3D versus 2D steering in patient anatomies: A comparison using hyperthermia treatment planning

Figures & data

Figure 1. The AMC-8 phased-array 70 MHz waveguide system. The system consists of two rings of four waveguides. Every waveguide has a separate water bolus that provides superficial cooling of the patient and coupling of the incident electromagnetic field into the patient. The two bottom waveguides share the same water bolus. The distance between the two rings is adjustable.

Table I.  Tissue properties in the electromagnetic and thermal simulations.

Tissue type	σ (S/m)	εr (−)	ρ(kg/m^3)	c (J/(kg K))	k (W/(m K))	Wb (kg/(m^3s))
Inner-air	0.0	1.0	1.29	10000^†	0.024	0
Bone	0.05	10	1595	1420	0.65	0.12
Fat-like	0.06	10	888	2387	0.217	1.1
Muscle-like	0.75	75	1050	3639	0.56	3.6
Tumour	0.74	65	1050	3639	0.56	1.8

Data are taken from Gabriel et al. [15] and the ESHO Taskgroup Committee [16]. σ is the electric conductivity, ε_r the relative electric permittivity, ρ the density of the tissue, c the heat-capacity, k the thermal conductivity and W_b the volumetric tissue perfusion rate.

^†The heat capacity of air is increased by a factor of 10 to allow a larger time step during simulation. The effect on the temperature distribution is negligible.

Table II.  Overview of patient characteristics for the selected patients.

Patient	GTV (cc)	Vfat (cc)	Vmuscle (cc)	DLR (cm)	DAP (cm)
1	139.3	9797	18135	37	24
2	88.9	15417	11048	40	25
3	26.9	17012	12087	38	29
4	52.5	24682	10309	48	26
5	75.1	24042	14245	44	29

V_fat and V_muscle are the volume of fat- and muscle-like tissues respectively. D_LR and D_AP are the largest distance measured for each patient in the left-right and the anterior-posterior direction, respectively.

Figure 2. Coronal cross-section of the waveguide configuration of the AMC-4 (left) and AMC-8 (right) phased-array system and the water boluses. Waveguide aperture size equals 20.2 × 34.3 cm². The distance between the two rings of the AMC-8 system (ring-to-ring distance (RRD)) can be varied and by default in this study is 1.5 cm.

Figure 3. The normalised ratio of target- and normal tissue SAR for the AMC-4 and the AMC-8 system for different axial positions after optimisation for patients 1 to 5. The SAR ratios are normalised by the SAR ratio found for the AMC-4 system under optimal positioning. Antennas were not allowed to contribute more to the total delivered power than 40%, or less than 10% for the AMC-4 system, and no more than 25% or less than 5% for the AMC-8 system.

Figure 4. The temperature gain for T₁₀, T₅₀ and T₉₀ when moving from the AMC-4 to the AMC-8 system for patients 1 to 5. The applied settings resulted from temperature-based optimisation. For the AMC-8 system a 1.5-cm ring-to-ring distance was applied.

Figure 5. Total absorbed power for the AMC-4 and the AMC-8 system for patients 1 to 5. The applied settings result from temperature-based optimisation. For the AMC-8 system a 1.5-cm ring-to-ring distance was applied.

Figure 6. Cumulative temperature−volume histograms of the temperature distribution resulting from temperature-based optimisation for the five patients. The inserts zoom in to the 43−45°C range of the histogram. Indicated in the lower left corner of each plot is the patient number.

Figure 7. An example of the resulting temperature distribution (right) after temperature-based optimisation with the corresponding coronal slices (left) for the AMC-4 system (top) and the AMC-8 system (bottom).

Figure 8. T₉₀ and T₅₀ for different axial shifts (z-axis) of patients 1 to 5. For every shift the T₉₀ and T₅₀ were evaluated for two sets of amplitudes and phases. The first is the result of optimisation with the patient modelled at the correct/actual position (unconnected markers). The second results from optimisation without shifting the patient (connected markers). The figure shows that the AMC-8 system has the capability of correcting for changes in the axial position of the patient, contrary to the AMC-4 system.

Figure 9. Differences in T₉₀ after temperature optimisation for the AMC-8 system using varying ring-to-ring distances. Results are relative to the results for a ring-to-ring distance of 1.5 cm. The figure on the left shows the change in T₉₀ applying optimised settings for a 1.5 cm ring-to-ring distance at larger distances without adjustment of settings. The figure on the right shows the T₉₀ optimising for the actual distances relative to the optimal T₉₀ for a ring-to-ring distance of 1.5 cm.

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