Adenoid cystic carcinoma of the trachea

Key points

• Second most common primary tracheal malignancy (after SCC); most common salivary gland-type tracheal tumour
• Hallmark: extensive submucosal and perineural spread - tumour extends far beyond the visible mass
• Smooth or lobulated intraluminal polypoid/circumferential mass, often deceptively well-defined
• Slow-growing but relentless; late haematogenous metastases (lung most common)
• Surgical margins frequently positive due to submucosal extent - wide resection required

Epidemiology & risk factors

• Peak age: 40-60 years; slight female predominance
• No association with smoking (unlike tracheal SCC)
• Arises from submucosal glands (minor salivary gland origin)
• Also seen in major salivary glands, paranasal sinuses, lacrimal glands

Pathophysiology

• Originates from intercalated duct cells of submucosal seromucinous glands
• Three histological patterns: cribriform (classic, "Swiss cheese"), tubular, solid
• Solid pattern carries worst prognosis
• Perineural invasion is near-universal - tracks along nerve sheaths for long distances
• Submucosal spread extends longitudinally well beyond the macroscopic tumour margin

Clinical features

• Progressive dyspnoea, wheeze, stridor - often misdiagnosed as asthma for months/years
• Haemoptysis (less common than in SCC)
• Dysphagia if oesophageal involvement
• Slow progression; symptoms appear late due to indolent growth
• Recurrence common even after apparent complete resection (perineural spread)

Imaging

CT

• Smooth or lobulated circumferential/eccentric tracheal wall thickening
• Intraluminal polypoid mass with narrowing of the tracheal lumen
• Longitudinal extent on coronal/sagittal reformats exceeds the focal mass - key finding reflecting submucosal spread
• Homogeneous soft tissue density; moderate enhancement post-contrast
• May extend through the tracheal wall into mediastinal fat
• Calcification uncommon (cf. TPO, chondrosarcoma)
• Lymphadenopathy uncommon at presentation

MRI

• T1: isointense to muscle
• T2: intermediate to high signal
• Better than CT for assessing submucosal longitudinal extent and perineural spread
• Useful for surgical planning

Bronchoscopy

• Smooth, well-vascularised submucosal mass
• Overlying mucosa often intact (submucosal origin)
• Biopsy may be superficially negative due to intact mucosal cover - deep biopsy required

Differentials

• Tracheal SCC - exophytic/irregular, older male smokers, upper trachea, more aggressive locally
• Mucoepidermoid carcinoma - also salivary gland type, younger patients, more bronchial than tracheal
• Tracheobronchial amyloidosis - diffuse circumferential thickening, often calcified, no focal mass
• Relapsing polychondritis - spares posterior membranous wall, cartilage destruction/calcification
• Carcinoid - endobronchial > tracheal, well-vascularised, younger patients

Management

• Surgical resection with wide margins is mainstay (tracheal sleeve resection)
• Adjuvant radiotherapy standard due to high rate of positive margins
• Chemotherapy role limited; reserved for metastatic/unresectable disease
• Indolent course - prolonged survival even with incomplete resection or metastases
• 5-year survival ~60-75%; 10-year survival drops significantly (late recurrences)