Low-grade papillary schneiderian carcinoma, a unique and deceptively bland malignant neoplasm: report of a case

Low-grade papillary Schneiderian carcinoma with TP53 mutation: a case report and review of the literature

BACKGROUND

Low-grade papillary Schneiderian carcinoma (LGPSC) is a relatively new entity of the sinonasal tract and is characterized by a bland morphology simulating sinonasal papilloma, invasive growth pattern with pushing borders, and aggressive clinical behavior with multiple recurrences and metastatic potential. Recently, DEK::AFF2 fusions were identified in LGPSC. However, some LPGSCs lack DEK::AFF2 fusion, and the molecular features of these tumors have not been clarified.

CASE PRESENTATION

A 69-year-old man presented with a discharge of pus from his left cheek. Computed tomography revealed a mass involving the left maxillary sinus, ethmoid sinus, and nasal cavity with the destruction of the orbital wall. The biopsy specimens showed that the tumor had a predominantly exophytic, papillary growth and did not have an apparent stromal invasion. The tumor was composed of multilayered epithelium that showed bland morphology with a round to polygonal shape, abundant eosinophilic cytoplasm, and uniform nuclei. Dense neutrophilic infiltrates were focally present. Immunohistochemically, CK5/6 was strongly and diffusely positive, and p16 was negative. p63 was mainly positive in the basal layer, and EMA was predominantly expressed in the outermost cell layer. DNA-based targeted sequencing showed TP53 R175H mutation, whereas neither EGFR nor KRAS mutation was identified. Reverse transcription polymerase chain reaction and fluorescence in situ hybridization revealed no DEK::AFF2 fusion.

CONCLUSIONS

We describe the first case of TP53-mutant LGPSC and review the literature. LGPSC is a genetically heterogeneous entity, and the recognition of this rare entity and comprehensive assessment of clinicopathological and molecular findings are crucial for the correct pathological diagnosis and clinical management.

DEK::AFF2 Fusion Carcinomas of Head and Neck

A novel DEK::AFF2 fusion carcinoma was recently described in 29 patients who originally presented with non-viral-associated nonkeratinizing squamous cell carcinoma. The tumors occurred at multiple sites in the head and neck including in the sinonasal tract, middle ear, and temporal bone. This tumor behaves aggressively involving adjacent vital structures, frequently recurs, and is inclined to develop lymph node and distant metastasis. This review aims to summarize the demographic, clinical, pathologic, immunophenotypic features, and pattern of molecular alterations as well as to discuss the differential diagnosis of DEK::AFF2 fusion carcinoma.

Nasopharyngeal low-grade papillary schneiderian carcinoma with cervical metastasis

Low-grade papillary Schneiderian carcinoma (LGPSC) is a rare and newly described type of cancer arising from the Schneiderian epithelium. Owing to cellular atypia, it is difficult to differentiate this type from other papillomas and malignancies. Although this condition remains unclear, it is associated with mortality and recurrence. Therefore, treating physicians should be aware of the possibility of LGPSC for prompt diagnosis and treatment. In this article, we present an additional case of nasopharyngeal LGPSC with cervical lymph node metastasis and reviewed the 14 cases reported thus far in the literature. A 76-year-old female was referred to our department for detailed examination of nasopharyngeal and cervical lymph node tumors detected by positron emission tomography-computed tomography. Based on the biopsy of the nasopharyngeal tumor, we suspected LGPSC. Considering the clinical course and pathological findings, the patient was diagnosed with cervical lymph node metastasis through neck dissection. We performed radiotherapy for the primary lesion of the nasopharynx, which led to the disappearance of the tumor. After 13 months following the radiotherapy, the patient died from a recurrence of retroperitoneal liposarcoma without the recurrence of LGPSC.

Nuclear expression of AFF2 C-terminus is a sensitive and specific ancillary marker for DEK::AFF2 carcinoma of the sinonasal tract

