Perineural Involvement: What Does it Mean?

Perineural Infiltration: A Comprehensive Review of Diagnostic, Prognostic, and Therapeutic Implications

ABSTRACT

Perineural infiltration refers to a neoplastic cell involvement in, around, and through the nerves. It is considered as one of the neoplastic dissemination pathways. Thus, its identification is crucial to establish the prognosis of some malignant skin neoplasms, such as squamous cell carcinoma, and explains the locally aggressive behavior of cutaneous neoplasms, such as microcystic adnexal carcinoma. We have conducted a review of malignant and benign skin tumors in which perineural infiltration has been described, and we also discuss some histopathological findings that may simulate perineural infiltration.

Unraveling the Mysteries of Perineural Invasion in Benign and Malignant Conditions

Perineural invasion (PNI) is defined as the dissemination of neoplastic cells within the perineural space. PNI can be a strong indicator of malignancy and is linked to poor prognosis and adverse outcomes in various malignant neoplasms; nevertheless, it can also be seen in benign pathologic conditions. In this review article, we discuss various signaling pathways and neurotrophic factors implicated in the development and progression of PNI. We also describe the methodology, benefits, and limitations of different in vitro, ex vivo, and in vivo models of PNI. The spectrum of presentation for PNI can range from diffuse spread within large nerves ("named" nerves) all the way through localized spread into unnamed microscopic nerves. Therefore, the clinical significance of PNI is related to its extent rather than its mere presence or absence. In this article, we discuss the guidelines for the identification and quantification of PNI in different malignant neoplasms based on the College of American Pathologists (CAP) and World Health Organization (WHO) recommendations. We also describe benign pathologic conditions and neoplasms demonstrating PNI and potential mimics of PNI. Finally, we explore avenues for the future development of targeted therapy options via modulation of signaling pathways involved in PNI.

Nerve Density and Neuronal Biomarkers in Cancer

Simple Summary

Researchers have shown that tumor biomarkers and increased nerve density are important clinical tools for determining cancer prognosis and developing effective treatments. The aims of our review were to synthesize these findings by detailing the histology of peripheral nerves, discuss the use of various neuronal biomarkers in cancer, and assess the impact of increased nerve density on tumorigenesis. This review demonstrates that specific neuronal markers may have an important role in tumorigenesis and may serve as diagnostic and prognostic factors for various cancers. Moreover, increased nerve density may be associated with worse prognosis in different cancers, and cancer therapies that decrease nerve density may offer benefit to patients.

Abstract

Certain histologic characteristics of neurons, novel neuronal biomarkers, and nerve density are emerging as important diagnostic and prognostic tools in several cancers. The tumor microenvironment has long been known to promote tumor development via promoting angiogenesis and cellular proliferation, but new evidence has shown that neural proliferation and invasion in the tumor microenvironment may also enable tumor growth. Specific neuronal components in peripheral nerves and their localization in certain tumor sites have been identified and associated with tumor aggressiveness. In addition, dense neural innervation has been shown to promote tumorigenesis. In this review, we will summarize the histological components of a nerve, explore the neuronal biomarkers found in tumor sites, and discuss clinical correlates between tumor neurobiology and patient prognosis.

Perineural Pseudoinvasion: An Unusual Phenomenon in Nonmalignancies

Perineural invasion (PNI) is characterized as tumoral or nontumoral cells invading in or around the nerves. The neural invasion is considered as a histopathologic characteristic for malignancy and is considered a mechanism for its spread. Both of these patterns usually portend a poor prognosis and very often are markers to prompt additional treatment. There are also some nonmalignancies representing PNI, including benign neoplasms, mimicking lesions, and disorders, such as chronic pancreatitis and endometriosis. The previously recommended terms are PNI, spread, or infiltration. To distinguish PNI in malignancies from that in nonmalignancies, we propose the term "perineural pseudoinvasion" to convey their nonmalignant behavior. Despite the low prevalence, awareness of this benign pseudoinvasion is necessary to avoid aggressive treatment and its misdiagnosis with malignancies. We conducted a systematic search in PubMed and Scopus databases up to December 2015 to find articles reporting PNI in nonmalignancies. After screening, 63 articles were identified as relevant. There were also 2 review articles discussing PNI in nonmalignancies. We aim to present an overview of the perineural pseudoinvasion and to discuss the previously published review articles.

Neurotrophin Receptors and Perineural Invasion: Analyses in Select Lineage-Unrelated Cutaneous Malignancies With a Propensity for Perineural Invasion

In this chapter, we parse the literature on neurotrophins that have been implicated in the pathogenesis of perineural invasion (PNI) in select lineage-unrelated malignancies. We also detail evidence linking neurotrophins and their receptors (TrkA, RET, p75NGFR, and NCAM) to the pathogenesis of PNI in desmoplastic melanoma and cutaneous squamous cell carcinoma-both malignancies with an established propensity for PNI. Lastly, the clinical potential of neurotrophins as receptors for targeted therapies is explored.

