Licentiate thesis: The development of a proton grid therapy

by Thomas Henry (Stockholm University, Department of Physics)

Wednesday 7 Jun 2017, 09:00 → 12:00 Europe/Stockholm

CCK Lecture Hall (Byggnad R8. Karolinska Sjukhuset)

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 target volume. 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.