S580 I. J. Radiation Oncology d Biology d Physics Volume 75, Number 3, Supplement, 2009

which may allow for decreased PTV margins and higher total doses. We investigated daily prostate localization using availableimaging techniques coupled with implanted fiducial markers. Patient setup errors were determined for fiducial-based prostate lo-calization using planar kV radiographs and conebeam CT (CBCT). The utility of CBCT in prostate radiation therapy was also as-sessed.

Materials/Methods: Patients were aligned to the treatment isocenter initially by laser alignment to skin tattoos, followed by or-thogonal kV radiographs to visualize both implanted fiducial markers and bony anatomy, and then by CBCT to visualize both fi-ducial markers and soft tissue. For daily setup using planar kV radiographs, couch shifts were performed based on matching fiducialmarker position to the planning DRRs. Twice weekly, CBCTs were acquired immediately following kV setup to verify both theinitial setup and measure any residual error in patient positioning. Couch shifts were then performed based on matching fiducialmarker position (and verified by soft tissue architecture when clearly visible) to the planning CT. All shifts were recorded for post-treatment analysis.

Results: The average couch shifts required to correct the initial laser alignment using orthogonal kV radiographs (fiducial markermatch) were 3.7 mm, 3.7 mm and 4.2 mm in the vertical, longitudinal and lateral directions, respectively (n = 14 patients). Theresidual setup errors following kV localization, as determined by CBCT (fiducial marker and soft tissue match), were 1.2 mm,0.8 mm and 0.7 mm in the vertical, longitudinal and lateral directions, respectively (n = 10 patients).

Conclusions: Daily image guidance using planar kV radiographs and implanted fiducial markers allows for more accurate prostatelocalization during radiation therapy. The residual setup errors following kV localization, as determined by CBCT, were small,which demonstrates the overall high accuracy of kV localization when implanted fiducial markers are present. CBCT may notbe beneficial for prostate localization during dose escalation when fiducial markers are present.

Author Disclosure: P.E. Clancy, None; B.W. Schuller, None; L.M. Sroczinski, None; A.E. Hirsch, None.

2889 Efficiency and Clinical Workflow of Delivering IMRT to the Prostate within 2 mm Tolerances

S. E. Tropper1, D. Khan2, C. A. Mantz3

121st Century Oncology, Scottsdale, AZ, 221st Century Oncology, Santa Monica, CA, 321st Century Oncology, Cape Coral, FL

Purpose/Objective(s): To report the efficiency of treating prostate cancer patients in routine clinical practice to extremelytight PTV margins and patient-realignment thresholds using real time electromagnetic localization and tracking. Inter- andintra-fraction prostate motion necessitates the use of substantive PTV margins to compensate for uncertainties in target posi-tion. Real time electromagnetic tumor localization and tracking ensures a high degree of accuracy and enables PTV marginreduction. Margin reduction results in less radiation to adjacent bladder and rectum and reduces the acute adverse effects oftreatment (data included in companion abstract). We report the clinical user experience of localizing and treating patientswithin a 2 mm tracking limit.

Materials/Methods: 63 patients with localized prostate cancer who opted for primary IMRT were enrolled on an IRB approvedprospective multi-institutional clinical study. Patients underwent intraprostatic implantation of transponders and were treated withIMRT to a nominal dose of 81 Gy in 1.8 Gy fractions to prostate and proximal SV using a posterior PTV margin of 3 mm. Treatmentwas delivered while patients remained within the 2 mm tracking limits, and beam delivery was interrupted when the prostate movedoutside the limit for 0-5 seconds depending on physician guidance. Beam delivery resumed when the target either spontaneouslyreturned to within limits or after the couch was manually re-aligned. Data was collected on the number of fractions with interven-tions. Interventions included pausing the treatment beam and waiting for spontaneous realignment (INT1), realigning the treatmentcouch manually (INT2), and other (INT3).

Results: 27 patients have received 1075 treatment fractions. One or more interventions were reported in 64.5% (693/1075) andnone in 35.5% (382/1075) of the fractions. Of these 693, 4.3% (30) were INT1 , 95.3% (660) were INT2, and 0.4% (3) wereINT3. Scheduled treatment timeslots were 15 minutes.

