Metastasizing Pleomorphic Adenoma of the Salivary Gland: A Rare Case of Lung Metastasis and Treatment Challenges
Pleomorphic adenoma is the most common benign tumor of the salivary glands, typically growing slowly with minimal risk to health. But when this “benign” tumor spreads to other organs without turning cancerous first, it becomes a medical enigma. A 25-year-old man’s case, detailed by researchers from Qilu Hospital of Shandong University, illustrates this rare phenomenon—called metastasizing pleomorphic adenoma (MPA)—and the challenges of treating a condition that behaves like cancer but looks benign.
The Patient’s Journey: Recurrence, Surgery, and Unexpected Lung Nodules
The man’s story began at 16, when he was diagnosed with pleomorphic adenoma (PA) in his right parotid gland (a salivary gland near the jaw). He underwent local mass resection, but the tumor returned two years later. A second surgery removed the recurrent mass and preserved his facial nerve. Five years after that, he noticed small, growing nodules on his right face and neck. A third surgery followed: an enlarged resection of the recurrent PA, facial nerve preservation, and tissue patch implantation.
It was during pre-operative computed tomography (CT) for this third surgery that doctors found a surprise: multiple well-defined, non-calcified nodules in both lungs. The patient had no respiratory symptoms—no cough, shortness of breath, or reduced exercise tolerance—and the nodules were entirely asymptomatic.
Testing Confirms: Benign Tumor, Malignant Spread
To investigate the lung nodules, doctors used F-18 fluorodeoxyglucose positron emission tomography (FDG-PET/CT)—a scan that uses a sugar tracer (FDG) to spot active, fast-growing cells. The results were telling: the nodules had low FDG uptake, a sign typically linked to benign (non-cancerous) growths. A needle biopsy confirmed the worst: the lung nodules were PA, meaning the tumor had metastasized (spread) from the salivary gland to the lungs.
Microscopic analysis revealed the classic features of PA: a mix of spindle cells, mucoid tissue, and cartilage-like components. Immunohistochemistry—a test that checks for protein markers in cells—further supported the diagnosis: tumor protein p63, cytokeratin5/6, cytokeratin7, smooth muscle actin, and S-100 protein were all positive (consistent with PA). The proliferation marker Ki-67 was negative, meaning the cells were growing slowly—another sign of a benign process.
The Treatment Dilemma: No Clear Path for MPA
Treating local PA is straightforward: surgical resection is the gold standard. But incomplete removal (like enucleation, where only the tumor is pulled out) can trigger recurrence—why the man’s third surgery used an enlarged resection to remove more surrounding tissue.
MPA, however, is uncharted territory. When PA spreads to the lungs as multiple nodules, surgery is nearly impossible. Chemotherapy—designed to kill fast-growing cancer cells—is not recommended here: MPA looks benign under a microscope and grows slowly, so chemotherapy has no target. The FDG-PET/CT results reinforced this: high FDG uptake (a sign of active tumor growth) is usually required for chemotherapy to work, and these nodules had none.
Radiotherapy is often used after surgery to prevent recurrence, but it’s not standard for isolated PA or young patients with single recurrences. And while some studies show chemotherapy helps with carcinoma ex-pleomorphic adenoma (CEPA)—a cancerous version of PA—MPA is different: it spreads like cancer but retains benign features.
Why Did This Happen? Clues from Research
Research offers some context. A 2015 systematic review found 72.8% of MPA cases involve prior PA recurrence—exactly what this patient experienced. The most common sites for MPA spread are bone (36.6%), lungs (33.8%), and neck lymph nodes (20.1%).
Doctors speculated repeated surgeries might have contributed to the lung metastasis, though this remains a theory. What’s clear is that MPA is a paradox: it behaves like cancer (spreading to distant organs) but lacks the abnormal cell growth of true cancer. It’s sometimes classified as a “malignant” PA, alongside CEPA and carcinosarcoma—but its rarity means there’s little data on prognosis. The only known negative prognostic factor? Multiple metastases, which this patient had.
What’s Next? Watchful Waiting and Unanswered Questions
For this patient, the only viable plan was long-term follow-up. With no symptoms and slow-growing nodules, watchful waiting made sense. His case highlights a critical gap in medicine: we know MPA exists, but we don’t know how to stop it. There’s no consensus on treating the primary tumor or metastases, no standard for surveillance, and no way to predict who will develop MPA after PA recurrence.
Kai-Xin He, Meng-Meng Peng, Xin Li, Wei Li, and De-Dong Ma—from the Department of Respiratology and Critical Care Medicine, Department of Nuclear Medicine, and Biomedical Engineering Institute at Qilu Hospital of Shandong University—reported this case in the Chinese Medical Journal in 2020. The team emphasized the need for more research to untangle MPA’s biology and improve care for patients facing this rare condition.
References
- Knight J, Ratnasingham K. Metastasising pleomorphic adenoma: systematic review. Int J Surg. 2015;19:137–145. doi.org/10.1016/j.ijsu.2015.04.084
- Valstar MH, de Ridder M, van den Broek EC, et al. Salivary gland pleomorphic adenoma in the Netherlands: a nationwide observational study of primary tumor incidence, malignant transformation, recurrence, and risk factors for recurrence. Oral Oncol. 2017;66:93–99. doi.org/10.1016/j.oraloncology.2017.01.004
- Sharon E, Kelly RJ, Szabo E. Sustained response of carcinoma ex pleomorphic adenoma treated with trastuzumab and capecitabine. Head Neck Oncol. 2010;2:2–4. doi.org/10.1186/1758-3284-2-12
- Witt RL, Eisele DW, Morton RP, Nicolai P. Etiology and management of recurrent parotid pleomorphic adenoma. Laryngoscope. 2014;125:1–6. doi.org/10.1002/lary.24964
- He KX, Peng MM, Li X, Li W, Ma DD. A case of multiple pulmonary metastasizing pleomorphic adenomas of the salivary gland. Chin Med J. 2020;133:235–236. doi.org/10.1097/CM9.0000000000000599
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