Advanced squamous cell carcinoma refers to tumors that have either spread to lymph nodes or distant organs, or grown so extensively in their original location that standard surgery or radiation alone is unlikely to cure them. In the skin, roughly half of these advanced cases respond to newer immune checkpoint inhibitors, a treatment class that did not exist for this cancer a decade ago. But the path from an early, curable lesion to an advanced one involves specific biological steps, and understanding those steps helps make sense of why certain treatments work, why some tumors resist them, and where the field is heading.
What “Advanced” Actually Means
Squamous cell carcinoma (SCC) arises in the flat cells lining the skin, mouth, throat, esophagus, and lungs. Most SCCs of the skin are caught early and cured with surgery. The term “advanced” covers two distinct situations. Metastatic disease means tumor cells have reached regional lymph nodes or distant sites like the lungs or bones. Locally advanced disease means the primary tumor has grown so large or invaded so deeply that surgery, radiation, or a combination cannot reliably eliminate it, even though it has not technically spread to distant organs.1PubMed Central. Risk Factors and Diagnosis of Advanced Cutaneous Squamous Cell Carcinoma
Locally advanced disease is harder to pin down than metastatic disease, because there is no single agreed-upon definition. In practice, the label gets applied after a multidisciplinary team decides the tumor cannot be managed with conventional local treatments. Metastatic disease, by contrast, is confirmed by imaging or biopsy showing cancer in a new location.
Risk Factors That Push SCC Toward Advanced Disease
Not every SCC behaves aggressively. The ones that progress tend to share a cluster of features that clinicians use for risk stratification. The most consistently validated high-risk characteristics include a tumor diameter of two centimeters or larger, invasion beyond the fat beneath the skin, poor differentiation under the microscope, and perineural invasion, meaning cancer cells growing along nerves.2JAMA Dermatology. Evaluation of AJCC Tumor Staging for Cutaneous Squamous Cell Carcinoma and a Proposed Alternative Tumor Staging System Location matters too: tumors on the ear, temple, lip, and genitalia carry higher risk than those on the trunk or limbs.3PubMed. Cutaneous Squamous Cell Carcinoma: A Review of High-Risk and Metastatic Disease
Immunosuppression is another major accelerant. Organ transplant recipients on anti-rejection drugs develop aggressive SCCs at dramatically elevated rates. Recurrent tumors and rapidly growing lesions also carry worse prognoses. Some staging systems, like the one developed at Brigham and Women’s Hospital, attempt to capture more of these nuances than the older American Joint Committee on Cancer system, which initially left out factors like immunosuppression and recurrence.3PubMed. Cutaneous Squamous Cell Carcinoma: A Review of High-Risk and Metastatic Disease
How SCC Invades and Spreads
When SCC becomes aggressive, the tumor cells undergo changes that allow them to detach from their neighbors, move through tissue, and establish themselves elsewhere. A central process in this transition involves cancer cells losing their normal sticky, epithelial character and acquiring traits of mobile, mesenchymal cells. Researchers call this epithelial-mesenchymal transition, and it is marked by the loss of surface adhesion proteins like E-cadherin and the gain of proteins like vimentin that help cells migrate.4PubMed Central. Epithelial-mesenchymal transition in oral squamous cell carcinoma In head and neck SCC, activation of a signaling protein called Src drives this switch and correlates with more aggressive invasion and lymph node spread.5PubMed. Epithelial to mesenchymal transition in head and neck squamous carcinoma: association of Src activation with E-cadherin down-regulation, vimentin expression, and aggressive tumor features
One particularly troublesome route of spread is perineural invasion, where cancer cells grow along nerve fibers. This is not simply a matter of the tumor pushing through a physical gap around a nerve. Recent work shows active cross-talk between the tumor and nerve components, especially Schwann cells, which normally insulate nerves. The tumor secretes molecules that attract nerve growth toward it, and the Schwann cells in turn migrate toward the tumor, helping cancer cells travel along nerve pathways.6PubMed Central. Molecular and Cellular Mechanisms of Perineural Invasion in Oral Squamous Cell Carcinoma: Potential Targets for Therapeutic Intervention Recent research has identified specific signaling pathways involved, including crosstalk between a nerve-derived peptide called NPY and TGF-beta signaling in the tumor. Blocking the NPY receptor in animal models reduced tumor growth, metastasis, and perineural invasion.7PubMed Central. Crosstalk of NPY and TGFβ orchestrates the signaling to facilitate perineural invasion of oral squamous cell carcinoma
When cutaneous SCC does metastasize to distant sites, the lungs are the most common destination. In one surgical series, about 70% of patients with distant metastases had lung involvement, followed by bone metastases in roughly 30%.8PubMed Central. Oncological Outcome After Lymph Node Dissection for Cutaneous Squamous Cell Carcinoma
Key Molecular Drivers
Two molecular players stand out across SCC subtypes. The first is the NOTCH signaling pathway. Mutations in NOTCH1 or NOTCH2 appear in roughly three-quarters of cutaneous SCCs, making them the most common tumor-suppressor mutations specific to these cancers aside from TP53.9PubMed Central. Loss-of-function mutations in Notch receptors in cutaneous and lung squamous cell carcinoma These mutations disrupt the normal communication between a cell and its neighbors, removing a brake on growth. Sequencing studies show that NOTCH1 mutations arise early in the development of SCC, acting as a gatekeeper event rather than a late consequence of tumor evolution.10PubMed Central. NOTCH1 mutations occur early during cutaneous squamous cell carcinogenesis
The second major player is the epidermal growth factor receptor, or EGFR. When EGFR is overactive, it drives cell division, blood vessel formation around the tumor, and resistance to radiation therapy.11PubMed Central. The epidermal growth factor receptor (EGFR) in head and neck cancer: its role and treatment implications High EGFR expression in head and neck SCC is associated with worse outcomes, which is why blocking it became one of the first targeted-therapy strategies for this disease.