DEK::AFF2 carcinoma of the sinonasal tract is an emerging entity. The tumor is typically characterized by papillary proliferation of non-keratinizing squamous epithelial cells with monotonous cytologic features, which may mimic other sinonasal tumors. The confirmation of this gene fusion has thus far relied solely on next-generation sequencing, fluorescence in situ hybridization (FISH), or reverse transcription polymerase chain reaction (RT-PCR). This current study aimed to validate an immunohistochemical assay for AFF2 C-terminus as an ancillary marker. We first analyzed publicly available RNA sequencing data of sinonasal tumors from the national center for biotechnology information (NCBI) sequence read archive and identified 3 DEK::AFF2 carcinomas out of 28 sinonasal tumors. The gene expression of AFF2 was significantly higher in the fusion-positive cases compared to the wild-type tumors (p < 0.001), while DEK was not. We then optimized an immunohistochemical assay with an anti-AFF2 C-terminus antibody for ancillary diagnosis. Seventeen DEK::AFF2 carcinomas, including 11 cases with predominantly low-grade morphology and one showing glandular differentiation, as well as 78 DEK FISH-negative sinonasal tumors were evaluated by AFF2 immunohistochemistry (IHC). Sixteen of the 17 DEK::AFF2 carcinomas showed nuclear AFF2 expression in ≥30% of tumor cells, including one decalcified case that failed FISH and RT-PCR confirmation. The one case that was negative for AFF2 IHC in the tumor cells also lacked expression in the internal positive control. It was thus considered a failure of the IHC rather than a truly negative case and was excluded from the statistical analysis. All DEK FISH-negative sinonasal tumors were negative for nuclear AFF2 expression. The nuclear expression of AFF2 IHC showed 100% sensitivity and specificity for DEK::AFF2 carcinoma. Accordingly, AFF2 IHC is a highly sensitive and specific ancillary marker that distinguishes DEK-AFF2 carcinoma from the other sinonasal tumors with overlapping morphological features and may be an especially useful alternative for decalcified specimens.

DEK-AFF2 Carcinoma of the Sinonasal Region and Skull Base: Detailed Clinicopathologic Characterization of a Distinctive Entity

A novel DEK-AFF2 fusion was recently reported in 4 nonkeratinizing squamous cell carcinomas of the sinonasal region and skull base, including 1 with exceptional response to immunotherapy, but it is not yet clear if this rearrangement defines a unique clinicopathologic category or represents a rare event. This study aims to characterize a larger cohort of carcinomas with DEK-AFF2 fusions to assess whether they truly constitute a distinctive entity. Among 27 sinonasal and skull base nonkeratinizing squamous cell carcinoma that were negative for human papillomavirus and Epstein-Barr virus, RNA sequencing identified DEK-AFF2 fusions in 13 cases (48%). Nine were centered in the nasal cavity, 2 in the middle ear/temporal bone, 1 in the nasopharynx, and 1 in the orbit. These tumors displayed recurrent histologic features including (1) complex endophytic and exophytic, frequently papilloma-like growth, (2) transitional epithelium with eosinophilic to amphophilic cytoplasm, (3) absent or minimal keratinization with occasional compact keratin pearls, (4) monotonous nuclei, and (5) prominent tumor-infiltrating neutrophils or stromal lymphocytes. This appearance not only overlaps with high-grade basaloid sinonasal carcinomas but also with benign papillomas and tumors reported as low-grade papillary Schneiderian carcinoma. However, DEK-AFF2 carcinomas showed frequent local recurrence, cervical lymph node metastases, and distant metastasis with 2 deaths from disease, confirming they are aggressive malignancies despite relatively bland histology. Overall, the distinctive molecular, histologic, and clinical features of DEK-AFF2 carcinomas suggest they represent a unique entity in the sinonasal region. This tumor merits increased pathologic recognition to better understand its prognostic and therapeutic implications.

DEK-AFF2 fusion-associated papillary squamous cell carcinoma of the sinonasal tract: clinicopathologic characterization of seven cases with deceptively bland morphology

A novel DEK-AFF2 fusion has been recently identified in four cases of basaloid to nonkeratinizing squamous cell carcinoma (SCC) in the sinonasal tract and middle ear with high-grade morphology. The exceptional response to immune checkpoint inhibitor in the first reported case highlights the potential clinical importance of identifying tumors with DEK-AFF2 fusions. We herein reported the first series of seven cases of DEK-AFF2 fusion-associated sinonasal SCC with deceptively bland morphology, including four cases of low-grade papillary Schneiderian carcinoma, which is a recently described tumor type with unknown molecular underpinnings. The DEK gene rearrangement was confirmed by DEK break-apart fluorescence in situ hybridization and DEK-AFF2 fusion transcripts were detected by reverse transcription polymerase chain reaction. In contrast to the previously reported DEK-AFF2 fusion-positive high-grade carcinomas, these tumors had a monotonous and bland morphology and were all initially diagnosed as sinonasal papilloma (SP) of various types, with or without dysplasia or carcinoma in situ. The tumor was characterized by mixed exophytic and inverted patterns, broad papillary fronds, acantholytic change, cellular monotony, dense neutrophilic infiltrates, and peripheral palisading. All tumors were diffusely positive for p40 or p63 and negative for NUT and p16. Molecular drivers associated with SP, including EGFR and KRAS mutations and both high and low-risk human papillomavirus infection, were negative in all cases. Although there was no overt stromal invasion or desmoplastic reaction in the initial specimens, these tumors tended to progress locoregionally through a prolonged clinical course and occasionally develop lymph node metastases, high-grade transformation, or extensively local destruction eventually leading to death. These justify more aggressive clinical management. Therefore, we propose the new terminology "DEK-AFF2 fusion-associated papillary SCC of the sinonasal tract" to better describe this clinicopathologically and molecularly distinct entity.

