Predictive value of SAR based quality indicators for head and neck hyperthermia treatment quality

Helmet Radio Frequency Phased Array Applicators Enhance Thermal Magnetic Resonance of Brain Tumors

Thermal Magnetic Resonance (ThermalMR) integrates Magnetic Resonance Imaging (MRI) diagnostics and targeted radio-frequency (RF) heating in a single theranostic device. The requirements for MRI (magnetic field) and targeted RF heating (electric field) govern the design of ThermalMR applicators. We hypothesize that helmet RF applicators (HPA) improve the efficacy of ThermalMR of brain tumors versus an annular phased RF array (APA). An HPA was designed using eight broadband self-grounded bow-tie (SGBT) antennae plus two SGBTs placed on top of the head. An APA of 10 equally spaced SGBTs was used as a reference. Electromagnetic field (EMF) simulations were performed for a test object (phantom) and a human head model. For a clinical scenario, the head model was modified with a tumor volume obtained from a patient with glioblastoma multiforme. To assess performance, we introduced multi-target evaluation (MTE) to ensure whole-brain slice accessibility. We implemented time multiplexed vector field shaping to optimize RF excitation. Our EMF and temperature simulations demonstrate that the HPA improves performance criteria critical to MRI and enhances targeted RF and temperature focusing versus the APA. Our findings are a foundation for the experimental implementation and application of a HPA en route to ThermalMR of brain tumors.

Advanced Radio Frequency Applicators for Thermal Magnetic Resonance Theranostics of Brain Tumors

Thermal Magnetic Resonance (ThermalMR) is a theranostic concept that combines diagnostic magnetic resonance imaging (MRI) with targeted thermal therapy in the hyperthermia (HT) range using a radiofrequency (RF) applicator in an integrated system. ThermalMR adds a therapeutic dimension to a diagnostic MRI device. Focused, targeted RF heating of deep-seated brain tumors, accurate non-invasive temperature monitoring and high-resolution MRI are specific requirements of ThermalMR that can be addressed with novel concepts in RF applicator design. This work examines hybrid RF applicator arrays combining loop and self-grounded bow-tie (SGBT) dipole antennas for ThermalMR of brain tumors, at magnetic field strengths of 7.0 T, 9.4 T and 10.5 T. These high-density RF arrays improve the feasible transmission channel count, and provide additional degrees of freedom for RF shimming not afforded by using dipole antennas only, for superior thermal therapy and MRI diagnostics. These improvements are especially relevant for ThermalMR theranostics of deep-seated brain tumors because of the small surface area of the head. ThermalMR RF applicators with the hybrid loop+SGBT dipole design outperformed applicators using dipole-only and loop-only designs, with superior MRI performance and targeted RF heating. Array variants with a horse-shoe configuration covering an arc (270°) around the head avoiding the eyes performed better than designs with 360° coverage, with a 1.3 °C higher temperature rise inside the tumor while sparing healthy tissue. Our EMF and temperature simulations performed on a virtual patient with a clinically realistic intracranial tumor provide a technical foundation for implementation of advanced RF applicators tailored for ThermalMR theranostics of brain tumors.

Field and Temperature Shaping for Microwave Hyperthermia: Recent Treatment Planning Tools to Enhance SAR-Based Procedures

The aim of the article is to provide a summary of the work carried out in the framework of a research project funded by the Italian Ministry of Research. The main goal of the activity was to introduce multiple tools for reliable, affordable, and high-performance microwave hyperthermia for cancer therapy. The proposed methodologies and approaches target microwave diagnostics, accurate in vivo electromagnetic parameters estimation, and improvement in treatment planning using a single device. This article provides an overview of the proposed and tested techniques and shows their complementarity and interconnection. To highlight the approach, we also present a novel combination of specific absorption rate optimization via convex programming with a temperature-based refinement method implemented to mitigate the effect of thermal boundary conditions on the final temperature map. To this purpose, numerical tests were carried out for both simple and anatomically detailed 3D scenarios for the head and neck region. These preliminary results show the potential of the combined technique and improvements in the temperature coverage of the tumor target with respect to the case wherein no refinement is adopted.

