Perineural invasion (PNI) is the process by which cancer cells migrate along, around, or through nerves in the body. Rather than spreading through blood vessels or lymphatics alone, tumor cells exploit the nerve sheath environment as a route for local and distant invasion.
PNI is observed across multiple cancers but is especially common in prostate and pancreatic cancers. In prostate cancer specifically, it can be detected in a substantial proportion of patients at biopsy and in the majority of radical prostatectomy specimens, depending on how it is defined and measured.
Tumor cells in PNI lesions are more aggressive than typical prostate cancer cells. They show increased levels of Ki-67 (a marker of cell proliferation), EGFR, CD74, and higher vascular endothelial growth factor (VEGF) expression, along with greater microvascular density and new blood vessel formation around nerves.
This review synthesizes current knowledge on how PNI develops at the molecular level, what its clinical and prognostic significance is for prostate cancer patients, and why inconsistent research methodology has made drawing firm conclusions difficult.
Early theories attributed PNI to the nerve's perineural space serving as a low-resistance channel for tumor cell spread. However, the nerve sheath layers composed of collagen and basement membrane actually present a highly resistant barrier, and this simple mechanical explanation is no longer considered sufficient.
Modern understanding emphasizes that cancer cells actively create a perineural niche, a specialized microenvironment around nerves that benefits tumor cell survival, proliferation, and metabolism while simultaneously stimulating nerve growth. This bidirectional interaction is central to PNI pathogenesis.
In vitro co-culture experiments combining human prostate cancer cells, stromal cells, and mouse dorsal root ganglia showed that this cellular combination increases tumor colony growth, promotes neurite outgrowth, and induces PNI formation, confirming that multiple cell types cooperate through autocrine and paracrine signaling to drive the process.
The role of inflammation in PNI is actively debated. One mouse model showed that prostatitis-induced inflammation enhanced tumorigenesis and PNI, while a retrospective study of 1,399 prostate cancer biopsies found that both acute and chronic inflammation were associated with less PNI, highlighting the biological complexity of this relationship.
Several neural signaling pathways fuel PNI in prostate cancer. Inhibition of beta-adrenergic or muscarinic receptors prevents cancer progression. Neural progenitor cells expressing doublecortin (DCX+) migrate from the brain into prostate tumors, initiating neurogenesis that promotes tumor growth. The transcription factor Snail in prostate cancer cells enhances adherence to nerve cells and promotes neurite outgrowth.
The CCL2-CCR2 axis is a key chemokine pathway in PNI: nerves release the inflammatory chemokine CCL2, which binds to CCR2 receptors on cancer cells and accelerates PNI through MAPK and Akt pathway activation. Neurotrophic factors including nerve growth factor (NGF), GDNF, and brain-derived neurotrophic factor (BDNF) also guide cancer cells toward nerves and support their survival in the perineural niche.
The TGF-beta/SMAD signaling pathway promotes epithelial-mesenchymal transition (EMT), a process through which cancer cells lose their epithelial characteristics and become more migratory. TGF-beta activity correlates with PNI levels in prostate cancer and also creates an antiapoptotic paracrine loop in which tumor-derived TGF-beta upregulates caveolin-1 in nerve perineurium, which in turn protects tumor cells from cell death.
Non-coding RNAs also regulate PNI. The long non-coding RNA OGFRP1 correlates with advanced tumor stages and PNI by binding miR-124-3p, while microRNAs miR-224, miR-301a, and miR-454 are positively associated with PNI and miR-130a is downregulated and inversely correlated with PNI. These findings add a post-transcriptional regulatory layer to the complex molecular landscape of PNI.
PNI is consistently associated with adverse clinicopathological features including higher Gleason score, elevated PSA levels, and extraprostatic extension. A meta-analysis of 19 studies comprising over 13,400 patients found that PNI was associated with a 1.4-fold increased risk of biochemical recurrence after radical prostatectomy and a 1.2-fold increased risk after radiotherapy.
Several systematic reviews and additional meta-analyses have confirmed this association with biochemical recurrence-free survival, the most commonly used clinical endpoint in prostate cancer follow-up. Biochemical recurrence, defined as a rising PSA level after treatment, is an early signal of disease relapse and the need for additional therapy.
PNI has also been shown to predict bone metastasis in some studies and is considered an independent predictor of cancer progression and disease-specific death in others. However, individual studies report conflicting findings, with some showing PNI as an independent predictor and others finding that its significance disappears after adjusting for Gleason score and PSA level in multivariate analysis.
The clinical impact of PNI may depend on its extent rather than simply its presence or absence. Studies quantifying PNI found that patients with more than three PNI foci had an independent risk of biochemical recurrence, and that only nerves with at least 50 percent of their circumference surrounded by tumor cells showed a significant correlation with tumor grade categories.
The primary obstacle to definitive conclusions is the lack of a consensus PNI definition. Some researchers define PNI as tumor cells invading in, around, and through nerves, while others require at least one-third of the nerve circumference to be involved, and still others count only nerves fully encircled by tumor cells. This definitional variation directly affects how many patients are classified as PNI-positive, with reported rates ranging from 3.9 percent in biopsy specimens under stringent criteria to 75 percent in prostatectomy specimens under liberal criteria.
The type of tissue sample examined is a major source of variability. Needle biopsies cover only a small fraction of the prostate and may not include peripheral nerve-rich zones, producing false-negative results. The same patient can show PNI in only 7.4 percent of biopsy cores but 52.1 percent of prostatectomy tissue, illustrating how the detection method profoundly affects reported prevalence and downstream prognostic conclusions.
Most studies simply classify patients as PNI-positive or PNI-negative, a dichotomous approach that loses important quantitative information. Studies that counted the number of PNI foci or the number of involved nerves per high-power field found stronger and more consistent associations with clinical outcomes than those using simple positive-negative classification.
Statistical methodology is another critical issue. PNI frequently predicts outcomes in univariate analysis but loses significance in multivariate analysis when Gleason score, PSA level, and clinical stage are included as covariates. This raises the fundamental question of whether PNI is an independent prognostic driver or a correlated indicator of tumor aggressiveness that is already captured by established staging variables.
The authors propose that a standardized PNI definition and mandatory reporting in pathology records would be a critical first step toward generating comparable, clinically actionable data. A practical reporting approach distinguishing no PNI, single PNI focus, and multiple PNI foci is considered feasible for routine pathology and would provide more informative clinical stratification.
Improved PNI detection is achievable by supplementing standard hematoxylin and eosin (H&E) staining with nerve-specific S-100 protein staining, which has been shown to reduce missed PNI detection and improve measurement accuracy. AI-based automated nerve detection in digital pathology images could further standardize assessment across institutions.
Regarding treatment implications, PNI is not currently a contraindication for nerve-sparing radical prostatectomy or active surveillance. Some evidence even suggests that PNI-positive patients undergoing bilateral nerve-sparing surgery have a lower progression risk, indicating that the presence of PNI alone should not drive treatment decisions without considering its extent and distribution.
The molecular targets identified in PNI research, including NGF, CCL2-CCR2, the SEMA3C-cMET axis, and PD-L1 on tumor-associated nerves, are emerging as potential therapeutic targets. The finding that nerve-associated PD-L1 may suppress immune responses in the tumor microenvironment suggests that PNI could influence the efficacy of immune checkpoint inhibitor therapies and is a promising direction for future clinical investigation.