- Seen in middle-aged adults (peak age 30–60 years, 2:1 female predominance; accounts for 70–80% of all parotid gland tumours and is the most common salivary gland neoplasm overall), presenting with a painless, slow-growing, firm, mobile mass at the angle of the mandible or pre-auricular cheek.
- Caused by a benign biphasic neoplasm composed of epithelial/myoepithelial ductal elements and abundant myxoid/chondromyxoid stroma:
- 1.Pseudocapsule & Pseudopods: Surrounded by a fibrous pseudocapsule penetrated by microscopic finger-like tumour projections ('pseudopods'); simple surgical enucleation leaves residual pseudopods behind and causes a high rate of multifocal recurrence.
- 2.Malignant Transformation ('Carcinoma ex Pleomorphic Adenoma'): Carries a 5–10% cumulative risk of malignant degeneration if left untreated over 10–15 years, heralded clinically by sudden rapid growth, pain, skin fixation, or facial nerve (CN VII) palsy.
Imaging Findings
- Ultrasound (First-Line Salivary Gland Modality):
- –Classic Appearance: Well-circumscribed, sharply marginated, lobulated hypoechoic mass with prominent posterior acoustic enhancement and a bosselated contour, most commonly in the superficial lobe of the parotid gland.
- –Doppler & Calcification: Typically hypovascular or shows sparse peripheral basket-like vascularity on Colour Doppler; long-standing lesions may show focal punctate dystrophic calcification.
- MRI Parotid & Neck (Gold Standard for Deep Lobe & Facial Nerve Mapping):
- –T1WI: Well-defined, homogeneously hypointense-to-isointense lobulated mass contrasting sharply against the hyperintense fatty parotid parenchyma.
- –T2WI ('Lightbulb Bright' Hallmark): Intensely hyperintense ('lightbulb bright') on T2WI and STIR due to abundant myxoid/chondromyxoid stroma, classically delineated by a complete, thin T2-hypointense fibrous capsule.
- –T1 C+ & Dynamic Contrast-Enhanced (DCE) MRI: Demonstrates slow, progressive, persistent enhancement (Type A time-intensity curve with gradual wash-in and no early washout).
- –DWI / ADC: Shows facilitated diffusion (no diffusion restriction) throughout the myxoid stroma; a new focal area of marked diffusion restriction, disruption of the T2-dark capsule, or infiltrative margins is a critical red flag for Carcinoma ex Pleomorphic Adenoma.
- –**SWI / T2* GRE: Typically shows no blooming artifact** in uncomplicated tumours; occasional coarse calcific blooming in chronic lesions, whereas irregular hemorrhagic blooming suggests malignant transformation.
- –Facial Nerve Plane & Stylomandibular Tunnel: The intraparotid facial nerve courses immediately lateral to the retromandibular vein; deep-lobe tumours extend medially through and widen the stylomandibular tunnel into the prestyloid parapharyngeal space ('dumbbell tumour').
- CT & Plain Radiograph (X-Ray) Findings:
- –CT Neck: Well-circumscribed lobulated soft-tissue mass with delayed contrast enhancement; best demonstrates coarse dystrophic calcifications and bony remodelling/widening of the stylomandibular tunnel.
- –Plain Radiograph (X-Ray): Rarely required except to exclude radiopaque submandibular/parotid duct calculi (sialolithiasis); chronic calcified pleomorphic adenoma can occasionally mimic a salivary calculus on plain film.
Differential Diagnosis
| Differential Diagnosis | Key Distinguishing Features |
|---|---|
Warthin Tumour | Seen in older male smokers (50–70 years); classically in the parotid tail, bilateral/multifocal in 10–15%; shows T1-bright proteinaceous cysts, low-to-intermediate T2 signal, marked diffusion restriction on DWI, and rapid washout on DCE-MRI. |
Mucoepidermoid / Adenoid Cystic Carcinoma | Low-grade mucoepidermoid carcinoma may mimic benign lesions; high-grade malignancies show ill-defined infiltrative margins, low T2 signal, diffusion restriction, and perineural spread along CN VII (stylomastoid foramen) or CN V3 (foramen ovale). |
Carcinoma ex Pleomorphic Adenoma | Long-standing parotid mass with sudden rapid enlargement or facial palsy; MRI shows focal loss of the T2-hypointense capsule, invasive margins, necrosis, and new focal diffusion restriction within a previously T2-bright mass. |
Schwannoma of Intraparotid Facial Nerve | Tubular mass tracking directly along the intraparotid CN VII canal and stylomastoid foramen; may show the 'target sign' on T2WI without widening the stylomandibular tunnel. |
Important Points to Remember
- Diagnostic Workup (UK NICE / RCR Guidelines): High-resolution Ultrasound combined with US-guided Fine-Needle Aspiration Cytology (FNAC) or core needle biopsy is the mandatory first-line investigation; MRI is performed prior to surgery for large (>3 cm), deep-lobe, recurrent, or suspected malignant lesions.
- Surgical Standard (Never Enucleate): Treated with superficial or total conservative parotidectomy with intraoperative facial nerve monitoring; simple enucleation is contraindicated because microscopic pseudopods extending through the capsule lead to multifocal recurrence ('grape-like' clusters) in the surgical bed.
- Rule of Salivary Gland Malignancy: While 80% of parotid tumours are benign (predominantly pleomorphic adenoma), the proportion of malignancy increases inversely with gland size (~50% malignant in submandibular gland, 70–80% malignant in sublingual and minor salivary glands).