Microwave Hyperthermia of Brain Tumors: A 2D Assessment Parametric Numerical Study

Due to the clinically proven benefit of hyperthermia treatments if added to standard cancer therapies for various tumor sites and the recent development of non-invasive temperature measurements using magnetic resonance systems, the hyperthermia community is convinced that it is a time when even patients with brain tumors could benefit from regional microwave hyperthermia, even if they are the subject of a treatment to a vital organ. The purpose of this study was to numerically analyze the ability to achieve a therapeutically relevant constructive superposition of electromagnetic (EM) waves in the treatment of hyperthermia targets within the brain. We evaluated the effect of the target size and position, operating frequency, and the number of antenna elements forming the phased array applicator on the treatment quality. In total, 10 anatomically realistic 2D human head models were considered, in which 10 circular hyperthermia targets with diameters of 20, 25, and 30 mm were examined. Additionally, applicators with 8, 12, 16, and 24 antenna elements and operating frequencies of 434, 650, 915, and 1150 MHz, respectively, were analyzed. For all scenarios considered (4800 combinations), the EM field distributions of individual antenna elements were calculated and treatment planning was performed. Their quality was evaluated using parameters applied in clinical practice, i.e., target coverage (TC) and the target to hot-spot quotient (THQ). The 12-antenna phased array system operating at 434 MHz was the best candidate among all tested systems for HT treatments of glioblastoma tumors. The 12 antenna elements met all the requirements to cover the entire target area; an additional increase in the number of antenna elements did not have a significant effect on the treatment quality.

ESHO benchmarks for computational modeling and optimization in hyperthermia therapy

BACKGROUND

The success of cancer hyperthermia (HT) treatments is strongly dependent on the temperatures achieved in the tumor and healthy tissues as it correlates with treatment efficacy and safety, respectively. Hyperthermia treatment planning (HTP) simulations have become pivotal for treatment optimization due to the possibility for pretreatment planning, optimization and decision making, as well as real-time treatment guidance.

MATERIALS AND METHODS

The same computational methods deployed in HTP are also used for in silico studies. These are of great relevance for the development of new HT devices and treatment approaches. To aid this work, 3 D patient models have been recently developed and made available for the HT community. Unfortunately, there is no consensus regarding tissue properties, simulation settings, and benchmark applicators, which significantly influence the clinical relevance of computational outcomes.

RESULTS AND DISCUSSION

Herein, we propose a comprehensive set of applicator benchmarks, efficacy and safety optimization algorithms, simulation settings and clinical parameters, to establish benchmarks for method comparison and code verification, to provide guidance, and in view of the 2021 ESHO Grand Challenge (Details on the ESHO grand challenge on HTP will be provided at https://www.esho.info/).

CONCLUSION

We aim to establish guidelines to promote standardization within the hyperthermia community such that novel approaches can quickly prove their benefit as quickly as possible in clinically relevant simulation scenarios. This paper is primarily focused on radiofrequency and microwave hyperthermia but, since 3 D simulation studies on heating with ultrasound are now a reality, guidance as well as a benchmark for ultrasound-based hyperthermia are also included.

Suitability of eigenvalue beam-forming for discrete multi-frequency hyperthermia treatment planning

PURPOSE

Thermal dose delivery in microwave hyperthermia for cancer treatment is expected to benefit from the introduction of ultra-wideband (UWB)-phased array applicators. A full exploitation of the combination of different frequencies to improve the deposition pattern is, however, a nontrivial problem. It is unclear whether the cost functions used for hyperthermia treatment planning (HTP) optimization in the single-frequency setting can be meaningfully extended to the UWB case.

