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

Abstract

Inconsistency is observed in comparing assessment data of applicators for endocavitary hyperthermia (EHT) with microwaves (MW) and radiofrequency (RF) obtained using the standard method of inserting bare applicators in phantom tissues. MW antennae exhibit overall average penetration depths of approximately 6 mm, excluding hot spots. RF radiators exhibit penetration depths of not more than approximately 3 mm, a value too low considering the superior penetration of the RF plane wave radiation. Assuming that a mismatch at the RF radiator–tissue interface is causing the poor energy transfer of RF energy, we developed new RF radiators with controlled dielectric matching interfaces for evaluating the potential of RF radiation in EHT and in interstitial hyperthermia (IHT) treatments. We designed, developed and assessed 27.12 MHz, 8 mm OD inductive and capacitive devices of novel and existing designs, each provided an optimised bi-layer matching interface. The assessment results reveal features such as customisable length and shape, independence of insertion depth, uncritical air gap, longitudinal heating uniformity, outstanding penetration depths (19–20 mm) and high SAR gradients at both radiator ends – i.e. prostatic urethra ends – for added safety. These data clear the way for the development of pre-optimised EHT inductive and capacitive RF applicators. Evidence of positive effects of high near-fields density in cavity microenvironments is given. Such devices show potential for more effective prostatic hyperplasia treatments and for improving the feasibility of more adequate treatment planning and thermal dosimetry of interstitial and transurethral hyperthermia treatments of prostate carcinoma.

• Tor Vergata RF hyperthermia applicators
• endocavitary RF hyperthermia
• interstitial RF hyperthermia
• radiofrequency hyperthermia
• BPH
• prostate carcinoma

Introduction

Conventional hyperthermia treatment for benign prostatic hyperplasia (BPH) and prostate carcinoma is commonly performed with endoscopic modalities favoured because access by the heating devices is minimally invasive to the prostatic urethra, which crosses the prostate gland axially. For endoscopic endocavitary hyperthermia (EHT) treatments that employ microwave (MW) antennae [1–19], these goals are heavily hampered by intrinsic electromagnetic (EM) limitations of the MW radiation, technology limits, and problematic optimisation procedures [5–13].

The requirements for transurethral BPH treatment are moderately demanding, and many types of MW devices are employed with relatively unsatisfactory results [20]. Transurethral hyperthermia treatment of prostate carcinoma has the special requirements of adequate penetration and uniform temperatures in the range of 42°–44°C over the whole prostate gland, accurate treatment planning and thermal dosimetry, and the need for sharp specific absorption rate (SAR) gradients at both ends of the prostatic urethra to protect the external sphincters. To partly achieve these requirements, the use of a MW transurethral device combined with a transrectal device is also proposed [7].

Analysis of the performance of MW endoscopic EHT devices in terms of understanding the reasons for their limitations reveals incongruous results. The fast growing alternative treatment modalities for the prostate, including high-temperature [20], interstitial [21–22], and tissue-destructive devices [23–31] with their technological limitations and their poorly defined targets and goals, suggest the inadequacy of the EHT applicator paraphernalia. Endoscopic MW catheterised antennae, used for transurethral and transrectal EHT treatment of prostate disease [1–8] and for endoluminal treatment of lesions in larger cavities such as the rectum, oesophagus, cervix, vagina, and bladder, among others [9–19], exhibit average penetration depths not more than circa (c.a.) 6 mm, hot spots excluded. This common penetration depth does not appear coincidental, considering that the lengths of the MW antennae range from 5 to 150 mm, with diameters (OD) from 4 to 20 cm; that the antennae are implemented at different MW frequencies, with different air gaps and cooling fluid layers; and that they are used with catheters of different thicknesses and dielectric constants.

Experimental investigations of the potential of RF radiation at 27.12 MHz for EHT, performed using non-catheterised inductive helix radiators [32] and catheterised capacitive intraluminal devices [29] were found to deliver penetration depths of 3 mm c.a.; these findings are in conflict also with the established greater penetration of plane waves RF radiation versus MW radiation [33].

