Assessing response in bone metastases in prostate cancer with diffusion weighted MRI

Objectives To determine whether changes in ADC of bone metastases secondary to prostate carcinoma are significantly different in responders compared with progressors on chemotherapy. Methods Twenty-six patients with known bone metastases secondary to prostate carcinoma underwent diffusion-weighted MRI of the lumbar spine and pelvis at baseline and 12 weeks following chemotherapy. RECIST assessment of staging CT and PSA taken at the same time points were used to classify patients as responders, progressors or stable. ADC (from b = 0,50,100,250,500,750 smm(-2)) and ADC(slow) (from b = 100,250,500,750 smm(-2)) were calculated for up to 5 lesions per patient. Results Mean ADC/ADC(slow) in lesions from responders and progressors showed a significant increase. Although the majority of lesions demonstrated an ADC/ADC(slow) rise, some lesions in both responders and progressors demonstrated a fall in ADC beyond the limits of reproducibility. Conclusions Mean ADC is not an appropriate measure of response in bone metastases. The heterogeneity of changes in ADC is likely to be related to the composition of bone marrow with changes that have opposing effects on ADC.