Abstract
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 D98 degraded 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.
Highlights
Lung cancer is the leading cancer cause of death in the United States with over 158,000 estimated deaths in 2015 [1]
After 2010, with the development of proton therapy technique, most of new proton centers under construction will be equipped with Pencil Beam Scanning (PBS) only
Less effort has been put into the investigation of the dosimetric outcome and planning strategies of using Single Field Uniform Dose (SFUD) for mobile lung tumor which has been one of the most popular proton PBS planning strategies implemented in clinic
Summary
Lung cancer is the leading cancer cause of death in the United States with over 158,000 estimated deaths in 2015 [1]. In the early 2000, several institutions have been investigating the benefits of using proton beam therapy for lung cancer based on PassiveScattering (PS) technique [2] [3]. After 2010, with the development of proton therapy technique, most of new proton centers under construction will be equipped with Pencil Beam Scanning (PBS) only. In contrast to more popularity of implementing the novel PBS technique in clinic, the treatment planning strategy for lung mobile tumors is still not clear due to the uncertainties related to the proton range and tumor motion. Less effort has been put into the investigation of the dosimetric outcome and planning strategies of using Single Field Uniform Dose (SFUD) for mobile lung tumor which has been one of the most popular proton PBS planning strategies implemented in clinic
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