J Chu

Conformal photon-beam therapy with transverse magnetic fields: a Monte Carlo study.

This work studies the idea of using strong transverse magnetic (B) fields with high-energy photonbeams to enhance dose distributions for conformal radiotherapy.EGS4 Monte Carlo code is modified to incorporate charged particle transport in B fields and is used to calculate effects of B fields on dose distributions for a variety of high-energy photonbeams. Two types of hypothetical B fields, curl-free linear fields and dipole fields, are used to demonstrate the idea. The major results from the calculation for the linear B fields are: (1) strong transverse B fields (>1 T ) with high longitudinal gradients (G) (>0.5 T/cm ) can produce dramatic dose enhancement as well as dose reduction in localized regions for high-energy photonbeams; (2) the magnitude of the enhancement (reduction) and the geometric extension and the location of this enhancement (reduction) depend on the strength and gradient of the B field, and photon-beam energy; (3) for a given B field, the dose enhancement generally increases with photon-beam energy; (4) for a 5 T B field with infinite longitudinal gradient (solenoidal field), up to 200% of dose enhancement and 40% of dose reduction were obtained along the central axis of a 15 MV photonbeam; and (5) a 60% of dose enhancement was observed over a 2 cm depth region for the 15 MV beam when B=5 T and G=2.5 T/cm . These results are also observed, qualitatively, in the calculation with the dipole B fields. Calculations for a variety of B fields and beam configurations show that, by employing a well-designed B field in photon-beam radiotherapy, it is possible to achieve a significant dose enhancement within the target, while obtaining a substantial dose reduction over critical structures.

Calculation of monitor units for a linear accelerator with asymmetric jaws.

A simple approach was developed that calculates the output factors and tissue maximum ratio of an asymmetric field at any point within the open field, and specifically both at the central axis (when the jaws do not shadow it) and at the effective center of the open field, using the existing tables for symmetric fields and the multidepth profile information for the largest available field size (either open or with a wedge present). Days method was adapted to calculate the effective values of the usual field-size-dependent parameters. This approach makes these parameters also dependent on the location of the calculation point relative to the field edges in an asymmetric field. This algorithm was tested by comparing its predictions with measurements of asymmetric and half blocked fields, with and without wedges, in a water phantom at different depths and off-axis distances. The agreement between calculated and measured dose rate is within 1%-3% even in highly asymmetric fields for both 6- and 18-MV photons.

Determination of penumbral widths from ion chamber measurements

We have investigated methods of reconstructing beam profiles in the penumbral region using a set of axially symmetric chambers, differing only in the detector radius. In principal, the transfer functions, or kernels, of such chambers should be functions of radius only. Three chambers of radii 0.297, 0.556, and 0.714 cm have been used. The transfer functions of the chambers can be determined by deconvolving the profiles measured with each detector with the PPMC profile. The results indicate that the transfer functions can be parametrized accurately as a Gaussian cutoff at 1.75(r), with (r) the average radius of the chamber. Deconvolution of the measured profiles with the transfer functions yields a profile that agrees with the PPMC profile to +/- 0.5 mm over the 20-80% penumbra.

TU‐EE‐A3‐06: Evaluation of a Volumetric Display for Radiation Therapy Treatment Planning

