Proceedings of the 51st Annual ASTRO Meeting S579

2886 A Dose-guided, Volumetric Target Localization Technique for Prostate IGRT

C. Ma, G. Shan, W. Hu, Q. Xu, J. Fan, I. Mohamed, L. Chen

Fox Chase Cancer Center, Philadelphia, PA

Purpose/Objective(s): Different imaging techniques have been used to facilitate patient setup and target localization for advancedprostate radiotherapy, which either utilizes implanted fiducial markers or 3D anatomical structures to precisely align the treatmentisocenter. These techniques work well for translational, interfractional organ motion but may result in poor target coverage for somecases where large rotational organ motion or deformation occurs. This work investigates the feasibility of the use of 3D dose dis-tributions to match the target volume to improve target coverage and critical structure sparing.

Materials/Methods: Fifteen previous treated prostate patients were recruited for this retrospective study. A Siemens CT-on-rails system was used for image-guided (contour based) target localization for IMRT of prostate cancer for these patients.CT-on-rails scans were performed before and after the IMRT treatment once a week under local IRB approval. Dose reconstruc-tions for 98 post-treatment CT-on-rails scans were performed using the actual MUs and MLC leaf sequences. The isodose dis-tributions and dose volume histograms were compared with those of the original treatment plans. Target localization was alsoperformed using the prescription isodose surface from the original treatment plan to match the target volume and a new isocentershift was applied in the dose reconstruction. The dose distributions were evaluated against the original treatment plans and thereconstructed dose distributions using the traditional contour-based target localization technique based on our clinical plan eval-uation criteria.

Results: Due to interfractional organ rotation and/or deformation a therapist can only achieve a best match of the anatomical struc-tures between the planning CT and the treatment CT. Our results show that using the standard target localization procedure basedon contour/anatomy matching, 7.1% of the 98 treatment fractions exhibit poor target coverage (Dmin\65 Gy). For the rectum,27.6% fractions violated our rectal criterion of V65\17% and 26.5% fractions violated the criterion of V40\35%. After the iso-center realignment based on 3D dose/target volume matching, all the fractions delivered .65Gy to the target, and the percentagesof fractions that violated the rectal criteria were reduced to 14.3% and 18.4%, respectively.

Conclusions: The current IGRT procedure for prostate treatment is still not ideal if only contour/anatomy matching is used for iso-center alignment due to poor soft-tissue contrast, residual translational/rotational organ motion and organ deformation (rectal andbladder filling). Target localization based on 3D dose/target volume matching provides better target dose coverage and critical struc-ture sparing.

Author Disclosure: C. Ma, None; G. Shan, None; W. Hu, None; Q. Xu, None; J. Fan, None; I. Mohamed, None; L. Chen, None.

2887 Setup Uncertainty Analysis for Intensity-modulated Radiotherapy for Soft Tissue Sarcoma of the Thigh

S. L. Berry, J. G. Mechalakos, K. M. Alektiar

Memorial Sloan Kettering Cancer Center, New York, NY

Purpose/Objective(s): Use of the proper setup uncertainty margins in radiation therapy planning is important to ensure that theclinical target volume (CTV) is covered while preserving as much normal tissue as possible. Setup uncertainty has been exten-sively studied in the head and neck and pelvic regions, however, data is only now starting to emerge for extremity irradiation.Treatment of the thigh represents unique challenges in positioning and immobilization due to its ability to undergo many trans-lations and rotations relative to the rest of the body. We evaluate three dimensional (3D) setup errors and report CTV to plan-ning target volume (PTV) margins for soft tissue sarcoma patients undergoing intensity modulated radiation therapy (IMRT) tothe thigh.

Materials/Methods: Twenty-one patients immobilized in an Alpha cradle mold and treated via IMRT to the thigh between 2006and 2008, a total of 114 pairs of orthogonal megavoltage portal images, were analyzed retrospectively using a template basedmatching technique. Individual setup errors, maximum and minimum displacements, and population based systematic (S) and ran-dom (s) errors were calculated. Setup margins were then determined using published margin recipes.

Results: The mean translational setup errors (and range max to min) from all data were 1 (+21 to -15), 0 (+19 to -10), and 1 (+8 to-6) millimeters (mm) in the right-left (RL), anterior-posterior (AP), and superior-inferior (SI) directions respectively. The meanrotational errors from all data were -0.1� (+3.5� to -9�) and 0.2� (+6.5� to -4.5�) in the cranial and sagittal planes respectively.In the RL, AP, and SI directions, the standard deviation of the patient systematic errors was 4, 3, and 2 mm and the root mean squareof the patient random errors was 4, 4, and 2 mm. This corresponds to a CTV to PTV margin of 12.5 (RL), 10.7 (AP), and 6.1 (SI)mm using the van Herk margin recipe formulation.

Conclusions: Our institutional experience in IMRT for soft tissue sarcoma of the thigh provides useful data for the quantification ofsetup errors for this anatomical site. The observed population systematic and random errors for the thigh are comparable to reportsin the literature for pelvic sites with similar immobilization methods. A CTV to PTV expansion of # 12.5 mm should result inadequate CTV coverage for this patient population.

Author Disclosure: S.L. Berry, None; J.G. Mechalakos, None; K.M. Alektiar, None.

2888 Assessment of Patient Setup Error in Prostate Radiation Therapy using Fiducial-based Image Guided

Radiation Therapy with kV Onboard Imaging and Conebeam CT

P. E. Clancy1, B. W. Schuller1,2, L. M. Sroczinski1, A. E. Hirsch1,2

1Boston Medical Center, Boston, MA, 2Massachusetts General Hospital, Boston, MA

Purpose/Objective(s): Dose escalation in prostate radiation therapy requires accurate target localization during the entire course oftreatment. The advent of onboard imaging (OBI) technology allows for daily prostate localization in either two or three dimensions,