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Primary tumors of the trachea are rare and constitute only 2% of all respiratory tract tumors [1]. Of these, malignant tumors are more common than benign tumors and account for 60–83% in adults [2]. Adenoid cystic carcinoma, which was formerly named “cylindroma” and “adenocystic carcinoma,” occurs mainly in the salivary glands or central airways, such as the trachea and the main bronchi; it is a low-grade malignancy that is the second most common tracheal malignancy at histology (33%) after squamous cell carcinoma (48%) [3].

With the advent of MDCT, rapid scanning throughout the thorax and multiplanar reconstruction without loss of z-axis information are possible; therefore, visualization of the airways has become easier. The purpose of this pictorial essay is to describe the helical CT findings of adenoid cystic carcinomas of the airways and to correlate these findings with the histopathology.

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Adenoid cystic carcinomas are usually first recognized without sex predilection in patients in their 40s [4], and smoking does not affect the incidence [5]. Symptoms in patients with adenoid cystic carcinoma are usually related to airway obstruction. Dyspnea, cough, stridor, wheezing, and hemoptysis are the most common complaints. Unfortunately, these signs and symptoms often lead to a misdiagnosis as asthma or bronchitis. Recognizing adenoid cystic carcinoma is important because patient prognosis is improved with early treatment [6].

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Adenoid cystic carcinomas usually arise in the lower trachea (Figs. 1A, 1B, 1C and 2A, 2B, 2C). Others are found in the mainstem bronchi (Figs. 2A, 2B, and 2C); lobar bronchi (Figs. 3A, 3B and 4A, 4B, 4C); or, rarely, in the segmental bronchi (Figs. 5A, and 5B) and extrathoracic trachea (Figs. 6A, 6B, and 6C).

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Fig. 1A. —Adenoid cystic carcinoma in 33-year-old man. Unenhanced transaxial CT scan (2.5-mm collimation) obtained at level of thoracic inlet shows tracheal tumor of irregular contour and broad base with both intraluminal (arrow) and extraluminal (arrowheads) components. More than 180° of tracheal circumference is involved.

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Fig. 1B. —Adenoid cystic carcinoma in 33-year-old man. Sagittal reformatted image (2.5-mm thickness) shows both intraluminal (arrow) and extraluminal (arrowheads) components of tracheal tumor and longitudinal extent of lesion.

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Fig. 1C. —Adenoid cystic carcinoma in 33-year-old man. Photomicrograph of transaxial section of gross specimen shows intraluminal (arrows) and extraluminal (arrowheads) components of tumor. (H and E, ×1)

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Fig. 2A. —Adenoid cystic carcinoma in 60-year-old man. Unenhanced transaxial CT scan (3-mm collimation) at level of distal trachea that was obtained using mediastinal window settings shows lobulated and broad-based intraluminal soft-tissue tumor (arrow) in anterolateral aspect of distal trachea. Also note marked wall thickening (arrowheads). More than 180° of circumference is involved.

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Fig. 2B. —Adenoid cystic carcinoma in 60-year-old man. Coronal reformatted CT scan (2.5-mm thickness) shows both intraluminal and extraluminal (arrowheads) growth of tumor. Longitudinal extent of lesion involving distal trachea (solid arrow) and right main bronchus (open arrow) is clearly visualized.

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Fig. 2C. —Adenoid cystic carcinoma in 60-year-old man. Photomicrograph of longitudinal section of gross surgical specimen shows intraluminal (arrows) and extraluminal (arrowheads) tumor growth (predominantly, intraluminal growth) with multisegmental involvements of tracheal cartilage. (H and E, ×1)

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Fig. 3A. —Adenoid cystic carcinoma in 41-year-old man. Coronal reformatted image of enhanced CT scan (2.5-mm thickness) shows soft-tissue mass (arrow) obliterating right upper lobar bronchus and attendant atelectasis of right upper lobe (arrowheads).

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Fig. 3B. —Adenoid cystic carcinoma in 41-year-old man. Enhanced transaxial CT scan (2.5-mm collimation) obtained at level of liver dome shows metastatic lesions (arrows) in liver. Metastases were proven with percutaneous liver biopsy.

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Fig. 4A. —Adenoid cystic carcinoma in 40-year-old man. Enhanced transaxial CT scan (2.5-mm collimation) obtained at level of left inferior pulmonary veins shows soft-tissue lesion (arrow) encircling bronchus intermedius. Right lower lobe (arrowhead) is partially atelectatic.

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Fig. 4B. —Adenoid cystic carcinoma in 40-year-old man. Coronal reformatted CT image (2.5-mm thickness) shows longitudinal extent of lesion (arrows) encircling bronchus intermedius. Right lower lobe (arrowhead) is partially atelectatic.

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Fig. 4C. —Adenoid cystic carcinoma in 40-year-old man. Photomicrograph of longitudinal section of bronchus intermedius shows intraluminal growth of tumor (arrows), obliterating distal bronchial lumen of bronchus intermedius. (H and E, ×1)

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Fig. 5A. —Adenoid cystic carcinoma in 43-year-old man. Enhanced transaxial CT scan (2.5-mm collimation) obtained at level of middle lobe bronchus shows soft-tissue tumor (arrow) filling lingular segmental bronchus.

