Ⅰ. INTRODUCTION
First described by Abrams et al. in 1973 1), necrotizing sialometaplasia (NSM) is a benign, self-limiting inflammatory process primarily affecting the minor salivary glands. The clinical significance of NSM lies in its diagnostic potential to mimic malignant neoplasms, such as squamous cell carcinoma (SCC) and mucoepidermoid carcinoma, both clinically and histologically 2-4). NSM primarily affects adults, with the highest incidence generally observed in middle age, and it exhibits a slight male predilection 5,10). Although the hard palate is by far the most commonly affected site, representing the vast majority of reported cases 5,10), NSM can occur in any anatomical site harboring salivary gland tissue, including the retromolar pad, lips, tongue, buccal mucosa, and upper respiratory tract 5-8,14). The diagnostic challenge is significantly amplified when the lesion manifests in an atypical location. In such cases, a high clinical suspicion of malignancy may lead to misdiagnosis, potentially resulting in unnecessary and overly aggressive surgical interventions 9,15). The retromolar trigone is an exceptionally rare site for NSM. This report presents a case of NSM occurring exclusively in the mandibular retromolar trigone, highlighting the clinicopathologic features essential for dis- tinguishing this benign reactive process from malignancy and ensuring appropriate conservative management.
Ⅱ. CASE REPORT
A 30-year-old female with an unremarkable medical history was referred to the Department of Oral and Maxillofacial Surgery, Kyungpook National University Dental Hospital (KNUDH), in May 2025. The patient's chief complaint was a painful ulcer in the right mandibular retromolar trigone, which had persisted for two weeks. According to the referral letter from the local dental clinic, the patient initially presented with mild swelling, a deep periodontal pocket, and gingival detachment at the distal aspect of the mandibular right second molar. To address this, a periodontal flap surgery involving a distal wedge incision and subsequent flap elevation under local anesthesia was performed two weeks prior to her visit to our department. However, delayed wound healing at the surgical site prompted further evaluation.
Panoramic radiography showed no evidence of osseous erosion or other abnormalities in the affected area (Figure 1A). Intraoral examination revealed an irregular ulcer in the right mandibular retromolar trigone, with surrounding erythema and partial induration (Figure 1B). These findings raised a high clinical suspicion of a malignant neoplasm, particularly squamous cell carcinoma (SCC).
To establish a definitive diagnosis and exclude malignancy, an incisional biopsy was performed under local anesthesia, yielding a specimen approximately 5 mm in diameter. Histopathological examination of hematoxylin and eosin (H&E)-stained sections demonstrated that the underlying lobular architecture of the salivary gland was relatively well preserved (Figure 2A). Chronic inflammatory cell infiltration was observed in the surrounding minor salivary gland acini and ducts (Figure 2B). At high magnification, prominent squamous metaplasia of the ductal epithelium was noted (Figure 2C). Crucially, the metaplastic squamous cells exhibited no cytologic atypia or dysplasia. The surrounding stroma showed granulation tissue formation with chronic inflammatory infiltrate (Figure 2D). Based on these histopathological features, the lesion was diagnosed as necrotizing sialometaplasia (NSM).
Considering the benign nature of NSM, no further surgical intervention was performed. The patient was informed about the self-limiting course of the disease and received conservative management. At the one month follow-up, the mucosa showed marked improvement, approaching a normal appearance. By six months, the lesion had completely resolved with full restoration of the mucosal integrity and no signs of recurrence (Figure 1C).
Ⅲ. DISCUSSION
The pathogenesis of necrotizing sialometaplasia (NSM) is widely attributed to ischemic infarction of the salivary gland lobules 5,16), typically precipitated by factors such as local trauma, dental injections, or surgical manipulation 5,10,11,19). In the present case, the onset of symptoms and impaired wound healing following a periodontal flap operation strongly suggest an iatrogenic etiology, where the preceding surgical procedure likely triggered the ischemic event.
The clinical significance of NSM lies in its potential to mimic malignancy, a challenge that is primarily rooted in its histopathological features. The combination of pseudoepitheliomatous hyperplasia (PEH) of the overlying epithelium and extensive squamous metaplasia of the ducts can render NSM virtually indistinguishable from invasive squamous cell carcinoma (SCC), especially in small incisional biopsy specimens 10,12,13,17). Therefore, differentiation from malignancy hinges upon specific histopathological criteria: the preservation of the native lobular architecture despite extensive necrosis, and the bland cytologic features of metaplastic squamous cells, which lack dysplasia or atypical mitoses 5,10). The biopsy findings in this patient were consistent with these diagnostic criteria. In addition to SCC, the differential diagnosis must encompass mucoepidermoid carcinoma (MEC) and pseudoepitheliomatous hyperplasia (PEH) 1,10,12). Unlike MEC, which typically demonstrates a proliferation of mucinous, intermediate, and epidermoid cells often forming cystic spaces with variable cytologic atypia, NSM maintains the overall lobular architecture of the salivary glands without true neoplastic proliferation 1,5,10). On the other hand, PEH is a reactive epithelial proliferation commonly overlying inflammatory lesions and can closely mimic SCC or NSM. However, PEH lacks the extensive coagulative necrosis of salivary acini and the prominent squamous metaplasia of salivary ducts that are highly characteristic of NSM 10,17).
This case is particularly significant due to its exceedingly rare anatomical location. The manifestation of NSM in the mandibular retromolar trigone amplifies the diagnostic complexity because this specific site is frequently affected by oral SCC but is extremely rare for NSM 17). Furthermore, while most cases of NSM are unilateral, as seen in this patient, the literature also describes even more perplexing clinical presentations, such as bilateral occurrences, which can further mimic multi-focal malignancy 18). This case represents a classic diagnostic dilemma where a rare benign entity mimics a common malignancy in a location where the latter is frequently encountered.
In conclusion, this report presents an exceptionally rare case of NSM involving the mandibular retromolar trigone. As a self-limiting condition, NSM typically resolves with conservative management, as demonstrated by the complete resolution observed in this patient 20). Once a definitive diagnosis is established, further surgical intervention is unnecessary. This case highlights the necessity for clinicians and pathologists to include NSM in the differential diagnosis of persistent, ulcerative, or indurated lesions in the retromolar trigone. Precise histopathological evaluation is paramount to ensure appropriate management and prevent unnecessary surgical complications for this benign reactive process.