Conclusions: Patients exhibit variable and unpredictable inter- and intra-fraction variation in prostate motion that required inter-vention during treatment delivery to maintain patient alignment. Nonetheless, one can efficiently treat patients with extremely smalltracking thresholds and narrow PTV margins within a 15 minute time slot using electromagnetic tracking. Data submitted in a com-panion abstract demonstrates the clinical advantage of this highly focused approach for minimizing acute adverse effects of prostatecancer IMRT treatment.

Author Disclosure: S.E. Tropper is currently enrolling in a Clinical Study sponsored in part by Calypso Medical Technologies Inc.,C. Other Research Support; D. Khan is currently enrolling in a Clinical Study sponsored in part by Calypso Medical TechnologiesInc., C. Other Research Support; C.A. Mantz is currently enrolling in a Clinical Study sponsored in part by Calypso Medical Tech-nologies Inc., C. Other Research Support.

2890 Comparison of Achievable Dose Reduction in CBCT for the Prostate using Intensity-weighted and

Conformal Region of Interest Imaging Techniques

E. Pearson, S. Cho, X. Pan, C. A. Pelizzari

University of Chicago, Chicago, IL

Purpose/Objective(s): Increased frequency of CT imaging can increase setup accuracy and prevent misplaced therapeutic dosedue to interfraction organ motion. This is especially important for sites in which the motion can be large and is unpredictable,such as the prostate. Adaptive radiation therapy (ART) with daily imaging is one method to increase the accuracy of dose deliveryto the prostate. However with increased CT frequency there is also an increase in non-targeted dose from the kV x-rays used forimaging. A promising dose reduction strategy is conformal region of interest (ROI) imaging, whereby only the target and imme-diately surrounding structures are illuminated from any projection angle. This leads to an interior reconstruction problem for whichthere is currently no stable reconstruction technique but is an active area of research. Alternatively, instead of blocking the rays thatdo not directly illuminate the ROI they can be heavily filtered at the source, thereby keeping patient exposure low but still providing

Proceedings of the 51st Annual ASTRO Meeting S581

information allowing for stable reconstruction. This intensity-weighted method also provides sufficient anatomic data in the outerregion for image registration, which is critical for accurate dose summation in an ART treatment scheme.

Materials/Methods: Intensity-weighted ROI imaging varies the spatial beam intensity by introducing filters in the kV x-ray beam.The results reported here are for 3mm copper filters which the authors have previously shown allows for full volume reconstructionwith sufficient SNR to enable accurate rigid body registration of the skeletal structure. For conformal imaging radiopaque leadblades were used to shape the beam. The imaged ROI was taken to be a 10cm diameter cylinder along the central axis, whichfor an average patient is expected, on any given day, to include the entire prostate and the highest dose regions of the bladderand rectum. Two-dimensional dose distributions were measured using Gafchromic film placed between slabs 33 and 34 of a stan-dard anthropomorphic RANDO Man phantom (The Phantom Laboratory, Salem, NY).

Results: Conformal ROI imaging was able to reduce the dose to the region outside the ROI by 50-70% and the dose to the ROI by25% due to reduced scatter from the outer region. Intensity-weighted ROI imaging achieved 75-90% of the dose reduction of con-formal imaging in the outer region while reducing dose to the inner region only half to two-thirds as much.

Conclusions: Measurements indicate that for prostate imaging intensity modulation can provide nearly as much dose reduction asconformal illumination while still providing information of the surrounding structure, enabling stable reconstruction and registra-tion required for accurate dose summation for ART treatment schemes.

Author Disclosure: E. Pearson, None; S. Cho, None; X. Pan, None; C.A. Pelizzari, Varian Medical Systems, Inc., B. ResearchGrant.

2891 A Dosimetric Comparison of Free-breathing (FB) and Deep Inspiration Breath Hold (DIBH) in Patients

Undergoing Lung Cancer Radiotherapy

V. Marchand1, S. Zefkili1, J. Desrousseaux1, C. Dauphinot1, P. Giraud1,2

1Institut Curie, Paris, France, 2Hopital Europeen Georges Pompidou, Paris, France

Purpose/Objective(s): To evaluate the potential benefit of deep inspiration breath hold (DIBH) technique comparing to freebreathing (FB) radiotherapy in a homogenous population of patients with lung cancer.

