A Case of Multiple Pulmonary Metastasizing Pleomorphic Adenomas of the Salivary Gland
Pleomorphic adenoma (PA) is the most common benign neoplasm of the salivary gland, characterized by slow progression and low morbidity. Despite its benign nature, it is exceedingly rare for PA to metastasize before undergoing malignant transformation. This rarity has led to a lack of consensus regarding the epidemiology, development mechanisms, treatment, and prognosis of metastasizing pleomorphic adenomas (MPA). According to a systematic review by James Knight, 72.8% of cases reported local recurrence of PA before MPA development. MPA typically spreads to the bone (36.6%), the lung (33.8%), and neck lymph nodes (20.1%). This case report focuses on a rare instance of pulmonary MPA of the salivary gland.
The patient in question was a 25-year-old man who presented with multiple bilateral well-defined non-calcified nodular lesions in the lung, identified during a pre-operative computed tomography (CT) scan. This imaging was conducted due to the second recurrence of PA in the right parotid gland. The patient’s history with PA began at the age of 16 when he was first diagnosed with PA of the right parotid gland and underwent a local mass resection. Two years later, the PA recurred, necessitating a second surgery involving right parotid mass resection and facial nerve anatomy. After another five years, the patient noticed multiple mung-bean-like granular nodules on the right side of his face and neck. As these nodules grew over time, he underwent an enlarged resection of the right parotid PA, facial nerve anatomy, and tissue patch implantation. Following this operation, the patient was referred to our hospital due to abnormal findings on his CT scan.
Despite the presence of multiple pulmonary nodules, the patient remained asymptomatic with no respiratory symptoms or decreased exercise tolerance. An F-18 fluorodeoxyglucose positron emission tomography (FDG-PET)/CT scan revealed that the multiple nodules in the lungs had a low uptake of FDG, suggesting characteristics of a benign neoplasm. Subsequent pathological results from a needle biopsy confirmed this diagnosis. Microscopic examination showed a mixture of spindle tumor cells on a background of mucoid and cartilage-like components, typical of PA. Immunohistochemistry results further supported this diagnosis, showing positivity for tumor protein p63, cytokeratin5/6, cytokeratin7, smooth muscle actin, soluble protein-100, cytokeratin, and negativity for antigen KI67.
The management of PA typically involves surgical resection, which is the first choice of treatment without controversy. However, enucleation can sometimes lead to recurrence, necessitating enlarged resection. When PA progresses to MPA, there is no established consensus on how to manage the primary lesion and metastatic lesions. Surgical resection of multiple pulmonary metastatic lesions is challenging due to their multiplicity. Histologically benign and low proliferative properties of MPA do not support the use of chemotherapy. In this case, the FDG-PET/CT scan showed no abnormally high uptake in the pulmonary metastasizing lesions, further discouraging the use of chemotherapy. While some cases have reported the effectiveness of chemotherapy in treating carcinoma ex-pleomorphic adenoma (CEPA), MPA, despite its benign histology, exhibits malignant behavior and is sometimes classified as a form of malignant PA, which includes CEPA, carcinosarcoma, and MPA.
Radiotherapy is often considered an auxiliary means of preventing recurrence after surgery but is not recommended for an isolated primary lesion or necessarily indicated for an isolated recurrence in younger patients. The local recurrence of PA appears to be significantly related to the development of MPA. However, there is a lack of studies on the prognosis of MPA, and it is known that the presence of multiple metastases is an independent negative prognostic factor. This case is somewhat unique in the uptake of FDG and the mixed tumor component compared to most reported cases. Based on the patient’s surgical history, it is speculated that the multiple pulmonary MPA may have resulted from surgical manipulation.
Managing benign MPA is particularly challenging when the patient is asymptomatic and the lesions are multiple and low proliferative. Reported cases often necessitate long-term follow-up without a better treatment option. In this case, a prolonged follow-up of the patient is planned to gather more evidence on the management of MPA. The patient’s consent was obtained for reporting his clinical information and images in the journal, ensuring anonymity and confidentiality.
In conclusion, this case highlights the rare occurrence of pulmonary MPA of the salivary gland and the challenges in its management. The benign histology yet malignant behavior of MPA complicates treatment decisions, especially in asymptomatic patients with multiple low-proliferative lesions. Long-term follow-up and further research are essential to better understand and manage this rare condition.
doi.org/10.1097/CM9.0000000000000599
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