What is the recommended procedure for conducting a Fine Needle Aspiration Cytology (FNAC) of the parotid gland?

How to Conduct FNAC of the Parotid Gland

For parotid gland masses, perform ultrasound-guided fine needle aspiration cytology (FNAC) using a 21-25 gauge needle with real-time ultrasound visualization, though ultrasound-guided core needle biopsy (USCB) with an 18-gauge needle should be strongly considered when FNAC is non-diagnostic or when specific tissue architecture is needed for diagnosis. 1, 2, 3

Pre-Procedure Imaging and Assessment

Perform neck ultrasound as the initial imaging modality to characterize the parotid mass, assess size, location, and relationship to surrounding structures including the facial nerve. 1 Ultrasound is superior to clinical examination alone in 31% of cases for defining lesion characteristics. 2

• Add CT with intravenous contrast if there is suspicion of adjacent bone involvement or deep lobe extension. 1
• Consider MRI with diffusion sequences if perineural invasion or skull base involvement is suspected. 1

FNAC Technique

Patient Positioning and Preparation

• Position the patient supine with the neck slightly extended and head turned away from the side being biopsied.
• Apply proper skin disinfection according to institutional protocol before needle insertion. 4

Needle Selection

• Use 21-25 gauge needles for standard FNAC, as these provide adequate cellular material while maintaining maneuverability. 1, 5
• The 25-gauge needle offers better flexibility and is particularly useful for superficial or easily accessible lesions. 1

Ultrasound-Guided Approach

• Perform real-time ultrasound guidance throughout the procedure to visualize needle trajectory and ensure accurate targeting of the lesion while avoiding the facial nerve and major vessels. 2, 3
• Insert the needle at an appropriate angle (typically 25-45 degrees) under direct ultrasound visualization. 4
• Target the periphery of the mass rather than the center, as the center is often more necrotic and yields non-diagnostic tissue. 1

Aspiration Technique

• Begin without suction initially - advance the needle into the lesion and move it back and forth 10-15 times within the mass using short, rapid strokes. 1
• Use a "fanning" technique by repositioning the needle at 4 different areas within the mass, sampling from left margin to right margin to increase diagnostic yield. 1
• If initial passes yield scant material, apply gentle suction (5-10 mL syringe) on subsequent passes, though this increases specimen bloodiness. 1

Number of Passes

• Perform 2-4 needle passes as the mean number required for diagnostic material is approximately 3 passes, with diagnostic yield plateauing after 7-8 passes. 6
• If an onsite cytopathologist is available, continue passes until adequacy is confirmed. 1

Specimen Handling

• Express the aspirate onto glass slides using the stylet for controlled expression. 1
• Prepare both air-dried smears (for Diff-Quik staining) and alcohol-fixed smears (for Papanicolaou staining).
• Request ancillary testing (immunohistochemical or molecular studies) when initial cytology is indeterminate to support diagnosis and assess malignancy risk. 1

Interpretation and Risk Stratification

• Pathologists should report results using a standardized risk stratification scheme with particular attention to high-grade features. 1
• FNAC has an overall sensitivity of 84.6%, specificity of 96.4%, and accuracy of 94.2% for parotid masses. 5
• The positive predictive value for malignancy is 84.6% and negative predictive value is 96.4%. 5

When FNAC is Non-Diagnostic

A critical caveat: FNAC has a non-diagnostic rate of 25.8% for parotid masses. 3 When this occurs:

• Proceed directly to ultrasound-guided core needle biopsy (USCB) with an 18-gauge needle rather than repeating FNAC. 2, 3, 7, 8
• USCB provides superior diagnostic accuracy (96.4% for benign lesions, 100% for malignant lesions) compared to FNAC and has a much lower non-diagnostic rate of only 4.5%. 3
• USCB is particularly valuable when tissue architecture assessment is needed for accurate tumor grading and staging, and in cases of diffuse gland enlargement. 2, 7

USCB Technique (When Indicated)

• Use an 18-gauge cutting needle with a 15-mm throw or specimen notch. 8
• Perform 1-3 passes (mean 2.43) under real-time ultrasound guidance. 8
• USCB achieves 97% accuracy with 83% sensitivity and 100% specificity for differentiating malignant from benign masses. 8

Safety Considerations and Complications

• The complication rate is extremely low - subclinical hematoma occurs in approximately 2% of cases with no sequelae. 3, 8
• There is no clinically significant risk of tumor seeding with proper technique. 7
• Monitor the patient for 15-30 minutes post-procedure for signs of hematoma or facial nerve injury (extremely rare).

Clinical Decision Algorithm

1. All parotid masses: Start with ultrasound imaging and clinical assessment
2. If surgical treatment is being considered: Perform ultrasound-guided FNAC with 21-25 gauge needle
3. If FNAC is non-diagnostic or indeterminate: Proceed to USCB with 18-gauge needle rather than repeat FNAC
4. If USCB confirms malignancy: This enables appropriate surgical planning including extent of parotidectomy and neck dissection
5. If both FNAC and USCB are non-diagnostic: Consider intraoperative frozen section, though this should not be used as a first-line diagnostic technique 7

Important pitfall to avoid: Do not rely solely on clinical examination or imaging to determine management - tissue diagnosis significantly alters management in the majority of cases and prevents unnecessary surgery in patients with benign disease. 2