Experimental determination of the force required for insertion of a thermoseed into deep brain tissues

Our laboratories are developing a new technique for delivering localized hyperthermia to deep-seated brain tumors. In this technique, a spherical thermoseed is stereotactically navigated through the brain and tumour tissues via the noncontact application of an external magnetic force. The force required to produce motion of a 3 mm diameter sphere through in vitro brain tissues was measured to be 0.07 +/- 0.03 N. This result was obtained from a series of experiments performed on whole brain specimens extracted from adult canines. Data were also taken with a 3 mm x 3 mm cylinder and a 5 mm sphere. An experimental procedure simulating physiological conditions was developed prior to testing. Evaluations of systematic effects included determinations of the calibration uncertainties, tests of the dependence of the measured force on temperature, and studies of the effects of method of storage of the tissue specimens. The results obtained are compared with (and confirmed by) two different series of experiments performed in vivo on adult canines and with another series of experiments using brain phantom gelatin.