Validation of Circulating Tumor DNA Assays for Detection of Metastatic Melanoma

The detection of cell-free, circulating tumor DNA (ctDNA) in the blood of patients with solid tumors is often referred to as "liquid biopsy." ctDNA is particularly attractive as a candidate biomarker in the blood. It is relatively stable after blood collection, can be easily purified, and can be quantitatively measured with high sensitivity and specificity using advanced technologies. Current liquid biopsy research has focused on detecting and quantifying ctDNA to (1) diagnose and characterize mutations in a patient's cancer to help select the appropriate treatment; (2) predict clinical outcomes associated with different treatments; and (3) monitor the response and/or progression of a patient's disease. The diagnostic use of liquid biopsies is probably greatest in tumors where the difficulty and/or risk of obtaining a tissue specimen for molecular diagnostics is high (e.g., lung, colon). In metastatic melanoma, however, obtaining a tissue sample for molecular diagnostics is not typically a major obstacle to patient care plans; rather predicting treatment outcomes and monitoring a patient's disease course during therapy are considered the current priorities for this cancer type. In this chapter we describe an approach to the validation of ctDNA detection assays for melanoma, focusing primarily on analytical validation, and provide methods to guide the use of droplet digital PCR assays for measuring ctDNA levels in plasma samples.