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Table 2 Assumptions made for cfDNA and ctDNA in patient plasma

From: Utility of circulating tumor DNA in cancer diagnostics with emphasis on early detection

Assumption Justification References
Fetal DNA in maternal circulation is proportionally related to fetal and maternal weight It has been documented that, as maternal weight increases, the percent fetal DNA in maternal circulation proportionally decreases [32]
Circulating tumor DNA and circulating free DNA from normal tissues diffuse into the circulation at roughly equal rates and by similar mechanisms ctDNA and cfDNA levels are quite variable between normal individuals and patients with cancer; however, as tumor volume increases, the amount of ctDNA also increases, correlating with tumor burden [4, 16, 24, 25, 32, 48]
In early cancer stages, the amount of ctDNA will not significantly affect the amount of total cfDNA or the circulating genome equivalents In early stage cancer, the amount of ctDNA is only 0.1% or less, of total cfDNA; thus, it will not significantly increase the number of circulating genomes [10, 17, 26, 38, 39, 42, 43]
Tumors are spherical and their weight and cellularity are proportional to the volume of the tumor; a tumor of 1 cm3 has a wet weight of 1 g and contains approximately 109 cells Calculations are intended to be approximations in order to estimate the ratio of tumor/normal DNA in the circulation
Abbosh et al. [21] reported that a 10 cm3 tumor leads to 0.1% ctDNA in the circulation (see text for greater detail)
[21, 39, 43]