Case report: Secondary peristomal cutaneous squamous cell carcinoma following multimodal treatment for rectal cancer.

Introduction: This case report describes the rare development of a highly differentiated, keratinising squamous cell carcinoma on the skin of the anus praeter following multimodal therapy for rectal cancer. The case was successfully managed through structured follow-up care and interdisciplinary collaboration. Case presentation: We report on the complex case of a patient with rectal carcinoma, which was treated with neoadjuvant radiochemotherapy, followed by surgical resection and adjuvant chemotherapy. During treatment, a low anterior resection syndrome developed, resulting in the creation of an end colostomy. Verrucous skin ulcerations developed around the stoma, leading to the development of squamous cell carcinoma of the skin based on this chronic inflammation. Discussion: Secondary malignancies are a significant late complication of radiotherapy or combined radiochemotherapy. While high doses of radiation cause cell death, malignant transformations can be promoted in the low-dose peripheral areas of the radiation fields. In the present case, however, the chronic inflammatory skin change indicates inflammation-induced carcinogenesis, which is described in the literature as a central pathogenetic mechanism. Conclusion: This case highlights the importance of close follow-up care and a holistic approach to patient care. It also illustrates the importance of recognising and managing treatment complications. Furthermore, it raises awareness of the development of secondary malignancies and malignant transformations.