Characterization of Ferromagnetic Composite Implants for Tumor Bed Hyperthermia

Abstract

Hyperthermia therapy (HT) is becoming a well-recognized method for the treatment of cancer when combined with radiation or chemotherapy. There are many ways to heat a tumor and the optimum approach depends on the treatment site. This study investigates a composite ferromagnetic surgical implant inserted in a tumor bed for the delivery of local HT. Heating of the implant is achieved by inductively coupling energy from an external magnetic field of sub-megahertz frequency. Implants are formed by mechanically filling a resected tumor bed with self-polymerizing plastic mass mixed with small ferromagnetic thermoseeds. Model implants were manufactured and then heated in a 35 cm diameter induction coil of our own design. Experimental results showed that implants were easily heated to temperatures that allow either traditional HT (39-45°C) or thermal ablation therapy (>50°C) in an external magnetic field with a frequency of 90 kHz and amplitude not exceeding 4 kA/m. These results agreed well with a numerical solution of combined electromagnetic and heat transfer equations solved using the finite element method.

Keywords: Cancer treatment; hyperthermia; induction heating; thermal ablation; tumor bed implant.

Fig. 1.

Experimental setup: (a) Dual-wall measurement cell showing the location of a spherical implant at the center filled with a gel and surrounded by glass tubes for insertion of temperature sensors, and (b) positioning of four fiber optic temperature sensors in equatorial plane of implant.

Fig. 2.

Computational model of the implant and phantom shown in Figure 1 as well as the surrounding 5 coils. The coils generate the magnetic field required to activate the steel thermoseeds immersed in the 2–3 cm implant.

Fig. 3.

Modelling of magnetic field in the coil and phantom: (a) integration region of the computational model; (b) physics-controlled mesh; (c) magnetic flux density distribution in the coil; (d) magnetic flux density distribution in the phantom region.

Fig. 4.

Modeling of absorbed power per unit volume induced by a 90-kHz magnetic for different: (a) diameter (ϕ) thermoseeds (PLD for a single thermoseed); (b) mass fractions (φ_m) of the composite implants filled with 1-mm thermoseeds.

Fig. 5.

Temperature distribution around a 2-cm heated composite implant immersed in ultrasonic gel for 1-mm thermoseeds at different mass fractions φ_m. The different curves show the temperature profile at a given distance (d) between the temperature sensor and implant surface. Dots are used for experimental data and solid lines for numerical simulations.

Fig. 6.

Temperature distribution around a 3-cm heated composite implant immersed in ultrasonic gel for 1-mm thermoseeds at different mass fractions φ_m. The different curves show the temperature profile at a given distance (d) from the implant surface to the temperature sensor. Dots are used for experimental data and solid lines for numerical simulations.