ABSTRACT
Purpose
To evaluate the diagnostic accuracy of fine‐needle aspiration cytology (FNAC) for parotid gland lesions using the Milan System while eliminating verification bias, and to systematically compare risk of malignancy (ROM) between superficial and deep lobe lesions across all Milan categories.
Methods
A retrospective review was conducted of 332 patients who underwent ultrasound‐guided FNAC and subsequent surgical excision between 2019 and 2025. Cytology results were classified according to the Milan System. ROM values and diagnostic performance metrics were calculated for each category, with subgroup analysis according to lobe location. Final pathology results were available for all cases, ensuring complete diagnostic verification.
Results
Histopathology confirmed malignancy in 37 cases (11.1%), with significantly higher frequency in the deep lobe compared to the superficial lobe (24.6% vs. 7.9%, p < 0.001), representing a 3.1‐fold increased malignancy risk. Deep lobe lesions demonstrated mostly higher ROM across multiple Milan categories. Most notably, within the “benign neoplasm” category, deep lobe lesions exhibited a significantly higher ROM of 7.3% compared to 1.0% for superficial lobe lesions (p = 0.009), representing a 7.3‐fold increased malignancy risk even within this traditionally low‐risk cytological category. In contrast, high‐risk categories (suspicious for malignancy and malignant) showed similar ROM values regardless of anatomical location.
Conclusion
FNAC with Milan System classification provides reliable diagnostic performance for parotid lesions. However, anatomical location significantly influences malignancy risk, with deep lobe lesions carrying substantially higher ROM across multiple diagnostic categories.
Level of Evidence
3.
Keywords: fine‐needle aspiration, Milan System, parotid gland, risk of malignancy, verification bias
1. Introduction
Parotid gland tumors are the most common salivary gland neoplasms, with both benign and malignant subtypes. Fine‐needle aspiration cytology (FNAC) is widely used as a first‐line diagnostic tool due to its minimally invasive nature, low cost, and reasonable accuracy. However, interpretation of salivary gland cytology is challenging due to overlapping morphological features, and standardization is necessary for consistent reporting [1].
The Milan System for Reporting Salivary Gland Cytopathology (MSRSGC) provides a structured classification of cytological findings into seven diagnostic categories, each with an estimated risk of malignancy (ROM) [2]. The use of standardized reporting systems for cytopathology has made enormous gains in popularity during the past decade, and the Milan System has been widely adopted across European institutions [3, 4].
Current guidelines and knowledge gaps: The ESMO–EURACAN Clinical Practice Guideline offers key recommendations for managing salivary gland cancer, encompassing both clinical and pathological diagnosis; however, it notably lacks specific recommendations for lobe‐based diagnostic or management approaches. While preoperative cytologic diagnosis of parotid gland neoplasms was considered mandatory to decide which surgical procedure would be appropriate, current guidelines do not address whether ROM estimates should be adjusted based on anatomical location within the gland [5, 6]. Furthermore, the anatomical division of the parotid gland into superficial and deep lobes may influence diagnostic accuracy. Deep lobe lesions are more challenging to access and may yield fewer representative samples; however, data on lobe‐specific ROM values are scarce [7, 8, 9].
This study aimed to evaluate FNAC accuracy for parotid lesions using the Milan System, and to systematically compare ROM values between superficial and deep lobe lesions across all Milan categories—addressing a critical gap in current evidence‐based guidelines for parotid gland management.
2. Materials and Methods
2.1. Study Design and Patients
This retrospective cohort study included all patients who underwent ultrasound‐guided FNAC for parotid masses followed by surgical excision at our institution from 2019 to 2025, with histopathologic confirmation after removal. The study protocol was approved by the Institutional Review Board (Approval No.: TABED 1‐25‐1069). All procedures were conducted in accordance with the Declaration of Helsinki. The inclusion criteria were age ≥ 18 years, adequate cytological samples, and availability of histopathological results. Patients with revision cases, incomplete cytology or histopathology records, who had metastatic parotid disease, and pediatric cases were excluded. Anatomical location (superficial vs. deep lobe) was determined from imaging and surgical records.
