Prospects for radiofrequency hyperthermia applicator research. I – Pre-optimised prototypes of endocavitary applicators with matching interfaces for prostate hyperplasia and cancer treatments

Figures & data

Figure 1. New and existing prototypes of 27.12 MHz radiators developed for endocavitary hyperthermia (EHT) of small-radius cavities. (1A) the inductive helix (H) radiator; (1B) the new inverse capacitive current (C) applicator; (1C) the new inductive toroidal (T) radiator; and (1D) the existing helix U-10 radiator [32] included for comparison. Physical data for each radiator are provided in Table I.

Table I.  Radiators and heads physical data. Radiator circuits lie over the surface of a cylindrical insulating mandrel (Figure 1); L and OD are radiator length and diameter; the U-10 helix radiator data are retrieved from [32]. The T_AA, C_AW and H_AW interface includes a cannula forming inner and outer interstices to be filled with specific dielectric media (Figure 2); the C_AW length is inclusive of both electrodes; heads cross-sections are in Figure 3 and longitudinal sections in Figures 5–7. Data in mm.

Heads	Mandrel	Radiator			Cannula		Cavity	Interstices
	OD	Type	L	OD	ID	OD	ID	Inner	Outer
TAA	3.2	4 loop sides toroid	55	4.8	5.0	5.2	8.0	2.4/0.1	1.4
CAW	3.2	electrodes pair	72	4.0	5.0	5.2	8.0	0.9/0.5	1.4
HAW	4.7	Helix	55	4.9	5.0	5.2	8.0	0.15/0.05	1.4
U-10	4.7	Helix	38	4.9			4.9

Figure 2. Schematic view and reference system of an RF head in the phantom cavity. The bottom-tapped thin cannula covers the RF radiator to establish inner and outer interstices that are filled with the specific dielectric media combination of the matching interface.

Figure 3. Cross-sectional view of T_AA, H_AW and C_AW in the phantom cavity (not shown). The outlined components are: the insulating mandrels (grey sections) supporting the radiator structures, i.e., the four outer loop sides of the T-radiator, the helix winding of the H-radiator, and the capacitive electrodes of the C radiator. The head cannulae (dashed lines) separate the inner and outer interstices to be filled with air (A) and/or non-conducting water (W) in each radial specific matching sequence. The H, C, and T radiator prototypes are shown in Figure 1, physical data of the radiators and their matching interfaces in Table I, and longitudinal sections of all the heads in Figures 5–7.

Figure 4. Cavity phantom with the temperature measurement assembly [32].

Table II.  Time constant t of capillary-thermocouple assembly immersed in controlled water bath at 24 ± 0.1°C. Capillaries are of different materials: Teflon, poly-urethane, glass, and of different OD and thickness; t is the readout time at 95% of the exponential temperature-time curves. OD and ID are in mm, t in s.

	PTFE	PTFE	PTFE	Polyurethane	Glass	Bare
OD	2	2	0.7	0.6	0.7
ID	1.2	1.4	0.3	0.4	0.5
t	4.0	3.1	1.3	0.9	0.8	0.4

Figure 5. Radial iso-SAR normalized contours above the 50% SAR efficacy level for T-heads, each with a different matching interface consisting of a dual interstitial gap filled with either air (A) and/or non-conducting water (W) in different radial sequences: (top) T_AW head, (middle) T_WW head, (bottom) T_AA head. The T_AA is the only acceptable performer and the related D_1/2 parameter is provided in Table III. The heads longitudinal sections are depicted in the diagrams. The T_AA cross-section is shown in Figure 3 and the physical data in Table I.

Figure 6. Radial iso-SAR normalized contours above the 50% SAR efficacy level for H-heads, each with a different matching interface consisting of a dual interstitial gap filled with either air (A) and/or non-conducting water (W) in different sequences: (top) H_AW head, (middle) H_WW head, (bottom) H_AA head. The H_AW is the only acceptable performer and the related D_1/2 parameter is provided in Table III. The heads longitudinal sections are depicted in the diagrams. The H_AW cross-section is shown in Figure 3 and the physical data in Table I.

Figure 7. Radial iso-SAR normalized contours above the 50% SAR efficacy level for C-heads, each with a different matching interface consisting of a dual interstitial gap filled with either air (A) and/or nonconducting water (W) in different radial sequences: (top) C_AW head, (middle) C_WW head, (bottom) C_AA head. The C_AW is the only acceptable performer and the related D_1/2 parameter is provided in Table III. The heads longitudinal sections are depicted in the diagrams. The C_AW cross-section is shown in Figure 3 and the physical data in Table I.