Purpose: To evaluate an innovative volumetric display for radiation treatment planning applications. Method and Materials: A volumetric, auto‐stereoscopic display (Perspecta Spatial 3D, Actuality‐Systems Inc., Bedford, MA) has been integrated with the Pinnacle3 TPS for treatment planning. The Perspecta 3D display renders a 25 cm diameter volume that is viewable from any side, floating within a translucent dome. In addition to display all the 3D data exported from Pinnacle, the system provides a 3D cursor and beam placement tools. A 125 point, 5 cm spaced grid centered at isocenter was created in Pinnacle and transferred to Perspecta. A Perspecta 3D ruler verified distances between any two points on the 3D grid. Ten teletherapy beams with various gantry/couch combinations were generated on Pinnacle and verified on Perspecta display. Doses at the same grid points were also compared. CTimages from a QUASAR phantom in 3 orientations were used on Perspecta to confirm beam field size, divergence, etc. Results: In general, the Perspecta system accurately depicted all 3D data exported from Pinnacle. When measured by the 3D ruler, distances between any two points using Perspecta agreed with Pinnacle within the measurement error (typically <0.5 mm). Beam angles were verified through Cartesian coordinate system measurements and also upon rotating the phantom. Field sizes,collimator angles, and beam divergence were similarly confirmed. Isodose surfaces and dose values chosen at arbitrary locations in Perspecta agreed with Pinnacle within ± 2% in an absolute sense, which was governed by human error in coinciding the points. Conclusions: These preliminary results indicate that the Perspecta device is capable of displaying consistent data from the Pinnacle radiotherapytreatment planning system, and may become a valuable tool for visualization and quantitative evaluations in radiation oncology. Conflict of Interest Statement: Actuality Systems Inc. provided the 3D display used in this study.

WE-AB-207B-11: Optimizing Tumor Control Probability in Radiation Therapy Treatment - Application to HDR Cervical Cancer

PURPOSE The ultimate goal of radiotherapy treatment planning is to find a treatment that will yield a high tumor-control-probability(TCP) with an acceptable normal-tissue-complication probability(NTCP). Yet most treatment planning today is not based upon optimization of TCPs and NTCPs, but rather upon meeting physical dose and volume constraints defined by the planner. We design treatment plans that optimize TCP directly and contrast them with the clinical dose-based plans. PET image is incorporated to evaluate gain in TCP for dose escalation. METHODS We build a nonlinear mixed integer programming optimization model that maximizes TCP directly while satisfying the dose requirements on the targeted organ and healthy tissues. The solution strategy first fits the TCP function with a piecewise-linear approximation, then solves the problem that maximizes the piecewise linear approximation of TCP, and finally performs a local neighborhood search to improve the TCP value. To gauge the feasibility, characteristics, and potential benefit of PET-image guided dose escalation, initial validation consists of fifteen cervical cancer HDR patient cases. These patients have all received prior 45Gy of external radiation dose. For both escalated strategies, we consider 35Gy PTV-dose, and two variations (37Gy-boost to BTV vs 40Gy-boost) to PET-image-pockets. RESULTS TCP for standard clinical plans range from 59.4% - 63.6%. TCP for dose-based PET-guided escalated-dose-plan ranges from 63.8%-98.6% for all patients; whereas TCP-optimized plans achieves over 91% for all patients. There is marginal difference in TCP among those with 37Gy-boosted vs 40Gy-boosted. There is no increase in rectum and bladder dose among all plans. CONCLUSION Optimizing TCP directly results in highly conformed treatment plans. The TCP-optimized plan is individualized based on the biological PET-image of the patients. The TCP-optimization framework is generalizable and has been applied successfully to other external-beam delivery modalities. A clinical trial is on-going to gauge the clinical significance. Partially supported by the National Science Foundation.

SU‐FF‐J‐81: A Feasibility Study for Real‐Time Tomosynthesis‐Guided Rapid Arc Therapy

Purpose: To investigate the feasibility of real‐time mis‐alignment correction in Rapid Arc treatment and design a corresponding tomosynthesis acquisition protocol. Method and Materials: A CTimage set of an anthropomorphic pelvic phantom was used in the study. Simulated projection images were produced to resemble a simultaneous kV fluoro in Rapid Arc treatment. A modified Feldkamp algorithm was used to reconstruct the tomosynthesisimages. Various combinations of imagingreconstruction parameters including scan angle, angular interval, and slice thickness (mm) were tested: 1) 60°, 6°, 2.4; 2) 60°, 6°, 0.8; 3) 60°, 3°, 2.4; and 4) 30°, 3°, 2.4. A predefined 5 mm displacement in all three orthogonal directions modeled patient motion during treatment. After each successive tomosynthesis acquisition, registrations were performed between current reconstructions and reference images. The phantom position was corrected accordingly by shifting the treatment couch. Residual errors and their root mean square (RMS) values were recorded for evaluation. Results: The residual errors (L‐R, A‐P and S‐I directions in mm) for the 4 schemes after the first tomosynthesis acquisition were (1.2, 2.5, −0.2), (−1.2, 1.1, 0.0), (1.1, 1.9, 0.0) and (−1.2, 3.1, 0.0), and the corresponding RMSs were 1.6, 0.9, 1.3 and 1.9 respectively. The RMSs after a full arc delivery were 0.7, 0.5, 0.5 and 0.7. All schemes tested accurately corrected displacement in the SI direction after first acquisition. Scheme 2 performed better than scheme 1 at the expense of more computation time. By doubling projection numbers in scheme 1, scheme 3 improved correction ability in the L‐R direction. With a smaller 30° scan angle, scheme 4 was acceptable and will be improved after several acquisitions. Conclusions:Tomosynthesis scans can be used for real‐time mis‐alignment correction in Arc therapy after 30° gantry rotation.

