Proceedings of the 50th Annual ASTRO Meeting S615

Conclusions: Dose distributions for lung cancer patients were calculated using the in-house developed voxel dose-tracking methodbased on the FFID algorithm. This method provides a realistic dose distribution by incorporating both respiratory motion and lungdeformation. We found that the ITV-based IMRT plan overestimated the TCP. However, the dose-volume relationship of the lungdid not change significantly incorporating the organ motion and deformation except for the high dose volume.

Author Disclosure: H. Alasti, None; J. Chow, None; D. Markel, None; C. Lochovsky, None; D. Payne, None.

2993 Monte Carlo Evaluation of Stereotactic Body Radiotherapy (SBRT) Treatment Planning for Lung Tumors

T. Wu, K. Farrey, J. K. Salama, K. M. Yenice

Department of Radiation and Cellular Oncology, The University of Chicago Medical Center, Chicago, IL

Purpose/Objective(s): To evaluate the accuracy of SBRT treatment planning with tissue heterogeneity corrections for primary andmetastatic lung tumors using collapsed cone (CC), pencil beam (PB), and a new commercially available Monte Carlo (XVMC)algorithms.

Materials/Methods: Two patients with lung oligometastases (one patient had four isolated lesions) and one patient with a primarynon-small cell lung cancer were treated with 3D-conformal SBRT to 30-60 Gy in 3 fractions. Planning target volumes (PTVs),based on 4D-CT simulations, ranged from 8.1-61.1 cc (mean = 23.4 cc). XVMC algorithm was commissioned on the BrainLABIPlan system, which also had a PB algorithm. Target and normal tissue volumes on axial CT images were transferred from Pinnacleto IPlan via DICOM protocols. Both planning systems used the same CT electron density table for heterogeneity corrections. Plansusing non-opposed, non-coplanar 6MV beams were first generated in IPlan with PB calculations. Dose distributions were thenrecalculated by XVMC with the same dose grid size (overall statistical uncertainty \0.3%). CC calculations were subsequentlyperformed in Pinnacle with the same beam arrangements. Three dimensional dose distributions and dose-volume-histogram sta-tistics were evaluated for all algorithms.

Results: XVMC dose calculations showed that PB algorithm consistently overestimated isocenter dose by 2.5%-6.6%. PTVcovered by prescription dose (V100) was also overestimated up to 68% (when the lesion was adjacent to the chest wall). Iso-center dose predicted by CC closely agreed with that calculated by XVMC for all cases, except for the chest wall lesion, whereCC underestimated it by 4%. V100 calculated by CC was uniformly underestimated by up to 40% compared to XVMC pre-dictions. The largest deviation occurred for a 3cm-lesion, which was entirely surrounded by the lung tissue. CC underestimatedthe dose covering 95% of PTV (D95) by 2-3% of XVMC results. Percent lung volumes (excluding PTV) receiving 5, 10, and 20Gy were similar for all calculation algorithms within 3%. Maximum doses to 1 cc of esophagus and spinal cord were also com-parable.

Conclusions: Monte Carlo predictions have shown that SBRT dose calculations employing PB and CC algorithms are subject tolarge errors for small lung targets treated with SBRT. Generally PB overestimates the PTV coverage near margins, while CC un-derestimates it. The magnitude of the discrepancy varies significantly depending on the location of the target. Although the dis-crepancy is substantial for V100, D95 is still greater than 92% of prescription for all targets. To predict the volumes of normallung tissue receiving mid to low doses, the three algorithms provide equivalent results.

Author Disclosure: T. Wu, None; K. Farrey, None; J.K. Salama, None; K.M. Yenice, None.

2994 The Effect of Transponder Motion on the Accuracy of the Calypso Electromagnetic Localization System

M. J. Murphy1, R. Eidens2, E. Vertatschitsch2, J. Wright2

1Virginia Commonwealth University, Richmond, VA, 2Calypso Medical Technologies, Seattle, WA

Purpose/Objective(s): The Calypso localization system remotely detects miniature electromagnetic transponders implanted ina patient. The system is capable of continuously measuring the position of a treatment site during radiotherapy. If the site is movingduring respiration then the system can be used for dynamic tracking of the target. With this application in mind we have mademeasurements to see if there is any change in localization accuracy due to transponder position or motion.

Materials/Methods: Three localization transponders were mounted on a remote-controlled turntable that could move the tran-sponders along a circular trajectory at speeds up to 3 cm/sec. A stationary calibration established the coordinates of six pointson each transponder’s circular path. We then compared position measurements taken while the transponders were in motion ata constant speed to the stationary coordinates. To synchronize the position of the moving transponders to a known point on thecircular path we injected an RF noise pulse into the Calypso readout antenna when the turntable passed 0�. This produced outlierson the arc of moving transponder positions that could be used to compare the transponder positions in the direction of motion to thecalibrated stationary positions.

Results: The stationary localization uncertainty varied from 0.02 to 0.06 cm per axis in the (x,y) plane, which is consistent with theobservations of other researchers. We detected a small systematic trend in the stationary uncertainty as a function of transponderposition along an arc 11 cm long. Lung tumors do not move more than about 3 cm in the most extreme cases. Therefore the Calypsosystem will have less than 0.02 cm systematic change in localization accuracy over the full range of tumor position. There was nostatistically significant change in the transponder positions in either the radial or tangential direction when the transponders were inmotion.

Conclusions: If the Calypso system is used for real-time respiratory motion tracking, then tracking accuracy will be unaffectedby transponder motion, at least for the velocity and position range that will be encountered in clinical applications. If the tran-sponder is moving, its reported location corresponds precisely to its position at the midpoint of the readout interval, regardless ofspeed.

Author Disclosure: M.J. Murphy, Accuray Incorporated, E. Ownership Interest; R. Eidens, Calypso Medical Technologies, A. Em-ployment; E. Vertatschitsch, Calypso Medical Technologies, A. Employment; J. Wright, Calypso Medical Technologies, A. Em-ployment.