Adenoid cystic carcinoma (ACC) is a rare type of cancer that affects glandular tissues throughout the body. ACC is known for its distinctive growth patterns, including a tendency to spread along nerves and to distant sites even years after initial treatment.
Understanding Adenoid Cystic Carcinoma
Adenoid cystic carcinoma is a malignant epithelial neoplasm, meaning it originates from the cells lining glands. It accounts for approximately 10% of all salivary gland tumors and has an estimated annual incidence of 0.4 to 1.2 cases per million people. Under a microscope, ACC tumors exhibit characteristic patterns: cribriform, tubular, and solid. The cribriform pattern, often described as “Swiss cheese” due to its sieve-like appearance with small, cyst-like spaces, is the most frequent.
A defining feature of ACC is its propensity for perineural invasion, which is the spread of cancer cells along nerve fibers. This microscopic spread can be challenging to detect and contributes to the cancer’s high rate of local recurrence. ACC is also known for its ability to metastasize, or spread, to distant parts of the body, most commonly the lungs, even many years after the primary tumor has been treated. This slow but persistent nature often leads to delayed symptoms and a prolonged clinical course.
Where Adenoid Cystic Carcinoma Develops
ACC most frequently originates in the salivary glands, which include the major glands like the parotid, submandibular, and sublingual glands, as well as hundreds of minor salivary glands found throughout the mouth and throat. Within the head and neck region, other common sites of origin include the sinonasal tract, lacrimal glands, and the trachea.
Beyond the head and neck, ACC can also arise in various other glandular tissues throughout the body. Less common primary sites include the breast, skin, and cervix.
How Adenoid Cystic Carcinoma is Diagnosed
The diagnostic process for adenoid cystic carcinoma typically begins with a thorough physical examination and a detailed review of the patient’s medical history. Symptoms can vary depending on the tumor’s location and size, ranging from a painless lump to pain, numbness, or weakness in affected areas, such as facial muscles. Because ACC often grows slowly, symptoms may develop over an extended period before a diagnosis is considered.
Imaging studies are an important part of defining the tumor’s extent and detecting any spread. Magnetic Resonance Imaging (MRI) is particularly useful for evaluating soft tissue involvement and identifying perineural invasion. Computed Tomography (CT) scans provide detailed anatomical information and help in surgical planning. Positron Emission Tomography (PET) scans can be used to assess for distant metastasis and to detect primary tumors that might not be obvious.
A definitive diagnosis of ACC relies on a biopsy, which involves the microscopic examination of tissue. This can be performed through a fine-needle aspiration or an incisional biopsy, where a small piece of the tumor is removed. Pathological analysis confirms the presence of adenoid cystic carcinoma.
Treating Adenoid Cystic Carcinoma
Surgical removal with wide, clear margins is considered the main treatment for adenoid cystic carcinoma whenever possible. The infiltrative nature of ACC, especially its tendency to spread along nerves, often makes achieving completely clear surgical margins challenging.
Following surgery, radiation therapy is often used as an additional treatment, particularly when surgical margins are not clear or if perineural invasion is present. External beam radiation therapy is a common approach, and proton therapy may be used for more precise delivery. Radiation therapy can improve local control of the disease.
Systemic therapies, such as chemotherapy, targeted therapies, and immunotherapy, have a more limited role compared to surgery and radiation but are important for advanced or metastatic disease. Conventional chemotherapy agents have shown modest response rates. Targeted therapies focus on specific molecular pathways involved in cancer growth, often by blocking blood vessel growth to the tumor. Immunotherapy aims to activate the patient’s immune system to attack the cancer, though current trials for ACC have not consistently demonstrated significant benefits.
Long-Term Outlook and Follow-Up
The long-term outlook for adenoid cystic carcinoma is highly variable due to its distinctive biological behavior. While ACC is generally slow-growing, it has a significant potential for late recurrence and distant metastasis, often occurring many years, sometimes even decades, after initial treatment. The lungs are the most common site for distant spread, with the liver and bones also being potential sites.
For instance, five-year survival rates are generally favorable, ranging from 64% to 91%, but the 10-year survival rate can decrease to 37-65%, reflecting the impact of late recurrences. Factors such as tumor size, location, and the presence of a solid growth pattern or perineural invasion can influence the prognosis.
Lifelong monitoring with regular imaging studies, such as CT or MRI scans, and clinical examinations is therefore necessary. This helps detect any signs of recurrence or metastasis early.