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