Trapezoidal Magnetic Fields Boost Nanoparticle Hyperthermia Against Tumors in Mice

Magnetic hyperthermia, a cancer therapy that heats tumors from within using magnetic nanoparticles, works markedly better when the alternating magnetic field is shaped as a trapezoidal pulse rather than the conventional sinusoidal wave, according to a new study in a mouse glioblastoma model. In the experiments, tumors treated with nanoparticles and a trapezoidal pulsed alternating magnetic field shrank to nearly 20 percent below their starting size over four days, while tumors treated with the same nanoparticles under a sinusoidal field grew by about 42 percent and untreated controls grew by roughly 77 percent. The finding, published in Cancer Nanotechnology, suggests that the shape of the magnetic waveform itself, an often-overlooked parameter, can be engineered to squeeze more therapeutic effect out of the same field amplitude and frequency.

Magnetic hyperthermia relies on magnetic nanoparticles that convert electromagnetic energy into heat when placed inside an alternating magnetic field, typically oscillating between 100 kilohertz and 1 megahertz. Iron oxide nanoparticles, particularly superparamagnetic iron oxide nanoparticles, are favored for this role because they are biocompatible, colloidally stable, retain no remanent magnetization once the external field is removed, and already have an established clinical track record in imaging and drug delivery. Their weakness is efficiency: the conversion of magnetic energy into heat remains modest, which is why magnetic hyperthermia has generally been considered more effective as an adjuvant to radiotherapy or chemotherapy than as a standalone treatment. In clinical trials combining nanoparticle hyperthermia with radiotherapy, complete tumor elimination was not achieved.

Researchers have pursued two main routes to improve performance. The first is to engineer the nanoparticles themselves, tuning core size, composition, morphology and surface coatings to raise the specific absorption rate, the metric describing how efficiently the particles turn magnetic energy into heat. The second, exploited in this study, is to reshape the applied field. Previous work had shown that trapezoidal waveforms raise the specific loss power of nanoparticles compared with sinusoidal excitation, and that this translates into greater cell death in in vitro assays. Simulations had also indicated that non-sinusoidal waveforms should not endanger surrounding healthy tissue. What remained untested was whether the advantage survives in a living tumor.

The team behind the new study, based in Madrid, used a custom-built exposure system capable of generating six distinct waveforms, including sinusoidal, triangular, trapezoidal and square profiles, across the 100 kilohertz to 1 megahertz range. For the in vivo experiments they compared only trapezoidal and sinusoidal fields at 200 kilohertz, because generating square waves at the required amplitude caused the equipment to overheat. In the trapezoidal signal, the field holds a constant level for half of each period, with the remaining half occupied by linear rising and falling transitions of constant slope. The authors hypothesize that these steep transient regions, rather than the peak field values, drive enhanced heating by forcing the magnetic moments of the nanoparticles to realign more rapidly with the field.

The tumor model used was a heterotopic CT2A glioma model, in which glioma cells are injected subcutaneously into the dorsal region of adult male C57BL/6 mice. The authors acknowledge that intracranial implantation would mimic clinical glioblastoma more faithfully, but the subcutaneous approach allowed tumors to be measured non-invasively with calipers, simplified excision for histology, and avoided neurological complications. Tumors were allowed to grow for twelve days after cell injection, reaching roughly 30 cubic millimeters before treatment began. Ten mice were assigned to each of six groups: untreated controls; nanoparticles without a field; sinusoidal field alone; trapezoidal field alone; nanoparticles plus a sinusoidal field; and nanoparticles plus a trapezoidal field.

Treatment ran over three consecutive days under anesthesia. Ten minutes before each exposure, a concentrated nanoparticle suspension was injected directly into the center of the tumor, 10 microliters on the first day and 5 microliters on the following two days; control groups received equivalent volumes of buffered saline. Each mouse was then placed in the coil, with the tumor centered, and exposed to the field for one hour per day at 200 kilohertz and an intensity reported as 4.29 millitesla, corresponding to 3,422 amperes per meter. To keep the animals from overheating, the mice rested in a 3D-printed polylactic acid support perfused with water held at 37 degrees Celsius. Monitoring at four anatomical sites during a one-hour trapezoidal exposure showed body temperature drifting only from about 36.6 to a maximum of roughly 37.1 degrees Celsius, confirming thermal stability.