METHOD

We discuss the ability of the eigenvalue (EV) and a novel implementation of iterative-EV (i-EV) beam-forming methods to fully exploit the available frequency spectrum when a discrete set of simultaneous operating frequencies is available for treatment. We show that the quadratic power deposition ratio solved by the methods can be maximized by only one frequency in the set, therefore rendering EV inadequate for UWB treatment planning. We further investigate whether this represents a limitation in two realistic test cases, comparing the thermal distributions resulting from EV and i-EV to those obtained by optimizing for other nonlinear cost functions that allow for multi-frequency.

RESULTS

The classical EV-based single-frequency HTP yields systematically lower target SAR deposition and temperature values than nonlinear HTP. In a larynx target, the proposed single-frequency i-EV scheme is able to compensate for this and reach temperatures comparable to those given by global nonlinear optimization. In a meninges target, the multi-frequency setting outperforms the single-frequency one, achieving better target coverage and 0 . 5 ∘ C higher T 90 in the tumor than single-frequency-based HTP.

CONCLUSIONS

Classical EV performs poorly in terms of resulting target temperatures. The proposed single-frequency i-EV scheme can be a viable option depending on the patient and tumor to be treated, as long as the proper operating frequency can be selected across a UWB range. Multi-frequency HTP can bring a considerable benefit in regions typically difficult to treat such as the brain.

Standardization of patient modeling in hyperthermia simulation studies: introducing the Erasmus Virtual Patient Repository

Purpose: Thermal dose-effect relations have demonstrated that clinical effectiveness of hyperthermia would benefit from more controlled heating of the tumor. Hyperthermia treatment planning (HTP) is a potent tool to study strategies enabling target conformal heating, but its accuracy is affected by patient modeling approximations. Homogeneous phantoms models are being used that do not match the body shape of patients in treatment position and often have unrealistic target volumes. As a consequence, simulation accuracy is affected, and performance comparisons are difficult. The aim of this study is to provide the first step toward standardization of HTP simulation studies in terms of patient modeling by introducing the Erasmus Virtual Patient Repository (EVPR): a virtual patient model database.Methods: Four patients with a tumor in the head and neck or the pelvis region were selected, and corresponding models were created using a clinical segmentation procedure. Using the Erasmus University Medical Center standard procedure, HTP was applied to these models and compared to HTP for commonly used surrogate models.Results: Although this study was aimed at presenting the EVPR database, our study illustrates that there is a non-negligible difference in the predicted SAR patterns between patient models and homogeneous phantom-based surrogate models. We further demonstrate the difference between actual and simplified target volumes being used today.Conclusion: Our study describes the EVPR for the research community as a first step toward standardization of hyperthermia simulation studies.

Patient-Specific Planning for Thermal Magnetic Resonance of Glioblastoma Multiforme

Simple Summary

Hyperthermia was proven to enhance the efficacy of chemo- and radiation therapy treatment of glioblastoma multiforme, an aggressive brain tumor of poor prognosis. Despite good clinical results in other tumor types and locations, hyperthermia induced by electromagnetic waves in the radiofrequency range is not available so far for the treatment of brain tumors due to the highly sensitive surrounding tissue and lack of non-invasive therapy monitoring. ThermalMR integrates non-invasive diagnosis, therapy, and therapy monitoring in a single RF applicator device by employing radiowaves for magnetic resonance imaging, radiofrequency heating, as well as magnetic resonance thermometry. This work examines three optimization algorithms for hyperthermia treatment planning and up to ten RF applicator configurations for a cohort of nine patient models with glioblastoma multiforme. Clinical diversity is represented in target size and location and the inclusion of post-operative models. Our findings indicate the need and potential for patient-specific treatment planning and RF applicator design when targeting brain tumors.