2.2. Cytological Assessment
All FNAC procedures were performed under ultrasound guidance by experienced radiologists using a 23‐ to 25‐gauge needle. In line with standard protocols, aspirated material was smeared onto glass slides, air‐dried for Diff‐Quik staining, and alcohol‐fixed for Papanicolaou staining. Additional material was collected for cell block preparation when required. All cytology results were reviewed and categorized according to the Milan System for Reporting Salivary Gland Cytopathology (MSRSGC) into one of seven diagnostic categories: nondiagnostic, nonneoplastic, atypia of undetermined significance (AUS), benign neoplasm, salivary gland neoplasm of uncertain malignant potential (SUMP), suspicious for malignancy, and malignant.
2.3. Histopathologic Confirmation
Final histopathologic diagnoses were established by pathologists from surgical specimens, serving as the reference standard. Final pathology results were available for all cases, ensuring complete diagnostic verification.
2.4. ROM Analysis
For each Milan category, ROM was calculated as the proportion of histologically confirmed malignant cases among all cases in that category. ROM values were calculated without verification bias using the formula: ROM = (number of malignant histopathology cases/total number of cases in category) × 100. Separate ROM calculations were performed for each Milan category, with additional calculations for superficial lobe ROM = (superficial lobe malignant cases/total superficial lobe cases) × 100 and deep lobe ROM = (deep lobe malignant cases/total deep lobe cases) × 100.
2.5. Statistical Assessment
Categorical variables were compared using the chi‐square test or Fisher's exact test, as appropriate. Odds ratios (OR) and 95% confidence intervals (CI) were calculated to quantify differences in ROM between lobe locations. Statistical analyses were performed using IBM SPSS Statistics version 31.0 (IBM Corp., Armonk, NY, USA), with p < 0.05 considered statistically significant.
3. Results
A total of 332 patients were included in the study, comprising 174 males (52.4%) and 158 females (47.6%), with a mean age of 50.3 ± 14.1 years (range 18–79 years). Histopathological examination revealed 295 benign (88.9%) and 37 malignant (11.1%) lesions. The distribution of lesions according to anatomical location showed 267 cases (80.4%) in the superficial lobe and 65 cases (19.6%) in the deep lobe.
3.1. Malignancy Distribution by Anatomical Location
Malignancy was significantly more frequent in deep lobe lesions compared to superficial lobe lesions. Of the 37 malignant cases, 16 (43.2%) were located in the deep lobe, and 21 (56.8%) were located in the superficial lobe. When calculated as ROM per anatomical location, deep lobe lesions demonstrated a markedly higher ROM of 24.6% (95% CI: 15.1%–36.5%) compared to 7.9% (95% CI: 5.1%–11.7%) for superficial lobe lesions, representing a 3.1‐fold increased malignancy risk (95% CI: 1.7–5.6, p < 0.001) (Table 1).
TABLE 1.
Baseline characteristics and demographics of study population.
| Characteristics | Benign (n = 295) | Malignant (n = 37) | p * |
|---|---|---|---|
| Age (years) | |||
| Mean ± SD | 49.9 ± 14.0 | 53.7 ± 14.5 | 0.183 |
| Median (IQR) | 52 | 54 | |
| Range | |||
| Gender, n (%) | |||
| Female | 139 (46.9) | 19 (52.8) | 0.512 |
| Male | 157 (53.1) | 17 (47.2) | |
| Parotid lobe location, n (%) | |||
| Superficial lobe | 246 (83.4) | 21 (56.8) | < 0.001 |
| Deep lobe | 49 (16.6) | 16 (43.2) | |
| Histopathology, n (%) | |||
| Benign | 295 (100.0) | 0 (0.0) | — |
| Malignant | 0 (0.0) | 37 (100.0) | |
Note: Bold is statistically significant.
A p value < 0.05 was considered statistically significant.
