Perianal Bowen's disease treated with radiotherapy preserving anal function with a unique skin reaction considered as 'tumoritis'.

Bowen's disease (BD) is a form of intraepidermal squamous cell carcinoma (SCC), and it occasionally occurs on the perianal site. BD is often treated with surgical excision; however, sometimes surgical excision for perianal BD cannot preserve anal function. We report the case of a 72-year-old man presenting with perianal pain and BD. He was treated with Radiotherapy (RT) and preserved his normal anal sphincter function without any recurrence or late adverse event. Moreover, we observed the unique skin reaction called 'tumoritis', which is characterized by mucosal inflammation. Tumoritis indicates the true extent of the tumor and evaluating the tumor or lesion size based on the extent of tumoritis when performing RT is important.