Abstract

Thermal intervention is a potent sensitizer of cells to chemo- and radiotherapy in cancer treatment. Glioblastoma multiforme (GBM) is a potential clinical target, given the cancer’s aggressive nature and resistance to current treatment options. This drives research into optimization algorithms for treatment planning as well as radiofrequency (RF) applicator design for treatment delivery. In this work, nine clinically realistic GBM target volumes (TVs) for thermal intervention are compared using three optimization algorithms and up to ten RF applicator designs for thermal magnetic resonance. Hyperthermia treatment planning (HTP) was successfully performed for all cases, including very small, large, and even split target volumes. Minimum requirements formulated for the metrics assessing HTP outcome were met and exceeded for all patient specific cases. Results indicate a 16 channel two row arrangement to be most promising. HTP of TVs with a small extent in the cranial–caudal direction in conjunction with a large radial extent remains challenging despite the advanced optimization algorithms used. In general, deep seated targets are favorable. Overall, our findings indicate that a one-size-fits-all RF applicator might not be the ultimate approach in hyperthermia of brain tumors. It stands to reason that modular and reconfigurable RF applicator configurations might best suit the needs of targeting individual GBM geometry.

Effect of gastrointestinal gas on the temperature distribution in pancreatic cancer hyperthermia treatment planning

PURPOSE

In pancreatic cancer treatment, hyperthermia can be added to increase efficacy of chemo- and/or radiotherapy. Gas in stomach, intestines and colon is often in close proximity to the target volume. We investigated the impact of variations in gastrointestinal gas (GG) on temperature distributions during simulated hyperthermia treatment (HT).

METHODS

We used sets of one CT and eight cone-beam CT (CBCT) scans obtained prior to/during fractionated image-guided radiotherapy in four pancreatic cancer patients. In Plan2Heat, we simulated locoregional heating by an ALBA-4D phased array radiofrequency system and calculated temperature distributions for (i) the segmented CT (sCT), (ii) sCT with GG replaced by muscle (sCT₀), (iii) sCT₀ with eight different GG distributions as visible on CBCT inserted (sCT_CBCT). We calculated cumulative temperature-volume histograms for the clinical target volume (CTV) for all ten temperature distributions for each patient and investigated the relationship between GG volume and change in ΔT₅₀ (temperature increase at 50% of CTV volume). We determined location and volume of normal tissue receiving a high thermal dose.

RESULTS

GG volume on CBCT varied greatly (9-991 cm³). ΔT₅₀ increased for increasing GG volume; maximum ΔT₅₀ difference per patient was 0.4-0.6 °C. The risk for GG-associated treatment-limiting hot spots appeared low. Normal tissue high-temperature regions mostly occurred anteriorly; their volume and maximum temperature showed moderate positive correlations with GG volume, while fat-muscle interfaces were associated with higher risks for hot spots.

CONCLUSIONS

Considerable changes in volume and position of gastrointestinal gas can occur and are associated with clinically relevant tumor temperature differences.

Iterative time-reversal for multi-frequency hyperthermia

Time-reversal (TR) is a known wideband array beam-forming technique that has been suggested as a treatment planning alternative in deep microwave hyperthermia for cancer treatment. While the aim in classic TR is to focus the energy at a specific point within the target, no assumptions are made on secondary lobes that might arise in the healthy tissues. These secondary lobes, together with tissue heterogeneity, may result in hot-spots (HSs), which are known to limit the efficiency of the thermal dose delivery to the tumor. This paper proposes a novel wideband TR focusing method that iteratively shifts the focus away from HSs and towards cold-spots from an initial TR solution, a procedure that improves tumor coverage and reduces HSs. We verify this method on two different applicator topologies and several target volume configurations. The algorithm is deterministic and runs within seconds, enabling its use for real-time applications. At the same time, it yields results comparable to those obtained with global stochastic optimizers such as Particle Swarm.

