Objectives: To evaluate the usefulness of diffusion-weighted magnetic resonance for distinguishing thymomas according to WHO and Masaoka-Koga classifications and in predicting disease-free survival (DFS) by using the apparent diffusion coefficient (ADC).
Methods:Forty-one patients were grouped based on WHO (low-risk vs. high-risk) and MasaokaKoga (early vs. advanced) classifications. For prognosis, seven patients with recurrence at followup were grouped separately from healthy subjects. Differences on ADC levels between groups were tested using Student-t testing. Logistic regression models and areas under the ROC curve (AUROC) were estimated.
Results:Mean ADC values were different between groups of WHO (low-risk=1.58±0.20×10-3mm2/sec; high-risk=1.21±0.23×10-3mm2/sec; p<0.0001) and Masaoka-Koga (early=1.43±0.26×10-3mm2/sec; advanced=1.31±0.31×10-3mm2/sec; p=0.016) classifications.Mean ADC of type-B3 (1.05±0.17×10-3mm2/sec) was lower than type-B2 (1.32±0.20×10-3mm2/sec; p=0.023). AUROC in discriminating groups was 0.864 for WHO classification (cutpoint=1.309×10-3mm2/sec; accuracy=78.1 %) and 0.730 for Masaoka-Koga classification (cut-point=1.243×10-3mm2/sec; accuracy=73.2 %). Logistic regression models and two-way ANOVA were significant for WHOclassification (odds ratio[OR]=0.93, p=0.007; p<0.001), but not for Masaoka-Koga classification (OR=0.98, p=0.31; p=0.38). ADC levels were significantly associated with DFS recurrence rate being higher for patients with ADC≤1.299× 10-3mm2/sec (p=0.001; AUROC, 0.834; accuracy=78.0 %).Conclusions: ADC helps to differentiate high-risk from lowrisk thymomas and discriminates the more aggressive type-B3. Primary tumour ADC is a prognostic indicator of recurrence.
Key Points• DW-MRI is useful in characterizing thymomas and in predicting disease-free survival.• ADC can differentiate low-risk from high-risk thymomas based on different histological composition • The cutoff-ADC-value of 1.309×10-3mm2/sec is proposed as optimal cut-point for this differentiation • The ADC ability in predicting Masaoka-Koga stage is uncertain and needs further validations• ADC has prognostic value on disease-free survival and helps in stratification of risk