Cytological and histopathological findings of recurrent canine squamous cell carcinoma

Introduction

Squamous cell carcinoma (SCC) is a malignant epithelial neoplasm arising from keratinocytes and is recognized as an important cutaneous malignancy in dogs. SCC accounts for approximately 5% of cutaneous neoplasms in canines and commonly occurs in the oral cavity, nail beds, paw pads, nasal planum, ears, abdomen, extremities and perianal region, with a tendency to occur in sparsely haired or mucocutaneous areas (Begum et al., 2023; Schneider et al., 2021) [2] . The development of canine cutaneous SCC is multifactorial. Chronic ultraviolet (UV) exposure is an important environmental risk factor, particularly in poorly pigmented and sparsely haired skin, along with viral, immunological and alterations in cellular adhesion (Alves et al., 2022; Schneider et al., 2021) [1] . Tumor may present from relatively differentiated lesions with limited metastatic potential to more invasive tumours with a greater risk of regional spread (Schneider et al., 2021; Chandrashekaraiah et al., 2011) [3] . Cytology is useful for recognizing epithelial malignancy and guiding initial diagnostic decisions, whereas histopathology provides information on tissue invasion, differentiation, keratinization, stromal response and surgical margins (Begum et al., 2023; Nagamine et al., 2017) [2,6] . In canine SCC, histological evaluation of the invasive front may be particularly informative because the advancing tumour front can display a different grade from the superficial component (Nagamine et al., 2017). Local recurrence is clinically important because residual microscopic tumour at or near the surgical margin may permit regrowth. In a prospective study of surgically excised cutaneous malignant tumors, infiltrated and close margins were associated with substantially more recurrence than clean margins, supporting careful histological assessment of excision margins (Scarpa et al., 2012) [7] . The current describes a canine cutaneous SCC that recurred only 45 days after surgical excision and correlates the clinical appearance with cytological and histopathological findings. The case is relevant because the rapid recurrence illustrates tumor grading, assessment of local invasion and margin evaluation is necessary.

Materials and Methods

In the present case, a non-descript intact male dog aged four years was presented with a history of an ulcerated mass in the right shoulder region for approximately one month. Grossly the mass was erythematous, firm, raised and ulcerated with maggot infestation. Fine needle aspiration cytology (FNAC) was performed using 22-gauge needles, with smears stained using Giemsa stain. The cytological smears were initially assessed for staining quality and cellular adequacy. Cytological assessment focused on cellularity, cellular arrangement, nuclear and cytoplasmic morphology, pleomorphism, keratinization and associated inflammatory cells. Representative tumor tissues were fixed in 10% neutral buffered formalin, processed, and processed using routine paraffin embedding techniques. Thin sections measuring 4 µm were prepared with a rotary microtome and stained using the Hematoxylin and Eosin (H & E). Histopathological interpretation considered cellular architecture, degree of differentiation, keratinization, nuclear pleomorphism, stromal response and invasion.

Results

On clinical examination, 5 × 5 × 3 cm mass was observed at the right shoulder region with a history of gradual increase in size in the last 1 month (fig. 1and fig. 2). FNAC smears showed moderate cellularity. Neoplastic cells were round to polygonal and occurred predominantly in clusters, with occasional singly dispersed cells. The cells exhibited anisocytosis and anisokaryosis, an increased nuclear-tocytoplasmic ratio, distinct cytoplasmic borders, coarse chromatin and multinucleation. Occasional emperipolesis and a variable inflammatory background were also observed. Collectively, the cytomorphological features supported a diagnosis of epithelial malignancy consistent with carcinoma (fig. 5; fig. 6; fig. 7). The animal underwent surgical excision of the mass. Grossly, the cut surface of the excised tissue was ulcerated, white in color and friable in consistency (fig. 3). Histopathology revealed invasive nests of neoplastic squamous epithelial cells infiltrating the dermis and accompanied by a desmoplastic stromal response. The neoplastic cells had eosinophilic cytoplasm, vesicular nuclei, intercellular bridges and variable nuclear pleomorphism (fig. 8). Occasional keratin pearls were present (fig. 9). On the basis of the reported cellular organization, keratinization and stromal invasion, the primary lesion was diagnosed as moderately differentiated cutaneous squamous cell carcinoma. Despite surgical excision, recurrence occurred 45 days later cranial to the original lesion (fig. 4). Histopathological examination of the recurrent tissue revealed nests and cords of atypical squamous cells, keratin pearl formation and stromal invasion (fig. 10). These findings are suggestive of recurrent SCC and were supportive of persistence of locally invasive neoplastic cells and/or inadequate clearance at the primary site. The short recurrence interval is clinically significant. The recurrence in this case should not, however, be attributed to incomplete margins alone but also consider microscopic extension beyond the grossly visible lesion and the intrinsic local invasiveness of SCC.

