The thermal therapy (hyperthermia) method is attracting attention as a method for cancer therapy. As for this thermal therapy method, attention is focused upon the fact that a cancer cell or a cancerous tissue is more vulnerable to heat than a healthy cell. By heating the cancer-affected portion for example to 43 degree C. for a certain length of time, the cancer lesion can selectedly be necrotized. In this thermal therapy method, an aqueous sol of magnetic fluid comprising a complex of dextran or its derivative and magnetic iron oxide, dextran magnetite for example, is injected into the affected portion, to which a strong magnetic field is applied from outside to selectively heat the cancer lesion.
In such a method for therapy, in order to inductively heat the magnetic material having been injected into the affected portion, it is necessary to irradiate a strong magnetic flux from outside to the affected portion by using a magnetic field generation device. A magnetic field generation device preferable for such an inductive heating comprises a transformer, an inverter circuit connected to the primary side of the transformer and a series resonance circuit including a magnetic field generation coil and a resonance condenser connected to the secondary side of the transformer. The magnetic field generating device transmits through electromagnetic induction an alternate output of the inverter circuit connected to the primary side of the transformer to the series resonance circuit connected to the secondary side of the transformer. An alternate magnetic field is generated by applying a current of high frequency to the magnetic field generation coil. On this occasion, if the frequency of the alternate output transmitted to the series resonance circuit on the secondary side conforms with the resonance frequency of the series resonance circuit, the amount of current passing in the series resonance circuit is large enough to generate an alternate magnetic field of needed intensity at the magnetic field generation coil, making it possible to irradiate from the magnetic field generation coil a strong magnetic flux to the affected portion.
On the other hand, if the frequency of the alternate output transmitted to the series resonance circuit on the secondary side does not conform with the resonance frequency of the series resonance circuit, only a small amount of current passes in the series resonance circuit, failing to generate an alternate magnetic field of needed intensity at the magnetic field generation coil. Here, that the frequency of the alternate output transmitted to the series resonance circuit on the secondary side does not conform with the resonance frequency of the series resonance circuit means that the switching frequency of the inverter circuit does not conform with the resonance frequency of the series resonance circuit. In this case, the impedance on the primary side of the transformer becomes large, drastically reducing the electrical power transmitted from the primary side to the secondary side of the transformer as well as the amount of the current passing in the series resonance circuit, so that the intensity of the alternate magnetic field generated from the magnetic field generation coil is drastically decreased.
Inductance of the magnetic field generation coil may vary significantly according to factors like the size and the position of the affected portion. Accordingly, the true resonance frequency cannot be obtained by calculating the resonance frequency only based upon the design values of the magnetic field generation coil and the resonance condenser, which leads to a problem that a magnetic field of the needed intensity cannot be provided to the affected portion in cancer therapy. More importantly, according to various experiments and analyses concerning the series resonance circuit having been carried out by the inventor of the present invention, the inductance of the magnetic field generation coil and the capacity of the resonance condenser may vary from the design values according to environmental conditions such as temperature. A magnetic field of the needed intensity may not be generated during the whole therapy at the magnetic field generation coil if the switching frequency of the drive signal supplied to the inverter circuit is fixed based upon these design values. By thermal therapy, therefore, it is crucially important to make the switching frequency of the drive signal supplied to the inverter circuit follow accurately the resonance frequency of the series resonance circuit when the resonance frequency varies according to the size and the position of the affected portion and changes in the environmental conditions.
For the purpose of solving this problem, in the Japanese Patent Laid-open Publication No. 2002-360712 is disclosed a thermal therapy device in which the switching frequency of the drive signal supplied to the inverter circuit is conformed to the resonance frequency of the series resonance circuit. This thermal therapy device comprises a transformer, an inverter circuit connected to the primary side of the transformer, a series resonance circuit connected to the secondary side of the transformer which includes a magnetic field generation coil and a resonance condenser, and a PLL (phase locked loop) controlling unit for supplying a drive signal to the inverter circuit. The PLL controlling unit detects a current passing between the inverter circuit and the transformer (a current on the primary side) and generates a drive signal by PLL controlling in which the phase of the current on the primary side is compared with the phase of the drive signal so that the switching frequency of the drive signal to be provided to the inverter circuit is conformed with the resonance frequency of the series resonance circuit. With such a constitution, when the resonance frequency of the series resonance circuit varies according to changes in environmental conditions or the like, and the current on the primary side decreases, the PLL controlling unit generates the above mentioned drive signal in such a way that the current on the primary side has the same intensity as in the case that the frequency of the current on the secondary side conforms with the resonance frequency, in other words, in such a way that the switching frequency of the drive signal supplied to the inverter circuit conforms with the resonance frequency of the series resonance circuit. A current having the same frequency as the resonance frequency passes in the series resonance circuit. Therefore, if the resonance frequency of the series resonance circuit varies according to changes in environmental conditions or the like, the switching frequency of the drive signal provided to the inverter circuit can follow the resonance frequency.