Incongruous results are further evidenced by theoretical simulation in the very wide frequency range of RF through MW frequencies of plane-waves radially propagating inside cavities of 5 mm OD, in which the penetration depth results are almost independent of frequency over the entire RF-MW range [34].

A clue to rationalising these data is given by the latter result that could be ascribed in part to not having considered the near-field component of RF and MW fields. Thus, an analysis of near fields in small radius cavities appears to have priority in trying to better define the EM cavity microenvironment under investigation. Near fields exhibit a rate of decrease with distance that is much greater than that for far fields. Moreover, near fields are stronger for dipole lengths L ≪ λ and RF radiators produce more reactive near fields than do MW antennae [35]. For very short dipoles (L ≪ λ), the outer boundary of the reactive near fields is taken to exist to a radial distance r, given by 0 < r < 0.62 (L³/λ) from the antenna surface. Thus, the value of r for the RF radiation is of approximately one order of magnitude smaller than that for the MW radiation, even though the effective value of r also depends on the diameter and other dipole parameters and on the complex dielectric constant of the surrounding media [35]. Thus, it is plausible that the RF near-field high energy density plays a significant role in small-radius cavity microenvironments. At MW, the near-field energy is spread over a much greater distance from the antenna [35] and effects of dielectrics on the heating field of MW antennae for EHT are evidenced [36–39].

A preliminary analysis of cavity EM microenvironments shows that for catheterised MW antennae [1–19], assessment is performed by the common practice of insertion into a phantom bulk (i.e. in direct contact). This procedure is prone to uncontrolled mismatch and instability [8], depending on undefined cavity EM microenvironments. For non-catheterised RF helix radiators [32], uncontrolled fixed mismatch occurs.

The aim of the present investigation is to further pursue a previous attempt to evaluate the potential of RF radiators for prostate transurethral treatments [32] and validate the assumption that is the RF device–tissue mismatch that accounts for the observed incongruence in penetration. The matching of three models of RF radiators provided of stepwise adjustable bi-layer dielectric interface to manipulate the RF near fields within the EM microenvironments of small-radius cavities is investigated. The 27.12 MHz prototypes of radiator models considered are: (1) a novel open-mode toroidal inductive miniaturised radiator producing intense, highly symmetric and substantially pure H-fields, (2) a novel capacitive current device to help improving the limited penetration of 27 MHz interstitial and intraluminal capacitive current devices [29]; and: (3) a helix radiator similar to an existing helix radiator tested without interface [32], included for comparison purposes.

Materials and methods

Prototypes of novel RF radiators

Figures 1A–D show the 27.12 MHz prototypal structures of the RF radiators developed in the present investigation. The radiating circuits are constructed over the surface of the insulating cable mandrels (i.e. the flexible coaxial cable cylindrical nylon segments deprived of the shield) the lengths of which establish the active lengths of the radiators. Each radiator is part of a resonant circuit connected to the RF power source through a match-and-tune (M&T) external box (not shown) that includes a manually controlled tuning capacitor and matching air-transformer [40].

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.

Inductive H-radiator

The helix winding of the H-radiator (Figure 1A) is made of thin and narrow (2 × 0.1 mm) phosphor-bronze ribbon spiralled over the mandrel [32]. The radius R of the coil is much smaller than the 27.12 MHz wavelength, a condition exciting the normal EM mode of the coil winding, producing significant internal and external longitudinal heating components that contribute to the heating of exposed tissues [41]. This helix presents non-significant wavelength because the winding length is kept sufficiently short with respect to the RF wavelength. This H-radiator was developed using the same coaxial cable and same technology, and a similar length as the existing U-10 helix prototype [32], shown in Figure 1D for comparison. Physical data of both H-radiators 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

Inductive T-radiator

The radiating circuit of the T-radiator prototype (Figure 1C) consists of four symmetric and radially orientated rectangular loops, having the central conductor of the insulating mandrel as a common inner loop side. The four copper wire conductors are shaped as external loop sides and are soldered at the top to the central conductor tip, lie longitudinally on the surface of the insulating mandrel, and are soldered at the bottom to the cable shield end. Thus, the RF current from the tip of the central cable conductor splits into four currents that flow along the four parallel-connected outer loop sides to the cable shield end, producing an axial flux tube of azimuthal H-field that induces radial solenoidal currents underneath the exposed conductive layer of the cavity wall.