Design of Site-Specific Microwave Phased Array Hyperthermia Applicators Using 434 MHz Reduced Cavity-Backed Patch Antenna

Cancers of the neck, breast, and lower extremities are common malignancies diagnosed in India with a higher incidence of advanced-stage disease. Phased array (PA) applicators reported for hyperthermia treatment (HT) of the breast have small focal region and high cross-coupling, and those reported for lower extremities provide regional heating and limited steering. In this study, we present the numerical design of site-specific PA applicators for HT of large solid tumors in the neck, breast, and lower extremities using a miniaturized 434 MHz cavity-backed water-loaded patch antenna. The fabricated antenna has 38 × 36 mm2 aperture, more than 90% power coupling, 25 MHz bandwidth, and good agreement between simulated and measured specific absorption rate (SAR) in phantom. The site-specific applicators demonstrated less power reflection (<-17.9 dB) and cross-coupling (<-26.8 dB) for 5 mm inter-ring spacing. SAR indicators for 64 cc tumor at varying locations in simplified layered three-dimensional (3D) tissue models of the neck, breast, and leg showed average power absorption ratio (aPAratio ) ≥ 3.16, target to hotspot quotient (THQ) ≥ 0.57, 25% iso-SAR coverage (TC25 ) ≥ 81%, and 50% iso-SAR coverage (TC50 ) ≥51.8%. Simulation results of site-specific applicators for 3D inhomogeneous patient models showed aPAratio  ≥ 5.98, THQ ≥ 0.9, TC50  ≥ 86%, and 100% TC25 for all sites. It is concluded that the 434 MHz miniaturized cavity-backed patch antenna can be used to develop high-density PA applicators with 12-24 antennas for HT of large solid tumors (≥4 cm) in the neck, breast, and lower extremities with 3D steering ability and less cross-coupling (≤-26.8 dB). © 2020 Bioelectromagnetics Society.

Dual-Function MR-Guided Hyperthermia: An Innovative Integrated Approach and Experimental Demonstration of Proof of Principle

Temperature monitoring plays a central role in improving clinical effectiveness of adjuvant hyperthermia. The potential of magnetic resonance thermometry for treatment monitoring purposes led to several MR-guided hyperthermia approaches. However, the proposed solutions were sub-optimal due to technological and intrinsic limitations. These hamper achieving target conformal heating possibilities (applicator limitations) and accurate thermometry (inadequate signal-to-noise-ratio (SNR)). In this work, we studied proof of principle of a dual-function hyperthermia approach based on a coil array (64 MHz, 1.5 T) that is integrated in-between a phased array for heating (434 MHz) for maximum signal receive in order to improve thermometry accuracy. Hereto, we designed and fabricated a superficial hyperthermia mimicking planar array setup to study the most challenging interactions of generic phased-array setups in order to validate the integrated approach. Experiments demonstrated that the setup complies with the superficial hyperthermia guidelines for heating and is able to improve SNR at 2-4 cm depth by 17%, as compared to imaging using the body coil. Hence, the results showed the feasibility of our dual-function MR-guided hyperthermia approach as basis for the development of application specific setups.

The Potential of Adjusting Water Bolus Liquid Properties for Economic and Precise MR Thermometry Guided Radiofrequency Hyperthermia

The potential of MR thermometry (MRT) fostered the development of MRI compatible radiofrequency (RF) hyperthermia devices. Such device integration creates major technological challenges and a crucial point for image quality is the water bolus (WB). The WB is located between the patient body and external sources to both couple electromagnetic energy and to cool the patient skin. However, the WB causes MRT errors and unnecessarily large field of view. In this work, we studied making the WB MRI transparent by an optimal concentration of compounds capable of modifying T2* relaxation without an impact on the efficiency of RF heating. Three different T2* reducing compounds were investigated, namely CuSO4, MnCl2, and Fe3O4. First, electromagnetic properties and T2* relaxation rates at 1.5 T were measured. Next, through multi-physics simulations, the predicted effect on the RF-power deposition pattern was evaluated and MRT precision was experimentally assessed. Our results identified 5 mM Fe3O4 solution as optimal since it does not alter the RF-power level needed and improved MRT precision from 0.39 °C to 0.09 °C. MnCl2 showed a similar MRT improvement, but caused unacceptable RF-power losses. We conclude that adding Fe3O4 has significant potential to improve RF hyperthermia treatment monitoring under MR guidance.

