A 70-year-old female diagnosed case of rectal carcinoma (T3N2M0) received FOLFOX × 2 cycles followed by CAPOX × 2 cycles. She then underwent chemoradiation using capecitabine as a radiosensitiser, followed by laparoscopic tumor resection. The histopathology report showed tumour regression grade 2 (TRG2) response.

Subsequently, the patient received 4 cycles of CAPOX but developed grade 2 peripheral neuropathy, leading to modification of her treatment to capecitabine alone for an additional 2 cycles. A whole-body positron emission tomography–computed tomography (PET-CT) scan at this stage showed no evidence of disease. However, a liquid biopsy test detected the presence of two circulating tumor cells (CTCs).

An MRI of the abdomen and pelvis was conducted, revealing multiple live lesions (4 mm–6 mm) in segment IV/VIII of the liver, with no sign of local disease. To manage liver metastasis, the patient received 1 cycle of FOLFIRI while awaiting selective internal radiotherapy (SIRT), followed by 5 cycles of FOLFIRI. Three years later, her PET scans were observed to be completely normal.

This case highlights the critical role of CTCs as a biomarker for detecting minimal residual disease (MRD) or relapse. Without CTC monitoring, the liver metastasis—which was successfully treated with SIRT—would likely have been missed under standard cancer care guidelines. As of today, the patient is completely disease-free, underscoring the importance of thorough investigation using advanced CTC liquid biopsy biomarkers in managing rectal cancer with liver metastasis.