Discussion
DRG metastases are rare and may be incidental findings in surgery for herniated discs, dorsal root neurinoma or spinal meningioma. This has to be taken into account in unanticipated intraoperative findings, including normal disc anatomy and a thinned dural sheath of the nerve root. A common finding in this pathology is enlargement of the ganglion with splaying of the nerve fibers. Metastasis of pulmonary adenocarcinoma to the DRG has not previously been described but cases of trigeminal ganglion metastasis have. Our case report provides further information on the potential of this entity to spread to the border between the central and peripheral nerve systems.
No metastases to the nerve root itself have been described so far. The possible pathophysiological mechanisms of DRG metastasis were discussed in a description of a breast cancer DRG metastasis. Based on histopathological examinations, it was assumed that the fenestrated endothelium, in contrast to the non-fenestrated endothelium in the nerve root, facilitates DRG but not nerve root metastasis. However, the number of known cases is too small to detect similarities shared by the different entities described. It is noticeable that none of the patients described so far harbored or developed metastases in the central nervous system. DRG metastasis might require distinct genetic changes in the primary tumor to evolve.
Metastatic spread of different primary tumors into the DRG, including different species of carcinomas, has been described; however, it is likely that a significant number of cases remain undetected. Besides the low incidence of this entity, this may be explained by the reduction in neurologic deficits and pain due to nerve root decompression. In unclear cases, an exposition of the DRG with inspection and biopsy has to be performed. Although bone metastases of the spine with compression of neuronal structures are more common than DRG metastases, the latter should be considered as a rare differential diagnosis of a lumbar disc herniation or schwannoma for patients with a known malignancy and a radiological mass at the neuroforamen.
Surgery for these rare instances is complicated by their usually unanticipated occurrence. Lesions unclear on radiology, especially in patients with known malignancies, should result in an increase in presurgical diagnostic work-up, ideally including positron-emission tomography imaging. Intraoperative neurophysiological monitoring should be used in all unclear lesions. Previous reports indicate that clear delineation of tumor and nerve tissue or tumor capsules is usually not found, and subtotal resections were generally performed. Complete tumor removal with preservation of neurological function is usually not an option. The benefit and importance of intraoperative direct nerve stimulation has been shown in benign and malignant peripheral nerve tumors. Therefore, subtotal resection and nerve root decompression should be performed under neurophysiological monitoring to allow confirmation of the diagnosis and to improve clinical symptoms and quality of life in these patients. This is especially true with DRG metastases as they likely indicate systemic tumor spread and life expectancy is limited in these patients.
Advanced treatment options include resection of the process including the DRG with acceptance of neurologic deficts followed by local radiation therapy. We believe these options should be subject to the patients' discretion. Radical surgery may then be considered following complete tumor staging, including cerebrospinal fluid cytology.