Cutaneous squamous cell carcinoma (cSCC) represents the second most frequent skin cancer (20%), after BCC, with an incidence of approximately 28.9 per 100 000 inhabitants/year (1). In 1-5% of cases, it can give distant metastases, and such metastases are associated with an average survival of two years (2). Molecularly, cSCC arises from the accumulation of genetic and epigenetic alterations in keratinocytes over time that permit development of an invasive tumor. Frequently it is associated with mutations of tyrosin kinase receptors, certain cell cycle regulatory genes or RAS/MAPK and PI3K signalling pathways. These genetic alterations drive the development of malignant and premalignant lesions, that is accelerated when intrinsic defenses are compromised. The accumulation of genetic alterations, and development of malignant and premalignant lesions, is accelerated when intrinsic defenses are compromised. Examples include patients on chronic immunosuppression, or those with a heritable predisposition to cancer, such as the disease states of Xeroderma Pigmentosum or Bloom syndrome (3, 4).

The primary goal of cSCC therapy is to completely remove the tumor with maximal functional and cosmetic preservation. Surgical excision alone guarantees the successful treatment of cSCC, with a good prognosis and cure rates greater than 90% (5).

In patients not amenable to surgery, radiotherapy, eventually associated with chemotherapy, represents a valid and curative treatment strategy for cSCC (5). Local recurrences of cSCC frequently exhibit a more extensive, irregular subclinical infiltration pattern than primary tumors, which is difficult to assess using conventional bread-loaf sections. Local recurrences in the head and neck region are often desmoplastic lesions characterized by high recurrence rates (6, 7). It is not uncommon for patients with this tumor type to die from complications caused by local tumor infiltration or metastases (8). Risk factors associated with recurrence, metastases and disease-specific death are as follows: Breslow thickness >2 mm, diameter >20 mm, invasion beyond subcutaneous fat, perineural involvement, poor differentiation, immunosuppression, and localization at the ear, lip and cheek (9). Recently, immunotherapy has been approved in locally advanced and metastatic cSCC. Cemiplimab (human IgG4 antibody against PD-1) was studied, and a response was observed in 13 of 26 patients (50%; 95% CI = 30 to 70). Among the patients who had a response, the duration of response exceeded 6 months in 57% (10). Cemiplimab is currently the only approved systemic immunotherapy for cutaneous SCC in Europe (2). Advanced cSCC often has a high tumor mutation burden (TMB), which may be responsive to immunotherapy, but this treatment is often contraindicated as many patients are immunosuppressed or have a history of organ transplantation (11).

ECT is an alternative treatment for cSCC that are unsuitable for standard treatments or for recurrences after standard treatments, consisting of an intravenous (IV) or intratumoral (IT) injection of a chemotherapeutic agent, such as cisplatin or bleomycin, combined with locally applied electric pulses that permeabilize tumor cell membranes to increase its cytotoxicity. In some retrospective studies and one meta-analysis, it has been reported that 20-70% of patients treated with ECT presented a good local response and disease control, while in a prospective study EURECA on cSCC patients, the rate of complete response at the 2-month follow-up was 55%, with only a 4% rate of disease progression (12–15). The results of these studies have concurred with the inclusion of ECT in the clinical guidelines of several European countries, including the UK, Italy and Germany (5, 16, 17).

The International Network for Sharing Practice in ECT (InspECT) is a pan-European collaboration of centers encompassing different specialties that treat cutaneous malignancies. The group has published analyses on outcomes for specific diseases such as cutaneous metastases from MM (18), breast cancer (19), and cutaneous tumors in the head and neck region (15, 20), as well as articles investigating specific topics of importance such as pain management (21). This database permits the collection of an extensive amount of information on large cohorts of patients, conducting more detailed analysis and investigating clinically significant variables due to data being accrued in a homogenous way.

Cohort studies on ECT in cSCC are limited to 22 patients in an exclusive paper focused on cSCC (12) and 50 patients in a study including various nonmelanoma skin cancers of the head and neck district (14). In this study, we aimed to analyze data from a larger cohort of cSCC patients included in the InspECT registry. This analysis may outline ECT benefits and disadvantages and help identify the most appropriate indications and procedural modalities.

In the present study, the patients were treated with ECT with a percentage of complete response of 62%, partial response of 21% (OR=83%) and stable, controlled disease of 11%. Side effects directly related to treatment were mild for most patients, except 9 patients with severe ulceration. Similar results were obtained by Bertino et al. (15, 24) in the EURECA study, with a 55% complete response in cSCC patients, and by Campana et al., with a 41% CR and 85% OR.

The results obtained here are of significant interest because the patients were not eligible for any other standard treatment, and they could benefit from ECT, which is an effective treatment in the majority of cases reported here.

Furthermore, the local control of the disease was relevant, as the median time to local progression was 4.8 months (range 1-33.7), with 9.9% recurrences and 22.2% disease progression. When a complete response was obtained, patients remained free of local disease for a mean time of 9.7 ± 8.2 months. Among complete responders, the duration of CR exceeded 6 months in 88% of patients. In a similar cohort of patients with locally advanced or metastatic cSCC, the safety and efficacy of immunotherapy with intravenous cemiplimab (human IgG4 antibody against PD-1) were investigated. A response to systemic therapy was observed in 13 of 26 patients (50%; 95% CI = 30% to 70%). Among the patients who had a response, the duration of response exceeded 6 months in 57% (10). In 2020, Migden et al. (25) reported the clinical activity of cemiplimab in patients with locally advanced cSCC. An objective response was observed in 34 (44%; 95% CI 32-55) of 78 patients. The best overall response was ten (13%) patients with a complete response and 24 (31%) with a partial response. Comparing the adverse event profile with systemic immunotherapy and ECT, there were significantly fewer side effects after ECT. While ECT patients from the cohort study had a total of 11% adverse events and rates of up to 48% grade 3-4, severe adverse events were observed in up to 35% of the clinical studies on cemiplimab (25, 26). According to recent real-world analyses, data of up to 77% of mild adverse events and severe adverse events of up to 45% were described (27, 28). Although cemiplimab represents the only approved systemic immunotherapy for cutaneous SCC in Europe (1), the incidence of moderate and severe adverse events is not negligible.

