To study the feasibility of contouring the planning organ at risk volume for heart based on kilovolt cone-beam CT images

Objective: To study the feasibility of contouring heart planning organ at risk volume (PRV) on kilovolt cone-beam CT (CBCT) images, and measure the margins for the PRV of heart based on planning CT images. Methods: 15 early stage non-small cell lung cancer (NSCLC) patients accepted radiotherapy in our hospital were selected. For every case, CBCT scan was achieved before every fraction to correct the setup error, and the first 10 serious CBCT images were selected in present research. After contouring the heart on CBCT images according to the same standard, compared the differences of heart between CT and CBCT, analyzed the reproducibility of contouring the heart on different serious CBCT images, measured the margins in different axial direction from heart on planning CT to CBCT. Results: The heart volume of planning CT was smaller than CBCTs with statistical difference (588 cm³ vs. 717 cm³, P = 0.000). The volumes based on different serious CBCT images were similar for every patient (P = 0.999) and the coincidence of the hearts region on the same anatomical section between the first serious CBCT and others serious could reach to 0.985 ± 0.020 without statistical significant difference (P = 0.070). The mean margins for heart from planning CT to CBCT was (10.5 ± 2.8) mm, (5.9 ± 2.8) mm, (2.2 ± 1.6) mm, (3.3 ± 2.2) mm, (6.7 ± 1.1) mm and (4.5 ± 2.5) mm in the left, right, anterior and posterior direction; respectively. Conclusions: The PRV of heart contouring based on CBCT was feasible. If there were no CBCT, 11 mm in left direction, 6 mm in right direction, 3 mm in head-foot direction, 7 mm in the anterior direction, and 5 mm in posterior direction should be applied to obtain the PRV of heart. (authors)