1. Introduction

Salivary gland neoplasms are rare, comprising 1-4% ^[1] of all neoplasms and 3-5% ^{[2, 3]} of all head & neck neoplasm. Minor Salivary gland tumors are infrequent accounting for 10-15% of all salivary neoplasms with a broad range of histopathological types and growth patterns. ^{[4, 5]}.

According to several studies like Auclaire et al ^[6], Isaccson & Shear ^[7] the ethnicity & geographic location of population possibly affects frequency & distribution of salivary gland tumors. The considerable variability observed across studies is also in relation to different diagnostic criteria’s which does not always follow the 1991 WHO classification ^[8]. The relative frequency of individual tumor types in some surveys is biased in favor of malignant tumor as the data is drawn from major cancer centers ^{[9, 10]}.

Pleomorphic adenoma is the most common benign salivary gland tumors. Palate is the most common intraoral site (42.8-68.8%) followed by upper lip (10.1%), cheek (5.5%) ^{[2, 11, 12, 13]}, other rare sites are throat (2.5%) retromolar region (.7%) floor of mouth & alveolar mucosa ^[12].

A rare case of pleomorphic adenoma of minor salivary gland in retromolar region is described.

2. Case Report

A 30 yrs old male patient presented to department of Periodontics with a slowly growing painless swelling in the left mandibular posterior region of two and half year duration. Clinical examination revealed a 3x3 cm, firm, mass in retromolar area (Figure 1). There was no history of trauma, fever, disturbance of salivation or intraoral surgical procedure. The investigations revealed normal values. Panoramic radiograph showed no evidence of osseous involvement or calcification. The lesions was dissected & excised with safety margins under local anesthesia (Figure 2). Microscopically tumor islands were observed composed of epitheloid, plasmacytoid predominantly and spindle type neoplastic myoepithelial cells (Figure 3a). Ducts were seen scattered in stroma and tumor islands, the luminal side is lined with flat or cuboidal shaped cells and outer side with spindle cell type which are one to many in numbers. Outer layer of myoepithelial cells seems to be melting into stroma. Ducts were filled with eosinophilic coagulum. Stroma is mostly hyalinized and few areas appear fibromyxoid. Focal squamous differentiation with keratinization is seen (Figure 3b). Post operative follow up after six months shows uneventful healing (Figure 4).

Figure 1. Gross features of pleomorphic adenoma of retromolar area. Preoperative view showing round nodular growth in retromolar region

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Figure 2. Postoperative view showing sutures placed after excision

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Figure 3. Photomicrograph showing ducts with eosinophilic coagulum & surrounded by myoepithelial cells in hyalinised stroma. (a) Focal squamous differentiation with keratinization (H&E, x 100). (b)Tumor islands of myoepithelial cells in sheets and melting in stroma. Plasmacytoid hyaline myoepithelial cells (H& E, x 400)

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Figure 4. Post-operative view after six month followup showing normal retomolar trigone region

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3. Discussion

Pleomorphic adenoma is the most common minor salivary gland tumor in most studies ^{[6, 7, 13, 14, 15, 16, 17]}, with incidence ranging from 33% to 70 % of all tumors ^{[6, 7, 13, 14, 15, 16, 17]} & from 70.6% ^[18] to 100% ^{[10, 11, 17]}, of benign tumors. Issacson & Shear ^[7] reported 70%, Rivera Bastidas ^[18] about 85%, Japanese study revealed 57-62% ^[17], these incidences are higher than US ^{[6, 15, 20]} UK^[16] & Australia where the percentage range was from 38.1%-54% ^[6], suggesting a regional variation. Other less common benign tumor percentage varied according to different study background like Waldron et al ^[20] reported monomorphic 19%, Cystadenoma 9%, Saliadenoma papilleferum 2%. Most studies have shown those minor salivary gland tumors are more common in females than male ^{[7, 16, 17, 21]}. The tendency for female predominance has been reported to be especially marked in benign tumors, with male to female ratio of ranging from 1:1:8 ^[17] to 1:2.4 ^[18] Mean age of the patients of benign minor salivary gland tumor was around 51.8 yrs (range of 22-86 yrs). Pleomorphic adenoma is most common from 4-6 ^th decade with mean age of 43-46 yr ^{[11, 12]}. Isacsson & Shear ^[7] reported a peak frequency in the third decade, which is in accordance to our case. Though Lucas ^[22] reported of peak frequency in the 5^th – 6^th decade.

Minor salivary gland tumors are located anywhere in the upper aero digestive tract, but the most frequent site is the oral cavity & mainly the palate ^[23]. In the series of Waldron et al ^[20], palate (42.5%) was most common. Other studies suggested higher presence of minor salivary gland tumor in palatal region, representing 65-86% of all tumors ^{[4, 8, 13]}. Studies suggesting the percentage of pleomorphic adenoma in retromolar trigone was not found, but percentage of benign tumors in retromolar trigone according to different studies are 5.4% by Waldron et al ^[20], 11.1%, by Pons-Vincente et al ^[3], 4.4% by Lopes et al ^[23], suggesting that retromolar area is an uncommon site for benign minor salivary gland tumors. Most studies indicate for malignant neoplasm in the retromolar trigone, suggesting that our case is rare in location.

Painless intraoral swelling was the most common symptom. Clinically involved cervical nodes or distant spread was not seen. The range of duration of symptoms is usually slow with mean time of 45 month which coincides with our history of pain less swelling for more than 2 yrs. Less common clinical signs & symptoms includes ulceration & ill fitting dentures, change in sensation & difficulty in speech.

Differential diagnosis of the growth in retromolar trigone is commonly mucocele, fibroma, neurilemmoma, malignant salivary gland tumor like mucoepidermoid carcinoma, solitary fibrous tumor which is mistaken for salivary gland tumor or uncommon pleomorphic adenoma as available literature only points to .7% of cases ^[12].

Pleomorphic adenoma has a tendency for local recurrence & some cases undergo malignant transformation. Treatment of choice for pleomorphic adenoma in minor salivary glands is wide local excision with removal of periosteum or bone if involved. Simple enucleation is believed to lead to a high local recurrence rate & should be avoided (24). Recent date suggest that recurrences is seen cases where surgery could not be performed properly.

4. Conclusion

Pleomorphic adenoma of retromolar area is a rare neoplasm however due to high predilection of recurrence and malignant transformations of minor salivary gland tumors, long term follow up is necessary. The present case report & review of literature of minor salivary gland tumors brings to light the wide variation in presentation of minor salivary gland tumors. Thus medical & dental practitioners should take them into account in any differential diagnosis when assessing intraoral pathology.

Competing Interests

We certify that our manuscript is free of conflict of interest.

Funding

None.

Ethical Approval

Not required.

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