Rare Feline Gingival Extramedullary Plasmacytoma: Clinical Application of Electrochemotherapy for Local Control

ABSTRACT

Feline extramedullary plasmacytoma (EMP) is an uncommon plasma cell neoplasm typically regarded as biologically benign; however, its clinical behaviour may be heterogeneous. This case report describes an aggressive gingival EMP in a 14‐year‐old neutered male Turkish Angora cat, characterised by alveolar bone lysis and cytologically confirmed regional lymph node metastasis in the absence of systemic features consistent with multiple myeloma. Histopathological examination revealed an infiltrative round cell neoplasm with high mitotic index, and strong diffuse MUM‐1 immunoreactivity confirmed the diagnosis. As complete surgical excision was declined, and tumour regrowth occurred after repeated radiation therapy, electrochemotherapy (ECT) was selected for local tumour control. Bleomycin (15,000 IU/m² IV) was administered before pulse delivery, and three ECT sessions were performed at weekly intervals. The tumour volume decreased by more than 70%, which was consistent with a partial response, and only mild self‐limiting local adverse effects were observed. No systemic toxicity was observed. To the best of our knowledge, this is the first report describing the use of ECT for feline gingival EMP. This case highlights the potential biological heterogeneity of feline EMP and suggests that ECT may provide short‐term local tumour reduction and palliation when complete surgical excision is not feasible.

This case report describes an aggressive gingival extramedullary plasmacytoma (EMP) with regional lymph node metastasis in a 14‐year‐old cat. As complete surgical excision was unfeasible, three weekly electrochemotherapy (ECT) sessions using intravenous bleomycin were performed for local tumour control. The treatment achieved over 70% tumour volume reduction with minimal side effects, demonstrating that ECT can provide effective local palliation for feline EMP.

1. Introduction

Myeloma‐related disorders (MRDs) are a heterogeneous group of diseases characterised by the transformation and proliferation of plasma cells, resulting in a clonal neoplastic cell population. These disorders include multiple myeloma (MM), extramedullary plasmacytoma (EMP), IgM macroglobulinaemia and solitary osseous plasmacytoma (SOP) (Vail 2020). EMP is defined as a plasma cell tumour arising from soft tissues without evidence of bone marrow infiltration by neoplastic plasma cells.

In cats, MRDs are rare, accounting for less than 1% of all feline malignancies and approximately 1.9% of haematopoietic neoplasms. The true incidence of EMP remains exceedingly low and poorly defined (Vail 2020). The most commonly reported site of feline EMP is the skin; however, cases involving other anatomical locations, including the oral cavity, eyes, gastrointestinal tract, liver and brain, have also been described (Greenberg et al. 2004; Larsen and Carpenter 1994; Majzoub et al. 2003; Michau et al. 2003; Zikes et al. 1998). Cutaneous and oral EMPs are generally associated with a benign clinical course and are often detected incidentally owing to minimal clinical signs. In contrast, non‐cutaneous and non‐oral EMPs in dogs generally exhibit more aggressive biological behaviour, with regional lymph node metastasis reported more frequently (Borgatti 2022; Jimeno Sandoval et al. 2025; Vail 2020; Zikes et al. 1998). Although complete surgical excision is often curative for EMPs, progression to MM has been documented in some cases (Radhakrishnan et al. 2004), highlighting the biological heterogeneity of this disease.

Electrochemotherapy (ECT) is a locoregional anticancer treatment modality that facilitates the intracellular delivery of chemotherapeutic agents through the application of electric pulses following drug administration. By transiently increasing cell membrane permeability, ECT enables a marked increase in intracellular drug uptake, which has been reported to reach up to 1000‐fold enhancement. As drug entry occurs via a physical mechanism, this approach is applicable regardless of the histological tumour type. Furthermore, ECT may be combined with other treatment modalities to improve local tumour control while limiting damage to the surrounding normal tissues, thereby reducing treatment‐associated adverse effects (Ramos et al. 2024; Tellado et al. 2022). This report describes a feline case of gingival EMP treated with ECT for local tumour control.

