by Jason Wasserman MD PhD FRCPC
November 4, 2024
Salivary duct carcinoma is a cancer that starts in the salivary glands. The salivary glands are small organs that produce saliva to help chew, swallow, and digest food. This cancer is considered rare and tends to grow more aggressively than other types of salivary gland cancers. Salivary duct carcinoma commonly affects the parotid gland, the largest salivary gland, located near the jaw and in front of the ear.
Salivary duct carcinoma often appears as a fast-growing lump or swelling, usually near the jaw or in front of the ear. It can cause weakness or paralysis in parts of the face, making it hard to move certain muscles. Pain around the lump and swelling of the lymph nodes in the neck are also common signs. If the cancer develops from a benign tumour that has been there for a long time, it can suddenly start to grow much faster than before.
The exact cause of salivary duct carcinoma is not known. However, it is thought that changes or mutations in the cells of the salivary glands might cause them to grow uncontrollably, eventually forming a tumour. Risk factors can include age (it is more common in older adults) and possibly certain inherited conditions, although research is still ongoing to better understand these factors.
To diagnose salivary duct carcinoma, your doctor will typically start with an imaging test, such as an ultrasound, CT scan, or MRI, to assess the size and location of the tumour. If a suspicious area is found, a biopsy will be performed to take a small tissue sample. A pathologist will then examine this sample under a microscope to confirm the diagnosis. Sometimes, additional tests are done to help confirm that the tumour is salivary duct carcinoma and to rule out other types of salivary gland cancers.
Under the microscope, salivary duct carcinoma shows complex structures, often arranged in solid clusters or patterns resembling cribriform (with small, sieve-like spaces) and papillary-cystic (finger-like projections into a cystic space) formations. In many cases, dead cells are found in the tumour, a feature known as comedonecrosis, which is common in this type of cancer.
The cells tend to be large and have noticeable variations in size and shape. They also have visible nucleoli (small, dense spots within the nucleus), and their cytoplasm (the fluid inside the cell) is often pink, described as eosinophilic. It has features similar to apocrine glands (glands that produce sweat). Cancer cells can frequently be found invading small blood vessels and nerves. Sometimes, a nodule of a benign (non-cancerous) tumour called a pleomorphic adenoma is found nearby. In rare cases, salivary duct carcinoma may only be present within the ducts, known as “in situ” carcinoma.
After the tumour has been completely removed, it will be measured, and its size will be described in your report. Tumour size is important because it determines the pathologic tumour stage (pT).
In the context of a salivary gland tumour such as salivary duct carcinoma, extraparenchymal extension (EPE) is the spread of the tumour beyond the salivary gland into the surrounding tissues. This condition is often associated with a more aggressive form of cancer, indicating that the tumour can invade beyond its original site. The presence of extraparenchymal extension is associated with more aggressive tumours and a worse prognosis.
Extraparenchyma, extension impacts the pathologic stage but only for tumours arising from one of the major salivary glands (parotid, submandibular, and sublingual). Tumours with extraparenchymal extension are generally classified at a higher stage, reflecting their advanced nature and the associated challenges in treatment and management.
Lymphovascular invasion occurs when cancer cells invade a blood vessel or lymphatic vessel. Blood vessels are thin tubes that carry blood throughout the body, unlike lymphatic vessels, which carry a fluid called lymph instead of blood. These lymphatic vessels connect to small immune organs known as lymph nodes scattered throughout the body. Lymphovascular invasion is important because it spreads cancer cells to other body parts, including lymph nodes or the liver, via the blood or lymphatic vessels.
Pathologists use the term “perineural invasion” to describe a situation where cancer cells attach to or invade a nerve. “Intraneural invasion” is a related term that specifically refers to cancer cells inside a nerve. Nerves, resembling long wires, consist of groups of cells known as neurons. These nerves, present throughout the body, transmit information such as temperature, pressure, and pain between the body and the brain. Perineural invasion is important because it allows cancer cells to travel along the nerve into nearby organs and tissues, raising the risk of the tumour recurring after surgery.
In pathology, a margin is the edge of tissue removed during tumour surgery. The margin status in a pathology report is important as it indicates whether the entire tumour was removed or if some was left behind. This information helps determine the need for further treatment.
Pathologists typically assess margins following a surgical procedure, like an excision or resection, that removes the entire tumour. Margins aren’t usually evaluated after a biopsy, which removes only part of the tumour. The number of margins reported and their size—how much normal tissue is between the tumour and the cut edge—vary based on the tissue type and tumour location.
Pathologists examine margins to check if tumour cells are at the tissue’s cut edge. A positive margin, where tumour cells are found, suggests that some cancer may remain in the body. In contrast, a negative margin, with no tumour cells at the edge, suggests the tumour was entirely removed. Some reports also measure the distance between the nearest tumour cells and the margin, even if all margins are negative.
Lymph nodes are small immune organs located throughout the body. Cancer cells can travel from a tumour to these lymph nodes via tiny lymphatic vessels. For this reason, doctors often remove and microscopically examine lymph nodes to look for cancer cells. This process, where cancer cells move from the original tumour to another body part, like a lymph node, is termed metastasis.
Cancer cells usually first migrate to lymph nodes near the tumour, although distant lymph nodes may also be affected. Consequently, surgeons typically remove lymph nodes closest to the tumour first. They might remove lymph nodes farther from the tumour if they are enlarged and there’s a strong suspicion they contain cancer cells.
Pathologists will examine any lymph nodes removed under a microscope, and the findings will be detailed in your report. A “positive” result indicates the presence of cancer cells in the lymph node, while a “negative” result means no cancer cells were found. If the report finds cancer cells in a lymph node, it might also specify the size of the largest cluster of these cells, often referred to as a “focus” or “deposit.” Extranodal extension occurs when tumour cells penetrate the lymph node’s outer capsule and spread into the adjacent tissue.
Examining lymph nodes is important for two reasons. First, it helps determine the pathologic nodal stage (pN). Second, discovering cancer cells in a lymph node suggests an increased risk of later finding cancer cells in other body parts. This information guides your doctor in deciding whether you need additional treatments, such as chemotherapy, radiation therapy, or immunotherapy.
Pathologic staging is a system doctors use to describe the size and spread of a tumour. This helps determine how advanced the cancer is and guides treatment decisions. The pathologic stage is usually determined after the tumour is removed and examined by a pathologist, who analyzes the tissue under a microscope. For salivary duct carcinoma, staging is based on the “TNM” system, where “T” stands for the size and extent of the primary tumour, “N” refers to lymph node involvement, and “M” indicates whether the cancer has spread to other parts of the body.
The tumour stage describes the size of the tumour in the salivary gland and whether it has spread into nearby tissues.
The nodal stage indicates whether the cancer has spread to the lymph nodes, which are small glands that help the body fight infection. Lymph node involvement can increase the risk of cancer spreading further.
The prognosis, or likely outcome, for salivary duct carcinoma depends on several pathologic factors. Key factors include the size of the tumour, the extent to which it has grown into surrounding tissues, and whether it has spread to nearby lymph nodes or other parts of the body. Tumours that are smaller and confined to the salivary gland generally have a better prognosis than those that have spread beyond the gland. Other important features include lymphovascular invasion (cancer cells within blood vessels or lymph vessels) and perineural invasion (cancer cells surrounding nerves). Both factors can indicate a higher risk of the tumour spreading to other areas, worsening the prognosis.