SU-FF-I-29: Investigation of the Perspecta Display for 4D Visualization

Purpose: Recent advancements in radiologic imaging(IGRT) have acquired 4D anatomic data permitting characterization of organ motion towards improving radiotherapy delivery. For radiationoncology patients, images illustrating temporal migration or tumor motion as a result of innate biological function can provide significant benefit towards improving target accuracy and minimizing healthy tissue dose. This study examines the utility of the Perspecta Spatial 3D system (Actuality Systems Inc) to display dynamic 3D data in comparison to flat panel 2D displays. Method and Materials: The AqSim (Philips Medical Systems) CT scanner was used to obtain scans of a patient with lungcancer, and entered into the Pinnacle3treatment planning system (Philips Medical Systems). A clearly delineated lungtumor was contoured in each pertinent CT slice. Ten scans (64 slices each) were obtained during the breathing cycle. Data were viewed side‐by‐side on a flat panel display and the Perspecta 3D system for comparison. Results: The Perspecta display permitted simultaneous visualization of ten CT scans at ∼ 1 Hz per dataset which was similar to the natural breathing rate during image acquisition. Optimal static beam orientation for dynamic target coverage and OAR avoidance was more easily accomplished on the Perspecta than on the 2D display. Conclusion: The 3D Perspecta display successfully depicted anatomic motion, clearly indicating tumor and OAR motion. In comparison to the 2D flat panel display, the Perspecta display permitted the radiationoncology team to readily visualize the temporal nature of lungtumor location for consideration during treatment planning. This application could play an important role in defining and displaying 4D patient data, which was previously relegated to predominantly 2D RTP systems. Furthermore, breath‐hold and coached breathing techniques may be quantitatively evaluated using this method. Conflict of Interest Statement: Actuality Systems Inc. provided the 3D display used in this study.

WE-FG-202-01: Early Prediction of Radiotherapy Induced Skin Reactions Using Dynamic Infrared Imaging

PURPOSE To predict radiotherapy induced skin reactions using dynamic infrared imaging. METHODS Thermal images were captured by our homebuilt system consisting of two flash lamps and an infrared (IR) camera. The surface temperature of the skin was first raised by ∼ 6 oC from ∼1 ms flashes. The camera then captured a series of IR images for 10 seconds. For each image, a baseline skin temperature was recorded for 0.5sec before heat impulse. The temporal temperature gradients were calculated between a reference point (immediately after the flash) and at a time point 9sec after that. Thermal effusivity, an intrinsic thermal property of a material, was calculated from the surface temperature decay of skin. We present experimental data in five patients undergoing radiation therapy, of which 2 were Head & Neck, 1 was Sarcoma and 2 were Breast cancer patients. The prescribed doses were 45 - 60 Gy in 25 - 30 fractions. Each patient was imaged before treatment and after every fifth fraction until end of the treatment course. An area on the skin, outside the radiation field, was imaged as control region. During imaging, each patients irradiated skins were scored based on RTOG skin morbidity scoring criteria. RESULTS Temperature gradient, which is the temperature recovery rate, depends on the thermal properties of underlying tissue. It was observed that, the skin temperature and temporal temperature gradient increases with delivered radiation dose and skin RTOG score. The treatment does not change effusivity of superficial skin layer, however there was a significant difference in effusivity between treated and control areas at depth of ∼ 1.5 - 1.8 mm, increases with dose. CONCLUSION The higher temporal temperature gradient and effusivity from irradiated areas suggest that there is more fluid under the irradiated skin, which causes faster temperature recovery. The mentioned effects may be predictors of Moist Desquamation.

