Interstitial microwave transition from hyperthermia to ablation: Historical perspectives and current trends in thermal therapy

Figures & data

Figure 1. Results for three synchronous, uncooled antennas, 4.2 cm (diameter) × 5.5 cm (length) ablation in ex vivo bovine liver with three antennas at 915 MHz, spaced 1.3 cm apart in triangular array (10 min, 33 W/channel, synchronous phase, no cooling).

Figure 2. Results from three non-synchronous antennas, 2.5 cm (diameter) × 2.9 cm (diameter) × 6 cm (length) ablation in ex vivo bovine liver with three antennas at 915 MHz, spaced 1.3 cm in triangular array (10 min, 33 W/channel, non-synchronous phase, no cooling).

Figure 3. Single cooled antenna results, 2.9 cm (diameter) × 3 cm (diameter) × 6.1 cm (length) ablation in ex vivo bovine liver with 1 antenna at 915 MHz (10 min, 60 W, cooled antenna). A 10 mL/min flow rate was used. The left figure shows the insertion plane (arrows designate the track) and the right figure shows the perpendicular plane (arrow shows insertion).

Figure 4. Dual synchronous, cooled antenna results, 4 cm (diameter) × 4.6 cm (diameter) × 5.5 cm (length) ablation in ex vivo bovine liver with two antennas (see white arrows for track indication) at 915 MHz, spaced 1.5 cm apart (10 min, 60 W/channel, synchronous phase, cooled antennas). The left figure shows the insertion plane and the right figure shows the perpendicular plane (see white arrows for antenna locations).

Figure 5. Triple synchronous, cooled antenna results, 5.2 cm (diameter) × 5.5 cm (diameter) × 6 cm (length) ablation in ex vivo bovine liver with three antennas at 915 MHz. (10 min, 45 W/antenna, cooled antennas). The left figure shows the insertion plane and the right figure shows the perpendicular plane. Spacing was 2.5 cm between antennas. With spacing pushed out to 3 cm (not shown) the dimensions of the volume are 5.3 cm (diameter) × 5.1 cm (diameter) × 5.1 cm (length), with some loss of volume when compared to the 2.5 cm spacing runs.

Figure 6. Triple non-synchronous, cooled antenna results. Left: spacing = 2.5 cm. Cross-section of the ablation was 4.2 cm (minimum diameter) × 4.5 cm (diameter) × 5.4 cm (length) (10 min, 45 W/channel, non-synchronous phase, cooled antennas). There are treatment gaps of viable tissue between antennas where the ablation lacks sphericity. Right: spacing = 3 cm. Cross-section of the ablation was 3.6 cm (minimum diameter) × 4.5 cm (diameter) × 5 cm (length) (10 min, 45 W/channel, non-synchronous phase, cooled antennas). The treatment gaps are even larger when the antenna spacing is increased.

Table I.  Transition from hyperthermia to ablation with interstitial MW antennas.

Feature	Hyperthermia	Ablation
Power	5–15 W/channel	25–100 W/channel
Cooling	No	No (surgical)
		Yes (percutaneous)
High power generator	No	Yes
Multichannel	Yes	Either
Treatment	Adjuvant with radiation or chemotherapy	Heat alone*
Temperature endpoint	43°C	>55–60°C
Treatment length	60 min at 43°C (does not include 5–10 min of heating ramp-up)	4–10 min^**
Temperature maximum (at antenna)	45–50°C	150–200°C^***
		[54], [90]

*Radiation and/or chemotherapy may follow some ablation procedures, but many ablations are palliative and thus use only the heat application.

^**The thermal dose to necrose tissue above 60°C may only be a few seconds [118].

^***Limited only by antenna components and materials [54], [90].

Table II.  Summary of the experimental data with synchronous (S) and non-synchronous (NS) denoted. Synchronous runs showed larger volumes and higher efficiency ratios, denoting a larger volume for the same energy input. L1 and L2 are the two diameters, perpendicular to the insertion of the antennas. L3 is the length in the direction of insertion of the array.

