## 4 Discussion
Figure 3: Post-treatment magnetic resonance imaging (MRI) of a 63-year-old female patient with human-papillomavirus-positive carcinoma of the right palatine tonsil with ipsilateral lymph node metastasis (T2N1) recently treated (3 months before) with chemoradiotherapy (CRT). Post-CRT MRI showed linear fibrotic tissues in the right palatine tonsil (white striped arrows) with hypointense signal intensity on T2-weighted (**A**), T2-weighted fat-saturated, (**B**) and diffusion-weighted b800 images (**C**), and intermediate apparent diffusion coefficient values (\(1.44\times 10^{-3}\) mm\({}^{2}\)/s) (**D**). After gadolinium contrast agent injection, post-CRT fibrotic tissue showed no submucosal enhancement (**E**) and low K(trans) value (\(48.36\times 10^{-3}\) mm\({}^{2}\)/s) on dynamic contrast enhancement-perfusion weighted imaging (**F**). Ratio between K(trans) values of the primitive tumor and residual tissue after CRT, standardized with respect to K(trans) value of the ipsilateral trapezius (K(trans)post/pre/muscle), was 0.307. These findings are typical of post-CRT scar tissue. [PERSON] et al. [27] found ADC values of 1.42 \(\pm\) 0.23 \(\times\) 10\({}^{-3}\) mm\({}^{2}\)/s and 1.02 \(\pm\) 0.20 \(\times\) 10\({}^{-3}\) mm\({}^{2}\)/s in post-therapy changes of patients treated for tongue carcinoma and tongue carcinoma recurrence, respectively. Taking as a reference such ADC values, in our study, 24 patients with post-CRT changes showed ADC\({}_{\text{post}}\) values > 1.42 \(\times\) 10\({}^{-3}\) mm\({}^{2}\)/s (mean = 1.56 \(\times\) 10\({}^{-3}\) mm\({}^{2}\)/s), whereas in the remaining 8 patients with post-CRT changes, ADC\({}_{\text{post}}\) values (mean = 1.24 \(\times\) 10\({}^{-3}\) mm\({}^{2}\)/s; range = 0.96-1.35 \(\times\) 10\({}^{-3}\) mm\({}^{2}\)/s) were similar to ADC\({}_{\text{post}}\) values of all 5 patients with tumor persistence/recurrence (mean = 1.05 \(\times\) 10\({}^{-3}\) mm\({}^{2}\)/s; range = 0.78-1.32 \(\times\) 10\({}^{-3}\) mm\({}^{2}\)/s) (Figure 2A). [PERSON] et al. [7] found that mean ADC values in post-treatment HNSCC highly differed between post-radiation therapy inflammatory edema (1.75 \(\pm\) 0.34 \(\times\) 10\({}^{-3}\) mm\({}^{2}\)/s) and late fibrosis (0.98 \(\pm\) 0.26 \(\times\) 10\({}^{-3}\) mm\({}^{2}\)/s). These results may justify overlaps of ADC values between post-CRT and tumor recurrence both in our study and in other papers [28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42]. 18F-FDG PET/CT is frequently used for treatment response assessment. It shows high sensitivity but low specificity [43], especially in the first 6 months after treatment due to inflammation, granulation, and scar tissues [44]. In the present study, 18F-FDG PET/CT performed 3-6 months after ending the treatment was positive in 12 patients, but only 5 of them had tumor persistence/recurrence at the 2-year follow-up. Compared to 18F-FDG PET/CT, ADC can be also performed in the first months after CRT to assess treatment response, but false positives and negatives cannot be fully excluded. However, studies that used ADC values without taking into account DWI SI underestimated the accuracy of diffusion-weighted MRI [45]. Scar tissue generally displays low ADC values in combination with the hypointense signal on high b value DWI images due to the low number of resonant protons. Residual cancer usually shows low values on ADC maps too, but together with the hyperintense signal on DWI images [46]. The combination of DWI and morphologic MRI features yields better results than DWI alone [7; 31; 44; 47]. The evaluation of SI on T2 images in the current study agreed with the literature since masslike alterations with moderately high (i.e., intermediate) SI, diffuse alterations with high SI, and linear or triangular alterations with very low SI (similar to or lower than muscle) were suggestive for tumor persistence/recurrence, post-CRT inflammatory edema, and post-CRT fibrosis, respectively.