1

Introduction

Salivary gland tumors are uncommon, but this complicated anatomical location requires usually an accurate presurgical diagnosis to correctly select the least aggressive and most efficient treatment . Salivary gland neoplasms account for less than 3% of all tumors, most of them are benign and parotid gland is the commonest site. Malignant salivary gland neoplasms are rare comprising less than 0.5% of all malignancies, and about 5% of the cancers of the head and neck . Fine-needle aspiration cytology (FNAC) proved to be a useful and reliable tool in the preoperative diagnosis of salivary gland masses . In contrast, Das et al. reported, a diagnostic accuracy ranging from 80% to 91.1% . Also, FNAC may carry the risk of tumor cells spread which can lead to a higher degree of local recurrence, especially in pleomorphic adenomas and malignant lesions. Additionally, the differentiation between benign and malignant lesions might be not only difficult but also impossible . For instance, pleomorphic adenomas, of variable histological patterns, can be misinterpreted as cystic carcinoma. Therefore, many MR imaging techniques have been used to differentiate between benign and malignant salivary gland tumors .

Diffusion-weighted magnetic resonance imaging (DW-MRI) is an imaging technique showing molecular diffusion, cell size, density and integrity that influence the signal intensity seen on diffusion-weighted images. This technique is a helpful complementary tool to distinguish tumoral from non-tumoral tissue, and has several interesting applications in the evaluation of head and neck cancer especially in head and neck lymphadenopathy . DW sequences are sensitized to detect the Brownian (random) motion of water protons in biological tissues. The apparent diffusion coefficient (ADC) is a mathematical quantification of the extent of free diffusivity of water molecules . In normal tissues and in vasogenic edema, where there is unrestricted motion of water protons, no restricted diffusion can be measured. However, in cytotoxic edema and in tumors with high cellularity, there is restricted diffusion of water protons which can be measured quantitatively .

Early studies using echoplanar MR imaging have shown that head-neck squamous cancers can show restricted diffusion due to their high cellularity. Non malignant changes such as necrosis, fibrosis, inflammation and fibrosis have less cellularity as compared to viable tumor and hence show higher ADC values .

DWI has been demonstrated to have potential in determining different histological subtypes of salivary gland tumors . Habermann et al. reported significantly different ADC values in pleomorphic adenomas, Warthin tumors, and mucoepidermoid carcinomas .

Another study demonstrated that pleomorphic adenomas were distinguishable from all other entities on the basis of ADC values, with the exception of myoepithelial adenomas . However, there may be some instances with considerable overlap between these tumor types, where ADC values alone may not be enough to differentiate benign from malignant salivary gland tumors .

The aim of this study was to evaluate the added value of DW-MRI in characterization of salivary gland lesions.

2

Materials and methods

This is a prospective study carried out between January 2013, and March 2015, after approval of the Institutional Review Board of Ain Shams University Hospitals. An informed consent was signed by all participants. Patients scheduled to have any salivary gland surgical intervention, for all newly diagnosed benign or malignant lesions, were included. Patients who underwent previous biopsy, surgery or who were previously treated were excluded.

Prior to surgery, all participants had routine laboratory investigations, chest X-ray, contrast-enhanced CT, and DW-MRI. A consultant radiologist reviewed the scans to comment on the ADC value and ADC histogram. He was blind to the suspected pathology. Radiological findings were then compared to clinical and histological findings.

2

Materials and methods

This is a prospective study carried out between January 2013, and March 2015, after approval of the Institutional Review Board of Ain Shams University Hospitals. An informed consent was signed by all participants. Patients scheduled to have any salivary gland surgical intervention, for all newly diagnosed benign or malignant lesions, were included. Patients who underwent previous biopsy, surgery or who were previously treated were excluded.

Prior to surgery, all participants had routine laboratory investigations, chest X-ray, contrast-enhanced CT, and DW-MRI. A consultant radiologist reviewed the scans to comment on the ADC value and ADC histogram. He was blind to the suspected pathology. Radiological findings were then compared to clinical and histological findings.