1.1. Case Description

A 14‐year‐old neutered male Turkish Angora cat was referred to our hospital from a local veterinary clinic for the evaluation and management of a gingival mass. Gingival oedema surrounding the right maxillary canine had been noted approximately 18 months prior. The lesion was initially managed as gingivitis using antibiotics and anti‐inflammatory medications; however, progressive enlargement of the lesion was observed despite treatment. Partial surgical excision was subsequently performed at a local animal hospital, and histopathological examination revealed a round cell malignancy. No additional diagnostic evaluation was performed at that time, and the lesion was presumed to be a lymphoma. Palliative radiation therapy (PRT; 4 Gy × 5 fractions) was administered, resulting in a reduction in lesion size.

Six months later, rapid macroscopic regrowth of the mass was observed, and a second course of radiation therapy using the same protocol was initiated approximately 2 months prior to presentation at our hospital. In contrast to the initial response, minimal tumour reduction was achieved following the second course, with continued progression of the lesion.

On physical examination, a firm, poorly circumscribed gingival mass measuring 2.0 × 1.4 × 0.9 cm² was identified adjacent to the right maxillary canine tooth (Triadan 104), with active bleeding noted from the lesion (Figure 1A–C). Palpation of the peripheral lymph nodes revealed bilateral submandibular lymphadenomegaly (right, 1.1 × 1.0 cm²; left, 1.5 × 1.2 cm²).

FIGURE 1.

Gross and computed tomographic findings of a gingival mass in a cat with extramedullary plasmacytoma. (A–C) Oral examination revealed a firm, poorly circumscribed gingival mass adjacent to the right maxillary canine (Triadan 104) with active surface bleeding. (D) Contrast‐enhanced computed tomography image showing a partially ill‐defined, round, heterogeneous contrast‐enhancing mass arising from the gingiva at the level of the right maxillary canine. (E) Bilateral mandibular lymphadenomegaly identified using computed tomography. (F) Lysis of the adjacent alveolar bone associated with the gingival mass.

Diagnostic imaging was performed to assess the tumour size, margins, local invasiveness and presence of distant metastases. Computed tomography revealed a partially ill‐defined, round, heterogeneously contrast‐enhancing, isoattenuating mass arising from the gingiva at the level of the right maxillary canine. Concurrent lysis of the adjacent alveolar bone was observed. Enlargement of the bilateral mandibular lymph nodes was also observed (Figure 1D–F).

To evaluate the tumour origin and malignancy grade and establish a definitive diagnosis, fine‐needle aspiration (FNA) and cytological examination of the mass were attempted, and additional histopathological and immunohistochemical analyses were performed on the previously excised tissue sample. Cytological evaluation revealed low cellularity and an overall homogeneous cell population. The predominant cells were individually exfoliated round cells with nuclei more than twice the diameter of neutrophils. The nuclei were pleomorphic in shape (round to oval, indented and lobulated) and finely stippled to coarse chromatin. Mild‐to‐moderate anisocytosis and anisokaryosis were observed, along with occasional multinucleated cells. A small proportion (<10%) of inflammatory cells, including neutrophils and small lymphocytes, was present, and no infectious organisms were identified (Figure 2A). On the basis of the cytological features, differential diagnoses for a round cell neoplasm included plasma cell neoplasm, high‐grade lymphoma and histiocytic neoplasm.

FIGURE 2.

Cytologic, histopathologic, immunohistochemical and flow cytometric findings of the gingival mass and regional lymph nodes. (A) Fine‐needle aspiration cytology of the gingival mass showing a low‐cellularity smear composed of a homogeneous population of round cells. The cells exhibit round to oval, occasionally indented and pleomorphic nuclei, with mild to moderate anisocytosis and anisokaryosis. Some cells contain prominent nucleoli, and occasional multinucleated cells are observed. A single neutrophil is present in the upper right field. (B) Histopathologic examination of the gingival mass demonstrating a densely cellular, poorly demarcated, infiltrative round cell neoplasm with frequent mitotic figures. (C and D) Immunohistochemical staining showing strong and diffuse cytoplasmic immunoreactivity of neoplastic cells for MUM‐1, consistent with extramedullary plasmacytoma. (E) Cytologic evaluation of the submandibular lymph node revealed a predominant population of large, atypical round to ovoid cells with marked nuclear atypia, prominent and multiple nucleoli, binucleation and multinucleation, admixed with small to intermediate‐sized lymphocytes, with one abnormal mitotic figure observed in the field, supporting metastatic involvement of the regional lymph node. (F) Flow cytometric immunophenotyping of the lymph node demonstrating heterogeneous expression of CD5 and CD21, with a predominant population of CD5‐ and CD21‐double‐negative cells, supporting the metastatic involvement of extramedullary plasmacytoma in the regional lymph node. Scale bars: A, E = 10 µm; B, D = 50 µm; C = 200 µm.

