Purpose: The Brainlab's Robotics 6D couch is integrated with ExacTrac X‐ray 6D system to compensate the rotational deviations in patient positioning. The purpose of this study is to quantitatively demonstrate the magnitude of improving the rotational accuracy by the 6D system. Method and Materials: Two tests were conducted. A self‐made tool was first used to verify that the rotation angles given by the ExacTrac matched with the actual rotation angles measured by geometry calculation and a level. A wedge was used to raise one side of the test tool for various rotational angles. A Rando head phantom with four BBs implanted was used for the second test. One BB was at the isocenter, with two others 5cm laterally, and the fourth 5cm superior. The relative rotational angles of each BB to the isocenter were calculated from CT images. The phantom was randomly setup within 5° rotations. Portal films were taken after each Robotics 6D couch corrections. BBs positions in the portal films were measured and the rotations were calculated and compared with that in the simulation. Results: The difference between the results determined by X‐ray 6D fusions and actual angels for various setups were 0.113°±0.06° and 0.05°±0.09°, according to level measurement or geometry calculation, respectively, for the pitch direction. The corresponding differences were 0.23°±0.09° and 0.08°±0.09° for the roll. After the Robotics 6D couch correction, the actual rotation was 0.03°±0.05°. and 0.13°±0.04° for the pitch and roll directions. For the second test, the final setup rotation angles compared with the Simulations were 0.42°±0.24°, 0.30°±0.16° and 0.06°±0.02° for pitch, roll and yaw rotations, respectively. Conclusion: The study demonstrated that the rotational accuracy was in the acceptable limit to use in the clinic. This has a great potential to improve the clinical targeting accuracy of radiosurgery.

Purpose: To compare target volumes assessed via 4D and free‐breathing CT scans for patients treated with peripheral lung lesions. Method and Materials: The target volumes of five lung cancer patients imaged using 4D‐CT and treated with hypo‐fractionated SBRT (12 Gy/Fxn × 4Fxn) were retrospectively analyzed. For each patient 6‐to‐8 CT datasets were acquired between inhale and exhale respiratory phases on a Philips 16 slice 4D‐CT scanner. The GTV was segmented on each dataset using a maximum‐intensity‐projection (MIP) method and an ITV (ITV 4D ), representing the composite of GTVs, was formed. The ITV 4D was expanded uniformly 5mm to generate a PTV (PTV 4D ). The GTV was also contoured on the free‐breathing scan and expanded using population‐based margins of 5mm and 10mm in the axial and longitudinal planes, respectively, to form a free‐breathing‐based PTV (PTV FB ), following RTOG ♯0236. Finally, a target volume defined as a composite of GTVs contoured on only the inhale and exhale datasets was generated to form the ITV Inh_Exh . Results: For three of five patient images, PTV 4D was substantially larger than PTV FB (average increase of 33%; max.=65%). In one case the volumes were equivalent and in the remaining case PTV 4D was 11% smaller than PTV FB . Significant shape changes were also observed in some instances between PTV 4D and PTV FB suggesting that PTVFB was improperly designed. The ITV Inh_Exh was smaller than the ITV 4D in all cases (mean=31%; max.= 78%, smaller) suggesting that the inhale and exhale breathing phases sometimes fail to capture the largest extents of tumor motion in the respiratory cycle. Conclusion: Results suggest that, based on 4D imaging, the use of population‐based margin expansions may not adequately account for tumor motion of peripheral lung tumors. This may be of increased consequence in the SBRT setting, where the overall effects of motion may be escalated given the small number of fractions. Acknowledgement_NIH‐R01‐CA106770.