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Fig. 5B. —Adenoid cystic carcinoma in 43-year-old man. Photomicrograph of transaxial section of gross surgical specimen shows transluminal tumor with predominantly extraluminal growth pattern (arrows). L = bronchial lumen. (H and E, ×1)

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Fig. 6A. —Adenoid cystic carcinoma in 50-year-old woman. Enhanced transaxial CT scan (5.0-mm collimation) obtained at level of thyroid glands shows lobulated and broad-based soft-tissue lesion (arrows) in right posterolateral portion of trachea with both intraluminal and extraluminal components. Less than 180° of circumference is involved.

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Fig. 6B. —Adenoid cystic carcinoma in 50-year-old woman. Parasagittal reformatted CT image (5.0-mm thickness) shows longitudinal extent of lesion (arrow) mainly located in posterior portion of trachea.

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Fig. 6C. —Adenoid cystic carcinoma in 50-year-old woman. High-magnification photomicrograph shows sheets of small uniform cells arranged in classic cribriform growth pattern. (H and E, ×100)

Routine posteroanterior and lateral chest radiographs are usually interpreted as showing normal findings [7]; however, intraluminal filling defect with irregular, smooth, or lobulated contours may be identified on radiographs. In addition, the extraluminal component may be visualized if it is large enough to distort the normal mediastinal contour [8].

For the adequate evaluation of the airways on helical CT, imaging parameters of 1- to 3-mm section thickness, 1- or 2-mm reconstruction increment, total active detector length pitch of 1.3–1.5, 120 kVp, and 180–250 mA are used [9]. Using these image data sets, transaxial images, multiplanar reconstruction images, volume- and surface-rendered 3D images are acquired.

On CT, the tumor has a striking tendency toward submucosal extension that manifests as an intraluminal mass of soft-tissue attenuation with extension through the tracheal wall (Figs. 1A, 1B, 1C, 2A, 2B, 2C, and 6A, 6B, 6C), a diffuse or circumferential wall thickening of the trachea, a soft-tissue mass filling the airway (Figs. 3A, 3B, 4A, 4B, 4C, 5A, and 5B), or a homogeneous mass encircling the trachea with wall thickening in the transverse and longitudinal planes [10, 11]. The longitudinal extent of the tumor is greater than its transaxial extent [4] (Figs. 1A, 1B, 1C, 4A, 4B, 4C, and 6A, 6B, 6C). The tumors usually involve more than 180° of the airway circumference (Figs. 1A, 1B, 1C and 2A, 2B, 2C). They are variable in shape: polypoid (Figs. 5A, and 5B) or broad-based (Figs. 1A, 1B, 1C, 2A, 2B, 2C, 4A, 4B, 4C, and 6A, 6B, 6C). Their margins are also variable: smooth (Figs. 5A, and 5B), lobulated (Figs. 2A, 2B, 2C and 6A, 6B, 6C), or irregular (Figs. 1A, 1B, and 1C). Calcification within tumor is rare [10, 11].

Although the most common site of the tumor in the trachea has been reported to be the posterolateral wall (Figs. 6A, 6B, and 6C), the anterolateral wall (Figs. 1A, 1B, and 1C)—near the junction of the cartilage and the soft membranous parts of the trachea where mucous glands are most plentiful—may also be a common site of involvement [10, 11].

Adenoid cystic carcinoma in the extrathoracic trachea may directly invade the thyroid gland and tracheal cartilages [6]. Distant metastasis occurs late in the disease course (Figs. 3A and 3B). Metastasis to the regional lymph nodes may be present in up to 10% of primary tumors at the time of diagnosis [7]. With the tumor in the lobar or segmental bronchus, distal atelectasis (Figs. 3A, 3B and 4A, 4B, 4C) or obstructive pneumonitis may be present. Air trapping may also be present.

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Grossly, adenoid cystic carcinoma is usually smooth, firm, and well circumscribed (Figs. 1A, 1B, and 1C). It may present as an exophytic lesion (Figs. 5A, and 5B), often with poorly defined margins (Figs. 2A, 2B, and 2C). The surface of the tumor may be smooth or ulcerated. A bulky extratracheal mass may often be seen, and direct invasion of the thyroid gland does occur [7].

Microscopically, adenoid cystic carcinomas contain sheets of small uniform cells arranged in a classic cribriform growth pattern (Figs. 6A, 6B, and 6C) or in a tubular pattern. A solid component without cystic spaces is reported to be associated with more locally aggressive tumors, which exhibit a tendency to spread submucosally along perineural spaces or along the perineural lymphatics [7].

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Spizarny et al. [10] expressed a concern that CT is a poor indicator of tumor longitudinal extent and mediastinal organ invasion. However, with the use of helical CT data sets, multiplanar reconstructions have been shown to facilitate the assessment of patients with airway disease and are known to provide various advantages in terms of image quality [12]. The reformatted images help to assess both the intra- and extraluminal growth of the tumor and its longitudinal extent along the tracheal or bronchial wall by allowing the evaluation of extraluminal surrounding tissues (Figs. 1A, 1B, 1C, 2A, 2B, 2C, 4A, 4B, 4C, and 6A, 6B, 6C). Therefore, helical CT provides precise information about the extent of a tumor, which is important for planning a surgical resection. Tumor sizes and growth patterns and the presence of pulmonary parenchymal involvement identified on CT correspond to those seen at pathology [12]. However, helical CT is still a poor predictor of microscopic mediastinal fat or neural invasion [10].

In summary, adenoid cystic carcinomas, which are usually located in the intrathoracic distal trachea, tend to exhibit longitudinal extension and both intra- and extraluminal growth on helical CT. These findings correlate with the histopathologic findings in most patients. In many cases, CT can be used to assess the tumor extent and growth patterns.