Improvement of three-dimensional treatment planning models of small lung targets using high-speed multi-slice computed tomographic imaging

Purpose : To improve the reliability of the patient model and reduce treatment volume by acquiring multi-slice computed tomographic (CT) data with the patients’ single holding of breath at normal inhalation and exhalation. Methods and Materials : Seven patients with nine small peripheral lung cancer tumors underwent CT scanning under three respiration conditions using multi-slice CT: free breathing (FB), shallow inspiration (SI), and shallow expiration (SE). To compare the treatment plan created using the two-respiratory-phase images (SI + SE) with the plans created using only SE images or using only FB images, we attempted to calculate the true dosimetric characteristics for three-dimensional treatment planning taking respiratory movement into consideration. Minimum dose to the gross tumor volume (GTV) and ipsilateral lung dose-volume histogram (DVH) were calculated for the inhalation and exhalation positions of shallow breathing. Results : There was no significant difference between minimum doses of the GTV in the three treatment plans when using anteroposterior/posteroanterior parallel-opposed fields. However, there was a significant difference between the minimum doses of the GTV in the two-phase treatment plan and the minimum dose in the other treatment plans when using the four-field technique, consisting of shaped anterior, posterior, right and left lateral fields ( p = 0.03, 0.04). Comparison of the percent volume of ipsilateral lung receiving a dose exceeding 20 Gy (V 20) based on inhalation and exhalation CT data revealed that the V 20 of the two-phase plan was the smallest of the three treatment planning fields ( p < 0.001). Conclusion : Two-phase planning using multi-slice CT provides an immediate reduction in the amount of normal tissue treated and improved reliability of patient data for DVH modeling.