Photodynamic Therapy - Radiation Damage in Biomolecular Systems - page 448

27.2.2 X-Ray potentialized radiation treatments

Norman and co-workers first proposed the concept of radiation dose enhancement
in the nineties [ 25 , 26 ] using a simple irradiation scheme and a conventional scanner
x-ray source. Iodinated contrast media were injected to patients and accumulated
preferentially in the tumour interstitium owing to the increased permeability of
the blood-brain barrier caused by an invasive tumour growth [ 25 , 26 ]. At kilo
electron-volt photon energies, the high photoelectric cross sections of iodine result
in substantial interactions with the incident radiation. The high linear energy transfer
and short range of photoelectric interaction products lead to a localized dose
enhancement.
Several agents have been evaluated as potential radiosensitizers. One of the oldest
one is platinum; a metal present in chemotherapeutic agents and studied for its
radiosensitizing activity [ 27 , 28 ]. The efficiency of the treatment with platinum-
complexes has been assessed by synchrotron radiation in vitro and in vivo conditions
[ 29 - 33 ].
The use of monochromatic X-rays tuned at the optimal energy could significantly
improve the dose delivery [ 34 ]. Synchrotron sources, providing flux and tuneable
monochromatic X-rays are therefore ideal for this potentialized radiation treatments.
Gadolinium, a high-atomic number (Z) element has been recently investigated in
combination with synchrotron microbeam radiation therapy [ 35 - 37 ].
While stereotactic radiation therapy is a type of external radiation delivery
involving tumours treatments with focused beam, the synchrotron stereotactic ra-
diation therapy (SSRT) is defined by the additional presence of high-atomic number
(Z) element to reach a radiation dose enhancement specific to the tumour when
irradiated in stereotactic conditions [ 38 ]. SSRT is developed in details in chapter 26.

27.3 Photodynamic therapy

Aside from all the improvements in radiotherapy, such as the above mentioned
approaches of X- ray potentialized radiation treatments (stereotactic and microbeam
radiation therapy), there is a real need for safer therapies, to limit and reduce the
toxic effect of X-ray radiation on healthy tissues [ 39 , 40 ]. Another approach would
use radiation in a particular wavelength range that would not be toxic. Such a
method is photodynamic therapy (PDT). PDT shares several of particularities with
radiotherapy and its newest developments. First at all, it shares with the potentialized
therapy the use of a drug in combination with the radiation. Moreover, the radiation
used in PDT is administrated very locally. But PDT has particular and unique
features.
Indeed, PDT is attractive, because it combines elements, which are harmless
separately, namely a drug referred to as the photosensitizer, oxygen and an
electromagnetic radiation [ 41 ]. In PDT, visible light is used to electronically excite