3.2. Histopathological Findings
Among malignant tumors, mucoepidermoid carcinoma was the most common histologic subtype, accounting for 45.9% of all malignancies (17/37 cases), followed by ductal carcinoma (six cases), acinic cell carcinoma (five cases), adenocarcinoma (four cases), myoepithelial carcinoma (three cases), and adenoid cystic carcinoma (two cases). For benign lesions, pleomorphic adenoma was the most frequent diagnosis, representing 46.8% of all benign cases (138/295), followed by Warthin's tumor at 36.9% (109/295). Together, these two entities accounted for 83.7% of all benign diagnoses (Table 2).
TABLE 2.
Histopathological classification of parotid gland tumors according to Milan System.
| Nondiagnostic | Nonneoplastic | AUS | Benign | SUMP | Suspicious for malignancy | Malignant | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Superficial lobe | Deep lobe | Superficial lobe | Deep lobe | Superficial lobe | Deep lobe | Superficial lobe | Deep lobe | Superficial lobe | Deep lobe | Superficial lobe | Deep lobe | Superficial lobe | Deep lobe | |
| Pleomorphic adenoma | 6 | 2 | — | — | 1 | — | 106 | 18 | 1 | 2 | — | 1 | — | 1 |
| Whartin | 5 | 2 | — | — | 5 | — | 73 | 20 | 1 | 2 | 1 | — | — | — |
| Basal cell adenoma | 3 | — | — | — | 2 | — | 2 | — | — | — | — | — | — | — |
| Benign inflammation | 2 | — | 2 | — | 2 | — | 1 | — | — | — | — | — | 1 | — |
| Benign cyst | 1 | — | 3 | — | 2 | — | 3 | — | — | — | — | — | — | — |
| Benign myoephitelioma | — | — | — | — | — | — | 3 | — | — | — | — | — | — | — |
| Benign oncosytoma | 1 | — | — | — | — | — | 1 | — | 1 | — | — | — | — | — |
| Benign parotid gland | 3 | — | — | — | 2 | — | 4 | — | — | — | — | — | — | — |
| Lipoma | 1 | — | — | — | 1 | — | 6 | — | — | — | — | — | — | — |
| Mucoepidermoid carcinoma | 2 | 1 | — | — | — | — | 1 | 3 | — | — | 1 | 1 | 6 | 2 |
| Adenocarcinoma | — | 1 | — | — | — | — | 1 | — | — | — | — | 2 | — | — |
| Adeno cystic carcinoma | 1 | — | — | — | 1 | — | — | — | — | — | — | — | — | — |
| Acinic cell carcinoma | 1 | — | — | — | — | — | — | — | — | — | 3 | — | — | 1 |
| Ductal carcinoma | 1 | — | 1 | — | — | — | — | — | — | 2 | — | — | 1 | 1 |
| Myoephitelial carcinoma | — | — | — | — | — | — | — | — | 1 | — | — | 1 | — | 1 |
Abbreviations: AUS, atypia of undetermined significance; SUMP, salivary gland neoplasm of uncertain malignant potential.
3.3. Milan System Classification and Distribution
When FNAC results were classified according to the Milan System, the benign neoplasm category constituted the largest group with 242 cases (72.9%), followed by nondiagnostic with 33 cases (9.9%). The distribution of remaining categories was nonneoplastic six cases (1.8%), atypia of undetermined significance (AUS) 16 cases (4.8%), salivary gland neoplasm of uncertain malignant potential (SUMP) 10 cases (3.0%), suspicious for malignancy 10 cases (3.0%), and malignant 14 cases (4.2%). High‐risk categories (SUMP, suspicious for malignancy, and malignant) together accounted for 34 cases (10.2%) of all cases (Table 3).
TABLE 3.
Distribution of parotid gland lesions based on Milan System classification across superficial and deep lobes.
