A remnant never forgotten: the utility of circulating tumor DNA in treatment guidance of urachal cancer.

Urachal cancer is a rare malignancy of the urachus that is treated with surgical resection if localized and systemic chemotherapy for metastatic disease. Circulating tumor DNA (ctDNA) is a single-stranded or double-stranded DNA released by tumor cells into the blood and harbored the mutations of the original tumor, shedding new light on molecular diagnosis and monitoring of cancer. We report a case of resected localized urachal cancer with clear surgical margins and negative lymph node dissection but elevated ctDNA that progressed to metastatic disease. We also highlight the possibility of using ctDNA levels to assist in adjuvant therapy.