Differentiating benign from malignant parotid gland tumors is essential for adequate surgical planning. MRI is considered to be the best non-invasive method for diagnosis of parotid gland tumors. However, there is still overlap between benign and malignant tumors in terms of imaging appearance on conventional MRI. In the last years, the role of functional MRI (DWI and DCE-MRI) have been assessed in head and neck neoplasms to overcome the overlap in imaging appearance of benign and malignant tumors on conventional MRI [1].
The main findings in this study are that combined use DCE-MRI and DWI had high sensitivity and specificity in differentiating benign from malignant parotid gland tumors. Analysis of TIC had high sensitivity and specificity in differentiating benign from malignant parotid gland tumors. Pleomorphic adenoma could be easily differentiated from malignant parotid gland tumors on basis of measured ADC and TTP values. On the other hand, there is still overlap between Warthin’s tumor and malignant parotid gland tumors as regard the measured ADC value and TTP value. In this study, the only way to differentiate between Warthin’s tumor and malignant parotid gland tumors is the WR.
Several previous studies had investigated the role of DCE-MRI in evaluation of parotid gland tumors and found that analysis of TIC could reveal physiological characterizations of different tissues using blood flow in them so it may help in differentiation between benign and malignant parotid gland tumors [7,8,9,10].
Zheng et al. [7] evaluated salivary gland tumors based on the TIC classification by Yabuuchi et al. [6] and found that most cases of pleomorphic adenomas (88.9%) had type A TIC; all cases of Warthin’s tumor had type B TIC; regarding malignant parotid gland tumors, most of them (81.8%) had type C TIC, and only 18.2% had type B TIC. Lam et al. [8] also studied TIC in differentiation between benign and malignant parotid gland tumors and found that 96.2% of pleomorphic adenomas had type A TIC; type C TIC was observed in all cases of malignant parotid gland tumors except lymphoma. They concluded that TIC had 79% sensitivity, 95% specificity, and 91% accuracy. This matches with our results where all cases of pleomorphic adenomas in our study had type A TIC; all cases of Warthin’s tumor had type B TIC and most cases of malignant parotid gland tumors (66.7%) had type C TIC. In this study, analysis of TIC type had 100% sensitivity, 87.5% specificity, and 90% accuracy in differentiating benign from malignant parotid gland tumors.
Regarding TTP values, Aghaghazvini et al. [9] stated that pleomorphic adenoma had the higher TTP values (mean ± SD = 91.84 ± 108.13 S) when compared with Warthin’s tumor (37.00 ± 3.41 S) and malignant parotid gland tumors (mean ± SD = 82.80 ± 84.14 S). Elmokadem et al. [10] stated that pleomorphic adenomas had the highest TTP values (mean ± SD = 185.73 ± 90.66 S) when compared with Warthin’s tumor (mean ± SD = 65.45 ± 80.34 S) and malignant parotid gland tumors (mean ± SD = 79.65 ± 86.47 S) and this matches with our results.
In this work, there was no statistically significant difference between Warthin’s tumor and malignant parotid gland tumors as regard TTP value (P > 0.5); this is in agreement with Elmokadem et al. [10] who stated that there was no statistically significant difference between TTP value of Warthin’s tumor and malignant parotid gland tumors (P = 0.6).
Regarding WR values of Warthin’s tumor and malignant parotid gland tumor, Zheng et al. [7] stated that WR of Warthin’s tumor (57.5 ± 8.1%) was significantly higher than that of malignant parotid gland tumors (17.2 ± 13.2%). Gokce [5] performed analysis of several previous studies in literature and concluded that WR value of Warthin’s tumor was significantly higher than that of malignant parotid gland tumors. This matches with our results where we found that the WR was the most important predictor to differentiate between Warthin’s tumor and malignant parotid gland tumors (high WR of WT when compared with MT).
Mikaszewski et al. [11] stated that TTP < 120 s and WR < 30% could diagnose malignant parotid gland tumors with 89.5% sensitivity, 100% specificity, and 97.7% diagnostic accuracy. Tao et al. [12] found that TTP < 58 s and WR < 22% had 78.7% sensitivity, 84.2% specificity, and 82.4% accuracy in diagnosis of malignant parotid gland tumors. Ogawa et al. [13] stated that TTP < 105 s and WR < 30% had 73.1% sensitivity, 94% specificity, and 88.2% accuracy in diagnosis of malignant parotid gland tumors. This is in agreement with our results where we found that TTP of < 85 s could predict malignant parotid gland tumors with 88.9% sensitivity, 78.9% specificity, and 82.1% accuracy. WR of < 35.5% could predict malignant parotid gland tumors with 87.5% sensitivity, 85.7% specificity, and 86.7% accuracy
Regarding the measured mean ADC value, we found that there was significant difference between the mean ADC value of pleomorphic adenoma and malignant parotid gland tumors (P = 0.046) and this in agreement with several previous studies. Mikaszewski et al. [11] stated that pleomorphic adenomas had high mean ADC value (1.862 × 10−3 mm2/s) when compared with malignant parotid gland tumors (1.059 × 10−3 mm2/s). Tao et al. [12] found that mean ADC value of pleomorphic adenoma was 1.43 × 10−3 mm2/s and mean ADC value of malignant parotid tumors was 0.91 × 10−3 mm2/s. Abdel Razek et al. [14] stated that mean ADC values of pleomorphic adenoma and malignant parotid gland tumors was 1.35 × 10−3 mm2/s and 0.94 × 10−3 mm2/s respectively. Zang et al. [15] found that mean ADC value of pleomorphic adenoma and malignant parotid gland tumors was 1.57 × 10−3 mm2/s and 1.16 × 10−3 mm2/s respectively. This is in agreement with our results where the mean ADC value for pleomorphic adenoma was 1.38 × 10−3 mm2/s versus 1.08 × 10−3 mm2/s for malignant parotid gland tumors (P = 0.046).
In this work, we found that cut off mean ADC value of 1.3 × 10−3 mm2/s could differentiate pleomorphic adenoma from malignant parotid gland tumors with 88.8% sensitivity, 93.3% specificity, and 91.6% accuracy. This is in agreement with Celebi et al. [16] who stated that cut off mean ADC value of 1.315 × 10−3 mm2/s could differentiate pleomorphic adenoma from malignant parotid gland tumor with 82.1% sensitivity and 81.2 specificity. Mikaszewski et al. [11] stated that cut off mean ADC value of 1.267 × 10−3 mm2/s could differentiate pleomorphic adenoma from malignant parotid gland tumor with 95.8% sensitivity and 93% specificity. Zheng et al. [7] found that mean ADC value of 1.29 × 10−3 mm2/s had 100% sensitivity and 91.7% specificity in differentiating pleomorphic adenoma from malignant parotid gland tumor.
Several previous studies [7, 10,11,12, 14,15,16,17,18] found that there was no significant difference between the mean ADC value of Warthin’s tumor (mean ADC value ranged from 0.74 to 0.99 × 10−3 mm2/s) and malignant parotid gland tumors (mean ADC value ranged from 0.795 to 1.21 × 10−3 mm2/s). This is in agreement with our study.
In this study, the combined use of DCE-MRI and DWI could result in high sensitivity, specificity, and diagnostic accuracy in differentiating benign from malignant parotid gland tumors. This is in agreement with Zheng et al. [7] who stated that combined use of conventional MRI, DCE-MRI, and DWI could result in 90% sensitivity, 97% specificity, and 97% accuracy in differentiating benign from malignant parotid gland tumors.