| Nondiagnostic | Nonneoplastic | AUS | Benign | SUMP | Suspicious for malignancy | Malignant | p (chi‐square) | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| n (%) | Adjusted residual | n (%) | Adjusted residual | n (%) | Adjusted residual | n (%) | Adjusted residual | n (%) | Adjusted residual | n (%) | Adjusted residual | n (%) | Adjusted residual | ||
| Superficial lobe | |||||||||||||||
| Benign | 22 (81.5%) | −2.2 | 5 (83.3%) | −0.8 | 15 (93.8%) | 0.2 | 199 (99%) | 7.3 | 3 (75%) | −1.3 | 1 (20%) | −6.0 | 1 (12.5%) | −8.5 | < 0.001 |
| Malignant | 5 (18.5%) | 2.2 | 1 (16.7%) | 0.8 | 1 (6.3%) | −0.2 | 2 (1%) | −7.3 | 1 (25%) | 1.3 | 4 (80%) | 6.0 | 7 (87.5%) | 8.5 | |
| Deep lobe | |||||||||||||||
| Benign | 4 (66.7%) | −0.5 | — | — | — | — | 38 (92.7%) | 4.4 | 4 (66.7%) | −0.5 | 1 (20%) | −3.0 | 1 (16.7%) | −3.5 | < 0.001 |
| Malignant | 2 (33.3%) | 0.5 | — | — | — | — | 3 (7.3%) | −4.4 | 2 (33.3%) | 0.5 | 4 (80%) | 3.0 | 5 (83.3%) | 3.5 | |
Note: Bolds are statistically significant.
Abbreviations: AUS, atypia of undetermined significance; SUMP, salivary gland neoplasm of uncertain malignant potential.
3.4. Lobe‐Specific Milan System Analysis
Analysis of Milan System categories by anatomical location revealed significant differences in distribution patterns. Deep lobe lesions were notably absent from the nonneoplastic and atypia of undetermined significance (AUS) categories, likely reflecting the technical sampling challenges inherent to accessing deep lobes. The distribution across other Milan categories showed varying patterns between superficial and deep lobe lesions (Table 3).
3.5. Lobe‐Specific ROM Analysis
The most clinically significant finding emerged from the analysis of category‐specific ROM in the Milan System. Deep lobe lesions demonstrated consistently higher malignancy rates across multiple diagnostic categories compared to superficial lobe counterparts. In the nondiagnostic category, deep lobe lesions showed a ROM of 33.3% (2/6) compared to 18.5% (5/27) for superficial lobe lesions.
Most notably, within the Milan “benign neoplasm” category, deep lobe lesions exhibited a significantly higher ROM of 7.3% (3/41) compared to 1.0% (2/201) for superficial lobe lesions (p = 0.009), representing a 7.3‐fold increased malignancy risk even within this traditionally low‐risk cytological category.
For the SUMP category, deep lobe lesions showed a ROM of 33.3% (2/6) versus 25.0% (1/4) for superficial lobe lesions. In contrast, high‐risk categories (suspicious for malignancy and malignant) demonstrated similar ROM values regardless of anatomical location, with both superficial and deep lobe lesions showing ROM values of 80% for suspicious for malignancy and 83.3%–87.5% for malignant categories (Table 4).
TABLE 4.
Risk of malignancy (ROM) of superficial and deep lob parotid lesions.
| Superficial lobe | Deep lobe | p (chi‐square) | |||
|---|---|---|---|---|---|
| n (ROM) | Adjusted residual | n (ROM) | Adjusted residual | ||
| Nondiagnostic | 5 (18.5%) | −0.8 | 2 (33.3%) | 0.8 | 0.422 |
| Nonneoplastic | 1 (16.7%) | — | — | — | — |
| AUS | 1 (6.3%) | — | — | — | — |
| Benign | 2 (1%) | −2.6 | 3 (7.3%) | 2.6 | 0.009 |
| SUMP | 1 (25%) | −0.3 | 2 (33.3%) | 0.3 | 0.778 |
| Suspicious for malignancy | 4 (80%) | 0 | 4 (80%) | 0 | — |
| Malignant | 7 (87.5%) | 0.2 | 5 (83.3%) | −0.2 | 0.825 |
| Total | 21 (56.8%) | 16 (43.2%) | |||
Note: Bolds are statistically significant.
Abbreviations: AUS, atypia of undetermined significance; SUMP, salivary gland neoplasm of uncertain malignant potential.
