Proceedings of the 52nd Annual ASTRO Meeting S685

assurance phantom to establish a gold standard for the setup comparisons. The phantom was set up in the treatment room at fourdifferent positions. At each position the four setup procedures reported the shift necessary to bring it to the treatment isocenter.This established the best result one could expect when comparing the results of the four imaging/registration methods to oneanother.

Results: The shifts reported for the QA phantom differed by 1.5 to 4.0 mm per axis. These are the minimum uncertainties inherentin the four methods for the case of an absolutely rigid object with well-controlled imaging properties. The mean variation among thefour calculated patient setup shifts ranged from 3 to 12 mm for the 29 test cases, with a maximum difference ranging from 4 to 17mm. The correlation coefficient for any two of the four setup calculations ranged from 0.21 to 0.83. Most of the correlation co-efficients were less than 0.60, indicating that no two results were in significant agreement. There was no indication to favor anyone of the setup results as the most accurate.

Conclusions: The variability in setup shifts calculated for the patients was considerably greater than for the phantom. Thiswas likely due to the presence of non-rigid anatomical elements and the way the targeted anatomy was selected and framedfor each imaging modality prior to registration, rather than to technical details of the imaging and registration processes. Thesetup corrections reported here were obtained from commercially available hardware and software systems, each commis-sioned and used according to standard procedures. Any one of them could have been used to make patient adjustments priorto treatment. Their variability should therefore be viewed as indicative of the uncertainty in head/neck setup in actual clinicalpractice.

Author Disclosure: M. Murphy, None; J. Wu, None; S. Song, None; N. Dogan, None.

3075 Impact of Prostate Motion on Delivered Dose during Intensity Modulated Radiotherapy

E. Schreibmann1, J. Shelton1, P. J. Rossi1, V. A. Master2, A. B. Jani1

1Department of Radiation Oncology and Winship Cancer Institute, Emory University, 30306, GA, 2Department of Urology andWinship Cancer Institute, Emory University, 30306, GA

Purpose/Objective(s): To evaluate the impact of intrafraction prostate motion on delivered dose in the setting of intensity mod-ulated treatment delivery.

Materials/Methods: Seventeen men, with 24 separate prostate radiotherapy plans, were included. Treatments were deliveredwith intensity modulated (IMRT) or volumetric modulated arc (VMAT) radiotherapy. Intrafraction motion was recorded usingan electromagnetic tracking system (Calypso Medical) in three dimensions (AP, SI, RL) at 10 Hz. Shifts occurring during actualbeam-on time (range, 1,548-36,144 shifts/plan) were applied to the original PTV structure (prostate, seminal vesicles + margin)in relation to the original dose matrix using an original 4D dose calculation engine. For each shift the coverage at the prescrip-tion dose (V100) and the generalized equivalent uniform dose (gEUD) were normalized in comparison to the same values in theoriginal plan (DV100, DgEUD = planned-motion/planned). The relationship between direction and extent of motion to dosi-metric endpoints were evaluated for their correlations. Where the relationship was linear, Pearson’s correlation coefficient (r)was used.

Results: Statistically significant correlations were noted between all motion and dosimetric endpoints. AP, SI and RL motionshowed strongest association with DV100 (r = 0.51, -0.63, and -0.66, respectively). For the entire population, mean DV100and DgEUD were -1.74% (95% CI -1.75, -1.73) and -1.16% (-1.17, -1.13), respectively. For individual treatment plans,DV100 ranged from -10% to 0.7% (mean 2.2, SD 2.5) and DgEUD ranged from -6.6 to -0.2 (mean 1.9, SD 1.7). Thoughmost treatment sessions showed little change in delivered dose, large (. -5%) daily dose changes occurred in 11.8% ofthe time.

Conclusions: Prostate motion observed by electromagnetic tracking has been well documented. Our analysis uses a novel softwaretool to combine observed motion data during treatment delivery with treatment plan information to estimate the effect on delivereddose. The overall impact of intrafraction prostate motion in fractionated IMRT was small for the entire patient population. How-ever, interpatient and intrapatient variability was significant, and in some patients led to substantial dose reduction. Early detectionof dosimetric changes and their cause, facilitated by detailed motion data, may be beneficial in the development of effective adap-tive treatment strategies.

Author Disclosure: E. Schreibmann, None; J. Shelton, None; P.J. Rossi, None; V.A. Master, None; A.B. Jani, None.