Low Grade Papillary Sinonasal (Schneiderian) Carcinoma: A Series of Five Cases of a Unique Malignant Neoplasm with Comparison to Inverted Papilloma and Conventional Nonkeratinizing Squamous Cell Carcinoma

There have been a few case reports and one small series of low grade papillary sinonasal (Schneiderian) carcinomas (LGPSC) which mimic papillomas but have overtly invasive growth and which occasionally metastasize. We describe the morphologic, clinical, immunohistochemical, and molecular features of five patients with LGPSC compared with eight cases each of inverted papilloma (IP) and conventional nonkeratinizing squamous cell carcinoma (SCC) with papillary growth. All LGPSC were nested with predominantly pushing invasion, no stromal reaction, and frequent surface papillary growth. All consisted of one cell type only, with polygonal cells with round nuclei, no (or limited) cytologic atypia, low mitotic activity, and prominent neutrophilic infiltrate. One patient had slightly more infiltrative bone invasion, another lymphovascular, perineural, and skeletal muscle invasion, and a third nodal metastasis after 17 years. By comparison, IPs had bland cytology, neutrophilic microabscesses, mixed immature squamous, goblet cell, and respiratory epithelium, and extremely low mitotic activity. Nonkeratinizing SCCs had basaloid-appearing cells with nuclear pleomorphism, brisk mitotic activity, and apoptosis. All LGPSC were p63 positive. Mitotic activity and Ki67 indices were significantly higher for LGPSCs than IPs and significantly lower than NKSCCs, while p53 immunohistochemistry in LGPSC was identical to nonkeratinizing SCC and higher than for IP. Sequencing showed all five tumors to harbor a MUC6 mutation, one tumor to harbor CDKN2A and PIK3R1 mutations, and one tumor to harbor a NOTCH1 mutation. All LGPSC lacked EGFR and KRAS mutations and lacked copy number variations of any main cancer genes. At a median follow up of 12 months, two LGPSC recurred locally, and one patient died after massive local recurrences and nodal metastases. LGPSC is a distinct, de novo sinonasal carcinoma that can be differentiated from papillomas by morphology and selected immunohistochemistry.

Clinicopathological analysis of low-grade papillary Schneiderian carcinoma: report of five new cases and review of the literature

AIMS

Low-grade papillary Schneiderian carcinoma (LGPSC) is a rare and newly described entity of the sinonasal tract. The aim of this study was to evaluate the clinicopathological and molecular characteristics in order to identify typical features for differential diagnosis.

METHODS AND RESULTS

Of the 3000 cases of sinonasal tumour studied during a period of 6 years, five cases were reviewed and diagnosed as LGPSC. All five patients were female (mean age, 47.8 years; range, 18-64 years) and had undergone multiple surgeries (3-10 surgeries). Both the sinonasal tract and the middle ear were involved in four patients. Nodal metastasis occurred in two patients, and one patient developed a distant metastasis to the left lung. Histologically, tumours had branched and crowded papillae with pushing boundaries. Tumour epithelia were multilayered and arranged in an orderly pattern without cilia. No malignant cytological features were observed in any of the cases. Immunohistochemical findings revealed a scattered distribution of Ki67-positive cells and positive staining for epithelial membrane antigen, mainly in the outermost-layer cells. Human papillomavirus (HPV) DNA was found in two patients and genotyped as HPV type 16. Sanger sequencing did not reveal any epidermal growth factor receptor or Kirsten rat sarcoma viral oncogene homologue gene mutation in the five cases.

CONCLUSIONS

We report on five new cases of LGPSC, and confirm LGPSC as a new sinonasal carcinoma that behaves aggressively with metastatic potential. The combination of clinical behaviour and typical histological features can distinguish LGPSC from sinonasal papilloma and other carcinomas.