This short and parallel-connected circuit presents very low RF impedance and resistive losses, non-significant wavelength effects and parasitics, and does not require specific optimisation procedures. In EM terms, this structure could substantially be considered a scaled-down lumped-constant equivalent to the RF toroidal flux tube inductive applicator for external hyperthermia [43], [44], in which four very large circumferential apertures are made. The RF currents along the short longitudinal loop sides contribute to effective and safe heating as short strip inductors [40]. Physical data of the T-radiator are provided in Table I.

Inverse-capacitive C-radiator

The C-radiator prototype is shown in Figure 1B. The distal ring electrode is soldered to the tip of the central conductor at the ends of the insulating mandrel and the proximal ring electrode is soldered over the end of the cable shield. The C-radiator operates on the wall of cylindrical cavities in an inverse manner to the external capacitive devices developed for heating internal cylindrical loads. The C-radiator retains the configuration of the external devices in which the ribbon electrode widths are substantially smaller than the space between the electrodes to help limit the preferentially superficial heating of the in-between superficial tissue layer that is deleterious to the penetration [42], [44]. Significant wavelength effects and tissue overheating at the electrodes at the testing power level are not detected. Physical data of the C-radiator are provided in Table I.

RF-matching interfaces and the T, H, C heads

Prior to insertion into the 8-mm inner diameter (ID) hole in the phantom cavity, each radiator described above was covered with a bottom-tapped thin plastic cannula in order to create inner and outer cylindrical interstices after insertion in the cavity. These interstices were filled with a selected combination of dielectric media to constitute a bi-layer structure–tissue interface (Figure 2) that operates as EM-matching transformer. Matching was sought by stepwise modifications of radial combinations of the filling dielectric media.

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.

Non-conductive water and air were the only dielectric media used. Non-conductive water (W) was selected because it has a very high dielectric constant, minimal losses, is readily available, and can be easily set into any shape and thickness. Air (A) is an unavoidable part of the gap and its layer can also be easily sized and shaped. The very thin plastic cannula of fixed OD is physically compatible with all active structures, does not significantly interfere with the RF field, and establishes the precise inner and outer cylindrical interstices reported in Table I. Each radiator composed of a bi-layer interface is referred to as a heating head. The heads 8 mm OD were chosen to be compatible with prostatic urethrae and with interstices of practical and feasible thickness. Cross-sections of the T, H, and C heads are depicted in Figure 3. Diagrams of the longitudinal sections of these heads are shown in the Results section below. Physical data of the heads are reported in Table I.

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.

Cavity phantom

Figure 4 shows the phantom assembly from a previous study [32] that is also used for head assessments in the present study. The phantom consists of a poly-methyl-methacrylate box sized 700 × 800 × 1400 mm, filled with polycarbonate gel that is loaded with carbon black and aluminium powder to give σ ∼ 0.2 S/m and ϵ ∼ 85 (as measured with the HP4193A Vector Impedance Analyzer (Hewlett Packard, Palo Alto, CA) and equipped with a temperature-controlled coaxial cell. A wax layer was spread over the gel surface to limit water loss. A stand was used to vertically position the heads along the z-axis of the 8-mm-ID cylindrical cavity. The phantom reference system is shown in Figure 2.

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

A layer of 10 glass parallel capillaries (0.5 mm ID; 0.7 mm OD; Wilmad Glass, Buena, NJ) spaced at intervals of 10 mm, was supported by two x,z opposite-box own templates to provide 10 temperature measurement channels along the y-axis at x₀ = 5.5 mm from the cavity z axis. A further x,z template was mounted on the carriage of a linear guide (THK, Kofu, Japan) that was driven along the y axis in 4-mm steps by a controlled step-motor. A linear array of 10 T-type wire thermocouples (0.005 mm OD; Scientific Wire, London) protected by very thin steel tubes was supported by a further x,z mobile template supporting the thermocouples to slide inside the boxed capillaries. Table II shows heat diffusion data of capillary-thermocouple assemblies in which the selected glass assembly yields the smallest time constant.

