Background
Osseous lesion of the tongue is a rare entity and was originally reported as a lingual osteoma [
1]. Since the biological behavior of the lesion did not meet the criteria of a tumor, the term osseous choristoma was proposed and is currently widely used for this lesion [
2]. Choristoma is defined as a growth of normal tissues in an ectopic location [
3,
4]. So far, only 67 cases of osseous choristoma of the tongue have been reported in the literature [
5]. Here we present two additional cases of lesion in the tongue and discuss the epidemiology, clinical presentation, and debatable pathogenesis of this disease.
Discussion
Osseous choristoma of the tongue is extremely rare with only 67 cases reported in the English literature so far [
5]. Here we have described two additional cases of osseous choristoma; the clinical and histological findings obtained from the two cases enabled us to reach the final diagnosis. The age of patients with osseous choristoma has been reported to range from 5 to 73 years with a mean age of 28.7 years [
5]. The lesion is more prevalent in females than in males [
4,
5]. The tumor is located in the posterior third of the tongue near the foramen cecum, most commonly around the circumvallate papilla, and sometimes at the lateral border or middle thirds of the tongue [
5]. The size of the lesion varies from 3 mm to 5 cm [
5], and the duration ranges from 3 days to 50 years [
4]. The lesions present as a hard mass and may be either pedunculated or sessile. The mucosa covering the lesion usually appears as normal epithelial mucosa, although ulcerated mucosal covering has also been reported [
4]. The clinical findings including age, gender, size and macroscopic appearance of the two cases in the present study are similar to those reported earlier.
The etiopathology of osseous choristoma is still unknown. Two different hypotheses have been proposed; whereas one suggests that this is a type of malformation, the other concludes that it is formed as a result of trauma or chronic irritation [
6]. According to the malformation hypothesis, the lesion arises at the line of fusion of the first and third brachial arches between the anterior two-thirds and posterior one-third of the tongue [
1]. This hypothesis is consistent with the knowledge that certain normal osseous structures such as the incus and malleus, and the hyoid bone are derived from the first and third brachial arches, respectively. Pluripotent cells originating from these arches might have been trapped within this region and subsequently undergone ossification resulting in the development of osseous choristoma [
6]. In another study, the foramen caecum has been stated as the site of development of the anlage of the thyroid gland during embryonic life [
1]. Remnants of the undescended intraglossal thyroid tissue can lead to unusual osseous proliferation later in life [
7]. The trauma hypothesis, on the other hand, indicates that the posterior third of the tongue is highly susceptible to trauma and irritation, and an osseous lesion on the tongue may represent a reactive or posttraumatic center of ossification. The occurrence of “myositis ossificans” in other muscles may support this hypothesis [
8]. However, some studies have demonstrated normal bone formation in these lesions, in contrast to the irregularly formed bone lacking the Haversian system seen during traumatic ossification [
9]. Histological findings revealed normally formed bone in both cases in the present study and might therefore not support the trauma hypothesis.
The differential diagnosis of a protruding mass in the tongue can also be based on its location. When the lesion is on the dorsum of the tongue near the foramen caecum, it can be considered a benign tumor such as hemangioma, lymphangioma, teratoma, hamartoma, leiomyoma, thyroglossal duct cyst, lingual thyroid, mucocele, and pyogenic granuloma, or a malignant tumor such as rhabdomyosarcoma and epidermoid carcinoma. Lesions found on the margins of the tongue may be diagnosed as a traumatic neuroma, neurofibroma, schwannoma, fibroma, or cartilaginous choristoma. Pyogenic granulomas, mucoceles and cartilaginous choristomas have a greater predilection for the anterior region of the tongue. Other differential diagnoses include vascular malformation, fibroma, papilloma, salivary gland mixed tumor, osteochondroma, and other neoplasms such as osteogenic sarcoma or squamous cell carcinoma [
5]. Hamartoma is a similar morphological entity which is often confused with choristoma [
3]. Choristoma is a histologically normal tissue proliferation of a type that is normally not found in the anatomic site. On the other hand, hamartoma is a disorganized proliferation of mature tissues, composed of elements that are normally found in the specific location in which it develops, often with one predominating element [
3,
10]. The diagnosis of a lesion would therefore depend on its anatomic site. Since there is no bone tissue in the tongue, our cases were diagnosed as choristoma. Case 1 was initially diagnosed as a fibroma; the definite diagnosis was reached after histological examination of the lesion.
Surgical resection is the most common treatment for osseous choristomas [
4,
5], although potassium titanyl phosphate (KTP) laser has been used to excise the lesions [
11]. The post-surgical prognosis of lingual osseous choristomas is excellent, and only two cases of recurrence have been reported so far [
12]. In the present study, neither of the two cases showed any sign of recurrence.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
YA, BRA, HN and MS conceived the idea and wrote the manuscript for both the cases. YA, BRA, HN, JS, FH, MU and TM did the initial investigation, clinical diagnosis and treatment of the first case and provided the clinical data and case pictures. MN, RT, HN, MS, KY, JOI and TC did the initial investigation, clinical diagnosis and treatment of the second case and provided the clinical data and case pictures. All authors read and approved the final manuscript.