Upright and supine particle therapy of lung cancer: A 4D dosimetric comparison.

[BACKGROUND] A renewed interest in upright particle therapy is currently driven by the availability of upright positioning and imaging systems. The upright positioning system could enhance fixed beamlines for effective carbon ion treatments in central body regions, with a substantial cost and space advantage. In addition, few studies have suggested advantages in patient breathing and lung volume in an upright posture. Comparative dosimetric analyses are needed to determine the clinical viability of upright patient positioning for carbon ion therapy of thoracic cancers but are challenged by various sources of bias.

[PURPOSE] To provide a comprehensive analysis of all parameters influencing the comparison between upright and supine carbon therapy of thoracic patients through 4D dosimetric studies.

[METHODS] Paired upright and supine 4DCTs were available for six patients treated at the Northwestern Medicine Proton Centre (NMPC), under the Proton Collaborative Group (PCG) registry. Deformable image registration (DIR) between upright and supine CTs was performed on a region of interest (ROI) including the rib cage for target propagation, to avoid failure in DIR caused by thorax anatomical differences. DIR quality was evaluated on lung structures through Dice similarity coefficient (DSC) and average Hausdorff distance (AHD) metrics. Paired 3D plans were optimized on the originally contoured and propagated target volumes, to investigate the effect of segmentation differences. The impact of beam geometry choice was investigated by optimizing plans with a variety of treatment angles. Single-fraction and accumulated 4D doses were calculated with the research treatment planning system TRiP4D to analyze the impact of differences in breathing-induced tumor motion in the two postures. Plan quality between upright and supine plans were assessed through D, HI, and V for the internal target volume (ITV) and V16Gy(lung) and V20Gy(heart) for lung and heart, respectively.

[RESULTS] Restraining DIR on the ribcage ROI enabled successful DIR. Within the ribcage ROI an average AHD of 1.5 mm and DSC of 0.95 was achieved on the propagated lung structure. Position specific angle selection showed vertical posterior/anterior beams might not be optimal for upright treatments. Comparable 3D treatment quality was achieved for five patients, while an increase of 5 pp occurred in V20Gy(heart) and V16Gy(lung) of patient P6 in upright. The 4D study showed the different positions have clinically relevant impact, increasing D of 3 pp for one patient with halved motion amplitude in upright posture. In addition, robustness was similar between postures, even with a more conservative 5%/5 mm uncertainty setting for upright. When assuming only a fixed beam line is available, as is the case for most carbon ion centers, a comparable plan quality with 360° beam angle flexibility in upright position was observed.

[CONCLUSIONS] The presented work comprehensively evaluates the influence of various parameters on the comparison of upright and supine therapy of thoracic patients. A solid understanding of these parameters is paramount to reduce bias in future larger patient cohort studies on the viability of upright positioning. The final dosimetric comparison between postures highly depends on patient characteristic and the investigated parameter. More data are needed to provide a resilient comparison between postures.