Sinonasal papillomas: A single centre experience on 137 cases with emphasis on malignant transformation and EGFR/KRAS status in "carcinoma ex papilloma"

Among the three major histological subtypes of sinonasal papillomas, inverted (ISP) and oncocytic (OSP) sinonasal papillomas tend to undergo malignant transformation to carcinoma. However, criteria determining risk of recurrence and malignant progression have not been established. Recently, EGFR and KRAS mutations were detected to be characteristic for ISP and OSP, respectively. In this study, we analyzed 137 sinonasal papilloma cases (132 ISP and 5 OSP) for clinicopathological characteristics, frequency of recurrences/malignant transformation, and histological types and genetic features of carcinoma ex Schneiderian papilloma. OSP presented at a higher age than ISP (median, 75 vs. 57 years) and affected predominantly females. Overall frequency of recurrences and malignant transformation was 23.1% and 9.5%, respectively. Rates of recurrence (33.3% vs. 22.0%) and malignant transformation (33.3% vs. 8.8%) were higher in OSP compared to ISP, respectively. Carcinomas (n = 10) occurred mostly synchronously, more frequently in females and mainly associated with ISP (n = 9). Squamous cell carcinoma (SCC) was the most frequently associated malignancy. Concordant EGFR (in ISP/associated carcinoma) and KRAS (in the OSP/associated carcinoma) mutations were detected in all successfully analyzed matching papilloma/carcinoma pairs, confirming their shared clonal origin. Results of this large study are in line with recent studies showing frequent EGFR and KRAS mutations in sinonasal carcinoma ex Schneiderian papilloma. As the papilloma component might on occasion be missed on biopsy of synchronous carcinoma ex papilloma, EGFR and KRAS mutation testing represents a promising molecular surrogate for sinonasal "carcinoma ex papilloma", at the same time offering an opportunity for targeting mutant EGFR in this rare cancer type.

Low-Grade Papillary Schneiderian Carcinoma: Report of a Case with Molecular Characterization

In 2015, Lewis et al. first described low-grade papillary Scheneiderian carcinoma (LGPSC) of the sinonasal tract. Their case resembled a sinonasal papilloma clinically and histopathologically; however, invasion and metastasis resulted in the death of the patient despite absence of malignant cytologic features. Additional reports established LGPSC as a distinct entity and characterized its immunohistochemical profile. Diffuse expression of low molecular weight cytokeratins, positivity for p16 and p53 in at least 50% of cells, a high Ki-67 index, and absence of human papillomavirus (HPV)-DNA was observed across all reported cases. We report an additional case of LGPSC and describe the clinical, histologic, and immunohistochemical features. In contrast to sinonasal papillomas, the case was negative for HPV-DNA and showed no mutations in the EGFR and KRAS hotspot regions.

Cervical nodal metastasis after malignant conversion of sinonasal inverted papilloma: Report of a rare case and literature review

Malignant conversion of sinonasal inverted papilloma (SNIP) occurs in approximately ten percent of cases. These tumors are classically described as locally destructive, but without metastatic potential. Only four cases of malignant conversion with cervical nodal metastases have been described in the English literature. We present the rare case of a 61-year-old Caucasian male with a nasopharyngeal recurrence of malignant SNIP with cervical and retropharyngeal nodal metastases. The patient underwent endoscopic transpterygoid with nasoseptal flap reconstruction, followed by staged bilateral and retropharyngeal node dissection. Histopathology of the specimens demonstrated poorly differentiated invasive nonkeratinizing squamous cell carcinoma with inverted-type features. Three months after surgery, the patient suffered from C1-C2 fractures consistent with osteoradionecrosis and expired. Although the rate of malignant conversion of SNIP is low, this case highlights the need for aggressive, definitive treatment and surveillance.

Low-Grade Papillary Schneiderian Carcinoma of the Sinonasal Cavity and Temporal Bone

OBJECTIVES:

The aim of this study was to further characterize a newly described neoplasm, low-grade papillary Schneiderian carcinoma, occurring simultaneously in the sinonasal cavity and mastoid. Additionally, the authors review the only 2 similar cases within the literature and describe the common clinical features, radiographic findings, and pathologic characteristics of this exceptionally rare disease process.

METHODS:

Chart review for single patient, review of literature.

