Diffusion-weighted magnetic resonance imaging of thymoma: ability of the Apparent Diffusion Coefficient in predicting the World Health Organization (WHO) classification and the Masaoka-Koga staging system and its prognostic significance on disease-free survival

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 Masaoka-Koga (early vs. advanced) classifications. For prognosis, seven patients with recurrence at follow-up 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 -3 mm 2 /sec; high-risk = 1.21 ± 0.23 × 10 -3 mm 2 /sec; p < 0.0001) and Masaoka-Koga (early = 1.43 ± 0.26 × 10 -3 mm 2 /sec; advanced = 1.31 ± 0.31 × 10 -3 mm 2 /sec; p = 0.016) classifications. Mean ADC of type-B3 (1.05 ± 0.17 × 10 -3 mm 2 /sec) was lower than type-B2 (1.32 ± 0.20 × 10 -3 mm 2 /sec; p = 0.023). AUROC in discriminating groups was 0.864 for WHO classification (cut-point = 1.309 × 10 -3 mm 2 /sec; accuracy = 78.1 %) and 0.730 for Masaoka-Koga classification (cut-point = 1.243 × 10 -3 mm 2 /sec; accuracy = 73.2 %). Logistic regression models and two-way ANOVA were significant for WHO classification (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 -3 mm 2 /sec ( p = 0.001; AUROC, 0.834; accuracy = 78.0 %). Conclusions ADC helps to differentiate high-risk from low-risk 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 -3 mm 2 /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