Figure 8. Normalized longitudinal SAR profiles above the 50% SAR efficacy level for the T_AA, H_AW and C_AW heads constructed with data retrieved from the iso-SAR contours of Figures 5–7 and from Table I. The profiles of the HLDA dipole [8] and of the U-10 RF radiator [32] are added for comparison. Included are traces of the active lengths of the radiators and the trace of the 80% SAR level, which are used to evaluate the numerical parameters of the profiles: Z₅₀, L_eff, Q₈₀, d-G_Z, and p-G_z that are provided in Table III. Physical data of the heads are provided in Table I.

Table III.  RF heads heating features. The C_AW L_eff is inclusive of both ring electrodes and for both inductive H_AW and T_AA heads a pair of SAR maximum show up at each Z₅₀ end (Figure 8). The heating features of the RF U-10 radiator and of the MW HLDA, dipole and helix antennae are added for comparison: the HLDA D_1/2 values are taken off the hot spot and at the hot spot (in parenthesis). Data are in mm: precision: ±15% (this work), ±20% (other sources).

	RF			RF	MW
	CAW	HAW	TAA	U-10 [32]	HLDA [8]	Dipole [8]	Helix [8]
SARMAX	single	pair	pair	pair	single	split	single
Leff (%)	80	120	180	110	160	170	100
Q80 (%)	48	75	75	68	26
p-Gz (%SAR/mm)	1.4	2.9	2.2	3.5	0.7
d-Gz (%SAR/mm)	2.9	3.9	2.7	3.9	0.5
D1/2 (Max, mm)	20	19	19	3	6(15)	5	5
ΔT/ΔTMAX	66	33	100

Table IV.  Heating features qualitative synopsis of RF heads, RF radiators and MW applicators for EHT.

	MW dipole helix [8]	MW HLDA [8]	RF TAA [this work]	RF HAW [this work]	RF CAW [this work]	RF U-10 [32]
Radiators/frequency
Heating rate	+ +	+ +	+	−	+/−	−
Wavelength effects immunity	− −	− −	+ +	+/−	++	+/−
Length customisation	− −	− −	+ +	+/−	+	+/−
Shape customisation	− −	− −	+ +	+/−	+/−	+/−
EM matching optimisation	− −	− −	+ +	+/−	+/−	+/−
Mechanical radiator flexibility	− −	− −	+/−	+ +	−	+ +
Longitudinal features
Leff congruity with radiator length	+ +	+ +	+ +	+ +	+ +	+ +
SAR sharp decays at radiator ends	− −	− −	+ +	+ +	+ +	+ +
Heating field uniformity	− −	− −	+ +	+ +	+ +	+ +
Hot spot immunity	− −	− −	+ +	+ +	+ +	+ +
Radial features
Maximum D1/2 effectiveness	+	+	+ +	+ +	+ +	− −
Longitudinal D1/2 uniformity	− −	− −	+ +	+ +	+ +	+ +
Clinical features potential
Stability, dependability	− −	− −	+ +	+/−	+/−	+/−
Prostate side sphincter safety	− −	− −	+ +	+ +	+/−	+ +
Pre-optimised heating features			+ +	+ +	−	+ +
Treatment planning feasibility	− −	− −	+ +	+ +	−	+ +
Thermal dosimetry feasibility	− −	− −	+ +	+ +	−	+ +
Azimuthal directivity			+ +		+ +
Full air-gap			+ +
Penetration depth effect immunity	− −	− −	+ +	+/−		+/−
Matching and tuning stability	− −	− −	+ +	+ +	+/−	+ +

(+) high; (+ +) top; (−) low; (− −) very low.

Figure 9. Radial iso-SAR normalized contours of the 27.12 MHz U-10 helix radiator assessed by direct insertion in the phantom bulk, protected by a thin Mylar film, and retrieved for comparison from [32]. The U-10 helix radiator is shown in Figure 1 and the physical data in Table I. The D_1/2 parameter is provided in Table III.

Figure 10. Prototype of the 27.12 MHz pre-optimized 8 mm OD inductive H_AW head, equipped with the bi-layer interface (Figure 2), and externally protected with a thin cannula as exemplified for direct insertion into a cavity of compatibly resilient OD.

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