RF/Microwave Delivery Systems for Therapeutic Applications - RF/Microwave Interaction with Biological Tissues - page 295

side fashion, depicted in Figures 6.25 and 6.26. Note that we did not have a
hole punch to cut the carotid artery and jugular vein after creating the ansto-
mosis. So, we had to cut each vessel before we created the anastomosis and
then try to align the holes.
We noticed that the vessel surface needed to be kept dry for a better anas-
tomosis. We measured flow through the carotid artery before and after the
anastomosis. After the anastomosis, the flow rate was close to baseline.
However, the flow tended to decrease over a 1-h period. This flow decrease
was associated with the development of a thrombus in the vessel. At the end
of the study, we noticed that the hole in the jugular vein was not in line with
the hole in the carotid artery.
Technical problems do remain:

1. Microwave tissue welding does not work very well in a wet environment.
Possible solutions include using a small blower and turning up the power
on the microwave unit.
2. At present, we have no device for making the hole between the two
arteries.

6.9.2 Endoscopic Light Source and Microwaves for Photodynamic Therapy

Photodynamic therapy (PDT) is a treatment modality using a photosensitiz-
ing drug which is activated by specific wavelengths of light. The light-excited
drug then interacts with molecular oxygen to produce a toxic oxygen species,
known as singlet oxygen, which mediates cellular death. The appeal of PDT
in oncology is that the photosensitizers are absorbed to a greater extent in
tumor tissues as compared to normal tissues. In addition, PDT can be used
before, after, or concurrently with traditional cancer therapy modalities such
as chemotherapy, radiation therapy, or surgery. Photodynamic therapy has
been approved by the Food and Drug Administration for the treatment of
both obstructing and preinvasive lesions of the lung and esophagus. The
efficacy of these treatments, however, is limited by the supply of oxygen to
the treated tissue. The consumption of oxygen during PDT is a rapid
process that can quickly deplete available oxygen in the illuminated tissue
[136, 137].
The presence of oxygen is a critical factor in determining the effectiveness
of the photodynamic effect. The PDT of anoxic tissues produces no effect [138]
and the presence of hypoxia can diminish the PDT effect [139, 140].
Several approaches have been used to increase the availability of oxygen
to the treated tissue during PDT. Minimizing oxygen depletion is one such
approach that can be achieved either by reducing light fluence rate or by frac-
tionating light delivery. At high fluence rates, mathematical modeling indicates
that oxygen consumption can outpace the rate of oxygen diffusion from cap-
illaries, thus resulting in a decrease in the oxygen level in surrounding tissue.
Lowering the fluence rate has been shown in preclinical studies to improve