: Basal cell adenoma of esophagus , a rare tumor at an atypical site

Basal cell adenomas are rare, salivary gland tumors. These are a class of non-malignant tumors that have glandular epithelial tissue origin. Often described to have a disputed recurrence rate after excision, the prognosis of basal cell adenomas is generally good. However, their rarity makes their continuous monitoring vital. Basal cell adenomas normally cause problems exerting a pressure effect on the surrounding organs or to the originating area. Often due to its location and effect, differentiation between other tumors, e.g. basal cell carcinoma, adenocarcinoma and cystic carcinoma, must be done. We were able to treat a patient with complaints of sensation of regurgitation that was diagnosed as an esophageal mass on esophagogastroduodenoscopy and later as basal cell adenoma on histopathological examination. The patient was treated with endoscopic submucosal dissection successfully and has had no fresh complaints since. Here, we present a case of basal cell adenoma in the esophagus, a rare entity in itself.


Introduction
The National Cancer Institute, USA, describes adenoma as "a tumor that is not cancer.It starts in gland-like cells of the epithelial tissue (thin layer of tissue that covers organs, glands, and other structures within the body)" 1 .Basal cell adenomas are a rare type of benign glandular tumors, salivary in origin.Accounting for less than 1-2% of salivary gland tumors 2 , they are located usually superficial within the glandular body, and often a brownish appearance is observed 3 .First described by Kleinasser and Klein in 1967, they are termed as rare benign tumors with a high recurrence rate and, in general, good prognosis and recognized as an independent entity in the Second Edition of the Salivary Gland Tumors Classification of the World Health Organization 4 .Normally indigenous to (parotid) salivary glands, basal cell adenomas have also been reported in the buccal mucosa, palate and nasal septum.They are often seen in patients in their fifth-seventh decade of life, a contrast to other benign tumors 5 .Basal cell adenoma can be divided into four subtypes: solid, trabecular, tubular and membranous 6 .Histologically characterized by the presence of uniform and regular basaloid cells having two differenced morphologies, the tumor cells are intermingled.Among the two groups, one has characteristic constitution of small cells with little cytoplasm and intense basaloid nuclei located near the tumoral nests while another is made up of large cells with abundant cytoplasm and pale nuclei in the centre of tumoral nests.The tumoral nests are separated from the surrounding connective tissues by a basal membrane like structure, which surrounds them 5 .Immunologically speaking, they strongly express PAN CK and CK 5/6, along with Calponin, P-63 and weak staining of Ki-67.Vimentin, actin also stain positive in basal cell adenomas, along with S-100 (alpha subunit) in ductal cells and S-100 (beta subunit) in basaloid cells.

Case report
A 63-year-old woman of Chinese (Han) origin was referred to Taihe Hospital, Department of Digestive Medicine after esophagogastroduodenoscopy (EGD) in her county hospital due to a protuberant mass in the esophagus, 35 cm from the incisors.The patient had initially complained of sensation of regurgitation and was a non-smoker and non-drinker.There was no history of heartburn, chest pain, nausea, vomiting or diarrhea.She also had no complaints of hematemesis, hematochezia or melena.The patient was non-diabetic, non-hypertensive, with no history of renal disorders, cardiac diseases or tuberculosis.She had no remarkable family history (medically relevant) and no signs of genetic disorders.The patient had undergone appendectomy 13 years back and had been diagnosed and treated for intestinal obstruction 4 years previously.Laboratory investigation reports were normal.Upper abdominal and chest contrast-enhanced computer tomography (CECT) showed: lower esophagus and cardiac region of stomach wall hypertrophy (highly indicates presence of a tumor); right hepatic lobe diffuse (indicates small cyst present); B/L lung field showed some scattered fibrotic and calcified lesions.
EGD was repeated and showed a protuberant lesion in the esophagus, 3cm in size, 35 cm from the incisors (Figure 1).It also showed another protuberant lesion 0.7 cm in size in the gastric cardia.Multiple yellow spots (gastric xanthoma) were seen in the stomach.Pylorus, pyloric sphincter and duodenum were normal.Endoscopic ultrasound (EUS) of the protuberance in the esophagus showed that the lesion as hypoechoic, an originated from and limited to the submucosal layer with an area of more than 3 cm (Figure 1).No lymphadenopathy was found around the lesion.Radiofrequency ablation was done on the yellow spots.
With the working diagnosis of esophageal leiomyoma/polyp, the patient was scheduled for surgery.Endoscopic submucosal dissection (ESD) was done under general anesthesia (Figure 2).The surgery went well without complications.The protuberant mass was then sent for histopathological examination and the patient was kept for observation.
Follow-up EGD of the patient was performed two months later and showed no recurrence of the tumor.The patient has had no fresh complaints since the ESD procedure (Figure 4).

Discussion
Basal cell adenomas are uncommon variants of benign salivary gland tumor with varying recurrence rates.There have been very few reported cases of basal cell adenomas in ectopic sites.Its rarity makes comparison between two similar cases very difficult 7 .Basal cell adenomas are generally characterized by no regional node involvement, no calcification and no cystic component within the tumor 8 .
This case posed a challenge to our team since ectopic occurrences of basal cell adenoma have been reported very rarely in literature and is not a very common occurrence in itself.The prognosis of basal cell adenoma, when found at ectopic sites is not clear and thus poses a challenge to predict the outcome even after intervention.
Differentiation between benign and malignant basal cell adenoma is vital since they share the same site and characteristics.However, the mechanism and rate of progression is vastly different.Histology is not alone enough to differentiate or predict the progression, hence immunohistology plays a vital role in differentiation.Immunohistological staining with CD5/6, Keratin, Alpha-1 antichymotrypsin, CEA, S-100, Vimentin, CD117, Actin must be done to differentiate it from other benign and malignant tumors.
Whenever encountered and suspected, differentials must be established in favor of basal cell adenoma.Pleomorphic adenoma, adenoid cystic carcinoma, basaloid squamous carcinoma must be excluded first with the help of gross microscopic examination and immunohistological staining.Surgical excision remains the first and foremost approach of choice for treatment of basal cell adenoma.However, during removal, it is mandatory not to disturb the tumor capsule to prevent recurrence.Moreover, the patient must be kept under regular follow-up to monitor malignant transformation as well as recurrence.

Consent
Written informed consent was obtained from the patient for the publication of this case report and any associated images.
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Figure 2 .Figure 3 .
Figure 2. Left and middle panels: Endoscopic submucosal dissection (ESD) removal of the mass under evaluation.Right panel: Closure of the defect by hemoclips.(ESD by single-use Olympus Dualknife TM , tip thickness-0.3mm.)

Figure 4 .
Figure 4. Site of endoscopic submucosal dissection (ESD) in follow-up after 2 months.The scar from ESD procedure is indicated by the red arrow.