|Year : 2013 | Volume
| Issue : 1 | Page : 33-36
Pleomorphic adenoma: Choice of radiographic imaging modality - Computed tomography or magnetic resonance imaging? Illustration through a case report
Shalu Rai1, Malik Rohit1, Mandeep Kaur2, Prabhat Mukul1
1 Department of Oral Medicine and Radiology, Institute of Dental Studies and Technologies, Modinagar, Uttar Pradesh, India
2 Department of Oral Medicine and Radiology, Faculty of Dentistry, Jamia Milia Islamia, New Delhi, India
|Date of Web Publication||6-Apr-2013|
Department of Oral Medicine and Radiology, Institute of Dental Studies and Technologies, Modinagar,
Source of Support: None, Conflict of Interest: None
Introduction: Pleomorphic adenoma (PA) is the most common benign neoplasm of the major salivary glands arising primarily from the parotid gland. Computed tomography (CT) is one of the primary imaging modalities used to assess the tumors of salivary glands. However, magnetic resonance imaging (MRI) may provide additional information over CT. Case Report: We report the case of a 60-year-old male with a slowly enlarging, well-defined, round, painless, non-fixated, rubber-like swelling over the left ramus region below the ear, measuring about 4 × 4.5 cm, covering the lower border of the mandible near the angle. A provisional diagnosis of PA was given and CT and MRI were used to study the lesion. Discussion: Through this case, which was suspected to have undergone malignant transformation because of indistinct margins and focal hypodense areas on CT but was later confirmed to be a benign salivary gland tumor on MRI, we illustrate the role of CT and MRI as diagnostic aids in PA and emphasize on what should be the choice of imaging modality for parotid tumors.
Keywords: Computed tomography, magnetic resonance imaging, necrosis, neoplasm, parotid, pleomorphic adenoma, salivary gland tumor
|How to cite this article:|
Rai S, Rohit M, Kaur M, Mukul P. Pleomorphic adenoma: Choice of radiographic imaging modality - Computed tomography or magnetic resonance imaging? Illustration through a case report. Dent Hypotheses 2013;4:33-6
|How to cite this URL:|
Rai S, Rohit M, Kaur M, Mukul P. Pleomorphic adenoma: Choice of radiographic imaging modality - Computed tomography or magnetic resonance imaging? Illustration through a case report. Dent Hypotheses [serial online] 2013 [cited 2017 Mar 25];4:33-6. Available from: http://www.dentalhypotheses.com/text.asp?2013/4/1/33/110181
| Introduction|| |
Pleomorphic adenoma (PA) is the most common neoplasm of the major salivary glands. These tumors have their origin in epithelial and connective tissue; hence the term "pleomorphic," which describes the embryologic basis of these tumors. They have highly varying histomorphology, with mixed epithelial, myoepithelial, and mesenchymal areas. ,
Majority of PAs (92.5%) are found in the major salivary glands.  Of these (84.0%) are located in the parotid gland, and 8.0% arise in the submandibular gland. Only 6.5% of these tumors are found in the minor glands. 
Most cases of PA arise in the lower pole of the parotid gland, superficial to the plane of the facial nerve. Tumors that occur in the deep lobe may extend parapharyngeally through the space between the angle of the mandible and the styloid process and present with intraoral swelling and medial displacement of the tonsil and lateral pharyngeal wall. 
It has female predilection and a peak incidence at 40 years. PA shows a variegated pattern due to the presence of solid adenoid areas of epithelial differentiation and chondroid myxoid areas of mesenchymal-like differentiation. It is often surrounded by a capsule, which may not always be intact and may have satellite micronodules. 
Computed tomography (CT) is one of the primary imaging modalities used to assess tumors of the salivary glands. It allows the detection of lesions and assessment of their extension and characteristics as well as their relationships to nearby structures.  However, this technique has some limitations, and higher accuracy levels might be obtained through magnetic resonance imaging (MRI).