3076 Using Atlas-based Auto-segmentation in 4-dimensional CT-based Radiation Therapy Planning for Patients

with Locally Advanced Pancreatic Cancer

T. S. Bray1, J. Roeske1, M. Quinn1, M. Gao1, E. Loo1, D. Javier2, M. Siddiqui1, S. N. Nagda1

1Loyola University Medical Center, Maywood, IL, 2University of Illinois at Chicago College of Medicine, Chicago, IL

Purpose/Objective(s): Target volume delineation on 4D-CT scans in patients with locally advanced, unresectable pancreatic can-cer (LAPC) is becoming increasingly important as radiation therapy (RT) methods become more conformal. However, automatedmethods to generate the internal target volume (ITV), such as the maximum intensity projection (MIP), do not work well in thissetting owing to the low contrast of soft tissues in the abdomen. The goals of this study are to determine: 1) if the use of Atlas-Based Auto-Segmentation (ABAS, Elekta CMS, St. Louis, MO) software is feasible for segmenting the tumor and duodenumthroughout the 4D scans; 2) if the 0%+50% CT scans (as opposed to the entire 4D data set) can be used to reliably producethe ITV.

Materials/Methods: The 4DCTs from 5 patients (total of 50 scans) diagnosed with LAPC were evaluated. The gross tumor vol-ume (GTV) and duodenum (D) were contoured on the inhalation (0% phase) scan. ABAS software was then used to automaticallysegment these structures on the subsequent phases (10-90%). Contours generated by ABAS were edited independently by two in-vestigators and agreed upon. The ABAS-generated and edited contours were quantitatively evaluated using the Dice Similarity

S686 I. J. Radiation Oncology d Biology d Physics Volume 78, Number 3, Supplement, 2010

Coefficient (DSC) (StructSure, Standard Imaging Inc., Middleton, WI). DSC values close to 1 indicate good agreement whilevalues closer to 0 indicate poor agreement. Additionally, the ITV of the GTV and D using the sum of the 0% and 50% phaseof the breathing cycle vs. the sum of the 0% through 90% phase were also compared using the DSC scores.

Results: ABAS was able to generate contours throughout all respiratory phases for each of the patients considered. Compared tothe manually edited contours, the mean and range of DSC scores were 0.961 (0.890-0.997) and 0.967 (0.911-0.998), for D andGTV, respectively. Separately, a comparison of the ITV generated from all respiratory phases vs. the 0% + 50% scans alone showedgood agreement. The mean and range of DSC scores for the ITvs. were 0.971 (0.947-0.986) and 0.933 (0.899-0.947) for the GTVand D, respectively.

Conclusions: ABAS software successfully produced contours of both GTV and D in our cohort patients. ABAS-based contoursrequired minimal intervention and were associated with high DSC scores for both GTV and D. The 0% + 50% phase position ofGTV and D may be used to reliably assess the ITV, and can be used instead of contouring all phases of the breathing cycle. Bothmethods may be utilized to minimize planning time when using 4D-CT-based RT planning for LAPC.

Author Disclosure: T.S. Bray, None; J. Roeske, None; M. Quinn, None; M. Gao, None; E. Loo, None; D. Javier, None; M. Siddi-qui, None; S.N. Nagda, None.

3077 Geometric Uncertainties in High Precision Radiotherapy for Brain Tumors: A Prospective Study

S. Das, R. Isiah, R B., P S., S. John

Christian Medical College Vellore, Vellore, India

Purpose/Objective(s): To determine set-up errors for the immobilization devices (Gill-Thomas-Cosman (GTC) frame, BrainLABand Thermoplastic ray cast) used in high precision radiotherapy (stereotactic radiotherapy, 3-D conformal radiotherapy) for braintumors and to derive clinical-target-volume (CTV) to planning-target-volume (PTV) margins.

Materials/Methods: Total 30 subjects were included (10 subjects in each category of immobilization). In case of immobili-zation with GTC frame, daily relocation error was measured using depth helmet and measuring probe. Translational displace-ments in mediolateral (ML), craniocaudal (CC) and antero-posterior (AP) direction was calculated from the position and angleof the measuring portal. In case of Brain LAB and ray cast, error was assessed using electronic portal imaging device (EPID).Systematic and random errors in ML, CC and AP directions were calculated. CTV to PTV margin was calculated using ICRU62 (

P+ 0.7 s) Stroom’s (2

P+ 0.7 s) and Van Herk’s (2.5

P+ 0.7 s) formula, (

P= systematic error and s = random

error).

