Evaluating Tumor Dynamics Using Circulating Tumor DNA in the Rare Cancer Anorectal Malignant Melanoma: A Report of Two Cases

Abstract

INTRODUCTION: Anorectal malignant melanoma (ARMM) is an extremely rare and aggressive cancer that lacks specific tumor markers, making tumor dynamics difficult to monitor. Circulating tumor DNA (ctDNA), which contains cancer-specific gene mutations, has emerged as a promising biomarker for monitoring various malignant tumors. We herein report the clinical utility of ctDNA measurements in 2 patients with ARMM.

CASE PRESENTATION: Case 1: A 64-year-old woman diagnosed with clinical Stage III ARMM underwent laparoscopic abdominoperineal resection with lateral lymph node dissection. Pathological findings confirmed R0 resection, and genetic analysis identified a neuroblastoma RAS viral (v-ras) oncogene homologue (NRAS) mutation (p.G12S). She remained recurrence-free for 7 years postoperatively. In a longitudinal ctDNA study, the preoperative ctDNA level (28.7 copies/mL) became undetectable after curative resection and remained undetectable throughout follow-up. By contrast, lactate dehydrogenase (LDH) currently the only blood-based tumor marker for mucosal melanoma, including ARMM, remained within the normal range (124–222 U/L) both preoperatively and postoperatively. Case 2: A 69-year-old woman diagnosed with clinical Stage III ARMM underwent laparoscopic abdominoperineal resection with lateral lymph node dissection. Pathological findings confirmed R0 resection, and genetic analysis identified two KIT proto-oncogene receptor tyrosine kinase (KIT) mutations (p.Y553H and p.Y646C) in exons 11 and 13. One year postoperatively, lung metastases were confirmed via positron emission tomography-computed tomography, and nivolumab monotherapy was initiated. The lung metastases remained stable for 7 months. In a longitudinal ctDNA study, the preoperative ctDNA level (28.7 copies/mL) became undetectable following curative resection. However, ctDNA re-emerged (9.05 and 7.2 copies/mL, respectively) at the time of lung metastasis detection. After initiating nivolumab treatment, ctDNA again became undetectable. Throughout the clinical course, the LDH level remained consistently within the normal range.

CONCLUSIONS: We demonstrated the utility of ctDNA monitoring in patients with ARMM, a rare tumor. This method has the potential to be applied to other rare tumors if tumor-specific mutations can be identified.