We report a classical case of PA where MRI proved more accurate and diagnostic over CT and therefore, suggest that MRI be used as the preferred radiographic imaging modality over CT for salivary gland tumors.
| Case Report|| |
A 60-year-old male presented to our institution with a slowly enlarging, 2-year-old swelling over the left ramus region below the ear, measuring about 4 × 4.5 cm, covering the lower border of the mandible near the angle. The left ear lobe was deflected and the overlying skin appeared normal. Palpation revealed a round, painless, well-defined, rubber-like mass, non-fixated to superficial, and deep structures [Figure 1]. No intraoral extension of the swelling could be seen. The medical history was unremarkable. No other abnormalities were found on further examination. Routine laboratory investigations were normal. A provisional diagnosis of PA of the left parotid gland was made on the basis of history and clinical features. Warthin's tumor, benign adenomas (monomorphic, basal cell, canallicular), oncocytoma, low-grade mucoepidermoid carcinoma, and carcinoma ex PA were considered for differential diagnosis.
A CT scan of the area demonstrated well-defined, large hypodense, heterogeneous mass with poorly defined anteromedial margin in the left parotid gland, approximately 2.4 (anteroposteriorly) × 2.5 (transversly) cm, bulging the skin, subcutaneous tissue and parotid fascia outwards and averting the masseter muscle [Figure 2]a. A small extension of the lesion was seen in the deep lobe of the parotid gland. Variable areas of low attenuation were seen on the posterior aspect of the superficial lobe, suggestive of cystic degeneration or seromucinous collections [Figure 2]b. No bony destruction was noted.
Heterogeneity and areas of low attenuation on CT, with poorly defines anteromedial margin could be suggestive of a malignant transformation. Therefore, an MRI was subsequently performed.
|Figure 1: Pleomorphic adenoma of the left parotid gland. A 60-year-old male with a round, painless, rubber-like swelling over the left ramus region below the ear, covering the lower border of the mandible near the angle (white arrows)|
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|Figure 2: Axial computed tomography scan (encircled area enlarged) showing, (a) well-defined, hypodense, heterogeneous mass in the left parotid gland (white arrow) with poorly defined anteromedial margin (black arrow); (b) variable areas of low attenuation seen on the posterior aspect of the superficial lobe (white arrow)|
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MRI showed a well-defined mass predominantly hypointense on T1-weighted images. Foci of low signal intensity were seen in the lower pole of the gland suggestive of cystic degeneration [Figure 3]a. On T2-weighted images, a eterogeneously hyperintense mass was seen with a star-like low signal foci in the center suggestive of area of fibrosis/clacification. A hyperintense foci in the lower pole of the gland could also be seen on T2-weighted images [Figure 3]b. MRI revealed a sharply marginated lesion showing cystic signal characteristics with internal debris on T1 and T2 sequences.
|Figure 3: Axial magnetic resonance imaging showing, (a) a well-defined hypointense mass with low signal intensity foci in the lower pole of the left parotid gland (white arrow) on T1 image, (b) a heterogeneously hyperintense mass with a star-like low signal foci in the center (white arrow) and a hyperintense foci in the lower pole of the left parotid gland (black arrow) on T2 images|
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A radiographic diagnosis of benign salivary gland tumor was suggested, which was later confirmed by fine needle aspiration cytology, showing presence of red blood cells and chronic inflammatory cells along with few acinar cells and myoepithelial cells on H and E stained smear.
Patient was referred to the Department of Oral and Maxillofacial Surgery for total parotidectomy with preservation of facial nerve. However, patient refused treatment and was consequently put on follow up at regular intervals.
| Discussion|| |
Pleomorphic adenomoa (PA) presents as a slow growing, firm and mobile swelling usually occurring in the posterior inferior aspect of the superficial lobe of the parotid gland. When present in the submandibular glands, it may be felt as a well-defined palpable masses difficulty to distinguish from malignant neoplasms and indurated lymph nodes. PA rarely ulcerates the overlying skin or mucosa.  Histopathologically it appears as a combination of gland-like epithelium and mesenchyma-like tissue in varying proportions.  Histomorphological diversity is the hallmark of PA.
Malignant potential of PA ranges from 1.5% in first 5 years and 9.5% after 15 years. Long-standing lesions have a higher risk of turning malignant.  Malignant transformation must be suspected with the presence of clinical features such as pain, ulceration, fixity, spontaneous bleeding, and facial nerve palsy. Radiographically, an evidence of invasion or a small heterogenous tumor may be suggestive of a malignant transformation.
