Biomedical Engineering Reference
In-Depth Information
26.3.2
Minibeam Radiation Therapy (MBRT)
As explained in Sect.
26.3.1
, the thin microbeams (and their associated small beam
spacing) need high dose rates, only available at synchrotrons nowadays. This limits
their widespread clinical implementation. In addition, the high lateral scattering
produced by beam energies higher than 200 keV would lead to healthy tissue sparing
loss [
40
]. The requirement of low-energy beams limits the dose penetration to the
tissue. To overcome those difficulties, A. Dilmanian et al. [
29
] proposed the so called
Minibeam Radiation Therapy (MBRT). They have hypothesized that beams as thick
as 0.68 mm keep (part) of the sparing effect observed in MRT [
29
]. Moreover, from
MRT preclinical studies there are indications that a wider beam results in a higher
tumoricidal effect [
39
]. In addition, the use of higher beam energies is feasible in
MBRT [
47
], resulting in a lower entrance dose to deposit the same integral dose
in the tumor. The dose profiles of minibeams are not as vulnerable as the ones of
microbeams to beam smearing from cardiac pulsations, therefore high dose rates are
not needed and it is conceptually possible to extend this technique by using modified
X-ray equipment.
An original method was developed and tested at the ESRF ID17 biomedical
beamline to produce the minibeam patterns [
48
]. It utilizes a specially developed
high-energy white-beam chopper whose rotation is synchronized with the vertical
motion of the target moving at constant speed. Each opening of the chopper
generates a horizontal beam print.
In parallel, a dosimetry characterization of MBRT was performed [
48
]. The good
agreement between Monte Carlo simulations and the experimental measurements
opened the door to the biological studies in MBRT. Recently, interlaced minibeams
were produced by slightly modifying the duty cycle of the chopper (53%). In this
configuration, two orthogonal arrays interlace at the target. A quasi-homogeneous
dose distribution in the tumor is achieved while the healthy tissue still benefit from
the spatial fractionation of those submillimetric beams.
Preclinical trials in MBRT have already started at the ESRF and they are ongoing.
Several radiobiology studies (in vitro and in vivo) showed that MBRT widens the
therapeutic window for gliomas: extremely high dose tolerances of healthy rat brains
accompanied by a factor three gain in mean survival time of treated tumor bearing
rats was observed [
49
]. This indicates that MBRT might allow the use of higher
and potentially curative doses in clinical cases where the tolerance doses of healthy
tissues impose a limit on the dose delivered to the tumor if conventional therapy is
used. Improvement of the outcome is expected by using image-guidance, chemo-
radiotherapy, etc. in future studies.
Other preclinical research opportunities for MBRT are the creation of small
lesions in the submillimeter range to mitigate Parkinson disease or epilepsy using
minibeams [
29
].