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

SAR data are obtained with the temperature–time method using an RF power generator (Henry Radio, Los Angeles, CA) and a dual-channel RF Wattmeter (Model 43, Bird Electronic, Bellevue, WA). The readouts of the temperature jumps Δt following the power pulses (15 W, 60 s) were obtained through a thermocouple multi-channel interface (MDP-8250; Mess Technik, Ortenburg, Germany). The five Δ_ti (i = 0–5) readouts at radial distances r_i(x₀, y_i) were normalised to the Δt_MAX readout of the whole set. The Δt_MAX readout is close to similar to the 7.4°C obtained with the same protocol in assessment of the RF U-10 radiator [32].

Radial {R, z} iso-SAR contours of the T-head, H-head, and C-head in Figures 5–7 were obtained from readouts and normalisation of the arrays of Δt values on the x₀, y, z measurement plane through the MDP-8250, and by further graphic processing (Golden Software™, Golden, CO) after symmetrisation (not shown). The radial R, z iso-SAR normalised contours of the U-10 radiator illustrated in [32] are retrieved for direct comparison. Matching of the heads did not change significantly during power operations or at different insertion depths, and the reflected power remained below 3%. The phantom box is not shielded.

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.

Results

The acceptance criteria for the heads include adequate longitudinal heating uniformity and penetration, absence of insertion depth effects, stability of heating features, and reproducibility. Figures 5–7 show the normalised iso-SAR contours; it is clear that only the T_AA, H_AW, and C_AW heads show acceptable heating features. Each head was tested twice in succession under equivalent heating conditions with the selected A-A, W-W, and A-W media combinations.

Figures 5–7 illustrate the iso-SAR normalised contours of the triple interface tests of the selected T_AA, H_AW, and C_AW heads. Figure 8 shows the longitudinal normalised SAR profiles constructed with points of the z-axis and the related points of the normalised SAR contours from 50%SAR to 100%SAR on the R,z planes of Figures 5–7. The heating parameters provided in Table III are introduced for comparison among the features of the heads and with those obtained from other representative EHT devices.

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

The following parameters are introduced from the 50%SAR profiles shown in Figure 8. The L_eff ratio of the longitudinal extension Z₅₀ of each 50%SAR profile to the related radiator length L is a measure of the relative length of the effective heating field. The Q₈₀ ratio of the longitudinal extension Z₈₀ of the 80%SAR level to the Z₅₀ extension is a measure of the longitudinal uniformity of the effective heating field. The distal (d-G_Z) and proximal (p-G_Z) longitudinal SAR gradients of the effective heating field at the Z₅₀ ends – together with the pair of weak SAR_MAX at the Z₅₀ ends in some of the 50% SAR profiles – help to determine the sharpness of the SAR clearance at the Z₅₀ ends (information of clinical interest regarding the safety of the prostate distal and proximal sphincters). The conservative penetration depth D_1/2 parameter – i.e. the maximum of the radial half-power distance – is determined from the 50% iso-SAR contours shown in Figures 5–7. The corresponding parameters for the HLDA dipole and the U-10 are obtained from Debicki et al. [8] and Franconi et al. [32], respectively.

Discussion

The T_AA head – i.e. the simple T-radiator toroidal radiating structure (Figure 1C) – is the top-performing and most versatile and safe RF device, on account of full uncritical air gap, immunity from parasitics and no access tissues overheating, advantages due to the azimuthal H-field fluxtube [43] carrying highest congruent symmetry with the wall tissue and to the complementary safe and congruent heating of the parallel connected four longitudinal short loop side strip inductors [40].

