S702 I. J. Radiation Oncology d Biology d Physics Volume 75, Number 3, Supplement, 2009

defined as brain and spinal canal respectively. A 10-12 mm block margin with site-specific smearing was used within the VarianEclipse treatment planning system. For SU, shifts of 3 orders were created: 5 mm, 10 mm, and 20 mm. The shift was accomplishedby moving the beam isocenter relative to the patient. X represented Lt-Rt, Y sup-inf, and Z ant-post. For the Cr fields, we created SUin the X, Y, and Z vectors of 5, 10, and 20 mm. For the Th spine field, we created SU in the X vector only of 5, 10, and 20 mm. Forthe LS spine field, we created SU in the X and Y vectors of 5, 10, and 20 mm. For QA, we applied all SU in both directions from thesetup point. A total of 45 plans per patient were created and evaluated for TV coverage and NTT dose. If mean 95% TV coveragedecreased .5%, the plan was considered compromised.

Results: For Cr treatments, TV coverage was not compromised with 5 and 10 mm shifts in any dimension. An inf shift of 20 mmcompromised TV coverage. Lens dose was increased 50% with 10 mm shifts in the ant and inf direction. For Th treatment, only 20mm shifts in the left or right direction (X vector) compromised TV coverage. Ipsilateral lung dose was increased .50% with 10 mmshifts (e.g. Lt lung dose increased with a Lt-sided shift). For LS treatment, only 20 mm shifts in the Lt or Rt direction (X vector) and20 mm inf shifts compromised TV coverage. 10 mm shifts increased the relative dose to the kidney more than 100%, but the resultwas still within renal tolerance doses.

Conclusions: With respect to TV coverage, Pr-based CSI using photon-based treatment margins on bony anatomy perpendicular tothe beam axis yield robust plans which are resistant to SU up to 20 mm. With respect to NTT exposure, the dose to the lens & lungsmay be altered by shifts .10 mm. When considered in the context of random error associated with daily setup, this preliminary datamay serve as a basis for modest reductions in margins employed for Pr-based CSI.

Author Disclosure: D. Pourang, None; D. Indelicato, None; R. Marcus, None; Z. Li, None; D. Yeung, None; C. Morris, None;N. Mendenhall, None; W. Hsi, None; S. Keole, Procure Treatment Systems, F. Consultant/Advisory Board.

3158 GPU Accelerated Monte Carlo Simulation for Radiotherapy Dose Calculation

Y. Zhuge, H. Xie, J. Cheng, R. W. Miller

National Cancer Institute, NIH, Bethesda, MD

Purpose/Objective(s): The Monte Carlo (MC) method has been the most accurate for radiation dose calculation so far. However,the large amounts of computing time necessary for employing MC technique have limited its use in clinical practice. GraphicProcessor Units (GPUs) have recently demonstrated substantial parallel computing power to accelerate scientific computing appli-cations. The purpose of this study is to speed up MC simulation for photon/electron radiation dose calculation by using GPU on anormal desktop PC, and to test the feasibility of replacing the Cluster of Workstations (COW) that is high cost, not widely available,and not easy to use for MC dose calculation.

Materials/Methods: We developed our program that is functionally equivalent to standard EGS4 by using a new data-parallel, Cbased CUDA programming language API for NVIDIA graphic boards. The hardware employed was a NVIDIA GeForce 9800GX2 installed on a normal desktop PC. The NVIDIA 9800 GX2 has 32 Multiprocessors with 8 processors each adding up to256 processors in total, and 1 GB of global memory. We use 256 blocks and each of them has 256 threads so in total one gridhas 65536 threads. Each thread processes one particle history for photon/electron energy reduction and dose deposition, so thereare 65536 particle histories being parallel calculated with one time kernel function call. Kernel function is executed in a serial wayuntil all particle histories are processed.

Results: We tested our program on a simple two-layer water phantom (20 cm x 20cm x 20cm) by simulating 20MeV,15MeV, and 6MeV parallel electron beam, each involving 10e5, 10e6, and 10e7 histories (respectively). The depth dosecurves produced from our program showed a very good agreement with those from the EGSnrc. But for efficiency, our pro-gram significantly improved the performance and outperformed than the EGSnrc on the COW of six personal computers inour lab.

Conclusions: This study shows that MC simulation for dose calculation is very suitable to be implemented by using CUDA in-terface for NVIDIA GPUs on a normal desktop PC. The performance can be significantly speed up by this new technique. Thesimulation of clinical linear accelerator on patient data needs to be further investigated.

Author Disclosure: Y. Zhuge, None; H. Xie, None; J. Cheng, None; R.W. Miller, None.

3159 Introducing New Technology into the Clinic: The First Year of Experience in Proton Therapy with

a Uniform Scanning System

M. M. Fitzek1, A. F. Thornton1, A. Chang1, K. Shahnazi1, M. Sullivan1, M. Wolanski1, C. Allgower1, V. Anferov2,D. Nichiporov2, V. Derenchuk2

1Midwest Proton Radiotherapy Institute, Bloomington, IN, 2Indiana University Cyclotron Facility, Bloomington, IN

Purpose/Objective(s): The introduction of new and unique technology into routine clinical practice needs critical attention to itsfunctionality and constant improvement. We wish to summarize some parameters relevant to clinical practice from our experienceof the first year of clinical operation of a treatment room with a unique proton beam scanning system at the Midwest ProtonRadiotherapy Institute (MPRI) constructed by Indiana University Cyclotron Facility.

Materials/Methods: We reviewed our clinical and technical experience of the first year of use of a uniform scanning protonbeam at MPRI since patient treatment began in March 2007. We examined our records for technical events and patient chartsfor information on diagnoses, delivered fractions, reliability of the scanning system, the gantry, and the robotic patientpositioner.

Results: During the first year of operation, we have treated 49 patients on the proton beam scanning gantry. The distribution ofdiagnoses was as follows: 14 had prostate cancer, 13 had mesenchymal tumors of the skull base or spine (including chordomas,chondrosarcomas, giant cell tumors), 9 had primary central nervous system tumors, 6 had pediatric tumors of various histologies, 7had miscellaneous tumors including difficult to treat metastatic situations. The number of delivered fractions with this system in thefirst year was 1648. The average use increased from 44 hours per week during the first 6 months to 54 hours per week during the