Purpose: To quantitatively evaluate four different Proton SFUD PBS
initial planning strategies for lung mobile tumor. Methods and Materials: A
virtual lung patient’s four-dimensional computed tomography (4DCT) was
generated in this study. To avoid the uncertainties from target delineation and
imaging artifacts, a sphere with diameter of 3 cm representing a rigid mobile
target (GTV) was inserted into the right side of the lung. The target motion is
set in superior-inferior (SI) direction from ?5 mm to 5 mm. Four SFUD planning
strategies were used based on: 1) Maximum-In-tensity-Projection Image (MIP-CT);
2) CT_average with ITV overridden to muscle density (CTavg_muscle); 3) CT_average
with ITV overridden to tumor density (CTavg_tumor); 4) CT_average without any
override density (CTavg_only). Dose distributions were recalculated on each
individual phase and accumulated together to assess the “actual” treatment. To
estimate the impact of proton range uncertainties, +/?3.5% CT calibration curve
was applied to the 4DCT phase images. Results: Comparing initial plan to the
dose accumulation: MIP-CT based GTV D98 degraded 2.42 Gy (60.10 Gy vs 57.68 Gy).
Heart D1 increased 6.19 Gy (1.88 Gy vs 8.07 Gy); CTavg_tumor based GTV D98
degraded 0.34 Gy (60.07 Gy vs 59.73 Gy). Heart D1 increased 2.24 Gy (3.74 Gy vs
5.98 Gy); CTavg_muscle based initial GTV D98 degraded 0.31 Gy (60.4 Gy vs 60.19
Gy). Heart D1 increased 3.44 Gy (4.38 Gy vs 7.82 Gy); CTavg_only based Initial
GTV D98 degraded 6.63 Gy (60.11 Gy vs 53.48 Gy). Heart D1 increased 0.30 Gy
(2.69 Gy vs 2.96 Gy); in the presence of ±3.5% range uncertainties, CTavg_tumor
based plan’s accumulated GTV D98 degraded to 57.99 Gy (+3.5%) 59.38 Gy (?3.5%),
and CTavg_muscle based plan’s accumulated GTV D98degraded to 59.37 Gy (+3.5%)
59.37 Gy (?3.5%). Conclusion: This study shows that CTavg_Tumor and
CTavg_Muscle based planning strategies provide the most robust GTV coverage.
However, clinicians need to be aware that the actual dose to OARs at distal end
of target may increase. The study also indicates that the current SFUD PBS
planning strategy might not be sufficient to compensate the CT calibration
uncertainty.
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