Carbon ion grid therapy spares healthy tissue
Delivering radiation in comb-like arrays of beamlets rather than a solid beam, grid-based radiotherapy exploits the dose-volume effect to spare healthy tissue in the beam’s path. Successfully realised, the approach could enable repeat treatments and is a potential strategy for increasingly popular hypofractionated treatments that deliver larger, and potentially more harmful, doses per fraction.
While the bulk of grid therapy research has focused on X-rays, in new work, the tissue sparing potential of carbon ions has been demonstrated by a Japanese-Swedish collaboration, using simulations. In a methodological advance, first author Toshiro Tsubouchi of Osaka University and colleagues also devised “goodness” criteria, enabling quantitative comparisons of different grid setups (Med. Phys. 45 1210).
The researchers had the particular goal of sparing tissue near deep-seated tumours. “It’s in these [organs] in which the most severe side effects appear after radiotherapy and radiosurgery … where the high dose volumes spread out from the target,” said Albert Siegbahn, senior author and physicist at Stockholm University.
Dosimetrically, carbon ions are a promising candidate, as a significantly lower beam divergence than photons or protons helps preserve the dose valleys between beamlets at depth. In the current study, for example, a nominal 3 mm wide beamlet was 3.3 mm wide at a depth of 9 cm.
Additionally, while beamlets less than a millimetre wide have dominated research to date, carbon ion beamlets of millimetres wide have two key advantages. They can be generated with existing clinical spot-scanning technology and are more robust to geometric uncertainties such as organ motion. A drawback, however, is a significant drop in normal tissue tolerance as beamlet width increases. Consequently, Tsubouchi and his collaborators examined grids using both a 0.5 mm wide beamlet and a 3 mm wide beamlet.
The researchers framed their investigation as an optimization problem, seeking the beamlet separation that minimized the valley-peak dose ratio (VPDR) 5 mm from the target. Simultaneously, two further criteria stipulated that the target dose should be uniform and higher than the entrance dose.
Grid arrangements
The team carried out Monte Carlo simulations for a 2-cm cubic target located in the centre of a 20-cm cubic water phantom. The target was irradiated with spread-out Bragg peaks with four different grid arrangements, ranging from a single grid to an orthogonal “crossfiring” of two pairs of opposed, interlaced grids.
Using a single grid, the researchers found that spacings of 1.0 mm (0.5 mm beamlet) and 3.2 mm (3 mm beamlet) that achieved uniform target coverage provided minimal dose sparing close to the target. For example, 5 mm from the target, VPDR values exceeded 0.9.
In contrast, the four-grid arrangement resulted in significantly lower doses outside the target. Here, uniform target coverage was achievable using greater spacings of 2.4 and 6.4 mm, for the 0.5 and 3 mm beamlets respectively. They resulted in VPDR values of 0.22-0.24 near the target.
Based on their findings, Tsubouchi and his collaborators are developing the technique further. “From a theoretical point of view, we are pretty confident,” said Alexander Valdman, co-author and radiation oncologist at Karolinska University Hospital in Stockholm. “We know that we can deliver a safe dose to the target while maintaining the grid pattern down to the target and preserving the tissue.”
In the first instance, the authors see brain tumour cases not cured by conventional radiotherapy as the cohort most likely to benefit from a clinical trial. Here, critical structures in the brain are likely to have already received a significant dose, contra-indicating additional, conventional treatment. Fixed intracranial anatomy and immobilization that minimize geometric uncertainties would also make accurate grid placement less challenging. “This is where grid therapy could really shine,” said Siegbahn.
In ongoing work, the authors are investigating the physical implementation of carbon ion grid therapy in experiments. The researchers are also developing ways to evaluate and compare carbon ion grid therapy with conventional treatments.
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