Comparison of supine and upright postures in carbon ion radiotherapy for pancreatic cancer: An in silico planning study

Background Conducting carbon ion radiotherapy (CIRT) in upright posture has several technical advantages over that in conventional supine posture for pancreatic cancer treatment. However, few studies have investigated effects of upright positioning on anatomy and CIRT dose distributions. Purpose This study compared anatomy and dose distributions in CIRT for pancreatic cancer between supine and upright postures. Methods Fourteen pairs of computed tomography (CT) images obtained from asymptomatic volunteers in these postures were analyzed. Clinical target volume (CTV), organs‐at‐risk (OARs), and gastrointestinal gas contours were delineated within a planning volume for each CT image, and a CIRT dose distribution was calculated. The anatomical differences between these postures were evaluated by comparing volumes and 3D rendered images of the contours, while the dosimetric differences were assessed with 12 dosimetric metrics. Moreover, a proximal overlap volume of the OARs and possible change in water‐equivalent path length due to gastrointestinal gas were calculated for all coplanar beam angles to compare ranges of applicable beam angles and potential range uncertainty between these postures. Results The bowel contour volumes in the upright posture were more than 46% smaller than those in the supine posture because the bowel was shifted. The upright posture provided a smaller dose to the bowel and higher dose coverage to the CTV than the supine posture. The possible water‐equivalent path length changes due to the stomach and duodenum gas decreased at most beam angles in the upright posture. Conclusions The upright CIRT had several anatomical and dosimetric advantages for pancreatic cancer treatment over conventional supine CIRT. This technique will be useful in applying the patient‐specific beam angle arrangement and precise dose delivery with reduced range uncertainty.