Histopathological examination performed to further differentiate the round cell neoplasms revealed a densely cellular, unencapsulated, poorly demarcated and infiltrative round‐cell neoplasm. Moderate anisocytosis and anisokaryosis were observed, with occasional binucleated and multinucleated cells, respectively. The tumour demonstrated high proliferative activity, with more than 20 mitotic figures per 10 high‐power fields (Figure 2B). Immunohistochemical staining was performed to establish a definitive diagnosis. Scattered, sparsely distributed cells within the tumour microenvironment showed faint‐to light‐positive staining for CD3 (T‐lymphocyte marker) and Pax5 (B‐lymphocyte marker); however, these cells were interpreted as non‐neoplastic reactive lymphocytes rather than neoplastic cells. In contrast, the neoplastic cell population demonstrated strong and diffuse positive immunoreactivity for MUM‐1 (plasma cell marker) (Figure 2C,D). On the basis of the morphological features and immunophenotypic profile, the gingival lesion was diagnosed as a plasma cell neoplasm consistent with EMP.

To evaluate the possibility of MM among the MRDs, additional systemic evaluations were performed. Radiographic examination revealed no lytic bone lesions other than localised osteolysis adjacent to the gingival mass. Urinalysis revealed a urine protein‐to‐creatinine ratio (UPC) of <0.20, and the sulfosalicylic acid (SSA) test was negative, indicating the absence of overt proteinuria. Serum biochemistry analysis (Catalyst One, IDEXX Laboratories, Westbrook, ME, USA) revealed no evidence of hyperglobulinaemia (3.2 g/dL; reference range, 2.8–5.1 g/dL). Although bone marrow evaluation was not performed, complete blood count (ProCyte Dx, IDEXX Laboratories, Westbrook, ME, USA) and blood smear analyses revealed no abnormalities. Although the available findings were not suggestive of MM, serum protein electrophoresis and bone marrow examination were not performed; therefore, systemic clonal plasma cell proliferation could not be fully excluded, and systemic staging remained incomplete.

Given the presence of bilateral submandibular lymphadenomegaly, further evaluation was performed to assess potential metastatic involvement. FNA and cytological examinations of the lymph nodes, followed by flow cytometric immunophenotyping, were performed. Cytological evaluation revealed a predominant population (>60%) of large atypical round cells with marked malignant features, admixed with small‐to‐intermediate‐sized lymphocytes (Figure 2E). For flow cytometric analysis, antibodies against CD21 (B‐cell marker; clone CA2.1D6, RPE‐conjugated, Bio‐Rad Antibodies, MCA1781R), CD5 (pan–T‐cell marker; clone FE1.1B11, FITC‐conjugated, Bio‐Rad Antibodies, MCA2038F), CD4 (helper T‐cell marker; clone VPG34, FITC‐conjugated; Bio‐Rad Antibodies, MCA1346F) and CD8 (cytotoxic T‐cell marker; clone VPG9, RPE‐conjugated; Bio‐Rad Antibodies, MCA1347R) were used. Notably, CD21‐ and CD5‐double‐negative cells comprised approximately 60% of the analysed population, corresponding closely to the proportion of predominant atypical cells identified by cytology (Figure 2F). In addition, CD21⁺ B cells, as well as CD4⁺ or CD8⁺ T cells, were heterogeneously observed within the population, and the overall immunophenotypic pattern made lymphoproliferative disease less likely. These findings strongly support the metastatic involvement of EMPs.

Complete surgical excision is the treatment of choice for EMP, and maxillectomy is recommended; however, the owner declined the procedure owing to its invasive nature. Given the limited response to repeated radiation therapy, ECT was subsequently selected as a less invasive treatment aimed at local tumour reduction. The potential benefits, risks and alternative treatment options were discussed with the owner, and written informed consent was obtained before initiating ECT. At that time, the owner declined immediate systemic therapy targeting suspected metastatic lesions and preferred close monitoring.