T_AA exhibits top penetration (D_1/2 ∼ 19 mm) with dumbbell-like SAR profiles and high side protrusions of L_eff over L, with the SAR_MAX close to the proximal and the distal radiator ends producing high p-G_Z, d-G_Z SAR terminal gradients, and the top longitudinal heating Q₈₀ value (Table III, Figures 5 and 8). Because the T_AA loop sides can be adjusted to any length and easily shaped and optimised, even to a non-uniform radius, in the absence of wavelength effects and parasitics, they can easily be shaped to match the contour of different body cavities. Short and moderately flexible T-radiators such as the prototype of Figure 1C can be shifted back and forth along the cavity retaining stable external tuning and matching during the heating.

The T_AA loop sides may be grouped in an azimuthal direction – by arranging correspondingly larger apertures along opposite loop sides – to focus RF energy for directional treatment of a localised wall lesion. Umbrella shaped top loop sides are candidates for end-fire heating of large cavity bottoms. Flexible and cooled T_AA loopsides can be implemented with thin small scale metallic expansion pipes (i.e.: bellows). To the T_AA highest heating rate and to the substantial absence of wavelength effects substantially contribute the parallel connection of the short radiating strips and the load-radiator conformal symmetry.

H_AW heads can administer uniform and stable RF heating fields at variable insertion depths, together with dependable heating parameters similar to those of the T_AA head. The side protrusions are not significant and the D_1/2 is 19 mm. (Table III, Figure 8). H_AW produces iso-SAR contours similar to T_AA; however, only with the contribution of the dielectric interface (Figure 6), likely due to the complex modes of helix resonators [41]. H_AW heads are suitable for treatment requiring very flexible radiators. Optimised combinations of length and pitch are obtained for most targets provided that coiled conductor lengths are much smaller than the RF wavelength [32]. The high winding losses, the complex H-field modes of the spiralled winding [41] and the length-pitch ratio [32] are major contributors to the lowest H_AW heating rate (Table III).

The C_AW head iso-SAR contours (Figure 7) show a distinct SAR_MAX close to the proximal electrode that causes a substantial protrusion of the distal electrode beyond Z₅₀. The L_eff value is small, and the p-G_Z, d-G_Z, and Q₈₀ values are similar to those of the inductive heads; the D_1/2 is 20 mm in line with the other heads (Table III; Figures 7 and 8), a datum that asks for a reconsideration of the capacitive RF energy transfer mechanism. The C_AW heating rate is acceptable.

Customisation of C_AW heads includes setting the separation between electrodes required to include the target using as much as possible narrow electrodes [42],[44], and providing the bi-layer interface conformal to the wall cavity surface. The latter may be implemented with conformal double catheter flexible balloons. Equipped with conformal electrodes, C_AW appears to have potential for heating wall segments of body cavities that have quasi-cylindrical symmetry and variable radius and length (e.g. the urethra, rectum, oesophagus, bladder, and uterus). For large cavities, C_AW heads are eligible for focused treatment of localised wall lesions with conformal patch electrodes at opposite extents of lesions on wall sites of limited curvatures. Patch electrodes offer advantages over the existing RF capacitive devices for treating localised wall lesions consisting of an intraluminal electrode facing one or more external ground electrodes only in those directions in which transverse current pathways are made possible by the site anatomy [45].

Prospects for the development of RF devices for EHT and IHT

The heating features of the RF heads are compared with those of existing MW and RF devices for EHT and IHT. A quantitative synopsis of the main heating features of RF heads and of representative RF and MW devices is presented in Table III.

RF heads versus MW antennae for EHT

Comparison data were obtained from a recent comprehensive study regarding catheterised MW antennae for EHT that includes helix and dipole antennae and also the hybrid HLDA dipole – a strongly hybridised dipole with attached lumped constant inductive loads – that delivered the best performance [8]. The retrieved HLDA normalised SAR profile appears in Figure 5, and related parameters appear in Table III, together with those substantially similar of MW helix and linear dipole antennae.

Figure 8 shows that the strong HLDA hybridisation produces an intense hot spot resulting in a very poor Q₈₀ longitudinal quality index (26%) (Table III), indicating that most of the MW energy is concentrated in approximately one third of the antenna length. Comparison of HLDA parameters with those of the RF heads (Table III) shows that while the L_eff values are comparable, the HLDA terminal SAR gradients p-G_Z and d-G_Z are significantly smaller, causing a decrease of the heating effectiveness outside the hot spot [8]. For HLDA, the hot spot D_1/2 is approximately 15 mm, which decreases approximating common values of MW helix and dipole antennae, hot spots excluded [8].

