Abstract
A 78-year-old woman was diagnosed with lower rectal cancer extending to the dentate line. She underwent preoperative chemoradiotherapy, which resulted in tumor shrinkage and clinical complete response (cCR), leading to the implementation of the "Watch and Wait" strategy. At the 21-month surveillance mark, pruritus and erythematous lesions appeared around her anus, which were histologically diagnosed as secondary Paget disease with invasion based on biopsy specimens. Because the primary lesion maintained cCR, local excision of the skin lesion was performed, and negative resection margins were confirmed by pathological examination. At the 4-year postoperative follow-up, neither local recurrence nor distant metastases were observed. This case suggests that although the primary focus disappeared entirely with the use of chemoradiotherapy, microscopic perianal secondary Paget disease hidden outside the radiation field became apparent during surveillance. As total neoadjuvant therapy (TNT) becomes more widespread, the frequency of cCR is expected to increase. During the "Watch and Wait" period, vigilant observation of the perianal skin as well as the rectal region should be performed.