ECT was performed under general anaesthesia using an AgilePulse in vivo electroporation system (BTX, Holliston, MA, USA) with needle electrodes (model 47‐0070; BTX, Holliston, MA, USA; 6‐mm row spacing, 10‐mm needle length). Bleomycin was administered as an intravenous bolus via the cephalic vein at a dose of 15,000 IU/m², followed by the delivery of electric pulses according to the previously published ECT guidelines (Tellado et al. 2022). The electroporation parameters were as follows: pulse amplitude, 600 V; pulse duration, 0.1 ms; pulse interval, 50 ms; and eight consecutive pulses per treatment site.

ECT was administered weekly for three sessions. After completion of the third ECT session, tumour response was assessed before extraction of the adjacent mobile right maxillary canine tooth. At that time, the estimated volume of the gingival mass had decreased from 1.32 cm³ at initial presentation to less than 0.38 cm³ (Figure 3A–D). Following ECT, the patient exhibited no difficulty in eating or swallowing. Mild oedema, inflammation and focal necrosis were observed at the treatment site. Post‐treatment pain was assessed using the Feline Grimace Scale, and the recorded score was considered to warrant additional analgesia. Accordingly, gabapentin was administered at 10 mg/kg PO twice daily. Post‐procedural management also included a corticosteroid at 1 mg/kg PO once daily for its anti‐inflammatory effect and topical oral antisepsis with 0.05% chlorhexidine. The local adverse effects resolved within 1 week with supportive management. The mobile right maxillary canine tooth (Triadan 104) was subsequently extracted. After ECT and tooth extraction, no further increase in gingival mass size was observed. After thorough consultation, the owner elected to pursue close clinical monitoring rather than additional aggressive local therapy.

FIGURE 3.

Serial gross photographs demonstrating the response of the gingival extramedullary plasmacytoma to electrochemotherapy. (A) Gross appearance before treatment. (B) Tumour appearance after the first ECT session. (C) Tumour appearance after the second ECT session. (D) Tumour appearance after completion of three ECT sessions, showing a marked reduction in tumour volume.

Four months after the completion of ECT, the size of the gingival mass remained comparable to that immediately after treatment. However, progressive enlargement of the bilateral submandibular lymph nodes, suspected to represent metastatic disease, was noted (right: 2.6 × 1.9 cm²; left: 3.2 × 2.6 cm²). Additional systemic evaluation revealed no evidence of progression to MM. Given that regional lymph node involvement had been documented at the time of initial presentation, lymphadenopathy was considered consistent with the progression of metastatic disease rather than de novo systemic MM.

After consultation with the owner, written informed consent was obtained for the extra‐label use of cyclophosphamide, and systemic chemotherapy was initiated. The treatment consisted of cyclophosphamide (250 mg/m², PO, every 3 weeks) in combination with prednisolone (1 mg/kg, PO, once daily). After the first chemotherapy cycle, a reduction in lymph node size was observed (right: 2.4 × 1.6 cm²; left: 2.4 × 1.9 cm²). However, considering the cat's quality of life, further chemotherapy was discontinued at the owner's request, and the treatment plan was transitioned to hospice care, focusing on analgesia and inflammation control. The cat died 4 months later due to an unrelated disease.

2. Discussion

MRDs are rare in cats, and EMP is considered an exceedingly uncommon manifestation. Cutaneous and oral EMPs are generally regarded as having a benign clinical course, and complete surgical excision is often considered curative. However, the case presented herein demonstrated several features suggestive of more aggressive biological behaviour, including tumour‐associated lysis of the adjacent alveolar bone, a high mitotic index on histopathology and suspected regional lymph node involvement.

Importantly, the aggressive behaviour and metastatic potential of EMPs have been reported. Regional lymph node metastasis has been documented in cats with intraocular EMP in the absence of MM (Michau et al. 2003), and kidney metastasis has been described in a dog with oral EMP (Park et al. 2023), indicating that EMPs may exhibit heterogeneous biological behaviour. Taken together, these findings suggest that EMP may present with a wide range of clinical manifestations and should not be uniformly regarded as a biologically benign tumour.