Long-term analysis in our cohort demonstrates that at the 1 year of follow-up, 66% of patients had maintained control of their local disease, but patients with primary lesions showed the highest rate of local control with respect to patients with locally advanced disease; 80% vs. 49% (p<0.0001). This particular result indicates that treating ECT primary (recurrent/persistent) lesions was significantly more effective than treating locally advanced lesions. Bertino et al. obtained similar results (15) in their study on head and neck cSCC.

A large number of patients/lesions in this study allowed us to perform univariate and multivariate analyses of factors affecting the response to ECT treatment in cSCC for the first time. In the univariate model, early stage of disease (T1-T2 vs. T3-T4), low number of previous treatments, intravenous drug administration, absence of previous irradiation and small size (<3 cm) were significant predictive factors of OR. When combined in the multivariate model, only a low number of previous treatments, intravenous drug administration and small size showed a significant association with OR. A high number of previous treatments indicated a long-lasting or aggressive disease that was difficult to successfully treat. Other studies have shown that pretreated lesions have a lower probability of being successfully treated with ECT (18); Bertino et al. (15), with a similar analysis conducted on cutaneous lesions of the head and neck of various histology (BCC, SCC, MM), showed that naïve tumors showed increased effectiveness of ECT and that previous surgery as single treatment the least affected the outcome compared to (chemo) radiotherapy or multiple treatments. These results would agree with our observations, even if various types of tumors were considered instead of one single histological type. In our cohort, we observed an 86.2% OR in naïve patients, 82.3% in patients who underwent a single previous treatment, 75.6% in those submitted to 2 previous therapies, and 56.7% in patients who underwent more than 2 previous treatments (p=0.0129).

We observed in our study that the influence of intravenous administration of bleomycin was significant and led to a favorable outcome with respect to intratumoral administration (p<0.0001). This finding was not significant in other analyses reported in the literature (15, 22, 29, 30). A possible explanation may be due to the difference in the histotype of sSCC with respect to other diseases as follows: it can be hypothesized that the peculiarity of the anatomical characterization of cSCC tissue, due to the strongly irregular structure but high vascularization, may be in favor of the intravenous administration of bleomycin, which more homogeneously diffuses the drug in the tissue. This observation needs to be evaluated considering the low rate of patients treated with intratumoral drug administration, which was performed in only 17% of all cases, even in the small lesions. Furthermore, the percentage of preirradiated lesions in intratumorally treated patients was 25%, higher than that of intravenously treated patients (15%); prior data had inferred a trend showing that intravenous bleomycin was preferred in previously irradiated tissues, likely due to fibrotic tissue limiting the diffusion needed for successful intratumoral drug diffusion (31); further investigation on this matter is needed.

The significant influence of lesion size in the multivariate model was confirmed, as already reported in several studies (15, 19, 24, 30, 32–34) on various types of histology.

The analysis has the following limitations: these data are from a patient registry instead of clinical trials and thus represent a real-world heterogeneous mix of patients treated with differing intentions, some for purely palliative indications and others with definitive curative intent. Patient review was not standard practice in all the participating centers; nonetheless, the InspECT group was committed to improving the quality of clinical reports through regular monitoring and analyses of the database and the publication of dedicated guidelines for researchers (35). The heterogeneous number of patients provided by each center was because not all centers started to upload patients into the registry at the same time; some centers started uploading patients’ data since the beginning (2008), and the last centers entered the InspECT group in 2019 and thus only contributed to data collection a few patients.

In the present analysis, ECT showed antitumor activity and a favorable safety profile in patients with primary unresectable/recurrent/persistent, locally advanced cSCC for whom there was no widely accepted standard of care. Furthermore, in advanced tumors, a therapeutic strategy of ECT in combination with systemic treatment, such as immunotherapy, might be even more beneficial for the patient. In fact, a preliminary report from the InspECT group showed that ECT could increase the positive effect of immunotherapy in advanced and metastatic MM patients (36). This aspect should be prospectively investigated even in cSCC patients.

The raw data supporting the conclusions of this article will be made available by the authors, upon request.

The studies involving human participants were reviewed and approved by Ljubljana: National Ethics Committee of the Republic of Slovenia (102/09/14). Padova: Padua Veneto Institute of Oncology, Institutional Approval Prot. No. 006264 – 09/04/2018; Cork: Clinical Research Ethics Committee, University College Cork, Ireland ECM 4 (IIIII) 07/05/13 & ECM 4(qq) 29/03/18. The patients/participants provided their written informed consent to participate in this study.

Conceptualization: GB, AG, LC, FT, and GS. Methodology, data acquisition and analysis: GB, AG, LC, CK, HS, JG, TM, JC, PQ, EK, MM, BB, GP, FB, PM, JO, FR, AO, RP-J, GM, DM, BS, VS, WS, GB, and FT. Review and editing: GB, AG, FT, and GS. All authors have read and agreed to the submitted version of the manuscript.

This work was conducted without any direct financial support. INSPECT is an independent group of clinicians doing research in the field of electroporation and its annual meeting is financially supported by IGEA without any involvement in the contents of the meeting.

FT is an IGEA employer.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.