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Adenoid cystic carcinoma

Adenoid cystic carcinoma is the second most common malignancy of minor and parotid salivary glands, with the most common presentation in the sinonasal region. Clinical presentation varies by location but often includes local mass effect or perineural symptoms.

Key Images

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Look For First
  • Homogeneous enhancement on T1 post-gadolinium MRI in a salivary gland or sinonasal location
  • T2 signal characteristics: low-grade tumors slightly hyperintense, high-grade tumors markedly hypointense
  • Perineural spread along cranial nerve VII (facial nerve) on MRI
  • Well-defined margins in low-grade disease versus infiltrative borders in high-grade tumors
Key Image Findings
  • On MRI T1-weighted sequences, adenoid cystic carcinomas are hypo- to isointense relative to normal salivary gland tissue.
  • On T2-weighted sequences, low-grade tumors show slight hyperintensity, while high-grade tumors demonstrate marked hypointensity, reflecting increased cellularity and stromal fibrosis.
  • Homogeneous enhancement after gadolinium administration is a characteristic feature of both low- and high-grade subtypes.
  • Low-grade adenoid cystic carcinomas typically display well-defined, circumscribed margins that may simulate benign tumors.
  • High-grade adenoid cystic carcinomas show infiltrative, poorly demarcated borders reflecting aggressive biological behavior.
  • Perineural spread is a hallmark feature best appreciated on MRI, with propensity for invasion along the facial nerve (CN VII) and other cranial nerves.
  • Adenoid cystic carcinomas arise more commonly in minor salivary glands (55%) compared to major glands, with the sinonasal region being the most frequent site of origin.
  • The tumor demonstrates local aggressiveness with a known proclivity for perineural invasion, which influences staging, treatment planning, and prognosis.
Differential Diagnosis
  • Mucoepidermoid carcinoma: most common minor salivary gland malignancy; distinguished by variable internal heterogeneity, calcification, and different T2 signal patterns depending on grade.
  • Pleomorphic adenoma: benign mixed tumor with similar well-defined appearance in low-grade ACC; differentiation requires clinical context and absence of perineural spread.
  • Acinic cell carcinoma: low-grade tumor with usually better-defined margins and different histology; shows similar homogeneous enhancement but different T2 characteristics.
  • Carcinoma ex pleomorphic adenoma: history of long-standing benign tumor with recent rapid growth helps distinguish; may show similar infiltration but with preexisting adenoma component.
  • Squamous cell carcinoma: typically shows more heterogeneous enhancement and different location in sinonasal cavity; lacks characteristic perineural spread pattern of ACC.
  • Salivary duct carcinoma: high-grade malignancy with more aggressive imaging appearance, greater necrosis, and nodal involvement at presentation.
Discussion

Adenoid cystic carcinoma demonstrates a strong propensity for perineural invasion, which is a crucial prognostic factor and should be actively sought on MRI as it upstages disease and influences multimodal treatment planning.

The imaging appearance correlates with histologic grade: low-grade tumors may mimic benign lesions with well-defined margins and slight T2 hyperintensity, requiring clinical correlation and close follow-up to detect aggressive behavior.

High-grade adenoid cystic carcinomas show markedly reduced T2 signal intensity due to increased cellularity and desmoplastic stromal reaction, which can be a helpful imaging clue to grade assignment.

The homogeneous enhancement pattern helps distinguish adenoid cystic carcinoma from other salivary malignancies that may show heterogeneous enhancement due to necrosis, hemorrhage, or cystic change.

Minor salivary gland origin (~55%) is more common than major gland involvement, and sinonasal adenoid cystic carcinomas are the most frequent salivary origin tumors in this region.

Facial nerve involvement (CN VII perineural spread) has major surgical implications and requires detailed imaging description to guide head and neck surgical planning.

Reporting Pearls

When reporting adenoid cystic carcinoma, emphasize the presence or absence of perineural spread with specific mention of facial nerve involvement, describe the margins as well-defined versus infiltrative to indicate grade, and note the characteristic homogeneous enhancement pattern to support the diagnosis in your structured report.

Pitfalls
  • Mistaking low-grade adenoid cystic carcinoma with well-defined margins and slight T2 hyperintensity for a benign pleomorphic adenoma; clinical presentation, perineural spread, and imaging follow-up are essential to establish malignant diagnosis.
  • Underestimating the presence of perineural spread by not carefully evaluating the course of cranial nerve VII on thin-section MRI, which can result in incomplete surgical planning and undertreatment.
  • Confusing the markedly hypointense T2 signal of high-grade tumors with fibrotic benign lesions such as schwannoma; the combination of T1 hypo-intensity, marked T2 hypointensity, and homogeneous gadolinium enhancement is more typical of high-grade ACC.
  • Failing to recognize adenoid cystic carcinoma in the sinonasal region where it is the most common salivary origin malignancy; maintaining awareness of location-specific prevalence improves diagnostic accuracy.
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