High-tech imaging modalities are helpful in detecting metastatic neural spread of tumors, a process known as perineural spread (PNS), and thus can be instrumental in better managing the patient, says Barton Lane, M.D.
High-tech imaging modalities such as magnetic resonance imaging (MRI), positron emission tomography (PET) and computed tomography (CT) scans are all used to detect metastasis of malignant skin tumors.
“The perineural spread of skin malignancies to the head and neck region is important for diagnostic, prognostic as well as treatment implications” says Dr. Lane, professor of radiology, Palo Alto Health Care System, Palo Alto, Calif. Dr. Lane spoke at the 53rd annual meeting of the North American Clinical Dermatologic Society in Italy. “As the imaging signs seen in MRI, PET and CT scans may often be subtle, clinicians must be wary to carefully assess the radiologic images, as a missed diagnosis can have far-reaching consequences regarding prognosis and treatment strategies.”
Examining the spread
The perineural spread of malignancies will typically follow along the neural sheath via the endoneurium, perineurium or perineural lymphatics, Dr. Lane says, adding that even though PNS most commonly occurs in a retrograde manner or toward the central nervous system (CNS), PNS may also occur in an antegrade fashion as well.
There is a common misunderstanding between perineural spread and perineural invasion, Dr. Lane says. While PNS is a macroscopic metastatic spread of tumor along the nerve sheath, perineural invasion is a microscopic finding of tumor cells surrounding very small nerve branches that can’t be seen in these high-tech imaging techniques.
“Despite the fact that PNS is a well-known phenomenon, the metastatic spread of lesions via the neural sheath can often be silent, and the subclinical spread of tumor can often be missed by unsuspecting clinicians,” he says. “The finding of PNS can quickly turn a tumor thought to be resectable at first presentation into a nonresectable one and can possibly result in only palliative therapeutic options for the patient.”
In the head and neck region, dermatologic tumors most often associated with PNS include squamous cell carcinoma (SCC), basal cell carcinoma (BCC) and melanoma (particularly the desmoplastic variant), as well as lymphoma, adenoid cystic carcinoma (such as in the salivary glands) and, rarely, Merkel cell carcinoma (MCC), Dr. Lane says. Tumors that are at a higher risk for PNS include very large, aggressive and/or recurrent tumors, as well as those located on the lower lip, side of the nose and around the eye.
“Interestingly enough, the tendency for PNS of skin malignancies located on the head and neck varies according to the area where they are located,” Dr. Lane says. “Though it is still unclear why this occurs, the tendency of PNS appears to be associated with the level of neural innervation of the skin in a given anatomic area.”
The signs of PNS may occur years after initial presentation of the tumor, Dr. Lane says. Therefore, clinicians should remain wary of telltale signs and symptoms associated with PNS, including localized pain, paresthesias, numbness, motor weakness and cranial neuropathies. However, because PNS is frequently asymptomatic, the metastasis of tumor is often first realized when it is too late.
Though MRI, PET and CT scans are all used to detect PNS in the head and neck region, Dr. Lane says that MRI is an excellent modality for visualizing different soft tissues. Therefore, this technique used together with contrast medium is often chosen first for tumor screening purposes.
If the MRI is positive for tumor, the clinician could follow up with a CT scan to establish whether the tumor has spread to bony structures and to better stage the patient. A CT scan would also be the choice of technique in patients in whom the MRI scan is either technically unsatisfactory or contraindicated, such as those with pacemakers, Dr. Lane says.
PET scans are generally used for the detection of tumor to make sure there is no metastasis at some point in their management as well as for overall staging purposes, Dr. Lane explains. Moreover, the technique is often combined with a CT scan for high special resolution in the head and neck region.
Dr. Lane says the imaging signs of PNS include a thickening of a nerve, contrast enhancement along the nerve, loss of perineural fat, osseous erosion of neural foramen, denervation of muscles subserved by nerve(s), and PET-positive uptake along the course of a nerve.
Due to the small nuances and intricacies of the imaging signs and symptoms, as well as the complexity of metastases in the head and neck region, a multidisciplinary approach to patients is usually the optimal choice.
“In my opinion, a multidisciplinary approach is often needed in patients in whom a metastasis or PNS is suspected,” he says. “The collective thoughts and know-how of all these specialists can offer the patient a wider armamentarium of treatment modalities.” DT