4. Discussion
This retrospective cohort study aimed to evaluate the diagnostic accuracy of FNAC in parotid gland lesions using MSRSGC, with a particular focus on systematically comparing ROM between superficial and deep lobe lesions. This study demonstrates that anatomical location within the parotid gland significantly influences malignancy risk, with deep lobe lesions showing a 3.1‐fold higher overall ROM compared to superficial lobe lesions. The finding that even cytologically “benign” deep lobe lesions carry a 7.3% risk of malignancy challenges current Milan System ROM estimates. It suggests the need for location‐adjusted risk stratification in clinical practice. To our knowledge, systematic comparison of ROM values across MSRSGC categories based on lobar location has not been previously reported, representing a novel contribution to the literature.
The overall histopathologic spectrum and diagnostic approach observed in our series align closely with established evidence in the literature [10]. In our cohort, histopathological examination revealed that 88.9% of the 332 lesions were benign, and 11.1% were malignant, consistent with the prevailing literature, which indicates the predominantly benign nature of parotid gland lesions. FNAC is widely recognized as a minimally invasive, cost‐effective, and reliable method for the preoperative evaluation of parotid masses [11]. When performed under high‐resolution ultrasound guidance, FNAC is particularly recommended for deep‐lobe lesions or those with cystic components, as it facilitates precise needle placement and enhances diagnostic accuracy [5].
Previous studies have consistently reported high specificity (89%–100%), but variable sensitivity (70%–80%) for FNAC in detecting malignancy [10]. For example, Inohara et al. reported a sensitivity of 90%, specificity of 95%, and overall diagnostic accuracy of 94% for predicting malignancy [12]. Similarly, in a large series of 996 parotid tumors, Suzuki et al. found a sensitivity of 82.3%, specificity of 98.7%, and overall accuracy of 95.9% for FNAC in malignancy detection [1]. The consistent malignant‐to‐benign distribution in our cohort supports the reliability of FNAC for parotid lesion evaluation and validates our systematic approach to lobe‐specific ROM analysis.
4.1. The Milan System and ROM
The MSRSGC was introduced in 2018 to standardize the reporting of salivary gland FNAC results and provide an estimated ROM for each diagnostic category [2]. By facilitating clear communication between pathologists and clinicians, the MSRSGC aims to improve patient care. Since its adoption, notable improvements in FNAC performance metrics have been reported, with sensitivity ranging from 71% to 93% and specificity from 96% to 99% [13].
In a large meta‐analysis, Wang et al. reported ROM values of 11.4% for nondiagnostic, 10.9% for nonneoplastic, 30.5% for atypia, 2.8% for benign neoplasm, 37.7% for SUMP, 83.8% for suspicious for malignancy, and 97.7% for malignant categories [14]. Farahani and Baloch reported corresponding rates of 17%, 8%, 34%, 4%, 42%, 58%, and 91%, respectively [15]. In a pediatric cohort, Maleki et al. found ROM values of 5.9%, 9.1%, 35.7%, 3.29%, 31.8%, 100%, and 100% for the same categories [16]. By providing category‐specific ROM calculations and comparing them with published data, our study helps better to elucidate potential regional and institutional variations in the MSRSGC.
The nondiagnostic rate in our series was 9.9% (33/332), which is within the reported range (5%–15%) but toward the higher end. Repeat FNACs were not routinely performed when surgical management was already indicated based on clinical and radiological findings, reflecting real‐world practice where management integrates multiple diagnostic modalities. The higher nondiagnostic rate may reflect technical challenges of deep lobe sampling, as evidenced by the ROM of 33.3% (2/6) for deep lobe versus 18.5% (5/27) for superficial lobe lesions in this category.
