PTEN and neoplasm: However, blood-based testing is associated with technical limitations, such as low or absent circulating tumor DNA (ctDNA) resulting in low tumor fraction (TF; representing the fraction of circulating cell-free DNA originating from a tumor [19]) in some patients [13, 20], and reduced sensitivity in the detection of CNVs and rearrangements [21, 22], such as PTEN homozygous deletions and rearrangements [23, 24].