RESULTS:

The patient presented with bilateral nasal obstruction. Computed tomography revealed a left sinonasal mass with skull base hyperostosis, and follow-up magnetic resonance imaging showed a concomitant olfactory groove meningioma. Examination showed a bilateral, completely obstructing sinonasal mass with skip areas, and biopsy confirmed inverted papilloma (human papilloma virus strains 16 and 18 indeterminate). The patient underwent bilateral endoscopic sinus surgery, left medial maxillectomy, and left partial nasopharyngectomy. Given her multifocal disease, she was advised that she would require additional excision, but was lost to follow up. One year later she developed acute left facial paralysis. Magnetic resonance imaging demonstrated an enhancing mass in the left mastoid with enhancement along the Eustachian tube in addition to her known recurrent sinonasal disease. Simultaneous endoscopic sinus surgery and mastoidectomy were performed. Polypoid tissue was removed from the nasopharynx, mesotympanum, epitympanum, and retrofacial air cells. Immunohistochemistry showed that cells stained positive for p63 and dermCK and negative for synaptophysin. Morphologically, cells were bland, without classic stromal invasion, retaining their smooth, cystic, and papillary features, despite their increased depth within the tissue. Upon further review and consultation with an outside pathologist, a diagnosis of low-grade papillary Schneiderian carcinoma was made. The patient was referred for radiation therapy and is disease free at 3-month follow-up, with return of her facial function.

CONCLUSIONS:

This case represents the first report of concurrent low-grade papillary Schneiderian carcinoma of both the nasal cavity and mastoid. It emphasizes the importance of recognizing this new entity through pathologic analysis and suspecting it when the clinical course does not follow an expected pattern.

Oncocytic Schneiderian papilloma-associated adenocarcinoma and KRAS mutation: A case report

RATIONALE

Oncocytic Schneiderian papillomas (OSP) are an uncommon type of sinonasal papillomas that arise from the Schneiderian epithelium, accounting for only 6% of all sinonasal papillomas. Malignancies arising in OSP are rare and are almost always described as in situ or invasive squamous cell carcinoma, although mucoepidermoid, small cell carcinoma and sinonasal undifferentiated carcinoma have also been reported. To our knowledge, only 18 such instances have been reported in the medical literature.

PATIENT CONCERNS

Here, we report the case of an 81-year-old man presenting with a left sinonasal neoplasm, who had undergone 4 operations. The first postoperative pathology revealed a benign nasal polyp. The following several postoperative pathology revealed a novel, human papillomavirus-negative adenocarcinoma with increasing malignant features with each recurrence arising in an OSP. In addition, the most recent recurrences were associated with metastasis of cervical lymph nodes. And after the operation, the patient refused adjuvant radiotherapy. On 6-month follow-up after the last operation, the patient developed an in situ tumor recurrence 1 month after the fourth operation and refused to undergo surgery again.

DIAGNOSIS

Immunohistochemistry for Ki67, CK7, CK5/6, P53, and P63 showed a progression of malignancy. HPV assay presented the 21 most prevalent HPV types were negative. In addition, KRAS gene exon 2 G12C presented mutation in the OSP-associated adenocarcinoma.

INTERVENTIONS

During the whole course of the patient's disease, we performed four nasal endoscopic operations. And after the last operation, the patient refused adjuvant radiotherapy and KRAS-targeted therapy.

OUTCOMES

We are the first to describe adenocarcinoma arising in an OSP. To our surprise, from the first benign neoplasm to the second OSP-associated adenocarcinoma, it went through a long period of 10 years. However, after the adenocarcinogenesis, the differentiation of tumor became worse with the shorter interval of each recurrence.

LESSONS

Therefore, for elderly patients with unilateral nasal polyps, long-term follow-up is necessary. Once OSP turns into malignant, radical resection should be performed as much as possible to reduce the irritability of tumors.

Low-Grade Papillary Schneiderian Carcinoma: A Case Report

The nasal cavity and paranasal sinuses are covered with ciliated respiratory mucosa of ectodermal origin, known as Schneiderian epithelium, which can give rise to different types of sinonasal carcinomas. A 42-year-old woman with a history of nasal polypectomy 3 years previously presented with nasal obstruction and rhinorrhea. She was found to have a papillary mass involving the left nasal cavity, and the left maxillary and ethmoidal sinuses by radiologic examination. She underwent endonasal resection under the diagnosis of oncocytic papilloma. The resected specimen showed exuberant invasive growth of papillary or inverted architectures of epithelial cells. The neoplastic cells were very bland, showing a round to polygonal shape, low nuclear-to-cytoplasmic ratio, abundant oncocytic cytoplasm, uniform nuclei, indistinct nucleoli, and scarce mitosis. The overall features were identical to those of a recent report of a low-grade papillary Schneiderian carcinoma. The main differential diagnosis is Schneiderian papilloma, and awareness of this novel entity is important for its proper treatment.

