Parotid gland tumors form 80% of all salivary gland tumors.(1) According to studies, only 20% of parotid gland tumors are malignant, and the majority of the other 77.97% are benign in condition.(1)
The benign conditions include pleomorphic adenoma, Warthin tumor, myoepithelioma, basal cell adenoma, oncocytoma, and cystadenoma.(3)
What Percentage Of Parotid Tumors Are Malignant?
Mucoepidermoid Carcinoma – The most prevalent malignant major salivary gland tumor that arises in the salivary tissue but predominantly the parotid gland. It is the most common salivary neoplasm seen in children. Low-grade or well-differentiated tumors are usually benign, intermediate ones being more aggressive while high-grade, or undifferentiated tumors metastasize early to regional lymph nodes and carry a poor prognosis. Five-year survival shows variation between 86% for low-grade and 22% for high-grade tumors.(3)
Adenoid Cystic Carcinoma- This is overall the most frequent malignant salivary gland tumor that arises from any salivary gland, but is more common in the minor salivary gland tissues. It is found equally in both genders with prevalence in the sixth decade. It first presents as a slow-growing mass and then spreads along nerve sheaths. The patients mostly complain of facial pain, and in some cases, facial paresis is present. Local recurrence is common, and distant metastases can be seen in 30% to 40% of patients, usually in the lungs. Stage, I and II cancers, have good cure rates, but stage III and IV diseases carry a poor prognosis with low survival rates (as low as 15% to 50%) at ten years. Patients with lung metastases may live up to 5 years before giving in to the disease.(3)
Acinic Cell Carcinoma- This is the third most common cancer of the parotid gland. They grow slowly and usually do not metastasize to local nodes. However, they can recur or spread many years after apparent disease-free survival. This is reflected in excellent survival rates of 90% at five years, which drops to 55% at 20 years.(3)
Polymorphous Adenocarcinoma- It is mostly recognized as a tumor of the minor salivary glands on the soft palate that is rarely seen in the parotid. It is easily confused histologically with pleomorphic adenoma and adenoid cystic carcinoma. It is generally unpredictable and can cause perineural invasion and lymph node metastases.(3)
Squamous Cell Carcinoma- Most cases of SCC are metastases to the parotid from skin carcinomas. In the parotid, they may threaten local structures and so immediate surgical intervention is required. A delay will lead to significant complications in surgery, as tumors readily invade the skin and surrounding structures.(3)
Lymphomas- They develop in intra-parotid lymph nodes with high risk in patients of Sjogren syndrome. They can often be confused with Warthin tumor on cytology, and more extensive tissue samples are usually requested, and immunohistochemistry is required.(3)
Parotid gland tumors are a group of rare cancers but still pose significant health hazards due to their poor prognosis. A large population has shown metastases to different body parts owing to its large variety of histological types. Out of the majority of the people diagnosed with the tumor, 20% will go on to spread to distant parts, making it difficult to control and manage the malignancy. The survival rate in these patients is found to be around 4 to 7 months.(2)
Malignancy is common in patients with high-grade tumors making the overall survival rate of the patient almost negligible. However, with more studies being conducted on the disease, the overall survival rate and treatment regimens are sure to improve in the coming times.
Parotid Tumor Etiology
Parotid tumor cells are known to arise from excretory stem cells that give rise to mucoepidermoid and squamous cell carcinomas whereas intercalated stem cells cause pleomorphic adenomas, adenoid cysts carcinomas, oncocytomas, adenocarcinomas, and acinic cell carcinomas. A link has been found with cigarette smoking and alcohol consumption that leads to skin malignancies in the head and neck region that further metastasize to parotid glands. Radiation exposure was also seen to be linked with parotid gland malignancies.(3)
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