(P 241) ASSOCIATION OF ADC VALUES WITH DESMOID TUMOR SHRINKAGE DURING ACTIVE OBSERVATION

Objective: Although recent guidelines recommend wait-and-see policy for desmoid tumor (DT) as first management, it is difficult to predict the clinical course after initial diagnosis. Magnetic resonance imaging (MRI) is often used to evaluate the status of DT, and association between T2 signal intensity and progression during active observation has been reported. Apparent-diffusion-coefficient (ADC) value is quantitative parameter of diffusion-weighted-imaging (DWI) which reflects diffusion movements of water molecules in various tissues. Although changes in cellularity or fibrosis in DT may affect the diffusion of water molecules in the tissue, it is not clear if there is a relationship between changes in ADC values and the clinical course during active observation of DT. We investigated whether changes in ADC values are related to the clinical course of DT.

Methods: Eleven patients diagnosed as DT in our hospital between April 2013 and August 2022 with measurable ADC values on MRI were included in the study. There were 7 male and 4 female patients with a median age of 49.1 years (range, 17-78) at the time of initial diagnosis. The mean follow-up period was 37 months (range, 3-78). The origin sites were 5 abdominal and 6 extra-abdominal. The mean diameter of tumor at the first visit was 5.2 cm (range, 2.4-9). Tumors that continued to grow in size compared with the initial diagnosis were designated as group G, and those that eventually shrank in size at final periods were designated as group S. For these two groups, the tumor size that defined as the sum of the longest diameters of the horizontal section and the ADC value were investigated at the time of initial examination (period A), at the time of the greatest enlargement (period B), and at the time of the greatest shrinkage (period C), and their associations were statistically analyzed.

Results: There were 5 patients in group G and 6 patients in group S. In group G, the mean size (cm) at period A and B were each 10.2 and 15.3, and the mean ADC value (×10-3 mm2/sec) were each 1.525 and 1.575, with no significant difference. In group S, the mean size at period A, B and C were each 7.4，9.2 and 4.7, and the mean ADC values were each 1.511, 1.602, and 1.139.　There was a significant difference in the change in ADC values between at period A and C (p=0.024) and between at period B and C (p=0.033) (repeated-measures-ANOVA and Holm–Bonferroni method) (Figure 1). There were no significant differences in ADC values at period A or B between the group G and S or within each group （Figure 2）.

Conclusion: The ADC value is a numerical expression of the diffusivity of water molecules, with higher values indicating easier diffusion and lower values indicating harder diffusion. DT become lower signal than muscle on T2-weighted images with shrinkage, suggesting that the increase in fibrous matrix during shrinkage prevents diffusion of water molecules, resulting in a lower ADC value. Although this study could not demonstrate that ADC values are a predictor of shrinkage or enlargement, further studies are needed to confirm this hypothesis in more cases.