Solving the Time- and Frequency-Multiplexed Problem of Constrained Radiofrequency Induced Hyperthermia

Targeted radiofrequency (RF) heating induced hyperthermia has a wide range of applications, ranging from adjunct anti-cancer treatment to localized release of drugs. Focal RF heating is usually approached using time-consuming nonconvex optimization procedures or approximations, which significantly hampers its application. To address this limitation, this work presents an algorithm that recasts the problem as a semidefinite program and quickly solves it to global optimality, even for very large (human voxel) models. The target region and a desired RF power deposition pattern as well as constraints can be freely defined on a voxel level, and the optimum application RF frequencies and time-multiplexed RF excitations are automatically determined. 2D and 3D example applications conducted for test objects containing pure water (r_target = 19 mm, frequency range: 500–2000 MHz) and for human brain models including brain tumors of various size (r₁ = 20 mm, r₂ = 30 mm, frequency range 100–1000 MHz) and locations (center, off-center, disjoint) demonstrate the applicability and capabilities of the proposed approach. Due to its high performance, the algorithm can solve typical clinical problems in a few seconds, making the presented approach ideally suited for interactive hyperthermia treatment planning, thermal dose and safety management, and the design, rapid evaluation, and comparison of RF applicator configurations.

Quality indicators for hyperthermia treatment: Italian survey analysis

AIM

Nowadays, no Quality Indicators (QI) have been proposed for Hyperthermia treatments. Starting from radiotherapy experience, the aim of this work is to adapt radiotherapy indicators to Hyperthermia and to propose a new specific set of QI in Hyperthermia field.

MATERIAL AND METHODS

At first, radiotherapy quality indicators published in literature have been adapted to hyperthermia setting. Moreover, new specific indicators for the treatment of hyperthermia have been defined. To obtain the standard reference values of quality indicators, a questionnaire was sent to 7 Italian hyperthermia Institutes with a list of questions on physical and clinical hyperthermia treatment in order to highlight the different therapeutic approaches.

RESULTS

Three structure, five process and two outcome QI were selected. It has been possible to adapt seven indicators from radiotherapy, while three indicators have been defined as new specific indicators for hyperthermia. Average values used as standard reference values have been obtained and proposed.

CONCLUSION

The survey performed on 7 Italian centres allowed to derive the standard reference value for each indicator. The proposed indicators are available to be investigated and applied by a larger number of Institutes in which hyperthermia treatment is performed in order to monitor the operational procedures and to confirm or modify the reference standard value derived for each indicator.

Radiofrequency applicator concepts for thermal magnetic resonance of brain tumors at 297 MHz (7.0 Tesla)

Purpose: Thermal intervention is a potent sensitizer of cells to chemo- and radiotherapy in cancer treatment. Glioblastoma multiforme (GBM) is a potential clinical target, given the cancer's aggressive nature and resistance to current treatment options. The annular phased array (APA) technique employing electromagnetic waves in the radiofrequency (RF) range allows for localized temperature increase in deep seated target volumes (TVs). Reports on clinical applications of the APA technique in the brain are still missing. Ultrahigh field magnetic resonance (MR) employs higher frequencies than conventional MR and has potential to provide focal temperature manipulation, high resolution imaging and noninvasive temperature monitoring using an integrated RF applicator (ThermalMR). This work examines the applicability of RF applicator concepts for ThermalMR of brain tumors at 297 MHz (7.0 Tesla).Methods: Electromagnetic field (EMF) simulations are performed for clinically realistic data based on GBM patients. Two algorithms are used for specific RF energy absorption rate based thermal intervention planning for small and large TVs in the brain, aiming at maximum RF power deposition or RF power uniformity in the TV for 10 RF applicator designs.Results: For both TVs , the power optimization outperformed the uniformity optimization. The best results for the small TV are obtained for the 16 element interleaved RF applicator using an elliptical antenna arrangement with water bolus. The two row elliptical RF applicator yielded the best result for the large TV.Discussion: This work investigates the capacity of ThermalMR to achieve targeted thermal interventions in model systems resembling human brain tissue and brain tumors.