Abstract
To improve the dose homogeneity in patch-field proton therapy using beam current modulation. MCNPX simulations were performed for various configurations of the Mevion S250 beamline. Spread-out Bragg peak (SOBP) dose distributions were calculated, and lateral and distal dose profiles were extracted for through- and patch-field targets at depths ranging from 5.0 to 27.5 cm. We define 'dose non-uniformity' (DNU) as the difference between the maximum and minimum dose relative to the prescription observed in a patched dose distribution. DNU was first calculated for dose distributions combining through-field lateral dose profiles and with patch-field distal dose profiles, assuming a 'standard' delivery with constant beam current. Distal dose profiles were then optimized using beam current modulation to better complement the lateral dose profiles when combined into a patched dose distribution. The DNU for optimized dose distributions was then calculated and compared to the DNU found for standard delivery. Using standard deliveries, DNU was 10% or less when patching lateral profiles 12.5-17.5 cm deep. Significantly greater DNU was observed for patches outside of this range, sometimes exceeding 35%. Using optimized distal profiles, DNU was reduced to 10% or less for all lateral profiles deeper than 12.5 cm. Patches with lateral profiles shallower than 12.5 cm were not improved, as distal dose dropoff could not be improved to better match the sharp lateral dose dropoff. Optimizing beam current modulation can create distal profiles with more gradual dose falloff than found in a standard delivery, allowing optimized distal dose distributions to sum more homogeneously with lateral dose distributions. Thus, the hot or cold spots that often appear in patched dose distributions from standard deliveries can be mitigated by optimizing beam current. This method may also be applied to feathering methods or scanned beam deliveries to further improve patch-field dose homogeneity. Partially supported by MEVION Medical Systems, Inc.
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