BACKGROUND AND PURPOSE: The electric field and the concomitant heat 
(electrohyperthermia) can synergistically induce cell death in tumor tissue, due 
to elevated glycolysis, ion concentration, and permittivity in malignant 
compared with nonmalignant tissues. Here we studied the mechanism and time 
course of tumor destruction caused by electrohyperthermia.
MATERIAL AND METHODS: Bilateral implants of HT29 colorectal cancer in the 
femoral regions of Balb/c (nu/nu) mice were treated with a single 30-min shot of 
modulated, 13.56-MHz, radiofrequency-generated electrohyperthermia (mEHT). 
Tumors at 0, 1, 4, 8, 14, 24, 48, and 72 h posttreatment were studied for 
morphology, DNA fragmentation, and cell death response-related protein 
expression using tissue microarrays, immunohistochemistry, Western immunoblots, 
and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assays.
RESULTS: Modulated EHT treatment induced significant tumor destruction in HT29 
xenografts with a peak of a sevenfold increase compared with the untreated 
controls. The significant treatment-related elevation of DNA 
fragmentation--detected with TUNEL assay--and apoptotic bodies between 24 and 72 
h posttreatment was proof of a programmed cell death response. This was 
associated with significant mitochondrial accumulation of bax and 
mitochondrial-to-cytoplasmic release of cytochrome c proteins between 8 and 14 
h. Cleaved caspase-3 levels were low and mainly localized to inflammatory cells. 
The substantial cytoplasmic-to-nuclear translocation of apoptosis-inducing 
factor (AIF) and its 57-kDa activated fragment detected between 14 and 24 h 
after treatment indicated AIF as an effector for DNA fragmentation.
CONCLUSION: Modulated EHT treatment can induce programmed cell death-related 
tumor destruction in HT29 colorectal adenocarcinoma xenografts, which dominantly 
follows a caspase-independent subroutine.