3

MRI protocol

This study was done at MRI department at Ain Shams University Hospitals. A 1.5T MR imaging unit (Philips, Healthcare) was used with a surface coil from pituitary gland to the clavicles.

Sequences obtained were: axial T1WI (time of repetition TR/echo time TE = 975/15 ms), T2WI (time of repetition TR/echo time TE = 7945/110 ms) and STIR (time of repetition TR/echo time TE = 2310/60 ms), coronal T2WI (time of repetition TR/echo time TE = 3178/80 ms) and coronal STIR (time of repetition TR/echo time TE = 3879/80 ms). DWI (with 2 b-values of 400 and 800 s/mm ² ) was done. Parameters used were time of repetition TR/echo time TE = 4300/100 ms. ADC map and ADC histogram were done for each case. Axial and coronal post contrast T1 fat saturation images were taken whenever needed. Parameters used were: FOV 200-250 mm, slice thickness 4 mm, section gap 0.5-1 mm, matrix 192 × 256.

4

Statistical methods

Data were analyzed using MedCalc© version 14 (MedCalc© Software bvba, Ostend, Belgium). Normality of numerical data distribution was examined using the D’Agostino-Pearson test. Normally distributed numerical variables were presented as mean (standard deviation), and inter-group differences were compared using the Welch test, since equality of variance could not be assumed. Categorical variables were presented as ratio or as number (%), and differences were compared using the Pearson chi-squared test or Fisher’s exact test, when appropriate. Receiver-operating characteristic (ROC) curve analysis was used to examine the value of the ADC for discrimination between benign or malignant lesions. A two-sided p -value <0.05 was considered statistically significant.

5

Results

A total of 46 patients were included in this study. 15 (33%) were males and 31 (67%) were females. Their age ranged from 21 to 67 years (mean 49.8 ± 11.3). The final histopathology of the salivary gland lesions was benign in 25 cases (54%), and malignant in 21 cases (46%). The histological diagnoses of the benign lesions were pleomorphic adenoma (n = 8), Warthin tumor (n = 4), sialoadinitis (n = 4), hyperplasia (n = 2), papillary cystadenoma (n = 1) cystic lesions (n = 2), and calcular sialoadinitis (n = 4). The histological diagnoses of malignant tumors were lymphoma (n = 5), mucoepidermoid carcinoma (n = 5), adenoid cystic carcinoma (n = 8), carcinoma ex pleomorphic (n = 1), salivary duct carcinoma (n = 1), and acinic cell carcinoma (n = 1) ( Table 1 ). The ADC values of all cases ranged from 2.6 × 10 ⁻³ mm ² s to 0.4 × 10 ⁻³ mm ² s (mean 1.27 × 10 ⁻³ mm ² s ± 0.55). There was an insignificant difference in ADC value across the various benign lesions ( Fig. 1 ). The highest ADC value was in patients with parotid pleomorphic adenoma (2.6 × 10 ⁻³ mm ² s), whereas the lowest ADC value was seen in a patient with parotid non Hodgkin lymphoma (0.4 × 10 ⁻³ mm ² s) ( Fig. 2 ). There was no significant difference in ADC value across the various malignant lesions ( Fig. 3 ). The mean ADC value for the benign lesions was 1.67 × 10 ⁻³ mm ² s (±0.42),while that for malignant lesion was 0.8 × 10 ⁻³ mm ² s(±0.18) ( Fig. 4 ). There was no significant difference regarding sex, smoking habits between the groups. Age distribution, and ADC values were significantly different ( p = 0.003 and p = 0.0001, respectively).

Right submandibular calcular sialoadinitis. Axial T2 (A) and coronal STIR (B) with post contrast axial (C) T1 weighted images. DWI with ADC map (D). Mean ADC value of 1.3 × 10 ⁻³ (E).

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