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Physics of Treatment Planning Using Scanned Beams *
Published in Harald Paganetti, Proton Therapy Physics, 2018
In a study we performed a few years ago, it was found that by building a “spot-reduction” option into the optimization algorithm [51,52], the number of delivered Bragg peaks could be reduced by up to 85%. When employing the same approach for IMPT (multiple-field) optimization, the number of Bragg peaks could be reduced even more, as would be expected perhaps when comparing the 3D-IMPT and DET approaches. Interestingly however, when applying the spot-reduction approach to a simple cylindrical target volume, it was even found that DET is not necessarily the optimum approach for reducing Bragg peak numbers. Figure 16.15 shows the Bragg peak positions resulting from the spot-reduction approach for this case. As can be seen, only distal Bragg peaks in the central portion of the sphere are actually needed to ensure a homogenous dose across the target when planning using five IMPT fields. The more lateral distal peaks have been successfully removed. For this solution, 20% fewer Bragg peaks are required than for the DET solution! More sophisticated methods for reducing the number of pencil beams delivered have recently been published by van de Water et al. [53], also demonstrating the striking potential of such methods for significantly reducing the number of pencil beams required per field and plan.
Adapt2Heat: treatment planning-assisted locoregional hyperthermia by on-line visualization, optimization and re-optimization of SAR and temperature distributions
Published in International Journal of Hyperthermia, 2022
Due to the off-center location of the target, focusing to this location yields a risk of a hot spot in the belly. Since this is close to the heating focus, such a hot spot would be challenging to resolve without affecting the tumor temperature. Suppose the patient indeed indicates hot spot nr 22, i.e., central lower belly. Our clinical protocol would prescribe to reduce the power ratio of the top waveguide stepwise, until the pain disappears. Evaluation of this strategy in Adapt2Heat predicts that the power ratio should be reduced significantly to 0.5, to realize a substantial temperature reduction in the belly, in front of the target. However, the top antenna contributes considerably to heating the pancreatic tumor location, so this strategy is also likely to result in a reduction of the target temperature, which is indeed predicted; simulated T50 is reduced by 0.5 °C (Figure 6(C)). To avoid significant reduction of the target temperature, one could evaluate alternative strategies. For example: a phase shift of the bottom antenna, slightly moving the focus in posterior direction, would reduce the power delivered to the belly. A small reduction of the left and right phase slightly spreads the focus, thereby also reducing the hot spot temperature. To compensate for the loss of tumor heating as a result of these phase shifts, more power can be applied by the top and bottom waveguides, compared to left and right. Figure 6(C) shows that with power ratios top:bottom:left:right = 1:1:0.75:0.75 and phases top:bottom:left:right = 0:45:45:40 (°), a similar hot spot reduction is predicted, compared to the clinical steering strategy, but with better tumor heating.