The usefulness of mobile insulator sheets for the optimisation of deep heating area for regional hyperthermia using a capacitively coupled heating method: phantom, simulation and clinical prospective studies

Figures & data

Figure 1. (a) Pattern diagrams of the conventional setting in the phantom study. (b) The distributions of the temperature increase in the phantom for the conventional setting. The superficial area was cooled by the overlay bolus, while the deep area was uniformly heated (with the exception of the external border).

Figure 2. (a) Pattern diagrams of the insulator sheet with a hole in the centre, defined as setting number 1. The insulator sheet was inserted between the regular bolus and the cooled overlay bolus in both the upper and lower sides of the electrode. (b) The distribution of the temperature increase in the phantom with setting number 1.

Figure 3. (a) Pattern diagrams in the phantom for setting number 2. The insulator sheets were alternately arranged in a circular sector shape at an angle of 270° and were inserted between the regular bolus and cooled overlay bolus in both the upper and lower sides of the electrode. (b) Pattern diagrams of setting number 3 in which mobile insulator sheets were used. The insulator sheets, which were prepared in a circular sector shape, the same as in setting number 2, were manually rotated in parallel at an angle of 90° at 5 min intervals during heating. (c) The distribution of the temperature increase with setting number 2. (d) The distributions of the temperature increase in setting number 3 in which mobile insulator sheets were used.

Table 1. Dielectric and materials properties used for the simulations at 8 MHz.

Materials	Electric conductivity (S/m)	Relative permittivity	Mass density (kg/m^3)
Regular boluses (5% NaCl at 10 °C)	6.31	82	1000
Overlay boluses (0.5% NaCl at 5 °C)	0.57	85	1000
Agar phantom	0.59	81	1020
Bone	0.04	43	1908
Muscle	0.61	203	1090
Prostate	0.67	305	1045
Fat	0.05	33	911
Silicon rubber	1.00E-14	3	1000

Figure 4. (a) Pattern diagrams for the 3 D SAR simulation using the agar phantom. (b) SAR distribution in the phantom for the conventional setting. (c) SAR distribution in the setting number 1. (d) SAR distribution in the setting number 1 at an oblique angle. (e) SAR distribution in setting number 3 at position #1. (f) SAR distribution in setting number 3 at position #1 at an oblique angle.

Figure 5. (a) Segmented CT slice of the caudal area showing the bone (blue), muscle (yellowish green), prostate (red) and subcutaneous fat (SF) of the right ventral area (pink), left ventral area (white), right dorsal area (orange), left dorsal area (green) and other fat (yellow). (b) Colour scale bar of the SAR distribution. (c) SAR distribution in the 3 D anatomical model for the conventional setting. Constant potential was set at 117 V. (d) Sagittal view of the SAR distribution in the conventional setting. (e) SAR distribution in setting number 1. Constant potential was set at 166.7 V. (f) Sagittal view of the SAR distribution in setting number 1. (g) SAR distribution in setting number 3 at position #1. Constant potential was set at 166.7 V for setting number 3. (h) Sagittal view of the SAR distribution in setting number 3 at position #2. (i) SAR distribution in setting number 3 at position #3. (j) Sagittal view of the SAR distribution in setting number 3 at position #4.

Table 2. Three-dimensional SAR simulations using the 3 D patient model.