The choice of diagnostic technique plays a key role in accurate diagnosis of PA. CT is often the first study ordered. CT does tissue distinction on the basis of X-ray attenuation. As a result, it renders poor soft tissue distinction and finer details are lost. Bony extension/invasion can be well visualized on CT but sometimes it may become problematic in cases of benign PA where the outer margins of the tumor may appear indistinct and suggest malignant invasion of the surrounding tissue.  The risk of radiation exposure with every CT examination goes without saying. MRI is often ordered as a subsequent investigation for a detailed study. Apart from being a non-ionizing modality, it shows excellent soft tissue distinction on the basis of signal intensity characteristics. Hard tissues, however, are not visible. Clearly demarcated borders along with soft tissue extensions and perineural invasions, if any, can be very well visualized on MRI. Modern CT and MRI scanners are equally capable of establishing 3D relationship of the tumor to its surroundings.
Degree of confidence with computed tomography
Without using contrast, the lesion in our case showed an average Hounsfield value (HU) of 37.32 calculated avoiding the cystic areas, over a wide region of interest ranging from 240 to 295 mm 2 . Cystic area toward the inferior pole of the gland was sampled separately showing average HU of 17.27 in an area of 12-18 mm 2 .
Brunese et al, studies the efficacy of multiphasic contrast CT with 8 min delay to differentially diagnose PA from other parotid tumors by measuring the HU and considering the degree of enhancement of the lesion (low [<70 HU], moderate 70-90 HU], strong [>90 HU]) and degree of enhancement homogeneity (non-homogeneous, mildly-homogeneous, homogenous) based on calculated HU. HUs were later compared statistically with the histopathological diagnosis. Strong enhancement of the lesion showed a sensitivity of 61.11% and specificity of 100% and uniform enhancement showed sensitivity and specificity of 100%.  PA is poorly enhancing in early phase of contrast enhancement. Delayed images obtained after 5-10 min of contrast injection are often useful in the differential diagnosis of PA. Presence or absence of uniform enhancement pattern is the best accuracy parameter and may allow a certain differential diagnosis. The technique developed by Brunese et al, may serve as a technical improvement and may be regularly applied in CT examinations of parotid enlargements.
Certain CT features, such as lobulation, homogeneity, and delayed enhancement can suggest the diagnosis of PA, but these findings are not specific to the tumor. CT results may falsely suggest invasion of the surrounding tissue owing to poor soft tissue contrast. 
Degree of confidence with magnetic resonance imaging
The diagnosis of PA can be at least strongly suggested in most cases using MRI. It should be performed to ensure that a diagnosis of malignancy is not incorrectly made. MRI can demonstrate a well-defined capsule, even when the border appears irregular on a CT scans. MRI has no role in delayed imaging. T2-weighted images on MRI with increased signal intensity and proton density is highly suggestive of the diagnosis of Pleomorhic adenomabut should be reviewed suspicion as it may also indicate a carcinoma ex PA. 
Kakimoto et al, investigated CT and MRI of 50 cases of head and neck PMAs and concluded that tumor detectability rate was 77% on plain CT, 90% on axial contrast-enhanced CT and 88% on T2-weighted MRI. Tumor capsule was hardly detec on CT images, where as it was detec in most benign cases on MRI. The authors concluded that PAs should be evaluated on MRI over CT. 
In this case, the anteromedial margin of the tumor was poorly distinguishable and the tumor capsule could not be seen at all on CT. However, MRI was able to clearly visualize the tumor margins, including the anteromedial margin, and was able to demonstrate a fibrous capsule along the periphery of the tumor as a faint hypointense band on T1-weighted images.
Therefore, in summation, when PA becomes large, it may loose its homogeneous appearance and may have areas of fibrosis, necrosis and hemorrhage. Mass heterogenesity on CT/MRI is not a useful predictor of benign versus malignant neoplasm for large tumors. Best predictors of benign PA are presence of dystrophic calcifications, best seen on CT. Irregular borders on CT may not necessarily indicate a malignancy but further investigation with MRI may be required in such cases. Nerve involvement and perineural invasion is best evaluated on MRI. CT may be the first-ordered modality but MRI should be the preferred modality for salivary gland imaging.
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[Figure 1], [Figure 2], [Figure 3]