Methods, systems, and software are provided for estimating a circulating tumor fraction for a test subject. Sequence reads are obtained from a panel-enriched sequencing reaction, including sequences for a first plurality of cfDNA fragments corresponding to probe sequences and a second plurality of cfDNA fragments not corresponding to probe sequences. Bin-level coverage ratios are determined from the sequences. Segments are formed by grouping adjacent bins based on similar coverage ratios and segment-level coverage ratios are determined based on bin-level coverage ratios for bins in the segment. For each simulated circulating tumor fraction in a plurality of circulating tumor fractions, segments are fitted to an integer copy state by identifying the integer copy state that best matches the segment-level coverage ratio. The circulating tumor fraction for the test subject is determined using error optimization between segment-level coverage ratios and integer copy states across the simulated circulated tumor fractions.