Perineural growth of benign cutaneous sweat gland tumors: a hitherto unrecognized phenomenon unassociated with malignancy

BACKGROUND

Cutaneous intraneural reactive epithelial proliferations mimicking malignancy include epithelial sheath neuroma, re-excision perineural invasion and reactive neuroepithelial aggregates. Nevertheless, intraneural growth of benign sweat gland tumors has not been reported before.

AIMS

To report a predominantly intraneural proliferation of morphologically bland sweat gland tumors, describe their clinicopathological features and correlate them with survival.

RESULTS

We analyzed a spiradenoma and a hidradenoma with a prominent intraneural growth, occurring on the back of the 19-year-old woman and on the arm of the 53-year-old woman. Both lesions presented as a painful and slightly raised papule. After complete excision, an uneventful clinical course was observed during the follow-up period of 52 and 54 months. Pathologically, the most striking feature was an almost exclusive intraneural growth within the peripheral nerves of the deep dermis and subcutis.

CONCLUSION

We report for the first time the predominantly intraneural growth of benign sweat gland tumors. Although their histogenesis is unknown, perineural displacement due to previous surgery or trauma, as well as development from intraneural embryological epithelial remnants remains possibilities. Long-term follow-up of our patients suggests that intraneural growth of otherwise bland sweat gland tumors does not signify malignancy. Complete excision appears to be sufficient treatment procedure.

Perineural invasion in cutaneous squamous cell carcinoma: role of immunohistochemistry, anatomical site, and the high-affinity nerve growth factor receptor TrkA

Perineural invasion (PNI) has been recently added to the American Joint Committee on Cancer cutaneous squamous cell carcinoma (cSCC) staging criteria as a high-risk tumor characteristic and is purportedly more common in cSCCs of the head and neck (H&N). Expression of the high-affinity nerve growth factor receptor TrkA has been shown to be associated with PNI in noncutaneous neoplasms. Given this, we sought to ascertain the incidence of PNI in cSCCs using double immunostaining (DIS) and to investigate PNI's relationship with TrkA and established histopathologic prognosticators. Fifty-seven cSCCs from the H&N and 53 from non-H&N areas were immunohistochemically analyzed for PNI (DIS with S-100 and p63) and TrkA expression. Comparing H&N versus non-H&N areas, using hematoxylin and eosin, PNI was detected in 11% versus 6% cases, respectively, and, using DIS, in 23% versus 15%, respectively, with significant disagreement between both methods (κ = 0.47; P = .002). There was a 2.33-fold increase in PNI detection with DIS compared to hematoxylin and eosin (95% confidence interval, 1.12-4.87; P = .02). TrkA expression was 1.96 times more frequently observed in cSCCs from the H&N compared to those from non-H&N areas (P = .01). Regardless of site, TrkA expression was associated with decreased degree of differentiation (odds ratio, 6.46; P = .0006) and high-risk morphologic variants (odds ratio, 6.53; P = .002) but not significantly associated with PNI (P = .33). Increased PNI detection with DIS underscores the adjunctive utility of immunohistochemistry in microstaging. Significantly more common TrkA expression in cSCCs of the H&N argues in favor of heterogeneity among SCCs from different anatomical sites.

Perineural spread in head and neck malignancies: clinical significance and evaluation with 18F-FDG PET/CT

Certain tumors of the head and neck use peripheral nerves as a direct conduit for tumor growth away from the primary site by a process known as perineural spread. Perineural spread is associated with decreased survival and a higher risk of local recurrence and metastasis. Radiologists play an important role in the assessment and management of head and neck cancer, and positron emission tomography/computed tomography (PET/CT) with 2-[fluorine 18]fluoro-2-deoxy-d-glucose (FDG) is part of the work-up and follow-up of many affected patients. Awareness of abnormal FDG uptake patterns within the head and neck is fundamental for diagnosing perineural spread. The cranial nerves most commonly affected by perineural spread are the trigeminal and facial nerves. Risk of perineural spread increases with a midface location of the tumor, male gender, increasing tumor size, recurrence after treatment, and poor histologic differentiation. Focal or linear increased FDG uptake along the V2 division of the trigeminal nerve or along the medial surface of the mandible, or asymmetric activity in the masticator space, foramen ovale, or Meckel cave should raise suspicion for perineural spread. If FDG PET/CT findings suggest perineural spread, the radiologist should look at available results of other imaging studies, especially magnetic resonance imaging, to confirm the diagnosis. Knowledge of common FDG PET/CT patterns of neoplastic involvement along the cranial nerves and potential diagnostic pitfalls is of the utmost importance for adequate staging and treatment planning.