs) could identify subtle microstructural changes prior to morphological changes. In this study, we explored the use of novel whole-body MRI (WB-MRI) QIBs for nodal disease characterisation and treatment response monitoring in radio-recurrent prostate cancer (rPC). We showed signal fat fraction could discriminate between positive and negative nodes and that it can be used for response monitoring.