## 1 Introduction  
Non-Hodgkin lymphoma (NHL) represent less than 1% of all malignant tumors in the head and neck region, although they represent the second most common localization in the gastrointestinal tract [1, 2]. In most cases, NHL involve the cervical, axillary and inguinal lymph nodes, but approximately 40% present extra-nodal location, such as in the gastrointestinal tract and head and neck region, central nervous system and cuts [3, 4, 5]. In the head and neck region, the most commonly affected areas are the Waldeyer's ring, the posterior tongue, the nasopharynx, the hard palate, the parananal sinuses, the thyroid gland and the parotid gland. Diagnosis of NHLs in the maxillary sinus is rare [4, 6, 7]. [PERSON] et al. examined 34 cases with primary oral NHLs and described involvement primarily of the maxillary mucosa or maxillary bone structures, whereas maxillary sinuses were shown to be only secondarily affected [8]. Considering the prevalence of NHLs of the paranasal sinuses, their main location is the maxillary sinus, followed by the ethmoid, sphenoid and frontal sinuses [9].  
Besides its location, genetic origin is the most important prognostic factor of B-cell NHL. Based on the WHO (2017) statement, high-grade B-cell lymphoma (HGBL) has been defined within mature B-cell NHL as a separate group distinct from diffuse large B-cell lymphoma (DLBCL), which is predominant in the head and neck [10], other large B-cell lymphomas and Burkitt lymphoma (BL) [11]. Despite differences of the immunological and pathological characteristics of HGBL and BL or DLBCL, NOS, the morphology is similar [9; 12]. Therefore, the WHO 2008 statement defined these groups as B-cell lymphomas, unclassifiable, with features intermediate between DLBCL and BL, but this definition is no longer recommended [13]. HGBL is a special class of aggressive mature lymphomas, and it includes two subtypes: (1) double or triple hit HGBL lymphomas, and (2) high-grade B-cell lymphoma not otherwise specified (HGBL, NOS), or double-expressor lymphomas (DEL) [11; 14; 15; 16]. If MYC and BCL-2 and/or BCL-6 rearrangements are present, they are categorized as HGBL with MYC and BCL-2 and/or BCL-6 rearrangements. In exceedingly rare cases the lymphoma is not attributed to BL, DLBCL or any other form of HGBL, with MYC translocation and BCL-2 or BCL-6 amplification. These lymphomas are not double hit, but double-expressor lymphomas, with a poor prognosis [11; 12; 17] (Figure 1). The MYC gene encodes an oncogenic protein, which leads to a proliferation of immature B cells. The protein encoded by BCL-2 inhibits apoptosis. The expression of BCL-6, a transcriptional repressor, helps in the development of lymphoma [15; 18].  
Both subtypes, especially the HGBL, NOS, are rarely diagnosed [12; 19], and while other mature B-cell lymphomas such as DLBCL have an average of 70% complete remission and a high average 5-year survival rate of 60% [1; 20], the prognosis of HGBL, NOS is poor [21; 22; 14; 23]. Despite the very rapid spread, there are few clinical signs of NHL-affected jaws, and these signs may mimic benign inflammatory diseases, thus leading to a delay of the correct diagnosis [24; 25].  
Limited data are available on the clinicopathological features of HGBL, NOS lymphoma, as it is often included in reports with other types such as DLBCL, NOS [22]. The present article reports a rare case of HGBL NOS involving the maxillary sinus and maxillary bone in an adult male with atypical clinical presentation, mimicking a periapical inflammation. The report provides relevant information about the clinical features and therapeutic efficacy of the HGBL, NOS in the maxillary sinus and the difficulty of distinguishing the disease from a periapical inflammation.