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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]