The growth data separated cleanly into two patterns. Neither field alone, in either waveform, slowed tumor growth: by day 4, tumors in the sinusoidal and trapezoidal field-only groups had grown by 84 and 77 percent respectively, essentially matching untreated controls, and the animals showed no adverse effects. Nanoparticles without a field also failed to inhibit growth, with a 99 percent volume increase by day 4, a slight excess the authors attribute plausibly to nanoparticle accumulation or edema at the injection site rather than any therapeutic signal. The sinusoidal field combined with nanoparticles produced a trend toward smaller tumors, but the difference reached statistical significance only on the final day and only against the nanoparticle-only group.

The trapezoidal combination was in a different category. Not only did tumor growth stall; tumor size actually regressed, falling by nearly 20 percent below the day-1 baseline by the end of the experiment, and on days 3 and 4 the trapezoidal group differed significantly from every other group. When tumors were dissected and weighed on day 5, normalized to their starting volumes, the four control groups clustered between 2.2 and 2.8 milligrams per cubic millimeter, the sinusoidal group came in slightly lower at 2.1, and the trapezoidal group measured 1.1, an approximately 60 percent reduction relative to the nanoparticle-only control. Photographs of the excised tumors corroborated the caliper measurements, and the dark coloration of nanoparticle-injected tumors, with no visible darkening of adjacent healthy tissue, indicated that the particles stayed largely confined to the tumor mass.

Histology pointed to the cellular mechanism. Sections stained for heat shock protein 70, a marker of cellular stress, and for activated caspase-3, a canonical marker of apoptosis, showed only faint baseline signal in all control groups, consistent with the ordinary cell turnover of a proliferating solid tumor. In contrast, both markers were strongly upregulated in the magnetically treated tumors, most intensely in the trapezoidal group, where quantitative image analysis confirmed significantly higher expression than in every other condition. The authors interpret this as evidence that the trapezoidal field triggers a stress response in cells adjacent to the nanoparticle deposits that proceeds to apoptotic cell death, providing a cellular explanation for the shrinkage in tumor size and weight.

Important caveats remain. The study could not measure temperature inside the tumor, a limitation the authors highlight themselves: magnetic resonance thermometry is compromised by the susceptibility artifacts that nanoparticles create, and in vitro electrode-based monitoring does not transfer to a coil-based animal setup. Without intratumoral temperature profiles, the relative contributions of thermal and non-thermal mechanisms to the trapezoidal advantage cannot be disentangled, and no validated mathematical model yet links non-sinusoidal heating dynamics to cell death. The model was subcutaneous rather than intracranial, the follow-up lasted only days, and the field intensity used was far below that of some prior studies that achieved comparable results at higher amplitudes, underscoring the efficiency gain but also the preliminary nature of the data. Notably, the trapezoidal approach achieved its effect while respecting the established safety constraint linking field amplitude and frequency. The authors conclude that waveform engineering, combined with appropriately sized nanoparticles, can push magnetic hyperthermia beyond its conventional limits in this preclinical model, while further studies will be needed to assess translational potential for glioblastoma patients.

Subject of Research: Trapezoidal pulsed magnetic fields enhancing nanoparticle-mediated magnetic hyperthermia in a mouse glioblastoma model

Article Title: Enhancement of therapeutic efficacy of magnetic hyperthermia by trapezoidal pulsed magnetic fields in a glioblastoma mouse model

Article References: Rodríguez-García, M. R., Martínez-Montero, S., Serrano-Olmedo, J. J., & Ramos-Gómez, M. (2026). Enhancement of therapeutic efficacy of magnetic hyperthermia by trapezoidal pulsed magnetic fields in a glioblastoma mouse model. Cancer Nanotechnology, 17(1), Article 59. https://doi.org/10.1186/s12645-026-00417-7

Image Credits: AI Generated

DOI: 10.1186/s12645-026-00417-7

Keywords: magnetic hyperthermia, glioblastoma, magnetic nanoparticles, trapezoidal waveform, alternating magnetic field, mouse model, tumor ablation, apoptosis, HSP70, caspase-3, specific absorption rate, preclinical study