				Setting number 3
	Conventional setting	Conventional setting*	Setting number 1	Position #1	Position #2	Position #3	Position #4	Average**
Averaged SAR (W/kg)
Prostate	120.3	59.3	70.5	63.9	68.4	53.1	49.1	58.6
SF
Right cranial ventral	424.9	209.3	215.8	235.6	231.3	71.7	80.8	154.9
Left cranial ventral	419.2	206.5	207.6	69.6	220.2	234.5	79.1	150.9
Right cranial dorsal	481.7	237.3	225.1	82.6	82.8	214.2	197.0	144.1
Left cranial dorsal	462.1	227.6	218.4	208.4	78.5	81.6	195.5	141.0
Right caudal ventral	304.6	150.0	182.1	184.0	62.9	65.0	234.4	136.6
Left caudal ventral	295.2	145.4	171.5	59.8	60.8	183.5	219.8	131.0
Right caudal dorsal	403.4	198.7	199.6	68.7	201.8	208.5	62.5	135.4
Left caudal dorsal	382.3	188.3	192.4	201.8	197.4	67.7	59.2	131.5
Other fat	164.7	81.1	88.6	58.7	60.5	58.7	64.5	60.6
Bone	60.0	29.6	31.6	21.5	21.1	21.1	19.4	20.8
Muscle	209.6	103.2	105.1	76.6	89.2	77.1	69.3	78.0
SF/target ratio for averaged SARs
Right cranial ventral/Prostate	3.5	3.5	3.1	3.7	3.4	1.4	1.6	2.5
Left cranial ventral/Prostate	3.5	3.5	2.9	1.1	3.2	4.4	1.6	2.6
Right cranial dorsal/Prostate	4.0	4.0	3.2	1.3	1.2	4.0	4.0	2.6
Left cranial dorsal/Prostate	3.8	3.8	3.1	3.3	1.1	1.5	4.0	2.5
Right caudal ventral/Prostate	2.5	2.5	2.6	2.9	0.9	1.2	4.8	2.3
Left caudal ventral/Prostate	2.5	2.5	2.4	0.9	0.9	3.5	4.5	2.4
Right caudal dorsal/Prostate	3.4	3.4	2.8	1.1	3.0	3.9	1.3	2.3
Left caudal dorsal/Prostate	3.2	3.2	2.7	3.2	2.9	1.3	1.2	2.1
Maximum local SAR (W/kg)
Prostate	1393	686	1005	408	670	347	356	445
SF
Right cranial ventral	15310	7540	7169	7571	5046	2474	3038	4532
Left cranial ventral	24120	11880	10120	4019	5147	12380	4739	6571
Right cranial dorsal	13180	6490	7104	2383	3256	4688	7914	4560
Left cranial dorsal	12000	5913	7236	3987	2690	2098	9173	4487
Right caudal ventral	34700	17100	17180	16090	4266	5608	7356	8330
Left caudal ventral	34300	16900	16670	5662	4479	16480	7394	8504
Right caudal dorsal	21520	10600	9350	4222	6053	9166	5981	6356
Left caudal dorsal	17360	8551	8526	10200	6359	4410	6299	6817
Minimum local SAR (W/kg)
Prostate	14.3	7.1	10.3	15.5	13.8	12.1	11.6	13.3

3 D: three-dimensional; SAR: specific absorption rate; SF: subcutaneous fat.

* The constant potential was set at 117 V. For the others, constant potential was set at 166.7 V.

** The average of all positions in setting number 3.

Table 3. The thermal parameters in the HT sessions with the conventional setting and with setting number 1 (insulator sheets with a hole in the centre).

Thermal parameters	HT sessions with the conventional setting (n = 29)	HT sessions with setting number 1 (n = 19)	p*
Heating duration (min.)			.098
Mean	49.1	47.9
Median	50.0	50.0
SD	2.8	3.8
Range	40.0–50.0	37.0–50.0
Median T90 (°C)			.800
Mean	40.1	40.2
Median	40.4	40.8
SD	1.0	1.1
Range	37.8–41.5	38.3–41.5
Median T50 (°C)			.301
Mean	40.5	40.9
Median	40.8	41.3
SD	1.0	1.2
Range	38.1-41.6	38.5-42.4
Median T25 (°C)			.109
Mean	41.0	41.3
Median	41.3	41.8
SD	1.0	1.2
Range	38.7–42.2	38.8–42.8
CEM43°CT90 (min.)			.941
Mean	1.6	1.7
Median	1.2	1.6
SD	1.3	1.6
Range	0.0–5.4	0.0–5.2

* Mann–Whitney U test.

Table 4. the thermal parameters in HT sessions with the conventional setting and with setting number 3 (using mobile insulator sheets).

Thermal parameters	HT sessions with the conventional setting (n = 32)	HT sessions with setting number 3 (n = 21)	p*
Heating duration (min.)			.59
Mean	49.6	49.1
Median	50.0	50.0
SD	1.4	2.1
Range	44.0–50.0	44.0–50.0
Median T90 (°C)			.032
Mean	40.4	40.8
Median	40.6	40.8
SD	0.6	0.6
Range	39.1-41.5	39.7-41.6
Median T50 (°C)			.011
Mean	40.9	41.4
Median	41	41.2
SD	0.6	0.6
Range	39.7–42.0	40.0–42.1
Median T25 (°C)			.0075
Mean	41.2	41.7
Median	41.1	42
SD	0.6	0.6
Range	40.3–42.4	40.5–42.7
CEM43°CT90 (min.)			.025
Mean	1.3	2.5
Median	1.0	1.9
SD	1.1	2.0
Range	0.1–3.8	0.3–7.1

* Mann–Whitney U-test.

Table 5. Comparisons of the thermal parameters in setting number 3 (using mobile insulator sheets) by the order of the HT sessions.

	Median T90		Median T50		Median T25		CEM43°CT90
	Difference (°C)	p*	Difference (°C)	p	Difference (°C)	p	Difference (min.)	p
Second minus first	0.27	.19	0.38	.22	0.52	.08	0.97	.19
Third minus second	−0.46	.19	−0.54	.09	−0.78	.02	−1.51	.12
Fourth minus third	0.48	.13	0.58	.08	0.53	.08	1.44	.10
Fifth minus fourth	−0.47	.20	−0.59	.05	−0.47	.09	−1.20	.17

* Mann–Whitney U-test.