Licentiate thesis: The development of a proton grid therapy
Wednesday 07 June 2017
to 12:00 at
Byggnad R8. Karolinska Sjukhuset
CCK Lecture Hall
Thomas Henry (Stockholm University, Department of Physics)
Spatially fractionated radiotherapy, also known as grid therapy (GRID), has been used for more than a
century to treat several kinds of lesions. Yet, the grid technique remains a relatively unknown and
uncommon treatment modality nowadays. Spatially fractionated beams, instead of conventional
homogeneous fields, have been used to exploit the clinical finding that normal tissue can tolerate
higher doses when smaller tissue volumes are irradiated. This increase in tolerance with reducing
beam sizes is known as the dose-volume effect. Despite the fact that targets were given an
inhomogeneous dose distribution, good results in the form of shrinking of bulky tumors have been
observed. The biological processes responsible for this effect are still under discussion, with several
possible causes. However, numerous experiments on mice, rats and pigs have confirmed the existence
of this effect, which in turn motivate the present development of grid therapy.
While mainly photons have been used in grid therapy, proton- and ion-grid therapies are also emerging
as viable alternatives especially in light of the development of new particle therapy centers.
In this work, we used millimeter-wide proton beamlets to propose a new, innovative form of grid
therapy. Grids of proton beamlets were crossfired and interlaced over a target volume with the
intention to achieve two main objectives: (1) to keep the grid pattern (adjacent high and low doses)
from the skin up to the vicinity of the target while (2) delivering a nearly homogeneous dose to the
In a preliminary study, the possibility of using this new interlaced and crossfired geometry with
currently available beam sizes at a modern proton therapy center was explored in a proof of concept
study. A treatment planning system was used to re-plan two patients previously treated with photon
therapy. This study demonstrated the potential of the said geometry, with encouraging results on the
studied cases. The beam separation could be well preserved down to the target, while delivering a
nearly homogeneous dose to the targeted area (± 5-10 %).
Dose distributions for mm-wide proton beamlets were then calculated through Monte-Carlo (MC)
simulations, to study the depth-dose and lateral characteristics of such smaller beams. Beams with
widths of 0.5 to 3 millimeters, full width at half maximum (FWHM), were considered. Using these
MC data, a virtual grid irradiation with the proposed interlaced-crossfiring geometry was performed in
a 200x200x200 mm3 water tank, targeting a 20x20x20 mm3 cube at the center of the tank. Results
comparable to the first study were observed, with a well-defined grid pattern down to the vicinity of
the targeted cube, and satisfying dose coverage of the target in all cases (< ± 5 %).
The interlaced crossfiring geometry proposed here allows a good spatial separation between the beams
from the skin down to the target depth, which is anticipated to be important to improve the normal
tissue tolerance to high doses. However, a good dose coverage of the target, similar to what is obtained
in standard clinical radiotherapy, can still be achieved by crossfiring and interlacing the beam grids
from several incident directions. The proposed new grid method has the potential to redefine the way
grid therapy is used today, and could be used in the future.