Although the cat did not die as a direct consequence of the neoplasm, its clinical course exhibited more aggressive features than those typically associated with the generally benign behaviour of EMP. The relatively prolonged survival observed in this case may have been influenced by the therapeutic intervention. Accordingly, this case highlights that the prognosis of EMP may vary substantially among individual patients, underscoring the importance of close monitoring and proactive therapeutic management even after EMP diagnosis.

Information on factors that can reliably predict the clinical behaviour or prognosis of EMPs remains limited. In a retrospective study evaluating canine oral EMPs, the mitotic count was found to be significantly associated with tumour progression, whereas the degree of nuclear atypia was associated with tumour‐related death (Evenhuis et al. 2023). However, these findings should be interpreted with caution owing to the retrospective nature of the study, lack of standardised treatment protocols and limited sample size.

On the basis of a previously proposed morphological classification scheme for canine EMPs (Platz et al. 1999), feline EMPs are categorised into four histological subtypes. These range from the mature type, which closely resembles well‐differentiated plasma cells, to the polymorphous type, characterised by marked anisocytosis, polymorphous tumour cells and numerous giant cells, highlighting the considerable morphological heterogeneity of feline EMPs. However, due to the limited number of reported cases, the prognostic significance of this histological classification in feline EMPs remains unclear (Majzoub et al. 2003). Furthermore, previous studies in dogs have demonstrated that neither the analysis of MIB1 proliferation rates nor follow‐up outcome data support the use of morphological subtypes as a reliable tumour grading system (Platz et al. 1999). In the present case, the tumour exhibited a morphology most consistent with the cleaved subtype; however, whether this histological classification correlates with tumour progression, including metastasis or recurrence, warrants further investigation.

Reliable prognostic biomarkers have therefore not yet been established for feline EMP. Future multicentre studies should evaluate standardised histological variables, including tumour differentiation, mitotic count, nuclear atypia and Ki‐67/MIB‐1 proliferation indices, in larger feline EMP cohorts and determine whether these parameters are independently associated with local recurrence, metastasis or disease‐specific survival. Advanced molecular approaches, including molecular profiling and paired comparison of primary and metastatic lesions, may further help clarify the biological heterogeneity and patterns of progression of feline EMP.

Although uncommon, systemic progression from EMP to MM has been reported in cats (Radhakrishnan et al. 2004). Unlike EMP, MM is a systemic proliferative disorder involving neoplastic plasma cells originating in the bone marrow. In cats, MM is generally diagnosed when at least two of the following criteria are met: (1) paraproteinemia or monoclonal gammopathy, (2) radiographic evidence of osteolytic bone lesions, (3) Bence‐Jones proteinuria and (4) bone marrow infiltration by neoplastic plasma cells exceeding 5% (Hanna 2005; Vail 2020). MM is generally associated with less favourable outcomes than EMP (Radhakrishnan et al. 2004). In the present case, haematological and imaging evaluations were performed to assess the possibility of progression to MM; however, no supportive evidence was identified. Nevertheless, systemic staging was incomplete because serum protein electrophoresis and bone marrow cytology were not performed. Abnormal immunoglobulin production may occur despite the absence of hyperglobulinaemia (Kyle 1975). In addition, bone marrow involvement cannot be definitively excluded without bone marrow examination, even in the absence of circulating neoplastic cells in the peripheral blood. Therefore, systemic MM could not be completely excluded, and the diagnosis of primary gingival EMP should be considered presumptive. Despite these limitations, no overt clinical abnormalities suggestive of MM were observed during the continued monitoring.

Previous studies in humans support a multistep transformation model of myeloma, in which neoplastic transformation originates within the bone marrow, followed by progressive dedifferentiation and subsequent dissemination to extramedullary sites over time (Hallek et al. 1998). In contrast, feline MRDs appear to follow distinct pathogenic pathways. Notably, extramedullary involvement has been reported to be significantly more common in cats with well‐differentiated plasma cell neoplasms than in humans (90% vs. 20%, p < 0.0002), leading to the proposal of a primary extramedullary neoplastic transformation model for cats (Mellor et al. 2008). Accordingly, in cats, EMP is not necessarily a manifestation of advanced MM but may frequently arise as a primary lesion. Considering the clinical presentation, available diagnostic findings and previously proposed feline disease model, a primary gingival EMP was considered more likely than an extramedullary manifestation of progressive MM.

