## 1 Introduction  
Head and neck cancers represent the sixth most common cancer worldwide and a major cause of morbidity and mortality [1]. More than 90% of head and neck cancers are squamous cell carcinomas (HNSCC) arising from the mucosal surfaces of the oral cavity, naso-oropharynx, and larynx [2]. Crucial risk factors aligned with head and neck cancers include tobacco, alcohol consumption, and human papillomavirus (HPV) or Epstein-Barr virus infections [3].  
Chemoradiotherapy theory (CRT) has become more popular over the past decade because the organ preservation possibilities are higher with CRT as compared to surgery [4]. The relapse rate is still 50% (35-65%) in patients with advanced HNSCC [5] and reaches 25% in early-stage cancers [6]. Almost 90% of HNSCC recurrences following CRT develop within 2 years [7]; the early detection of tumor recurrence prompts curative salvage treatment and may allow the preservation of organ functions [6].  
The interpretation of post-treatment follow-up via imaging techniques is complicated by post-actinic edema, soft tissue necrosis, and fibrosis. Such post-treatment changes make it difficult to detect tumor recurrence within a distorted anatomy [8]. Biopsy with negative findings does not exclude HNSCC recurrence, and multiple biopsies may increase overall morbidity [6]. Therefore, in addition to clinical and histological parameters, other biomarkers are needed to stratify patients for optimal therapy [9].  
Magnetic resonance imaging (MRI) is an accurate technique for the assessment of deep tumor invasion and morphological tumor features [10], but it is not able to identify early locoregional recurrences, predict tumor response to treatment and monitor post-treatment changes [11; 12].  
Metabolic imaging with 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) has evolved as a tool for the post-treatment evaluation of HNSCC, but it is generally delayed for at least 12 weeks due to the potential false-positive results in early post-treatment inflammatory changes [13].  
Nowadays, a multiparametric approach employing MRI has been proposed with diffusion (DWI) and dynamic contrast-enhanced perfusion-weighted imaging (DCE-PWI) for the distinction between post-treatment changes and tumor persistence/recurrence [14; 15]. Moreover, MRI is ideally suited to serial scanning, reducing the use of ionizing radiations commonly emitted by CT examinations [16; 17; 18; 19].  
DWI with apparent diffusion coefficient (ADC) maps can theoretically differentiate between inflammation and neoplastic tissues since the water molecule diffusion is increased into inflammatory tissues (T2* loss of signal and high ADC values), whereas water molecules have restricted diffusion within neoplastic tissues (T2* signal maintenance and low ADC values) [20].  
DCE-PWI examines microvascular tumor tissue characteristics [21] and can potentially assess the reduction of tumor blood perfusion by means of K(trans), which represents the volume transfer constant from the vascular to the extravascular extracellular spaces [22; 23; 24; 25].  
We aimed to retrospectively differentiate post-CRT changes from tumor persistence/recurrence in the early follow-up of patients with primary naso-oropharyngeal carcinoma using multiparametric MRI with DWI and DCE-PWI sequences.