SU‐FF‐J‐17: Accuracy and Dosimetric Advantage of Target Localization Using Stereoscopic Image‐Guided Radiotherapy for Lung Cancer Treatment

Purpose: We analyzed the accuracy of stereoscopic image‐guided radiotherapy (S‐IGRT) with the ExacTrac® system (BrainLAB AG, Heimstetten, Germany), and demonstrated the dosimetric advantage for lung cancer treatment. Methods: The accuracy of target localization using the ExacTrac system was analyzed by re‐scanning ten patients immobilized in the Vac‐loc bag with BB tags put on the isocenter marks determined by the ExacTrac system. The re‐scanned CT data from each patient was fused and compared to the isocenter locations on the original CT used for the treatment planning. The Active Breathing Coordinator™ (Elekta, Norcross, GA) was employed to minimize the breathing motion effect on S‐IGRT. The dosimetric advantage of S‐IGRT was demonstrated by comparing the normal lung volume receiving 15%, 30% and 50% of the prescription dose (V15%, V30% and V50%) between the plans with smaller margins in the S‐IGRT and those with normal margins in the conventional radiotherapy for the ten patients. Results: The average isocenter shifts using S‐IGRT were within 3.4 ±1.7mm in the lateral, 3.6±1.9mm in the anterior/posterior, and 2.2±3.2mm in the superior/inferior directions. The added margins around the CTV to create the PTV were chosen to be 10mm in the superior/inferior and 5mm in radial direction for the lung cancer treatment planning in S‐IGRT. For the ten patients with CTV volume from 8.3cm3 to 43.4cm3 and lung volume from 2736cm3 to 3640cm3, the averages of V15%, V30% and V50% were 17.4%, 9.8%, 5.9% for the S‐IGRT plans, and 33.4%, 18.2%, 12.8% for the conventional radiotherapy plans with 2cm margins in the all directions. Conclusion: The S‐IGRT with the ExacTrac system provided highly accurate tumor localization. The margins from CTV to PTV in S‐IGRT treatment planning could be reduced significantly due to the accurate target localization which will reduce the lung volume receiving doses in the medium and low ranges.