Materials/Methods: Twenty-five patients with non-small-cell lung cancer treated by DIBH underwent an additional FB CT scan.Dose-volume histograms were generated and the DIBH and FB treatment plans were compared. The target volume was comparedusing coverage, homogeneity, and conformal indices. Organs at risk (OAR) were compared using dosimetric parameters like V5,V13, V20, V25, V37, mean dose (MD) for lungs, V40 and MD for the heart, and V50, MD and maximum dose (Dmax) for the esoph-agus. Biological indices i.e. the effective uniform dose (EUD) and the normal tissue complication probability (NTCP) were alsocalculated for OAR.

Results: Median age was 62. Right lung tumors (56% of patients) were mainly located in the upper lobe (79%), followed by thelower (14%) and the middle lobes (7%). Left lung tumors (44% of patients) were located mainly in the upper lobe (73%), followedby the lower lobe (27%). Histological types included adenocarcinoma (56%), squamous cell carcinoma (24%), and large cell car-cinoma (20%). Mean prescribed total dose was 62.7 Gy. Conformal index was improved with DIBH (0.67 vs. 0.58, p = 0.046) butcoverage and homogeneity indices were not significantly different between DIBH and FB treatment plans. Lung dosimetric param-eters were significantly improved using DIBH: MD (13 vs. 15 Gy, p = 10-4), EUD (8.2 vs. 9.9 Gy, p = 3.10-4), NTCP (1.9 vs. 4.8%,p = 10-3), V13 (31 vs. 38%, p = 2.10-3) and V5 (43 vs. 51%, p = 6.10-5) as well as V20 (25 vs. 31%, p = 0.01), V25 (22% vs. 27%, p =0.01) and V37 (12 vs. 16%, p = 0.03) were reduced. For heart, MD (14 vs. 17 Gy, p = 0.003), EUD (19 vs. 22 Gy, p = 6.10-4) and V40

(12 vs. 17%, p = 0.004) were significantly reduced with DIBH, V30 and NTCP were similar for both techniques. Concerning esoph-agus, DIBH significantly improved the MD (28 vs. 30 Gy, p = 0.007) and the V50 (25 vs. 30%, p = 0.003), whereas EUD, NTCP andDmax were not significantly changed.

Conclusions: DIBH improves target conformity index as well as heart and lung dosimetry in lung cancer patients treated withradiotherapy. The clinical implications of these findings must be confirmed by further studies.

Author Disclosure: V. Marchand, None; S. Zefkili, None; J. Desrousseaux, None; C. Dauphinot, None; P. Giraud, None.

2892 3D Trajectories for Limited-angle and Full-angle Dual-X-ray Conebeam CT Onboard Radiation Therapy

Machines

J. E. Bowsher, W. Giles, J. R. Roper, H. Li, T. Zhuang, F. Yin

Duke University Medical Center, Durham, NC

Purpose/Objective(s): A system of two orthogonal x-ray imaging chains mounted to a radiation therapy machine might providemany target localization and target tracking advantages as compared to single x-ray imaging systems. Here we consider the poten-tial benefit of displacing these two x-ray systems from one another axially, in order to increase the axial range of high-quality con-ebeam (CB) CT imaging.

Materials/Methods: In principle, 2D trajectories of a single x-ray focal spot through a single plane will not completely samplevolumes outside that plane. Complete CBCT sampling can often be achieved by 3D trajectories in which one or more focal spotscross multiple planes. Even for limited-angle (digital tomosynthesis) imaging, in which complete sampling is not an objective, 3Dtrajectory may improve the axial range of high-quality imaging. In order to evaluate 2D versus 3D trajectories, CBCT projectiondata were computer-simulated for a Defrise phantom, with each of two orthogonal CB x-ray imaging chains traversing (A) 60 de-grees and (B) 360 degrees, with the two focal spots (1) in the same plane and (2) axially displaced. Images were reconstructed fromthese 4 sets of projection data using iterative ordered-subsets methods (OSTR). Source-to-isocenter and source-to-detector dis-tances were 100 cm and 150 cm respectively. Detector was 30 cm wide axially.

Results: For the two 2D trajectories, 4 (A1) or 5 (B1) of the 1.2-cm-wide Defrise on/off patterns were fully recovered, indicatingabout a 6 cm range of top image quality axially. For the two 3D trajectories, 12 (A2) or 14 (B2) of the Defrise patterns were fullyrecovered, indicating about a 16 cm axial range of top image quality. Beyond these axial ranges, image quality increasingly