ACC arises more commonly in the minor salivary glands and in the seromucinous glands of the upper respiratory tract. Tracheal tumors mostly arise in the lower or upper third, with a tendency to originate at the lateral and posterolateral wall near the junction of the cartilaginous and membranous portions.[9
Pathologically these tumors characteristically grow into the airway lumen, forming a smooth surfaced, somewhat polypoid tumor; occasionally, growth is circumferential and annular. Submucosal extension, sometimes to a considerable distance from main tumor is not uncommon. Histologically, three patterns are seen; trabecular, cribriform and solid type. The cribriform pattern is most common consisting of uniform cells with relatively little cytoplasm arranged in well-defined nests of variable size. The cells in these nests are separated by well-defined cystic spaces containing a mucinous substance that stains strongly with alcian blue and weakly with Periodic Acid - Schiff (PAS).[10
Most ACCs are discovered in middle age with no gender predilection. The usual clinical presentation is directly related to the size and location of the tumor within trachea. These tumors may grow to near obstructive level and produce cough, hoarseness, wheezing, dyspnoea, hemoptysis and recurrent pneumonitis.[8
] ACC spreads by direct extension, perineural invasion and hematogenous metastasis. Lymphatic spread is uncommon.[10
] Direct extension of ACC of the laryngotracheal complex into the thyroid with clinical manifestation as a thyroid nodule has been rarely reported previously.[4
] In our case also, the tumor has infiltrated between the cartilaginous plates of trachea, muscle and soft tissues of the neck and formed rounded midline swelling in the neck simulating a thyroid tumor without any respiratory symptoms. A case of metastatic ACC of thyroid from an unknown primary presenting as thyroid swelling is also reported.[11
Extension of an ACC of the larynx and trachea to the thyroid with manifestation as a thyroid nodule is extremely rare. Idowu et al
] reported two cases of ACC arising in the laryngotracheal complex and involving the thyroid gland by direct extension. In both cases, the initial clinical manifestation was a suspected thyroid mass for which FNAC was performed and papillary carcinoma was cytological diagnosis in one case. Although FNAC is accepted widely as most accurate, sensitive, specific and cost effective diagnostic procedure in the assessment of thyroid nodules, difficulties might arise at times when classical features of ACC are absent or subtle.
Cytological features of ACC are high cellularity with cribriform or trabecular pattern. Cytoplasm is scanty and basaloid. Nucleus is oval to angulate with coarse chromatin, small indistinct nucleoli and no inclusions. The background element may show hyaline globules. These features usually differentiate ACC from other common primary thyroid neoplasms. If enough aspirates are obtained, immunohistochemical analysis is helpful. Primary thyroid neoplasm will express thyroglobulin, thyroid transcription factor-1, and/or calcitonin, while these antibodies will be negative in ACC and most other tumors of extra thyroid origin. In addition, myoepithelial components of ACC express muscle-specific actin and, occasionally S-100, which usually are absent in thyroid neoplasms.[6
The CT scan is a useful imaging procedure for ACC. It is highly accurate in the assessment of the tumor location, extra luminal extensions, carinal involvement and distant metastasis.[9
] With the use of helical CT data sets, multiplannar reconstructions have been shown to facilitate the assessment of patients with airways disease and are known to provide various advantages in terms of image quality. The reformatted images help to assess both the intra and extra luminal growth of the tumor and its longitudinal extent along the tracheal or bronchial wall by allowing the evaluation of extra luminal surrounding tissues.[12
] Therefore, helical CT provides precise information about the extent of a tumor, which is important for planning a surgical resection. Unfortunately, this investigation could not be done in our case.
In conclusion, while evaluating a thyroid nodule, one must be vigilant for tumors from extra-thyroidal sites and ACC should be included in the differential diagnosis of midline swelling neck. In patients with unusually aggressive cervical region tumors, a history of previous or distant malignancy, or atypical cytopathological features noted in the aspiration specimen, the possibility of a non-thyroidal neoplasm should be considered that might arise in adjacent structures, as illustrated by our case.