Mucous retention cyst (MRC) is not uncommon in the pathology of maxillary sinus, which should be differentially diagnosed from chronic maxillary sinusitis. The main stream of diagnosis usually depends on the clinical symptoms and radiological findings. Thus it was sometimes puzzling to confirm the histological features of mucous retention phenomenon in the antral mucosa, when the specimen was from a limited portion or much degenerated by inflammatory reaction. This study aims to define the histopathological features of MRC through reevaluation of MRCs (n=19) and maxillary sinusitis (n=65) diagnosed previously. The present study classified three types of MRC, i.e., an extravasation type, a luminal retention type, and a mixed type of MRC. The extravasation type MRC showed clear pseudocyst cavity under sinus mucosa with infiltration of foamy macrophage, and the luminal retention type MRC showed mucous retention in the luminal cavity of maxillary sinus accompanied with inflammatory reaction, and the mixed type MRC showed the both features of extravasation and luminal retention type MRCs. Resultantly, among nineteen cases of MRC only three cases belonged to the extravasation type MRC, eleven cases belonged to luminal retention type MRC, and three cases belonged to mixed type MRC, while two cases were turned out to be postoperative residual cysts of maxillary sinus. The MRCs examined in this study showed different pathological features from ordinary maxillary sinusitis, exhibiting the typical mucin retention phenomenon of extravasation type or luminal retention type with relatively mild inflammatory reaction with infiltration of mucin-pooled macrophages. However, the luminal retention type MRC was predominant among the MRCs (11/17, 64.7%) and each of the extravasation and mixed type MRCs was only three cases out of 17 MRCs (17.6%). The extravasation type MRC characteristically produces a pseudocyst by the overexpressions of matrix metalloprotease-3 (MMP-3) and connective tissue growth factor (CTGF). Because not only the pathogenetic mechanism but also the prognosis of MRC is different from chronic maxillary sinusitis, we suggest that the MRC of maxillary sinus should be classified into extravasation, luminal retention, and mixed types in the histological observations in addition to the clinical and radiological informations.

I. INTRODUCTION
 II. MATERIALS AND METHODS
 III. RESULTS
 IV. DISCUSSION
 V. REFERENCES

• 김연숙(Department of Dental Hygiene, College of Health, Cheongju University) | Yeon Sook Kim
• 안기용(Department of Oral Pathology, College of Dentistry, Kangnung-Wonju National University) | Ki Yong An
• 이석근(Department of Oral Pathology, College of Dentistry, Kangnung-Wonju National University) | Suk Keun Lee correspondence