CT and MR imaging assist nasopharyngeal tumor mapping

Cross-sectional imaging modalities have proved invaluable in mapping and staging nasopharynx tumors, according to a lecture at the 2006 European Congress of Radiology. Nasopharyngeal carcinoma has its highest incidence in southern China and Hong Kong. Many cases are also seen in immigrant populations originating from this region. Its occurrence has been linked to the Epstein-Barr virus, genetic predisposition toward the disease, and environmental factors.

Most nasopharyngeal tumors originate in the fossa of Rosenmuller, said Dr. Vincent Chong, an assistant professor of radiology at the National University of Singapore, at the ECR. Tumors confined within this area are classified as T1. But the cancer can also spread aggressively from its origin in several directions, as can be seen on CT and MRI.

"The accurate and precise staging of nasopharyngeal carcinoma depends very much on a good understanding of the anatomy," Chong said.

Tumors spreading anteriorly will move into the nasal cavity. Extension then may continue into the pterygopalatine fossa, which is usually filled with fat.

"When the pterygopalatine fossa is fat-filled, it has high signal on T1-weighted MRI and low density on CT. If imaging shows this fat has been replaced, it means there is tumor within the pterygopalatine fossa," he said.

Tumors reaching the pterygopalatine fossa can then enter the foramen rotundum and orbital apex. The cancer should then be classified as T4, indicating serious disease, he said.

Lateral spread can be equally serious. Once the mandibular nerve is infiltrated, the tumor can move via perineural spread through the foramen ovale into the intracranial cavity. Posterolateral spread can lead to infiltration of the carotid sheath, the final four cranial nerves, and the jugular foramen. Superior spread of nasopharyngeal carcinoma brings a significant risk of skull base erosion. For many years, radiologists regarded thin-section CT as the best way to demonstrate this damage. Research conducted in the 1990s showed that MRI could better visualize the tumor's replacement of bone marrow.

"In the skull base, CT continued to have this magical hold over people," Chong said. "But the best way to demonstrate early involvement is MRI."

Inferior spread of nasopharyngeal carcinoma leads to extension into the oropharynx or hypopharynx. Involvement of the oropharynx means the disease should be staged as T2. This can be difficult to determine on imaging without adequate anatomic landmarks. Use of the soft palate or hard palate is not always reliable. Instead, Chong recommends choosing fixed bone structures, such as the C1 or C2 cervical vertebrae.

Unlike most head and neck cancers, nasopharyngeal carcinoma is associated with a high prevalence of distant metastases. Involvement of the bones, lungs, and liver is common. Yet the brain is rarely infiltrated.

"As we begin to cure patients, or at least keep them alive longer, we are seeing more metastases," he said.