The optimisation of the MW device–tissue matching [1–19] involves laborious and critical cut-and-try adjustments of antennae [8], on account of large wavelength effects and parasitics due to the MW wavelengths being of the same order of magnitude as those of the devices (and targets). The customisation T_AA and C_AW for small radius cavities is not affected by significant wavelength effects and the design of these heads is practically target guided, whilst H_AW requires preliminary cut-and-try optimisation of helix pitch and length [32].

Table IV is a comprehensive qualitative synopsis of the main heating features of the RF heads and representative MW devices for EHT.

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.

RF heads versus bare RF radiators

Figure 1 shows the present helix H-radiator (Figure 1A) of the H_AW head and the similar existing RF helix radiator U-10 (Figure 1D). These radiators are developed using the same technology and their physical data are provided in Table I. The U-10 radiator is assessed on the phantom of Figure 4 protected by a thin Mylar film [32] whilst the H-radiator is provided with the A-W matching interface, and both deliver heating fields with comparable and excellent L_eff, p-G_Z, d-G_Z, and Q₈₀ parameters (Figures 6, 8, 9; Table III). However, whilst the D_1/2 of the U-10 is ∼3 mm, the D_1/2 of the H_AW is five-fold higher, unambiguously confirming the key role of the A-W interface in determining the high levels of RF energy transfer to deep tissues for helix radiators. The 27 MHz capacitive device implemented according to the interstitial multi-electrode current source (MECS) technology [29], consisting of a catheter over which a pair of substantially adjacent wide electrodes is painted and protected with a thin coating, exhibits a penetration depth about six times smaller than the C_AW head. Thus, C_AW heads of smaller OD may usefully be developed in arrays for interstitial hyperthermia treatments. The air-coupled and bare T-radiator exhibits superior features among all proposed and existing RF EHT devices.

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.

Pre-optimised RF heads

Pre-optimised RF heads can be developed to reproduce the stable optimised cavity EM microenvironment in the clinic. In particular, the full air-gap T-radiators are easy to be pre-optimised by customising length, size and apertures of the toroidal structure. Figure 10 exemplifies a prototype of a pre-optimised H_AW head externally protected by a RF-transparent 8-mm OD thin sleeve, reproducing the optimised microenvironment of cavities of compatibly resilient diameters. Pre-optimised heads potentially enable the development of customisable devices, improving the feasibility of both IHT and EHT treatment planning and thermal dosimetry for effective hyperthermia treatments while relieving clinicians of many critical on-field optimisation procedures.

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.

Conclusions

To exploit the features of these RF models, it is necessary to investigate the specific interactions in the EM microenvironment of each cavity. The evident positive and negative effects of different dielectric interfaces support the presence of high-intensity near-fields that appear to improve the heating efficiency besides indicating the need for theoretical near-fields modelling to help rationalise the disclosed phenomenology. It is also shown that the potential of the heating mechanisms of large RF devices for external hyperthermia extends over scaled down devices and that the 27 MHz radiation is adequate for effective EHT treatments.

More research is required concerning the number, thickness and diameter of interstices, the type and radial sequences of filling media and effects of dielectric properties of cooling fluids, catheters and mandrels. More research is also required regarding possible instabilities related to problematic contacts of the heads with the tissue. Nevertheless, the use of high permittivity and workable ceramics could be investigated with the aim of producing dependable and practical devices of different size and shape.

Prior to implementation in the clinical environment, further research and development is necessary to improve RF head technology with regard to versatility, dependability, and customisation; however, low frequency RF radiation shows promise as an effective EM field for hyperthermia treatment for BPH, and could improve planning and dosimetry in endoscopic as well as interstitial hyperthermia treatments of prostatic carcinoma.

Acknowledgements

The authors wish to thank Italo Vannucci, Paolo Necci, and Antonio Canichella for assistance and editing of figures.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.