Figure	Mode	Spacing (cm)	Number of antennas	L1 (cm)	L2 (cm)	L3 (cm)	Volume (cm^3)	Time (min)	Power/antenna (W)	Total power (W)	Total energy (kJ)	Efficiency = Volume/Energy
1	S	1.3	3	4.2	4.2	5.5	50.8	10	33	99	59.4	0.9
2	NS	1.3	3	2.5	2.9	6.0	22.8	10	33	99	59.4	0.4
3	–	–	1	2.9	3.0	6.1	28.2	10	60	60	36	0.8
4	S	1.5	2	4.0	4.6	5.5	53.0	10	60	120	72	0.7
5	S	2.5	3	5.2	5.5	6.0	89.8	10	45	135	81	1.1
6	NS	2.5	3	4.2	4.5	5.4	53.4	10	45	135	81	0.7
N/A	S	3.0	3	5.3	5.1	5.1	72.1	10	45	135	81	0.9
6	NS	3.0	3	3.6	4.5	5.0	42.4	10	45	135	81	0.5

N/A, not available.

Figure 7. Ablation shape scale definition for the plane perpendicular to the insertion axis (left) and parallel to the insertion axis (right).

Table III.  Detailed comparison between the largest spacing of 2.5 and 3 cm (relating to Figures 5 and 6) runs with three antennas in a triangular configuration. The synchronous runs had no D2 measurements since they were more spherical. In addition, the synchronous runs had no D values since the peak was central, not located in the vicinity of the antenna shafts.

Mode	Spacing (cm)	A apex to tip (cm)	B apex to girth (cm)	C central length (cm)	D apex to shaft peak (cm)	Minimum diameter (D1) (cm)	D2 (cm)	Maximum diameter (D3) (cm)
S	2.5	0.8	2.8	6.0	–	5.2	–	5.5
NS	2.5	0.4	2.6	5.2	5.4	4.2	4.5	5.1
S	3.0	0.4	2.5	5.1	–	5.1	–	5.3
NS	3.0	0.3	2.6	5.0	5.4	3.6	4.8	5.3

Figure 8. Three antenna simulations: synchronous versus non-synchronous. (A) The predicted SAR pattern with three synchronous antennas (see arrow) spaced 2.2 cm on a side with triangular cross section. (B) The predicted SAR pattern with three non-synchronous antennas (see arrow) spaced 2.2 cm on a side with triangular cross section.

Figure 9. Four antenna simulations: synchronous versus non-synchronous. (A) The predicted SAR pattern with four synchronous antennas (see arrow) spaced 2.4 cm on a side with square cross-section. (B) The predicted SAR pattern with four non-synchronous antennas (see arrow) spaced 2.4 cm on a side with square cross-section.

Table IV.  Steps in the transition from hyperthermia (HT) to thermal ablation (TA).

Item	Hyperthermia (HT)	Thermal ablation (TA)
Temperature measurements	Multipoint thermometry or scanning systems that take measurements at intervals as close as every 5 mm along a catheter are common. The scanning sweeps may be continuous throughout the 60 min treatment, including the typical 10 min warm-up and cool-down periods.	The main concern is ablation size and shape and not the temperature distribution throughout the entire heated tissues. Ablations are short and the temperature distribution is transient. Most commercial thermal ablation systems in clinical practice do not include temperature monitoring to enable assessment of temperatures.
Multiple applications	The goal was uniform temperature tumour coverage during a single treatment with antennas added accordingly in an array, even if 4, 5, 6, 8, or 12 antennas were needed to cover all of the volume with a single application.	One antenna is used if the volume is small. For larger tumours, either multiple antennas are used, or single antennas are used sequentially with overlap to cover the volume.
Antenna placement	10–20 min may be spent in the setting up of the antennas, where insertion catheters may be implanted, often with a template insertion grid for well-spaced, parallel placement. This is then carefully studied in 3D since brachytherapy doses have to be calculated.	Several ablations may be done with ultrasound, CT or MR guidance, where the needle is visualised as it is located inside the tumour. It is something of an art since there may be freehand placement with limited a priori knowledge of where the previous insertion and heating took place.
Treatment planning	Treatment planning may be done before treatment with carefully laid out spacing and placement of the antennas. The insertion plan often matched the plan for brachytherapy radiation treatment.	Ablation procedures would benefit by using the same planning techniques to achieve optimal insertion patterns for arrays or multiple serial treatments. Whether the antennas are inserted freehand under image guidance or a template is used, treatment planning can help guide the application pattern and number of antennas to be used. Typically, manufacturers have not included this feature.
Real time assessment	Temperatures are the result of mild heating and will not normally be assessable by imaging modalities. Thus, temperature maps are relied upon.	MR, CT or ultrasound can be used to assess the target tissue either during the ablation or immediately thereafter to look for viable regions which may require further treatment.

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