SU‐F‐T‐285: Evaluation of a Patient DVH‐Based IMRT QA System

PURPOSE To evaluate the clinical performance of a patient DVH-based QA system for prostate VMAT QA. METHODS Mobius3D(M3D) is a QA software with an independent beam model and dose engine. The MobiusFX(MFX) add-on predicts patient dose using treatment machine log files. We commissioned the Mobius beam model in two steps. First, the stock beam model was customized using machine commissioning data, then verified against the TPS with 12 simple phantom plans and 7 clinical 3D plans. Secondly, the Dosimetric Leaf Gap(DLG) in the Mobius model was fine-tuned for VMAT treatment based on ion chamber measurements for 6 clinical VMAT plans. Upon successful commissioning, we retrospectively performed IMRT QA for 12 VMAT plans with the Mobius system as well as the ArcCHECK-3DVH system. Selected patient DVH values (PTV D95, D50; Bladder D2cc, Dmean; Rectum D2cc) were compared between TPS, M3D, MFX, and 3DVH. RESULTS During the first commissioning step, TPS and M3D calculated target Dmean for 3D plans agree within 0.7%±0.7%, with 3D gamma passing rates of 98%±2%. In the second commissioning step, the Mobius DLG was adjusted by 1.2mm from the stock value, reducing the average difference between MFX calculation and ion chamber measurement from 3.2% to 0.1%. In retrospective prostate VMAT QA, 5 of 60 MFX calculated DVH values have a deviation greater than 5% compared to TPS. One large deviation at high dose level was identified as a potential QA failure. This echoes the 3DVH QA result, which identified 2 instances of large DVH deviation on the same structure. For all DVHs evaluated, M3D and MFX show high level of agreement (0.1%±0.2%), indicating that the observed deviation is likely from beam modelling differences rather than delivery errors. CONCLUSION Mobius system provides a viable solution for DVH based VMAT QA, with the capability of separating TPS and delivery errors.

TH‐CD‐207A‐08: Simulated Real‐Time Image Guidance for Lung SBRT Patients Using Scatter Imaging

PURPOSE To develop a comprehensive Monte Carlo-based model for the acquisition of scatter images of patient anatomy in real-time, during lung SBRT treatment. METHODS During SBRT treatment, images of patient anatomy can be acquired from scattered radiation. To rigorously examine the utility of scatter images for image guidance, a model is developed using MCNP code to simulate scatter images of phantoms and lung cancer patients. The model is validated by comparing experimental and simulated images of phantoms of different complexity. The differentiation between tissue types is investigated by imaging objects of known compositions (water, lung, and bone equivalent). A lung tumor phantom, simulating materials and geometry encountered during lung SBRT treatments, is used to investigate image noise properties for various quantities of delivered radiation (monitor units(MU)). Patient scatter images are simulated using the validated simulation model. 4DCT patient data is converted to an MCNP input geometry accounting for different tissue composition and densities. Lung tumor phantom images acquired with decreasing imaging time (decreasing MU) are used to model the expected noise amplitude in patient scatter images, producing realistic simulated patient scatter images with varying temporal resolution. RESULTS Image intensity in simulated and experimental scatter images of tissue equivalent objects (water, lung, bone) match within the uncertainty (∼3%). Lung tumor phantom images agree as well. Specifically, tumor-to-lung contrast matches within the uncertainty. The addition of random noise approximating quantum noise in experimental images to simulated patient images shows that scatter images of lung tumors can provide images in as fast as 0.5 seconds with CNR∼2.7. CONCLUSIONS A scatter imaging simulation model is developed and validated using experimental phantom scatter images. Following validation, lung cancer patient scatter images are simulated. These simulated patient images demonstrate the clinical utility of scatter imaging for real-time tumor tracking during lung SBRT.