Immune Checkpoint Inhibitors
The biggest shift in treatment for advanced cutaneous SCC has been the arrival of immune checkpoint inhibitors. These drugs release the brakes that tumors place on the immune system, allowing T cells to recognize and attack cancer cells. Cemiplimab, a PD-1 blocker, was the first approved specifically for advanced cutaneous SCC. In its pivotal trial, about half of patients with metastatic disease responded to treatment, and roughly half of those in the locally advanced expansion cohort did as well.12PubMed. PD-1 Blockade with Cemiplimab in Advanced Cutaneous Squamous-Cell Carcinoma Pembrolizumab, another PD-1 inhibitor, was subsequently approved for recurrent or metastatic cutaneous SCC that cannot be cured with surgery or radiation.13PubMed. Treatment approaches of advanced cutaneous squamous cell carcinoma
A meta-analysis pooling data from 13 studies covering nearly 1,000 patients confirmed a pooled response rate of about 47% and a disease control rate of roughly 64%. At one year, about three-quarters of patients were still alive.14PubMed Central. Immune checkpoint inhibitors in advanced cutaneous squamous cell carcinoma: A systemic review and meta-analysis Tumors on the head and neck and those expressing higher levels of PD-L1 showed better responses.14PubMed Central. Immune checkpoint inhibitors in advanced cutaneous squamous cell carcinoma: A systemic review and meta-analysis Side effects are real but generally manageable: about one in five patients experienced grade 3 or 4 adverse events, the kind that require medical intervention.
These drugs changed the landscape for a patient population that previously had no standard systemic therapy. Before checkpoint inhibitors, advanced cutaneous SCC was treated with chemotherapy regimens borrowed from head and neck cancer protocols, with modest and often short-lived responses.
Using Immunotherapy Before Surgery
An exciting extension of checkpoint inhibitor therapy is giving it before surgery, rather than only after a tumor has become inoperable. Early-phase trials of neoadjuvant cemiplimab in patients with resectable, locally advanced cutaneous SCC have shown striking results. In one pilot study, 55% of patients achieved a pathologic complete response, meaning no viable tumor was found in the surgical specimen. A larger phase II trial confirmed these findings, reporting a 51% pathologic complete response rate and an additional 13% with only minimal residual tumor.15PubMed Central. Neoadjuvant and adjuvant treatment strategies for locally advanced cutaneous squamous cell carcinoma: A comprehensive review If these results hold up in larger trials, neoadjuvant immunotherapy could allow less-extensive surgeries and preserve more normal tissue, particularly in sensitive areas like the face.
Targeted Therapy Against EGFR
Cetuximab, a monoclonal antibody that blocks EGFR, was the first targeted drug approved for head and neck SCC. It works by binding the outside of the EGFR receptor, preventing the growth signals that drive cell division. The FDA approved it both alongside radiation for locally advanced, potentially curable head and neck SCC and as a single agent for recurrent or metastatic disease that had progressed on other treatments.16PubMed Central. The role of cetuximab for the treatment of squamous cell carcinoma of the head and neck Adding cetuximab to a platinum-based chemotherapy backbone also improved survival for patients with recurrent or metastatic disease.17PubMed. Targeted therapies in squamous cell carcinoma of the head and neck
For cutaneous SCC specifically, cetuximab and other EGFR inhibitors are sometimes used when checkpoint inhibitors are not an option, such as in transplant recipients whose anti-rejection drugs make immunotherapy dangerous. The evidence base for EGFR inhibitors in cutaneous SCC is thinner than in head and neck SCC, largely consisting of small series and case reports, but the biological rationale is sound given the receptor’s role in driving these tumors.
