SU‐EE‐A2‐06: Benefit of 3D Image‐Guided Stereotactic Localization in the Hypofractionated Treatment of Lung Cancer

Abstract

Purpose: To investigated the benefit of image‐guided stereotactic localization for lung cancer patients treated with hypofractionated radiotherapy. Materials & Methods: A stereotactic body localizer (SBL) was used for patient immobilization, image registration among multi‐phase CT simulation, and image‐guided stereotactic localization. The simulation scans consist of 3 sets of CT images taken during free breathing and 2 breath‐holding scans (at maximum inhalation and exhalation). Target delineation was performed on all 3 sets of images and the combination of the targets forms a composite gross‐target volume (GTV). Treatment planning was performed on the planning‐target volume (PTV) using 3 mm margin to account for the presumed reliability of the CT localization. Prior to each treatment, a localization CT scan using a CT‐on‐rails was obtained. Couch shifts were made based on the changes of the stereotactic coordinates of three pre‐selected bony landmarks. In this retrospective treatment dose verification, we performed image fusion between the simulation CT scan and each pre‐treatment CT scan to obtain the same target and critical structure information. The same treatment plans were re‐loaded onto each pre‐treatment CT scan with their respective stereotactic coordinate system. The changes in dose distributions were assessed by dose‐volume histograms of the PTV and the critical structures before and after isocenter corrections which were prompted by image guided stereotactic localization. We compared D95, D99, and V95 for the PTV and GTV, and V20and V30 for the ipsilateral lung. Results: Our retrospective study for 10 patients with 40 dose reconstructions showed that the average D95, D99, and V95 of the PTVs are 92.1%, 88.1%, and 95.8% of the planned values before isocenter corrections. With the corrections, these values are all improved to 100% of the planned values. Conclusions: 3D image guidance is crucial for stereotactic radiotherapy of lung tumors when small margins are used.