Presentation Study of dose distribution difference between supine and prone positions in the carbon-ion treatment for the pancreatic tumor

三木, 健太朗

PurposeThe radiation sensitivity of surrounding gastrointestinal organs at risk (OARs) and their close anatomical relationships to the pancreas severely limit target volume margins. When using a fixed irradiation ports, treatment planning needs multiple CT datasets with different patient positions to irradiate from multiple beam angle. Usually, two CT datasets are acquired at supine and prone patient positions, and three or four beam angles are used for the treatment of pancreatic tumor. It needs to consider the variances of dose distribution due to positional morphological changes. Since patient couch can rotate around yaw axis, we can choose supine or prone in the case of horizontal beam irradiation. Therefore, we can select the better position to improve the planning dose distribution. We evaluated dose variations associated with positional morphological changes of organs.Materials and methodsWe retrospectively chose the datasets of 33 patients with pancreatic tumor who underwent CT at supine and prone patient position for carbon-ion therapy. Contours were appropriated from clinical treatment plan, a radiation oncologist manually contoured the gross tumor volume (GTV), planning target volume (CTV), and OARs (duodenum [D1-D2: 1st-2nd portionl; D3-D4: 3rd-4th portion] and stomach). The prescribed dose of 55.2 Gy (RBE) was delivered from four different beam angles (0°, 90°, 180°, and 270°). Dose distributions were calculated using datasets of supine and prone, separately. Patient collimator and compensating boli were designed for each beam. Dose assessments included D95 for GTV and PTV; dose to the most exposed 2 cc volume (D2cc) for the duodenum and stomach; V5 and V10 (volume of the target irradiated with greater than 5 and 10 Gy (RBE)) for the duodenum and stomach in the dose volume histograms (DVHs) for each beam.ResultDose distribution variances were observed on the most cases between supine and prone. For instance, the stomach located distal side of the tumor in the 90° beam on supine position, it moved proximal side on prone, resulting higher dose irradiation. The positional morphological changes were observed in duodenum also around 0.5 - 1.0 cm between supine and prone. The changes of organ position and dose distribution were various by each patient.ConclusionOptimization of beam angle selection include patient positions can improve the planning dose distribution for pancreatic tumor, sparing OARs.

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