Radiation and Platinum-Based Chemotherapy
For many locally advanced SCCs, especially in the head and neck, concurrent chemoradiation remains a cornerstone. The standard approach combines high-dose radiation with a platinum drug like cisplatin, typically given at intervals during the radiation course.18PubMed. Tirapazamine, cisplatin, and radiation versus cisplatin and radiation for advanced squamous cell carcinoma of the head and neck (TROG 02.02, HeadSTART): a phase III trial of the Trans-Tasman Radiation Oncology Group This combination works because platinum drugs make cancer cells more vulnerable to radiation damage. In older adults with locally advanced esophageal SCC, adding chemotherapy to radiation improved both overall and progression-free survival compared to radiation alone.19PubMed Central. Benefit of chemotherapy based on platinum with definitive radiotherapy in older patients with locally advanced esophageal squamous cell carcinoma
Early data also suggest that combining radiation with checkpoint inhibitors rather than chemotherapy could work for cutaneous SCC. A retrospective study found that concurrent radiation and immunotherapy was safe and effective, with tumor control that appeared more pronounced in locally advanced disease than in widely metastatic cases.20International Journal of Radiation Oncology, Biology, Physics. Efficacy and Safety of Concurrent Radioimmunotherapy for Locally Advanced and Metastatic Cutaneous Squamous Cell Carcinoma These are early findings from a small cohort, but they point toward a combination strategy that could become standard if confirmed in prospective trials.
Why Some Tumors Resist Treatment
Radiation resistance is a persistent problem in SCC. Radiation works by breaking DNA strands in cancer cells, and cells that can efficiently repair that damage survive treatment. Two of the most common molecular features of SCC, EGFR overexpression and TP53 mutations, both enhance DNA repair and help cells evade radiation-induced death.21PubMed Central. Radiation resistance in head and neck squamous cell carcinoma: dire need for an appropriate sensitizer This creates a frustrating paradox: the same mutations that make the tumor aggressive also make it harder to kill with one of the most widely used treatments.
The tumor microenvironment compounds the problem. Advanced SCCs recruit immunosuppressive cells and create conditions, including low oxygen levels and altered tissue architecture, that shield them from both radiation and immune attack.22Exploration of Immunology. Immunosuppressive microenvironment in oral cancer: implications for cancer immunotherapy This is one reason why combining treatments, using radiation to damage the tumor’s defenses while simultaneously activating the immune system with a checkpoint inhibitor, holds theoretical appeal.
HPV Status Makes a Real Difference
In oropharyngeal SCC (cancers of the tonsils and base of the tongue), whether the tumor is driven by human papillomavirus (HPV) fundamentally changes the prognosis. HPV-positive oropharyngeal cancers tend to occur in younger, otherwise healthy patients and carry a distinctly better outlook. One study found a seven-year overall survival rate of about 68% for HPV-positive cases compared with 51% for HPV-negative ones.23PubMed Central. Overall and disease-free survival in patients with HPV-positive and HPV-negative oropharyngeal cancer
The molecular landscapes of these two cancer types are strikingly different. HPV-negative oropharyngeal SCCs that retain normal TP53 frequently carry NOTCH mutations instead, while HPV-positive tumors have their own set of recurrent genetic alterations. NOTCH1 mutations in HPV-positive tumors were associated with worse survival, and SOX2 amplification in HPV-negative tumors likewise predicted poorer outcomes, suggesting these markers could eventually refine how aggressively a given tumor is treated.24PubMed Central. Identification of prognostic molecular biomarkers in 157 HPV-positive and HPV-negative squamous cell carcinomas of the oropharynx
There is a counterintuitive wrinkle in the treatment response: in lab studies, HPV-positive cancer cells were actually more resistant to cisplatin than HPV-negative cells, even though HPV-positive patients do better overall. The responses to radiation and cetuximab did not differ by HPV status in those same experiments.25PubMed. Treatment response of HPV-positive and HPV-negative head and neck squamous cell carcinoma cell lines The better survival in HPV-positive patients likely reflects a stronger immune response against the virus-driven tumor and a generally healthier patient population, rather than superior sensitivity to every drug.
Tumor Mutational Burden as a Guide to Immunotherapy
Not everyone with advanced SCC responds to checkpoint inhibitors, and clinicians want biomarkers that predict who will benefit. One promising metric is tumor mutational burden (TMB), a count of how many mutations a tumor carries. The logic is straightforward: more mutations mean more abnormal proteins on the cell surface, giving the immune system more targets to recognize once the brakes are lifted by immunotherapy.
