Background: Circulating tumor DNA (ctDNA) is tumor-derived fragment of DNA circulating in plasma. ctDNA has been used to detect minimal residual disease (MRD) in early-stage cancers after curative intent therapy. However, ctDNA based MRD concept is not investigated in metastatic disease. There is a lack of clarity around optimal timing to stop immune checkpoint inhibitors (ICI) in melanoma and lung cancer patients who have achieved durable response (DR) and MRD may have a role here to select right patients to stop therapy at an optimal time. Methods: This is prospective study on metastatic lung cancer and melanoma patients with DR treated with ICI. DR is identified as objective response (CR/PR) by modified WHO criteria lasting ≥6 months continuously. We utilized Signatera assay by Natera. Signatera uses tissue from original biopsy of the patients and develop multiplex PCR based personalized assay to detect MRD. The level of ctDNA is reported in mean tumor molecules per mL. We obtained ctDNA assay at baseline, month 3 and month 6 at their routine follow up visits. If applicable, ctDNA assay was done at the time of progression, and 3 months after resuming treatment. Results: We identified 29 melanoma and 17 Lung cancer (both small cell and non-small cell) patients with metastases and DR. Median age was 69 years in both cohorts. Median duration of ICI was 14 months in melanoma and 24 months in lung cancer cohort. ctDNA assay was obtained in 26 melanoma and 13 lung cancer patients. One small cell lung cancer patient had detectable ctDNA at baseline, although radiographically, the patient had DR. The rest of the patients in both cohorts had undetectable ctDNA. 2 melanoma patients developed detectable ctDNA on subsequent assays and their therapies were changed. One small cell lung cancer patient has persistently detectable ctDNA on subsequent assays but remained in DR. A total of 10 patients in both cohorts decided to stop their therapy following several -ve ctDNA assay. Conclusions: The role of ctDNA needs to be investigated in advanced/metastatic settings as MRD may have a role to identify patients who may benefit from treatment break. MRD surveillance may also support radiographic assessment.