How large is the periablational zone after radiofrequency and microwave ablation? Computer-based comparative study of two currently used clinical devices

Figures & data

Figure 1. (a) Moderate hyperthermic heating occurs in periablational zone and has been related with tumor cell activity [1]. This heating could affect either a tumor area outside the thermal coagulation zone (which is completely destroyed) or nearby micronodules. (b) Relation between percentage of living cells after heating and index Ω obtained from the Arrhenius damage model, which associates temperature with exposure time using a first-order kinetics relationship. Periablational zone was assumed to be between Ω = 0.6 and Ω = 2.1 (values derived from experimental data in [1], see text for details), while coagulation zone was defined by the Ω = 4.6 contour, which represents 99% probability of cell death.

Table 1. Characteristics of materials used in the computer model [14,15,18–20].

Material	σe (S/m)	${\epsilon }_r$	k (W/m·K)	ρ (kg/m^3)	c (J/kg·K)	σ (S/m)
Liver	1.8^(a)	44.3^(a)	0.502	1080^(b)	3455^(b)	0.19^(a)
				370^(c)	2156^(c)
Copper	5.87 × 10^7	1	385	9000	384
Alumina	0	10	30	3970	875
PTFE	1.6 × 10^−5	1.8	0.24	1200	1050
Stainless steal	1.74 × 10^6	1	16.2	8000	500
Plastic			0.026	70	1045	1 × 10^−5
Electrode			15	8000	480	7.4 × 10^6

σ_e: (effective) conductivity; ε_r: relative permittivity; k: thermal conductivity; ρ: density; c: specific heat; σ: electrical conductivity.

^(a)Measured at 37 °C, ^(b)for temperatures between 37 °C and 99 °C, ^(c)for temperatures higher than 100 °C.

Figure 2. Temperature distributions computed from RFA(Cool-tip applicator, pulsed protocol, 10 min) and MWA (Amica-Gen applicator, 60 W continuous application, 10 min) just after switch-off (a) and 10 min later (b). White lines represent limits of coagulation zone (Ω > 4.6) and periablational zones (0.6 < Ω < 2.1). (Scale in °C; MWA temperatures exceeded 110 °C).

Figure 3. Coagulation and periablational zone volumes in RFA/MWA simulations. Lighter color bars and bottom values are the volumes computed just after switch-off. Darker color bars represent the increase in volume 10 min after switch-off. Upper values give total volumes after this 10-min period.

Figure 4. Periablational/coagulation volume ratios in RFA/MWA simulations. Lighter color bars and bottom values are the ratios computed just after switch-off. Darker color bars represent the increased ratio 10 min after switch-off. Upper values show the ratio after this 10-min period.

Table 2. Comparison of RFA/MWA coagulation transverse diameters (in cm) computed by FEM (Finite Element Method) and reported in clinical and experimental studies (in vivo ablations only) [27–38].

	RFA	MWA
		60 W		80 W		100 W
	10 min (FEM)	5 min	10 min	5 min	10 min	5 min	10 min
	12 min (Clin/Exp)
FEM	2.56	2.73	3.20	3.06	3.33	3.43	3.78
Clin./ Exp.	1.8 [27], 1.85 [28] 2.0 [29], 2.6 [30,31]	2.5−4 [32] 2.2 [27]	3.1−4.1 [32] 3.1 [34] 2.35 [35]	3.12 [36] 3.5 [32]	2.37 [35] 3.8 [32]	2.85 [37] 3.4 [32] 3.3 [38]	4.9 [32]

Figure 5. Overlapping coagulation zones after 10 min of RFA (∼51 W) and MWA (60 W). Note that while the volumes offer similar values, the MWA transverse diameter is larger.

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