In lung SCC, higher TMB was independently linked to longer progression-free and overall survival in patients treated with checkpoint inhibitors, and the correlation was especially strong when immunotherapy was used alone rather than in combination with chemotherapy.26PubMed Central. Predictive values of genomic variation, tumor mutational burden, and PD-L1 expression in advanced lung squamous cell carcinoma treated with immunotherapy A meta-analysis focused on head and neck SCC found that patients with high TMB had more than twice the odds of responding to immunotherapy and a significant survival advantage.27PubMed Central. Tumor mutational burden predictability in head and neck squamous cell carcinoma patients treated with immunotherapy: systematic review and meta-analysis Cutaneous SCC, heavily caused by ultraviolet radiation damage, often carries an extremely high mutational burden, which may partly explain why checkpoint inhibitors work so well in this cancer type compared with many other solid tumors.
Liquid Biopsies and Circulating Tumor DNA
One of the hardest parts of managing advanced SCC is knowing whether treatment worked and catching recurrence early. Imaging has limitations: scarring from surgery or radiation can look like residual tumor, and small deposits of cancer can hide below the resolution of a scan. Circulating tumor DNA (ctDNA), fragments of tumor-derived DNA shed into the bloodstream, offers a complementary approach.
In head and neck SCC, personalized ctDNA assays that track mutations specific to a patient’s tumor have shown remarkable accuracy. One study reported per-patient sensitivity of 91% and specificity of 100% for detecting recurrence during follow-up. Patients who were ctDNA-positive after treatment had dramatically worse progression-free survival.28Clinical Cancer Research. Personalized ctDNA for Monitoring Disease Status in Head and Neck Squamous Cell Carcinoma A separate study found that ctDNA positivity after treatment preceded clinical recurrence by a median of seven months, a potentially valuable window for early intervention.29ESMO Open. Tumor-informed ctDNA assay to predict recurrence in locally advanced squamous-cell carcinoma of the head and neck (SCCHN)
These are still relatively small studies, and ctDNA monitoring is not yet part of routine practice guidelines for SCC. But the technology is maturing fast, and its ability to flag recurrence months before a scan picks it up could change how surveillance is done after treatment.
Field Cancerization and Why New Tumors Keep Appearing
Patients treated for one SCC often develop additional primary tumors in nearby tissue. This phenomenon, called field cancerization, occurs because the carcinogenic exposure, whether ultraviolet light, tobacco, alcohol, or a combination, damages a wide area of tissue, not just the spot where the first tumor emerged. The surrounding cells may carry precancerous molecular changes that have not yet produced a visible tumor.
Researchers have found that epigenetic markers, chemical tags on DNA that alter gene activity without changing the DNA sequence itself, rise in a stepwise fashion from healthy tissue through carcinogen-exposed tissue to the normal-appearing tissue of cancer patients. In one study of the upper digestive tract, methylation levels of four specific markers increased progressively with exposure to alcohol, betel quid, and cigarettes, and were higher still in the visually normal tissue surrounding a cancer.30Cancer Prevention Research. Revisit of Field Cancerization in Squamous Cell Carcinoma of Upper Aerodigestive Tract: Better Risk Assessment with Epigenetic Markers The practical implication is that curing one tumor does not eliminate the underlying field of damaged cells, which is why long-term surveillance matters even after a successful treatment.
Antibody-Drug Conjugates on the Horizon
For patients whose tumors progress through both chemotherapy and checkpoint inhibitors, the pipeline of new agents is growing. Antibody-drug conjugates (ADCs) pair a targeted antibody with a potent chemotherapy payload, delivering the drug directly to tumor cells while sparing much of the rest of the body. In head and neck SCC, a systematic review of early-phase trials found monotherapy response rates around 47% for ADCs, a notable figure in a population where virtually all patients had already failed standard treatments.31Cancer Treatment Reviews. Antibody-drug conjugates and bispecific antibodies in head and neck squamous cell carcinoma and nasopharyngeal carcinoma: A systematic review Bispecific antibodies, engineered to bind two targets simultaneously, are also in trials, though their response rates so far are more variable.
These agents come with real toxicity. Bone marrow suppression was common in ADC trials, with grade 3 or higher side effects in roughly a third to two-thirds of patients, and four treatment-related deaths were reported across the included studies.31Cancer Treatment Reviews. Antibody-drug conjugates and bispecific antibodies in head and neck squamous cell carcinoma and nasopharyngeal carcinoma: A systematic review Still, for a disease that was essentially untreatable once checkpoint inhibitors failed, having another line of therapy with meaningful response rates represents genuine progress. Ongoing work is focused on refining the drug-antibody combinations and figuring out how to sequence these agents with existing treatments to get the best outcomes.