Biomedical Engineering Reference
In-Depth Information
26.4
Stereotactic Synchrotron Radiation Therapy (SSRT)
SSRT consists in loading the tumor with a high atomic number (Z) element and
irradiating it with monochromatic X-rays from a synchrotron source (tuned at an
optimal energy) in stereotactic conditions. The high Z element injected in the patient
selectively accumulates in the brain tumor as a consequence of the permeability of
the blood-brain barrier due to invasive growing of the tumor. At energies of few tens
of keV, the high photoelectric cross section of high Z materials like Iodine, results in
a great number of photoelectric interactions. Due to the short range of the products
of those interactions there is an enhancement of the dose deposited locally in the
tumor. This leads to improved dose distributions when compared to conventional
high energy treatment [
50
,
51
]. The use of monochromatic X-rays optimises the
dose distributions with respect to a spectrum [
52
]. Hence, the synchrotrons are ideal
for this therapeutic modality since they provide very intense monochromatic X-rays.
Several preclinical studies were carried out and are still ongoing using monochro-
matic X-rays at the ESRF biomedical beamline. Two different approaches were
simultaneously developed. The first one is contrast enhanced Synchrotron Radiation
Therapy (SRT) with extracellular agents like Iodine [
51
,
53
]. A enhancement of
life span of around 200 % was achieved. The second one uses some chemotherapy
drugs containing platinum [
54
,
55
] or iodine compounds [
56
,
57
]. The studies using
platinum compounds reached a mean survival increase close to 700 %. Those drugs
penetrate the cell and bound to DNA. Due to the intrinsic toxicity of those drugs only
small concentrations (ppm) can be brought to the DNA and therefore it is not clear
whether there is a physical dose enhancement in this chemo-radiotherapy modality.
For the aforementioned reasons, the ESRF is planning the clinical trials following
the first avenue. With this aim different dosimetric aspects have been assessed and
will be described hereafter.
First, the beam energy providing the best balance between tumor treatment and
healthy tissue sparing in a human head had to be assessed. In SSRT Monte Carlo
simulations in anthropomorphic head phantoms showed that even if energies around
50 keV are the ones providing the highest dose enhancement factor in the tumor, 80
keV renders a good compromise between a high dose deposition in the target (up to
82 Gy) with healthy tissue doses within tolerable levels [
52
,
58
,
59
]. It is expected
that the net gain in deposited dose in the tumor (32 Gy) with respect to conventional
radiotherapy (50 Gy) will result in an increase of tumor control probability.
To be able to treat patients, a Monte Carlo based TPS has been developed
and benchmarked against experimental measurements [
60
]. The calculation engine
will be integrated in the ISOgray
TM
system sold by the French firm DOSIsoft
(Cachan, France). A three-step Monte Carlo simulation has been implemented in
order to compute the dose in the patient from the TPS, considering the features
of SSRT: particles of medium energies; beamline geometry; contrast media in
the target. Simulations were compared to measurements conducted under clinical
irradiation conditions. Measurements were performed using ionization chambers
