A-RMs are considered rare in the oral cavity and there are about 80 cases reported in the English-language literature [5]. The etiology of its pathogenesis is unclear and it may originate from the third or fourth branchial arch; it may also be related to genetic alterations in striated muscle development, as balanced translocations of chromosome pairs 15 and 17 have been identified in some patients; it has also been suggested that it may be associated with chronic irritation [6–9].
A-RM occurs mainly in the head and neck region. Patients with oral A-RM were predominantly male (75%), with a male to female ratio of 3:1; mean age was 57 years, with peak incidence between 60 and 70 years; cases occurred mostly in the floor of the mouth/submandibular region (49%-61.2%), followed by the base of the tongue (16%-20%), soft palate (9%-11.2%), buccal mucosa (3%-3.8%), and lips (3%-3.8%); some patients (17.5%) presented with multifocal lesions; lesion size ranged from a few millimeters to 15 cm [10]. The masses were mostly slow-growing, circumscribed painless masses with occasional rapid enlargement; compression of the surrounding tissues by the mass could cause symptoms such as hoarseness, dyspnea and dysphagia; on CT or MRI enhancement, a well-defined and clearly enhanced mass was seen [1, 11, 12]. In this paper, six cases were in middle-aged and elderly males who presented with multinodular masses in the floor of the mouth area, which were basically consistent with the clinical features of A-RM; while one case was in a 4-year-old girl, which is rare in children and less than 10 cases have been reported in the literature so far [4, 13–18].
A-RM is a dark brown or brownish-red mass, generally 1.5–7.5 cm in diameter, soft or rubbery in texture, lobulated or nodular in shape, and without obvious capsule. The tumor cells are surrounded by reticular fibers and have thin capillaries, which are arranged in a lobular pattern. The tumor cells have clear borders, large cell bodies, eosinophilic fine granular or vacuolated cytoplasm, and contain variable amounts of lipids and glycogen. PAS staining was positive, while glycogen was digested by diastase and PAS staining was negative after digestion. The tumor cells expressed desmin, MSA, Myogenin and Myoglobin, focally expressed vimentin, SMA and S-100 proteins and did not express CK and CD68 [1, 8, 11, 19]. In our study, all seven cases of A-RM immunohistochemical staining were strongly positive for desmin, Myogenin and Myoglobin, focally positive for SMA, scattered weakly positive for S-100 or not, and did not express CK, vimentin and CD68; together with the characteristic HE histology and cell morphology, they often led to a clear diagnosis.
A-RM should be diagnosed clinically differently from adenoid cystic carcinoma, granulosa cell tumor, nerve sheath tumor, dermatomal/epithelioid cyst, and vascular malformation. The patient has no obvious pain and discomfort or only pressure symptoms, and the disease duration is long, and the mass is still clear, which is not consistent with the malignant tumor characteristics of adenoid cystic carcinoma, and benign lesions are more likely; clinical examination of the mass is soft and clear, and there are no obvious adhesions with surrounding glands and tissues, so it needs to be differentiated from granulosa cell tumor and nerve sheath tumor, etc.; intraoperative realistic masses are found, and cystic lesions are excluded, and solid benign tumors are considered, pending pathological results.
Differential diagnosis: (1) Granular cell tumor: Both are inconspicuous masses without capsule, which occur in the head and neck, especially in the tongue, and both have eosinophilic polygonal cells. However, granular cell tumor cells have indistinct borders, relatively small cell bodies, and no vacuoles, cross-striations, or rod-shaped crystals in the cytoplasm; eosinophilic granules in the cytoplasm are resistant to diastase digestion, so PAS staining remains positive after digestion; immunohistochemical markers are strongly positive for S-100 protein and CD68 protein, but negative for desmin, Myoglobin, MyoD1, Myogenin, etc. [20]. (2) Hibernoma: It is a rare benign adipose tissue tumor and must be differentiated from A-RM if it is eosinophilic type. Hibernoma cells are small and round, eosinophilic and vacuolated cells are seen, with abundant, granular cytoplasm containing many lipid droplets of different sizes, without cross-striations and rod-shaped crystals; nuclei are small and central; mature adipocytes and adipoblast-like cells are seen; special staining is positive for oil red O and negative for PAS and PTAH; immunohistochemical markers show variable degrees of positive expression of S-100 protein, while desmin, Myoglobin, MyoD1 and Myogenin are negative. (3) Acinic cell carcinoma: It is more common in females and occurs in the salivary glands, with the parotid gland being the most common. The tumor is composed of densely arranged round or polygonal cells with cytoplasm rich in microbasophilic zymogen granules, and sometimes a large number of eosinophilic tumor cells can be seen; PAS is positive and resistant to diastase digestion; immunohistochemically labeled tumor cells express CK, Dog-1, NR4A3 and other proteins, and occasionally S-100 is positive, while desmin, Myoglobin, MyoD1, Myogenin, etc. are negative [21, 22]. (4) Rhabdomyosarcoma: It is the most common soft tissue sarcoma occurring in children and adolescents, mainly in the head and neck. The tumor is mostly composed of small round or short spindle-shaped cells, and sometimes eosinophilic red-stained material or cross-striations are seen in the cytoplasm of the tumor cells, suggesting a rhabdomyosarcoma origin; however, poorly differentiated tumor cells, marked pleomorphism and mitotic figures are common in rhabdomyosarcoma [23]. In this paper, one case was a 4-year-old child and expressed myogenic antibodies, so it had to be distinguished from well-differentiated rhabdomyosarcoma. In this case, the tumor had circumscribed borders, no adhesions to surrounding tissues, distinct tumor cell borders, cross-striations and vacuoles in the cells, no cellular anisocytosis or mitosis, PAS (+), PTAH (+), desmin (++), Myoglobin (+++), MyoD1 (+++), so the diagnosis of A-RM was confirmed.
The treatment for A-RM is complete surgical excision, and local extensive resection is not recommended. The tumor is not invasive or metastatic and has no malignant potential [3]. Incomplete resection may lead to recurrence, but even when it recurs, its growth rate is very slow, ranging from 10 years to 35 years [24–29]. In this paper, seven cases were followed up for 0.1 to 14 years without recurrence, respectively.