Pre-operative chemoradiation for sinonasal undifferentiated carcinoma of the anterior nasal cavity resected through a lateral nasal flap approach

INTRODUCTION

Sinonasal undifferentiated carcinoma (SNUC) is an exceedingly rare and aggressive tumor that carries a poor prognosis due to its non-specific presentation and advanced stage at time of diagnosis. Early detection and treatment are vital, with chemotherapy, radiation, and surgery all being viable options. The literature is sparse and there is no consensus for optimal treatment. In surgical candidates, the otolaryngologist must have a vast skill set in order to resect the tumor with wide margins and reconstruct the defect in hopes of returning the patient to their pre-morbid state.

METHODS

A 74-year-old female presented with a growing left nasal mass which was biopsied and found to be a sinonasal undifferentiated carcinoma originating from the anterior nasal cavity between the septum and upper lateral cartilage. The patient was treated with neo-adjuvant carboplatin with concurrent radiation, followed by resection through a lateral nasal flap. The defect was reconstructed with a contralateral septal hinge flap and septal cartilage graft with primary closure of the lateral nasal flap.

RESULTS

Intraoperatively, no skin or cartilage invasion was noted and as such, nasal skin was spared and utilized for primary closure. At a follow-up of 3 months, the patient had no evidence of recurrence and had a well healing repair site with satisfactory cosmesis.

CONCLUSIONS

Despite the aggressive nature of SNUC tumors, neo-adjuvant chemo-radiation and surgical intervention with functionally and aesthetically minded reconstruction can provide patients with improved outcomes and decreased morbidity.

Low-Grade Epithelial Proliferations of the Sinonasal Tract

Low-grade epithelial proliferations of the sinonasal tract include Schneiderian papillomas, respiratory epithelial adenomatoid hamartoma, seromucinous hamartoma and low-grade non-intestinal adenocarcinoma. There is considerable overlap in their clinical presentation, endoscopic appearance, and imaging features. Although well-described diagnostic criteria exist, a definitive diagnosis may be difficult to reach on a small biopsy. Schneiderian papillomas are divided into fungiform, inverted, and oncocytic types, each with characteristic clinical and morphological features. The latter two may progress to malignancy. The majority are still considered to be HPV-related. Two lesions are designated as hamartomas, but their pathogenesis remains uncertain, with inflammatory and neoplastic origins proposed. Respiratory epithelial adenomatoid hamartoma is increasingly being recognized for its association with chronic rhinosinusitis and olfactory cleft site of origin. Seromucinous hamartoma has gained attention in recent years and overlaps with both respiratory epithelial adenomatoid hamartoma and low-grade non-intestinal adenocarcinoma. Controversy surrounds their distinction, particularly from low-grade adenocarcinoma. The latter generally is cured by complete excision, with a 26 % risk of recurrence but rare metastases and deaths from disease.

Nasal Cavity and Paranasal Sinuses

The nasal cavity and paranasal sinuses occupy the top of the upper respiratory tract and form pneumatic spaces connected with the atmosphere. They are located immediately beneath the base of the cranium, where crucial vital structures are harbored. From this region, very much exposed to airborne agents, arise some of the more complex and rare benign and malignant lesions seen in humans, whose difficulties in interpretation make this remarkable territory one of the most challenging in the practice of surgical pathology.

Contents of this chapter cover inflammations and infections, polyps and pseudotumors, fungal and midfacial destructive granulomatous lesions, as well as benign, borderline, and malignant neoplasms. Among the neoplasms, emphasis is made on those entities characteristic or even unique for the sinonasal region, such as Schneiderian papillomas, glomangiopericytoma, intestinal- and non-intestinal-type adenocarcinomas, olfactory neuroblastoma, nasal-type NK-/T-cell lymphoma, and teratocarcinosarcoma. Moreover, recently recognized entities involving this territory, i.e., HPV-related non-keratinizing carcinoma, NUT carcinoma, and SMARCB1-deficient basaloid carcinoma, are also discussed in the light of their specific molecular findings. Furthermore, the text is accompanied by numerous classical and recent references, several tables, and 100 illustrations.