Salivary gland oncocytoma

• Well-circumscribed, solid mass with avid homogeneous enhancement on contrast-enhanced CT or MRI
• Relatively low T2 signal (iso- to hypointense relative to parotid tissue) on MRI despite benign nature
• Possible nonenhancing curvilinear central cleft on CT, representing an internal fibrous scar
• Well-defined noninfiltrative margins without perineural spread or nodal involvement

• Ultrasound: Ovoid or mildly lobulated, well-defined, usually homogeneous hypoechoic mass with often prominent vascularity; cystic or hemorrhagic change may introduce heterogeneity.
• CT: Sharply marginated, avidly enhancing solid mass that is frequently homogeneous but may show heterogeneous enhancement; a nonenhancing curvilinear central cleft represents internal fibrous scarring.
• T1 MRI: Mass is typically hypointense relative to the normally fatty parotid gland parenchyma.
• T2/STIR MRI: Characteristic iso- to hypointense signal relative to parotid tissue, attributed to high cellularity and low free-water content; signal intensity may increase with cystic or hemorrhagic degeneration.
• Postcontrast MRI: Usually demonstrates avid, relatively homogeneous enhancement with fat-suppressed T2 and postcontrast T1 sequences; some lesions become isointense to native parotid producing the distinctive 'vanishing parotid mass' appearance.
• DWI/ADC: Generally shows less diffusion restriction and higher ADC values (mean ~1.06 × 10⁻³ mm²/s) compared to Warthin tumor, though overlap occurs.
• PET/FDG and Tc-99m pertechnetate scintigraphy: Oncocytomas may be intensely FDG-avid and show strong radionuclide uptake despite benign histology, findings that are shared with Warthin tumor.
• No concerning features: Absence of perineural spread, extraparenchymal invasion, or suspicious lymphadenopathy supports benign diagnosis.

• Warthin tumor: Most important mimic; key distinguishing feature is that Warthin tumor typically shows greater DWI signal and lower ADC values, though substantial overlap in enhancement, multiplicity, FDG avidity, and radionuclide uptake makes histopathologic confirmation necessary.
• Other benign salivary neoplasms (pleomorphic adenoma, myoepithelioma): Can mimic oncocytoma imaging appearance; pleomorphic adenoma more likely shows fatty or myxoid components with higher T2 signal.
• Malignant salivary tumor: Distinguished by perineural invasion, extraparenchymal spread, ill-defined margins, fixation, and ipsilateral lymphadenopathy.
• Parotitis or inflammatory mass: Clinical context (acute vs. chronic presentation) and absence of systemic symptoms help differentiate; inflammatory lesions typically less sharply defined.
• Hemangioma: Usually shows more extensive flow voids and higher T2 signal; well-defined but typically more hyperenhancing on arterial phase.
• Lymphoma involving parotid: Often bilateral or multifocal with less avid enhancement and higher T2 signal than oncocytoma; associated systemic findings may be present.

Describe the lesion as a 'well-circumscribed, avidly enhancing parotid mass with relatively low T2 signal intensity and homogeneous enhancement, consistent with oncocytoma; note the absence of perineural spread, extraparenchymal invasion, and suspicious lymphadenopathy.' If present, specifically mention 'a nonenhancing central cleft, consistent with internal scarring.' Consider recommending histopathologic confirmation if clinical suspicion for malignancy exists.

• Mistaking the 'vanishing parotid mass' appearance (isointense to parotid on fat-suppressed T2 and postcontrast T1) for absence of a lesion; carefully review precontrast sequences and early arterial phase images.
• Assuming intense FDG avidity or strong Tc-99m uptake excludes benign diagnosis; both are characteristic of oncocytoma and do not indicate malignancy.
• Relying on imaging alone to definitively distinguish oncocytoma from Warthin tumor; substantial overlap in enhancement patterns, multiplicity, and metabolic activity necessitates histopathology.
• Overlooking subtle perineural invasion or extraparenchymal extension by not carefully reviewing axial and coronal T1 fat-saturated postcontrast sequences along facial nerve anatomy.