A Case of Pleomorphic Adenoma of the Submandibular Gland Radiologically Suspected to be Malignant

Abstract

Pleomorphic adenoma (PA) is the most common benign salivary gland tumor, typically arising from the parotid gland. PA of the submandibular gland is relatively uncommon and may present diagnostic challenges, particularly when imaging findings raise suspicion of malignancy. A 66-year-old woman presented with a painless mass in the left submandibular region. Imaging studies including unenhanced computed tomography (CT), magnetic resonance imaging (MRI), ultrasonography (US), and positron emission tomography (PET) revealed a lobulated mass with irregular margins, heterogeneous internal architecture, and partially disrupted capsular structures. PET showed abnormal fluorodeoxyglucose (FDG) accumulation with a maximum standardized uptake value (SUVmax) of 3.70. Based on these findings, malignancies such as adenoid cystic carcinoma or carcinoma ex pleomorphic adenoma (CXPA) were strongly suspected. The tumor was resected under general anesthesia with careful preservation of the capsule and excised en bloc together with the submandibular gland and a portion of the sublingual gland. Histopathological and immunohistochemical analyses revealed no evidence of malignancy, and a final diagnosis of PA was made. This case highlights the diagnostic difficulty of submandibular PA, especially when capsular structures appear ambiguous on imaging. It underscores the limitations of relying solely on imaging modalities and reaffirms the importance of integrating clinical history, imaging, and pathological findings to achieve accurate diagnosis.

Keywords: capsule; differential diagnosis; pleomorphic adenoma; salivary gland tumor; submandibular gland.

Figure 1. Panoramic radiograph

Panoramic radiograph shows no evidence of invasive bone resorption in the left submandibular region.

Figure 2. Unenhanced CT images (A: axial; B: coronal)

A lobulated mass with irregular margins is observed in the left submandibular gland. The inner border of the lesion is partially indistinct (white arrows), and the internal density is relatively homogeneous. CT: Computed tomography

Figure 3. MRI images (A: T1-weighted; B: fat-suppressed T2-weighted; C: DWI)

A mass lesion is observed in the left submandibular gland region. The lesion shows relatively homogeneous signal intensity on the T1-weighted image (A). On the fat-suppressed T2-weighted image (B), the lesion presents a lobulated shape with mixed low to high signal intensities and partially indistinct capsular structures (white arrow). The DWI (C) demonstrates heterogeneous high signal intensity. MRI: Magnetic resonance imaging; DWI: Diffusion-weighted imaging

Figure 4. Ultrasonographic images of the left submandibular gland (A: grayscale; B: color Doppler)

A lobulated, partially ill-defined (white arrow in A), heterogeneously hypoechoic mass is observed in the left submandibular gland region. Color Doppler imaging demonstrates abundant vascularity within and around the lesion (B).

Figure 5. PET images (A: axial, B: coronal whole-body)

Abnormal FDG uptake is observed in the left submandibular gland (white arrow in A), indicating a metabolically active lesion. No evidence of distant metastasis, including to the lungs, is seen. PET: Positron emission tomography; FDG: Fluorodeoxyglucose

Figure 6. Intraoperative and gross specimen images of the left submandibular gland region (A: surgical field; B: excised lesion)

The tumor is identified in the submandibular gland region during surgery (A). The excised lesion shows a lobulated contour (B; white arrow: tumor, yellow arrow: submandibular gland, black arrow: portion of the sublingual gland).

Figure 7. Histopathological and immunohistochemical features of the submandibular gland tumor

(A) Multinodular tumor composed predominantly of solid proliferation. (B) Ductal epithelial-like cells and spindle-shaped myoepithelial-like cells form glandular structures with myxoid stromal changes. (C) Some gland-forming tumor cells exhibit bizarre nuclear morphology without marked chromatin hyperplasia. (H&E staining; A: loupe view, B: low magnification, C: high magnification). (D, E) Nests of tumor cells infiltrate the surrounding adipose tissue to a depth of approximately 0.5 mm in areas where the capsule is indistinct. (F) Nests of tumor cells exhibit bizarre nuclei. (H&E staining; D: low magnification, E: intermediate magnification, F: high magnification). (G) Immunohistochemical staining for p53 shows no or weak expression in the tumor cells. The inset shows the positive control. (H) Ki-67 immunostaining demonstrates a low labeling index. (I) Elastica van Gieson staining reveals no evidence of vascular invasion by tumor cells. H&E: Hematoxylin and eosin