Detection and differential diagnosis of cancer relapse using circulating tumor DNA profiling in a patient with synchronous breast and rectal cancer

Background: In Chinese women, breast and colorectal cancers are highly prevalent. In the early stage, the primary treatment for these cancers is surgical resection. However, many patients develop a metastatic recurrence. Thus, tools that help estimate the risk of recurrence are critical. Although synchronous breast and rectal cancer is uncommon, estimating recurrence risk is even more challenging in patients with two histologically distinct malignancies. Methods: Next generation sequencing (NGS) allows the comprehensive detection of simultaneous genome abnormalities. NGS-based circulating tumor DNA (ctDNA) profiling is a new molecular technique that has demonstrated great potential in the detection and differential diagnosis of cancer relapse. Results: We present a 43-year-old female patient with synchronous breast and rectal cancer that was surgically removed 2 years prior. During regular follow-up, elevated carcinoembryonic antigen (CEA) levels were detected. ctDNA profiling revealed multiple somatic mutations that were identical to those found in rectal cancer samples. Thus, we suspected relapse of rectal cancer. Positron emission tomography-computed tomography (PET-CT) and pathogenic analysis confirmed lung metastasis of rectal cancer. Conclusions: This case demonstrated the utility of ctDNA profiling in the detection and differential diagnosis of cancer relapse in a patient with synchronous breast and rectal cancer.