ECT enhances intracellular drug delivery through electric pulses and may represent an effective alternative for tumours resistant to conventional chemotherapy or radiation therapy or when surgical excision is not feasible. Additionally, electric pulses induce vascular blockade, promoting the retention of chemotherapeutic agents within the tumour vasculature, resulting in direct damage to tumour blood vessels and reduced tumour blood supply within hours of treatment. Furthermore, electrical pulses may exert immunoadjuvant effects by promoting the release of damage‐associated molecular patterns (DAMPs) from cells, thereby inducing immunogenic cell death (Holanda et al. 2025). ECT has been used to treat various histologic tumour types, with reported objective response rates ranging from approximately 70% to 100% (Tellado et al. 2022).

ECT has been increasingly applied to treat various neoplastic conditions in dogs and cats. In cats, the reported indications include squamous cell carcinoma (SCC), incompletely excised soft tissue sarcomas and feline injection‐site sarcomas (Ramos et al. 2024). However, to the best of our knowledge, the use of ECT for feline EMP has not been previously reported, although several studies have described its use for canine EMPs. All dogs were histopathologically diagnosed with EMP and underwent ECT with either intravenous bleomycin or intratumoural cisplatin. Complete remission was achieved after one–two treatment sessions in all the patients. Mild adverse effects were observed but resolved within several days (de Castro Cunha et al. 2017; Santos dos Anjos et al. 2020). Although direct evidence in feline EMP is lacking, the favourable outcomes reported in canine EMPs, together with the broader use of ECT in feline oncology, supported the selection of ECT for local cytoreduction in the present case.

Nevertheless, evidence on ECT for primary feline oral tumours remains limited. In a retrospective study of 24 cats, only one had a primary oral SCC and achieved a complete response after one ECT session; although 67% of the overall cohort had local toxicity scores >2, toxicity was not analysed by tumour location (dos Santos et al. 2024). Therefore, poorer response or greater toxicity specifically attributable to oral tumours has not been established. Nevertheless, because post‐treatment oedema, pain, inflammation and necrosis may have substantial functional consequences in the oral cavity, careful patient selection, analgesia and close monitoring are warranted. In the present case, ECT was selected for short‐term local cytoreduction and palliation because maxillectomy was declined, the response to repeated radiotherapy was limited, and the lesion was accessible to the electrodes.

In the present case, three sessions of ECT were performed to reduce the tumour volume. In general, tumours measuring up to 3 cm³ are considered suitable candidates for ECT, regardless of histologic type or morphology (Tellado et al. 2022). In the present case, the tumour volume was 1.32 cm³, indicating that the lesion was within the appropriate size range for treatment. To assess the therapeutic response, the tumour volume was calculated using the formula V = abcπ/6, where ‘a’, ‘b’ and ‘c’ represent the length, width and thickness of the tumour, respectively. Following the completion of the three treatment sessions, the tumour volume decreased by more than 70%, indicating a treatment response consistent with at least a partial response.

This marked response contrasted with the limited response to the second course of palliative radiotherapy. In preclinical head and neck SCC models, radiosensitive and radioresistant tumours showed comparable responses to bleomycin‐based ECT, suggesting that reduced radiosensitivity does not necessarily confer resistance to this treatment (Zakelj et al. 2019). ECT also induces an immediate vascular lock followed by prolonged tumour vascular disruption through endothelial injury, thereby reducing tumour perfusion and oxygenation and promoting tumour necrosis (Sersa et al. 2008). These mechanisms may have contributed to the local response observed in this case; however, this interpretation remains speculative because tumour radiosensitivity and vascularity were not directly evaluated.

