Clinical Utility of ctDNA in Detecting Resistance Mechanisms and Delivering Precision Medicine to Cancer Patients

ctDNA stands for circulating tumour DNA. As ctDNA is released by tumour cells into the blood stream, taking a blood sample and analysing it for ctDNA, can provide a lot of useful information about a patient's cancer. In certain situations, ctDNA can be used to screen for or detect cancer early, to aid clinical decisions about which treatment to give a patient, to provide information about if a cancer has become resistant to treatment, or provide information about how much cancer may be left after treatment (residual disease). The aim of this trial is to establish the clinical utility of implementing ctDNA testing in cancer patients with a view to enhance the delivery of personalised care within the National Health Service in the United Kingdom (UK). One hundred patients will be recruited, with 20 from each of the following cancer types: * Non-small cell lung cancer * Gastrointestinal stromal tumours * Colorectal cancer * Biliary tract cancer * Ovarian cancer. Patients must be aged 18 or over, must have had progressive disease whilst receiving anti-cancer treatment, and must be being treated at The Royal Marsden. Patients will have a blood sample taken and analysed using the Marsden360 ctDNA test. The results of the test will be looked at by The Royal Marsden Genomic Tissue Advisory Board (GTAB), and for each individual patient, the GTAB will determine if having a ctDNA test helped to personalise their care by: * Aiding the identification of a genomically-matched standard of care therapy * Aiding the identification of a genomically-matched clinical trial (based in the UK) * Offering additional prognostic information not otherwise available through standard of care testing * Negating the need for a tissue biopsy.