Résumé:   
Il s’agit d’un homme âgé de 57 ans, qui consultait pour une obstruction nasale chronique et un épistaxis. L’examen endoscopique mettait en évidence de nombreuses lésions polypoides. La tomodensitométrie montrait un processus hétérogène, occupant la fosse nasale droite avec ostéolyse des cornets moyen et inférieur et rétention liquidienne du sinus maxillaire droit. Une résection chirurgicale était réalisée. L’examen anatomopathologique concluait à un papillome inversé nasal dégénéré en un carcinome sarcomatoide.

Introduction
Nasal inverted papilloma is a benign epithelial tumor that develops from schneiderian mucosa lining the nasal cavity [1]. His incidence is estimated at 0.4 cases per 100 000 people. It typically corresponds to an endophytic proliferation of transitional cells. It has the potential to recur and the tendency towards malignant transformation [2]. 
The aim was to determine clinical and histological features of malignant inverted papilloma and to discuss the differential diagnosis.
Case report
We present the case of a 57 year-old male, who presented with a chronic unilateral nasal obstruction. He complained of epistaxis and headaches. The clinical examination revealed a right nasal tumor. Intranasal endoscopy showed multiple polypoid lesions. The computed tomography exam revealed a heterogeneous mass of tissue density that occupied the right nasal cavity with osteolysis of the middle and lower cone causing fluid retention of the right maxillary sinus. The treatment was decided to consist in surgical resection. Frozen section showed a benign inverted papilloma. But the definitive microscopic examination revealed malignant areas. It showed hyperplastic aggregates of well-demarcated squamous and respiratory epithelium that growth endophytically [Figure 1]. These lesions were merely associated with carcinomatous component. This component showed squamous differentiation [Figure 2]. It was composed by nests and sheets of cells with pearl formation and intercellular bridges. Some other tumoral areas revealed undifferentiated component with infiltrative sheets of spindle shaped cells with markedly atypical nuclei and numerous mitotic figures [Figure 3]. Immunohistochemical study revealed that the sarcomatous area expressed Vimentin antigen. Actine, desmine, caldesmone and PS100 were negative revealing an   undifferentiated sarcomatous component. The diagnosis of malignant transformation of nasal inverted papilloma into sarcomatoid carcinoma was made.
Discussion:
Inverted papilloma specifically affects male subjects with a sex ratio of 3/1. The median age is 50 to 60 years [2]. Nasal obstruction is the most common presenting symptom [1]. The case presented with nasal obstruction and epistaxis.
This tumor has the potential to recur and exhibit malignant transformation [3]. The latter is possible in 3-15% of cases [4]. Malignant transformation is characterized by the presence of the following features: cellular pleomorphism, atypical mitoses, keratin pearls and stromal invasion [5]. Based on the literature, the rates of synchronous and metachronous carcinoma are 7,1 and 3,6% respectively. Most cases are infiltrating squamous cell carcinomas, followed by transitional cell carcinomas. Transformation into sarcomatoid carcinoma is a rare condition which represents approximately 5% of malignant cases. This tumor has a sarcomatous morphology and aggressive behavior [6]. The Wnt signaling pathway plays an important role in embryogenesis and carcinogenesis [7]. The differential diagnosis include nasal polyp with squamous metaplasia and adenoid epithelial respiratory hamartoma.
The complete resection is difficult and the prognosis is poor. Complete endoscopic resection and close follow up are recommended in nasal inverted papilloma. In case of malignant transformation, post-operative radiation or chemotherapy will be needed, especially in case of sarcomatoid carcinoma.
Conflict of interest: The authors declare that they have no conflict of interests

Figures

Figure 1: Pathological examination (Hematoxylin eosin x 10):  Inverted papilloma characterized by respiratory epithelium that growth endophytically

Figure 2: Pathological examination (Hematoxylin eosin x 20): Malignant transformation of the inverted papilloma

Figure 3: Pathological examination (Hematoxylin eosin x 40): Sarcomatoid component with infiltrative sheets of spindle shaped cells with markedly atypical nuclei and numerous mitotic figures

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