The phantom menace - a case of a parotid gland tumor.

60-year old female patient, non-smoker, was referred to the Department of Otolaryngology with left parotid gland tumour. The lesion was incidentally found during thyroid ultrasound (surveillance due to hypothyroidism). The patient was asymptomatic.

The lesion was reported on ultrasound as solid, hypoechoic and hypervascular. Fine Needle Aspiration Biopsy (FNAC) was inconclusive due to the scarce amount of acquired material, however, no signs of atypia were found. Multiparametric magnetic resonance imaging (MRI) was thus requested by the referring physician. The examination confirmed the presence of an ovoid, well-marginated lesion in the deep lobe of the left parotid gland, measuring 8x11x16mm. The tumour was heterogeneous, predominantly hypointense on T1-weighted images (T1-WI), hyperintense on T2-WI and fat-saturated T2-WI (Figs. 1a-c). Signs of restriction of diffusion were present with Apparent Diffusion Coefficient (ADC) value of 1.0x10^-3mm²/sec (Figs. 2a,b). Lesion enhanced vividly and on the basis of dynamic contrast-enhanced sequence type "b” Time-intensity Curve (TiC) was obtained (Fig. 3). No signs of facial nerve involvement or perineurial spread were noted. Regional lymph nodes were not enlarged.

Imaging findings combined with anamnesis and physical examination seemed to be indicative of a Warthin Tumor. The patient was scheduled for elective parotidectomy - superficial lobe of the gland and a solitary nodule within the deep lobe was removed, the function of the facial nerve was preserved. However, the postoperative histopathological report brought an unexpected diagnosis of salivary gland acinic cell carcinoma (AciCC) with a prominent lymphoid infiltrate of the stroma. Subsequently, the patient underwent a second surgery in order to obtain wide margins and radiation. Post-operative second look evaluation of MRI did not differ from the first impression.

AciCC is a slowly-growing, 3rd most frequent primary parotid malignancy with a prevalence in women [1]. It may be multicentric and bilateral. The prognosis is generally favourable (80%10-yr survival), however, local recurrence may be seen in up to 35% and distant metastases can be found in lungs and bones [1].

Differentiation between malignant and benign lesions is of the utmost importance in reference to treatment planning. Local excision or superficial parotidectomy is usually sufficient in benign lesions, total parotidectomy with risk of injury to the facial nerve is required in malignancy [2]. Clinical findings often show limited value with a few indicators of malignancy (e.g. facial nerve palsy). FNAC is inconclusive in up to 5% of the cases [3-6]. MRI is well established in detecting parotid gland tumours. The analysis of conventional sequences with DWI and DCE-MRI allows to differentiate various types of parotid tumours [7,8,9] with increasing diagnostic accuracy, however, some of the features may overlap between histologies. Radiological signs of malignancy include indistinct margins, involvement of nerves and/or lymphadenopathy. The well-defined lesion is usually benign. Analysis of DCE-MRI is based on an assessment of time-signal intensity curves obtained by placing regions of interest (ROIs) over the lesion [8,9,10] - Fig. 4. Estimation of ADC thresholds derived from the receiver operating characteristic curves remains a subject to research [8,11,12,13]. Yabuuchi et al. suggested thresholds of 1.4x10^-3mm²/sec to differentiate pleomorphic adenomas from carcinomas and 1.0x10^-3mm²/sec between Warthin Tumors and carcinomas [11].

Take-home messages:

1. Multiparametric MRI is an important examination in the assessment of parotid gland masses.

2. Radiologists should be aware that certain features may overlap across various histologies, particularly in case of Warthin Tumors and malignancies.

3. Low-grade malignancy may present as a well-circumscribed parotid mass.