Relative fat fraction of malignant bone lesions from breast cancer, prostate cancer and myeloma are significantly lower than normal bone marrow and shows excellent interobserver agreement

OBJECTIVES: To compare relative fat fraction (rFF) of active bone lesions from breast, prostate and myeloma malignancies and normal bone marrow; to assess its inter-reader agreement. METHODS: Patients with breast (n = 26), myeloma (n = 32) and prostate cancer (n = 52) were retrospectively evaluated. 110 baseline rFF maps from whole-body MRI were reviewed by two radiologists. Regions of interest for up to four focal active lesions in each patient were drawn on rFF maps, one each at the cervicothoracic spine, lumbosacral spine, pelvis and extremity. The mean and standard deviation of rFF were recorded. The rFF of normal marrow was measured in the pelvis for patients without diffuse bone disease (n = 88). We compared the rFF of malignant bone lesions and normal marrow using Mann–Whitney test. Interobserver agreement was assessed by interclass correlation coefficient. RESULTS: Malignant bone lesions showed significantly lower median rFF (13.87%) compared with normal marrow (89.76%) with little overlap (p < 0.0001). There was no significant difference in the median rFF of malignant lesions from breast (14.46%), myeloma (13.12%) and prostate cancer (13.67%) (p > 0.017, Bonferroni correction) and in the median rFF of bone disease according to their anatomical locations (p > 0.008, Bonferroni correction). There was excellent interobserver agreement (0.95). CONCLUSION: The low rFF of active bone lesions in breast, prostate and myeloma malignancies provides high image contrast relative to normal marrow that may be used to detect bone metastases. ADVANCES IN KNOWLEDGE: This study shows the importance of rFF towards detecting bone metastases.