Objectives:
To assess the test-retest reproducibility and intra/inter observer agreement of Apparent Diffusion Coefficient (ADC) measurements of myeloma lesions using WB-DW-MRI at 3T MRI.
Methods:
Following ethical approval, eleven consenting patients with relapsed multiple myeloma were prospectively recruited and underwent baseline WB-DW-MRI. For a single bed position, axial DWI was repeated after a short interval to permit test- retest measurements.
Mean ADC measurement was performed by two experienced observers. Intra and inter observer agreement and test-retest reproducibility were assessed, using coefficient of variation (CV) and interclass correlation coefficient (ICC) measures, for diffuse and focal lesions (small ≤10 mm and large >10 mm).

Results:
Forty seven sites of disease were outlined (23 focal, 24 diffuse) in different bed positions (pelvis = 22, thorax = 20, head and neck = 5). For all lesions, there was excellent intra observer agreement with ICC of 0.99 (0.98–0.99) and COV of 5%. For inter observer agreement, ICC was 0.89 (0.8–0.934) and COV was 17%. There was poor inter observer agreement for diffuse (ICC = 0.46) and small lesions (ICC = 0.54).
For test-retest reproducibility, excellent ICC (0.916) and COV (14.5%) values for mean ADC measurements were observed. ICCs of test-retest were similar between focal lesions (0.83) and diffuse infiltration (0.80), while ICCs were higher in pelvic (0.95) compared to thoracic (0.81) region and in small (0.96) compared to large (0.8) lesions.

Conclusions:
ADC measurements of focal lesions in multiple myeloma are repeatable and reproducible, while there is more variation in ADC measurements of the diffuse disease in patients with multiple myeloma.
Advances in knowledge:
Mean ADC measurements are repeatable and reproducible in focal lesions in multiple myeloma, while the ADC measurements of diffuse disease in multiple myeloma are more subject to variation. The evidence supports the future potential role of ADC measurements as predictive quantitative biomarker in multiple myeloma.