Discussion

Canine cutaneous SCC is characterized by a range biological behavior as well as etiological factors and presents a variation in gross appearance, microscopic appearance and histological grade (Schneider et al., 2021). In the present case, the clinical appearance of the mass with maggot infestation is compatible with a chronic ulcerated malignant cutaneous lesion. Myiasis may develop secondarily in ulcerated masses and should not be regarded as the primary diagnosis when the underlying lesion is neoplastic. The cytological findings were strongly supportive of epithelial malignancy. Clustering of polygonal cells, distinct cytoplasmic borders, anisocytosis, anisokaryosis, increased nuclear-to-cytoplasmic ratio and multinucleation are characteristic cytological features of SCC (Begum et al., 2023; Marques et al., 2020) [2,5] . A previous review of 17 canine SCCs similarly reported that malignant squamous cells occurring singly or in clusters, with pleomorphism, anisokaryosis and anisocytosis (Marques et al., 2020) [5] . Previous reports also suggested that well-differentiated tumors showed keratin pearls and moderately differentiated tumours formed cords or nests separated by fibrous stroma (Begum et al., 2023; Marques et al., 2020) [2,5] . Histopathology remains essential because cytology cannot fully characterize tissue invasion and margin status. In the present case, dermal invasion, desmoplasia, intercellular bridges and keratin pearl formation supported a diagnosis of moderately differentiated SCC. These findings are consistent with the literature in which the degree of squamous differentiation and tumour architecture are used for histological grading (Schneider et al., 2021; Marques et al., 2020). Assessment of the invasive nature is increasingly recognized as an important component of SCC grading. Nagamine et al. (2017) [6] reported that grading based on invasiveness could yield higher histological grades than grading based on the superficial component and suggested that epithelial mesenchymal transition-related changes may contribute to invasive behavior. The most striking feature of this case was recurrence 45 days after excision. Histological margin evaluation has been shown to be clinically useful in predicting recurrence of cutaneous malignant tumors. In a study by Scarpa et al. (2012) [7] , recurrence was markedly higher when margins were infiltrated or close compared with clean margins, and the authors recommended prolonged postoperative surveillance. The rapid recurrence in the present case therefore supports the need for submission of the entire excised lesion for careful margin evaluation and for follow-up examinations even when the primary tumor appears clinically removed. Recent studies also reinforce the prognostic value of histological grade in canine cutaneous SCC. A case-series study of inguinal SCC reported a substantially longer median survival in dogs with low-grade tumours than in those with the highest grade and concluded that histological grade and clinical staging were useful prognostic factors (Jiménez-Alonso et al., 2025) [4] . Molecular and immunohistochemical studies have investigated additional prognostic markers. Thus, conventional histopathological assessment remains central to routine diagnosis, while additional biomarkers should be considered adjunctive rather than replacements for morphological assessment.

Conclusion

The current case demonstrates recurrent canine cutaneous squamous cell carcinoma with close clinicopathological correlation. FNAC provided an early indication of epithelial malignancy, while histopathology established the diagnosis and demonstrated invasive squamous differentiation with keratin pearl formation. Recurrence within 45 days highlights the locally invasive potential of canine SCC and the importance of complete excision, meticulous histological margin assessment and continued postoperative surveillance. This illustrates that apparently successful initial surgery does not eliminate the need for postoperative monitoring, particularly when the tumor is ulcerated, infiltrative or incompletely characterized at the margins.

References

1. Alves DS, Cavalca AMB, Alves CEF. A critical review of the risk factors associated with canine squamous cell carcinoma development. Braz J Vet Pathol. 2022;15(1):110.
2. Begum S, Deka A, Tamuli SM, Dutta B, Buragohain M. Histopathological and cytopathological findings of canine squamous cell carcinoma. Haryana Vet. 2023;62(SI2):162-164.
3. Chandrashekaraiah GB, Rao S, Munivenkatappa BS, Mathur KY. Canine squamous cell carcinoma: a review of 17 cases. Braz J Vet Pathol. 2011;4(2):79-86.
4. Jiménez-Alonso, A. A., Pérez-Santana, C. G., CazorlaRivero, S. E., Rodríguez-Esparragón, F., & Rodríguez Grau-Bassas, E. (2025). Clinical Staging and Histopathological Grading of Inguinal Squamous Cell Carcinoma in Dogs: A Case Series Study. Veterinary medicine (Auckland, N.Z.), 16, 1-8.
5. Marques GR, Rocha LF, Vargas THM, Pulz LH, Huete GC, Cadrobbi KG, Pires CG, Sanches DS, Mota EFFM, Strefezzi RF. Relationship of Galectin-3 expression in canine cutaneous squamous cell carcinomas with histopathological grading and proliferation indices. J Comp Pathol. 2020; 178:16-21.
6. Nagamine E, Hirayama K, Matsuda K, Okamoto M, Ohmachi T, Uchida K, Kadosawa T, Taniyama H. Invasive front grading and epithelial-mesenchymal transition in canine oral and cutaneous squamous cell carcinomas. Vet Pathol. 2017;54(5):783-791.
7. Scarpa F, Sabattini S, Marconato L, Capitani O, Morini M, Bettini G. Use of histologic margin evaluation to predict recurrence of cutaneous malignant tumors in dogs and cats after surgical excision. J Am Vet Med Assoc. 2012;240(10):1181-1187.