The Correlation between Plasma Circulating Tumor DNA and Radiographic Tumor Burden

Abstract

Conventional blood-based biomarkers and radiographic imaging are excellent for use in monitoring different aspects of malignant disease, but given their specific shortcomings, their integration with other, complementary markers such as plasma circulating tumor DNA (ctDNA) will be beneficial toward a precision medicine-driven future. Plasma ctDNA analysis utilizes the measurement of cancer-specific molecular alterations in a variety of bodily fluids released by dying tumor cells to monitor and profile response to therapy, and is being employed in several clinical scenarios. Plasma concentrations of ctDNA have been reported to correlate with tumor burden. However, the strength of this association is generally poor and highly variable, confounding the interpretation of longitudinal plasma ctDNA measurements in conjunction with routine radiographic assessments. Herein is discussed what is currently understood with respect to the fundamental characteristics of tumor growth that dictate plasma ctDNA concentrations, with a perspective on its interpretation in conjunction with radiographically determined tumor burden assessments.

Figure 1

A model of tumor progression and ctDNA release. Panel 1: At the initiation stage of tumor proliferation, growth rate predominates, with minimal cell death, resulting in ctDNA concentrations ([ctDNA]) that are less than the lower limit of detection (LOD). Tumor proliferation leads to the formation of a nutrient-deprived necrotic core (gray cells). Panel 2: Cancer cells proliferate and increase in abundance. Cell death rate increases and low [ctDNA] may be detectable at this stage (brown region). Panel 3: At later stages of disease, the tumor microenvironment is fully established, and cell death rate increases dramatically. Increased cellular turnover allows the opportunity for the malignancy to differentiate and become heterogenous as invasive cancer cells appear (darkred regions). Panel 4: The initiation of therapy promotes tumor cell death and diminishes cellular proliferation, leading to rapid clearance of ctDNA due to the absence of proliferation. Panel 5: Residual proliferation after therapy allows for tumor subclones to reestablish themselves, including new therapy-resistant cells. Panel 6: New therapy-resistant subclones begin yielding ctDNA as proliferation increases (green region).

Figure 2

Two theoretical scenarios—disease recurrence and complete therapeutic response—comparing the changes in the rates of tumor proliferation and cell death (represented by ctDNA concentration [ctDNA]), both in arbitrary units. Given the relationship ΔTB ∝ Δr_g – Δr_d ([ctDNA]), where TB is the tumor burden, r_g is the proliferation rate, and r_d is the cell death rate, the relative tumor burden can be inferred at various stages after treatment and during disease surveillance, based on the relative rates of tumor proliferation and [ctDNA] release. Arrows indicate the time points relevant in the calculation of Δ. The theoretical lower limit of detection (LOD) of ctDNA is represented by the red line; theoretical radiographic LOD is represented by the blue line. A: Partial response with recurrence. Prior to treatment, proliferation is increased (Δr_g = 20), resulting in an increased rate of cell death (Δr_d = 5). The net effect is an increase in ctDNA, with a net increase in TB of 15 after 1 month. (A1): At 1 month after treatment, proliferation is partially reduced (Δr_g = −40), resulting in a reduction in cell death rate (Δr_d = −30), driving decreases in both TB (ΔTB = −10) and plasma [ctDNA]. (A2): Several months after treatment, proliferation remains reduced (Δr_g = 0) and the cell death rate normalizes to the proliferation rate (Δr_d = 0), resulting in persistently low but detectable quantities of plasma ctDNA without any change in TB (ΔTB = 0) relative to the previous month. (A3): Developing resistance to therapy allows for an increased rate of proliferation (Δr_g = 20), which drives an increase in cell death rate (Δr_d = 15), resulting in an acute increase in plasma ctDNA and a gradual increase in TB (ΔTB = 5) relative to the previous month. B: Complete response. As in A, prior to treatment, proliferation increases (Δr_g = 20), resulting in an increased rate of cell death (Δr_d = 5). The net effect is an increase in ctDNA, with an increase in TB of 15 after 1 month. (B1): At 1 month after treatment, proliferation is dramatically reduced (Δr_g = −110), resulting in a reduction in cell death rate (Δr_d = −70), such that plasma [ctDNA] are below the LOD, and TB is also reduced (ΔTB = −40). (B2): Several months after treatment, the rates of proliferation and cell death remain unchanged, resulting in persistently undetectable plasma [ctDNA] and no change in TB relative to the previous month. (B3): Complete response ablates proliferation (Δr_g = −20), and death rate persists (Δr_d = 0), resulting in continued concentrations of plasma ctDNA below the LOD and a reduction in residual TB (ΔTB = −20) relative to the previous month.

Figure 3

Graphical representation of the relationship between the rates of cell proliferation and death in the context of changes in tumor burden and tumor cell turnover. A: The diagonal line represents stable tumor burden as the rates of cell proliferation and death increase proportionately with each other. In the blue region, the cell death rate is high, while the proliferation rate is low, which reduces tumor burden. In the red region, the proliferation rate is high, while the cell death rate is low, which increases tumor burden. The shaded arcs represent zones of cellular turnover that are low (bottom left) and high (top right). The numbered points represent longitudinal radiologic measurements of disease status. B: 1 → 2, Disease burden remains stable despite an increased proliferation rate (Δr_g = Δr_d [ctDNA]; increasing in parallel); 2 → 3, the proliferation rate begins to surpass the cell death rate, resulting in increased radiographic tumor burden (Δr_g > Δr_d); 3 → 4, a complete response to therapy greatly reduces the rate of tumor cell turnover, resulting in a steep decrease in the rates of cell proliferation and death [Δr_g < Δr_d; Δr_d < lower limit of detection (LOD)]. C: 1 → 3, Clinical progression identical to that in the previous case; 3 → 4, a partial response to therapy leads to residual proliferation and continued cellular turnover (Δr_g < Δr_d; Δr_d > LOD); 4 → 5, continued cellular turnover generates subpopulations of highly proliferative cells, leading to tumor recurrence (Δr_g > Δr_d). Theoretical ctDNA LOD is represented by the red line; theoretical radiographic LOD is represented by the blue line.