Results: With GTC frame, mean displacement in AP axis is 0.15 mm (± 0.38), CC axis -0.17 mm (± 0.42) and ML axis 0.38 mm(± 0.56). The mean vector displacement for the population is 1.03 mm (± 0.34). The errors were within ±2 mm in ML direction in99.6% cases, in 99.2% cases in AP and 97.6% cases in CC direction. The random error was 0.73 mm in CC, 1.86 mm in AP and1.41 in ML axes and it was normally distributed. The margins (in mm) in ML CC and AP directions were 1.55, 0.93, 1.68 (ICRU62), 2.1, 1.35, 2.06 (Stroom’s) and 2.39, 1.56 and 2.25 (van Herk’s). In BrainLAB, mean errors in ML, CC and AP directions was0.14 cm (SD 0.27), -0.044cm (SD 0.151) and 0.059 cm (SD 0.150), respectively. The systematic errors in ML, SI and AP di-rections were 0.03 cm, 0.052 cm and 0.124 cm, respectively. The random errors in ML, SI and AP directions were 0.24 cm,0.143 cm and 0.106 cm, respectively. The margins (in cm) in ML CC and AP directions were 0.19, 0.15, 0.19 (ICRU 62),0.23, 0.20, 0.32 (Stroom’s) and 0.24, 0.23, and 0.38 (van Herk’s). In ray cast, the mean error in ML, SI, and AP directionswas 0.15 cm (SD 0.14), -0.015 cm (SD 0.326), and 0.15 cm (SD 0.1668). The systematic errors in ML, SI and AP directionswere 0.133 cm, 0.2831 cm and 0.0887 cm while random errors in the same directions were 0.160 cm, 0.1784 cm and 0.1429cm respectively. The margins (in cm) in ML CC and AP directions were 0.24, 0.40, 0.18 (ICRU 62), 0.37, 0.69, 0.27 (Stroom’s)and 0.44, 0.83 and 0.32 (van Herk’s).

Conclusions: Immobilization with GTC is superior to BrainLab device for brain. In 97-99% cases the error lies between ± 2 mm.Therefore, a margin of 2 mm around the tumor seems to be adequate. BrainLAB immobilization was better than thermoplastic raycast in reducing set up errors in radiotherapy for brain tumors.

Author Disclosure: S. Das, None; R. Isiah, None; R. B, None; P. S, None; S. John, None.

3078 An Analysis of Real-time Prostatic Fossa Motion during Radiotherapy

R. D. Foster, J. Anderson, T. Boike, D. Pistenmaa, L. Ouyang, T. Solberg

University of Texas Southwestern Medical Center, Dallas, TX

Purpose/Objective(s): While intrafraction motion of the intact prostate has been shown to be variable and unpredictable and mayhave dosimetric consequences for radiotherapy delivery, little is known about intrafraction motion of the prostatic fossa in patientswho have undergone a prostatectomy. Studies analyzing patient set-up errors for prostatic fossa localization have shown that im-planted markers agree with bony anatomy localization within 0.3-1.1 mm on average. However, if the prostatic fossa moves in-dependently of the bony anatomy, bony anatomy localization may not be sufficient for high precision radiotherapy or marginreduction for post-prostatectomy patients. The purpose of this study is to analyze the intrafraction motion of the prostatic fossausing implanted electromagnetic transponders to determine the probability that a static localization would find the prostatic fossadisplaced from its planned position.

Materials/Methods: Nine patients had electromagnetic transponders implanted in the prostatic fossa for the purpose of localiza-tion and tracking during radiation therapy, resulting in 320 tracking sessions available for analysis. The fraction of time the prostaticfossa was displaced . 3, . 5, and . 10 mm was calculated for each direction as well as the 3D position of the centroid of thetransponders. No deformation was taken into account. The fraction of tracking time that the prostatic fossa is displaced representsthe probability of it being displaced from its planned position if a static localization is obtained.

Results: For all tracking sessions, the average fraction of time that the 3D position of the prostatic fossa was displaced by morethan 3 mm was 8.8% and ranged from 4.30 to 18.8% for individual patients. For 5 mm displacement, the average was 0.86%