4.2. Comparison of Superficial and Deep Lobe Lesions
A key objective of this study was to assess the influence of lobar location (superficial vs. deep) on ROM within each MSRSGC category. Previous literature has indicated that deep lobe tumors showed higher malignancy rates than superficial lobe tumors. In a meta‐analysis, Aasen et al. reported a malignancy rate of 26.6% for deep lobe parotid tumors, significantly higher than that of superficial lobe tumors [6]. Anatomical inaccessibility of the deep lobe can pose challenges for obtaining adequate material, potentially affecting diagnostic accuracy. Seyhun et al. demonstrated that FNAC remains a safe and reliable tool for deep lobe tumors, reporting a sensitivity of 90.4% and a specificity of 77.7% [5]. In contrast, Altin et al. reported a higher sensitivity (80%) in tumors extending into the deep lobe [17]. In our cohort, 267 lesions (80.4%) were located in the superficial lobe, and 65 (19.6%) were located in the deep lobe, consistent with the general tendency for parotid tumors to arise predominantly from the superficial lobe. Overall, deep lobe lesions exhibited a 3.1‐fold higher ROM compared with those in the superficial lobe. In the “benign neoplasm” category, deep lobe lesions had a significantly higher ROM (7.3%; 3/41) than superficial lobe lesions (1.0%; 2/201) (p = 0.009), representing a 7.3‐fold increase in malignancy risk within a cytologic category traditionally considered low risk. In the nondiagnostic category, the rate of ROM was 33.3% (2/6) for deep lobe lesions versus 18.5% (5/27) for superficial lobe lesions. In the SUMP category, ROM was 33.3% (2/6) for deep lobe lesions compared with 25.0% (1/4) for superficial lobe lesions. Conversely, in high‐risk categories (“suspicious for malignancy” and “malignant”), ROM values were comparable between lobar locations (80% for suspicious for malignancy, and 83.3%–87.5% for malignant). This direct comparison of lobar ROM values provides clinically meaningful information for surgical planning and patient counseling.
In our cohort, several deep lobe tumors that yielded benign cytology were still considered for further management. These cases were not incidental findings; rather, additional evaluation or intervention was prompted by subtle clinical or radiographic features that raised concern for malignancy—such as ill‐defined margins, deep extension, or proximity to the facial nerve. This observation supports the notion that benign FNAC results in deep lobe lesions should be interpreted with caution and in conjunction with imaging and clinical findings. Given the potential increased malignancy risk in deep lobe lesions, FNAC results should be interpreted in the context of anatomical location to further optimize clinical decision‐making.
4.3. Study Limitations and Future Directions
Limitations of this study include its retrospective, single‐center design. While a single‐center approach ensures methodological consistency and uniform cytopathologic interpretation, it may limit generalizability. Another limitation is the relatively small number of malignant cases (n = 37), which may limit statistical power in some Milan categories. However, our deep lobe cohort provides sufficient power to demonstrate the clinically significant 7.3‐fold increased ROM in the benign neoplasm category (p = 0.009). Future research should validate these findings in larger, multicenter, prospective cohorts. Factors previously shown to influence FNAC performance include lesion size, number of slides prepared, and the experience of the performing clinician; larger lesion size and a greater number of slides have been identified as independent predictors of FNAC success, whereas submandibular gland lesions are associated with lower diagnostic yield [18]. Further investigation of these factors—particularly in deep lobe lesions and specific MSRSGC categories—would be valuable. The use of rapid on‐site evaluation and ancillary techniques (e.g., immunophenotyping, molecular testing) may further improve diagnostic accuracy and reduce nondiagnostic rates [19]. Additionally, ultrasound‐guided core needle biopsy has been suggested as a means of increasing tissue yield in selected cases [20].
5. Conclusion
Our findings suggest that FNAC results indicating benign neoplasms in deep lobe parotid tumors should be interpreted with caution, given the substantially higher malignancy potential observed in this anatomical location. This underscores the importance of integrating lobar location into preoperative risk assessment, even within cytologic categories traditionally considered low risk. Larger, multicenter studies with more diverse populations are warranted to validate these results and to provide stronger evidence for refining diagnostic and surgical decision‐making in parotid gland tumors.
Funding
The authors have nothing to report.
Consent
Patient consent was waived due to the retrospective nature of the study and anonymized data usage, as approved by the Institutional Ethics Committee.
Conflicts of Interest
The authors declare no conflicts of interest.
Şerifler S., Gül F., Öztürk A., et al., “Lobe‐Specific Risk of Malignancy in Parotid Gland Lesions: A Milan System‐Based Analysis,” Laryngoscope Investigative Otolaryngology 11, no. 1 (2026): e70344, 10.1002/lio2.70344.
Data Availability Statement
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.