Bleomycin is associated with both acute and delayed toxicity. The reported acute toxicities include fever, anorexia, mild vomiting and allergic reactions. Delayed toxicities may include dermatological effects, stomatitis and pulmonary toxicities, such as pneumonitis and pulmonary fibrosis (McCluskey 2023). Careful monitoring of the cumulative dose is essential to minimise the risk of pulmonary toxicity. The maximum cumulative dose in dogs has been reported to be 200,000 IU/m². However, a definitive maximum cumulative dose has not yet been established in cats (Tellado et al. 2022). In the present case, the cumulative bleomycin dose was less than 50,000 IU/m², and no clinically apparent systemic adverse effects attributable to bleomycin were observed during the available follow‐up period. Nevertheless, because a species‐specific maximum cumulative dose has not been established, the risk of severe and irreversible delayed toxicity, including pulmonary fibrosis, could not be quantified or fully excluded. This represents an important clinical knowledge gap and should be considered when evaluating the safety of repeated ECT sessions involving bleomycin in cats. Mild local oedema and inflammatory changes at the ECT‐treatment site were noted for several days following ECT but resolved within 1 week with supportive management. A previous study proposed a 5‐point local toxicity scale for ECT, according to which the present case corresponded to a score of 2 (oedema/necrosis <1 cm) (Lowe et al. 2017).

Importantly, the response to ECT should be interpreted in the context of the patient's overall disease course. Although the gingival mass remained macroscopically stable for 4 months after ECT, the preexisting bilateral mandibular lymph node lesions progressively enlarged during the same period. Therefore, ECT achieved short‐term local tumour reduction and palliation but failed to control the regional metastatic disease. This clinical course highlights the inherent limitation of ECT as a local treatment modality, as local tumour control does not necessarily prevent progression of regional or systemic disease, particularly in tumours with potentially aggressive biological behaviour. The local response should not be interpreted as curative or as evidence of overall disease control, given the high mitotic activity of the tumour and the presence of suspected regional lymph node involvement.

Accordingly, ECT may be considered as an alternative local treatment option when complete surgical excision is not feasible, rather than as an independent replacement for definitive surgery. In clinically appropriate cases, it may also be incorporated into multimodal treatment strategies, including neoadjuvant cytoreduction before function‐preserving surgery, adjuvant treatment of incompletely excised margins or combined local treatment with radiotherapy. When regional lymph node involvement is present, additional treatment directed at metastatic lesions and/or systemic therapy should also be considered. However, these multimodal approaches have not yet been evaluated in feline EMP and require further investigation.

In conclusion, EMP generally exhibits a benign clinical course, although a subset of cases may demonstrate aggressive behaviour, underscoring the importance of long‐term monitoring and individualised treatment planning. Currently, reliable prognostic factors for predicting the biological behaviour of EMP are lacking, highlighting the need for further investigation. This report describes the first application of ECT for unresectable feline EMP. ECT achieved a marked reduction in tumour volume with minimal adverse effects, suggesting that it may be a valuable short‐term local treatment option for feline EMP when complete surgical excision is not feasible.

Author Contributions

Seulgi Hwang: conceptualisation, data curation, formal analysis, investigation, visualisation, writing – original draft preparation. Hayoung Ryu: data curation, formal analysis, investigation, validation. Dong‐In Jung: methodology, project administration, supervision. DoHyeon Yu: methodology, project administration, supervision. Kyu‐Woan Cho: methodology, project administration, supervision. Hyeona Bae: conceptualisation, methodology, project administration, resources, supervision, writing – review and editing.

Funding

This research was supported by the Regional Innovation System & Education (RISE) programme through the RISE Center, Gyeongsangnam‐do and funded by the Ministry of Education (MOE) and the Gyeongsangnam‐do Provincial Government, Republic of Korea (2025‐RISE‐16‐001), and the Korea Institute of Planning and Evaluation for Technology in Food, Agriculture and Forestry (IPET) through the Agriculture and Food Convergence Technologies Program for Research Manpower development funded by the Ministry of Agriculture, Food and Rural Affairs (MAFRA) (grant number: RS‐2024‐ 00398561).

Ethics Statement

The authors confirm that this study complies with the journal's ethical policies as stated in the author guidelines. IRB/IACUC approval was not required, as no experimental procedures involving human or animal subjects were conducted. The authors confirm compliance with the ethical standards of the journal as outlined in the author guidelines.

Consent

Informed consent was obtained from the dog's owners for disclosure of case details.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data supporting the findings of this study are available from the corresponding author upon request.