SU-E-T-579: Dosimetric Benefits of Online Adaptive Replanning for Post-Operative Radiation Therapy of Prostate Bed.

PURPOSE: To demonstrate the dosimetric benefits of using an online adaptive replanning scheme to address interfractional variations in radiotherapy of prostate bed. METHODS: We have previously developed an online adaptive replanning tool (RealART, Prowess Inc.) aiming to address interfractional variations including organ deformation and rotation. Using this tool, we analyzed a total of 102 daily pre-treatment CTs acquired using an in-room CT (CTVision, Siemens) for 10 patients treated with post-operative IMRT of prostate bed. The PTV was assumed to be the volume enclosed by the 95% prescription isodose surface generated for the conventional four field box. On each daily CT set, contours of the PTV, rectum and bladder were generated by populating the planning contours using an auto-segmentation tool based on deformable registration (ABAS, Elekta) with manual editing. Four plans were generated and compared: (1) IGRT (repositioning) plan by copying the original plan with aligning the anterior rectal wall from the daily CT to that of the planning CT, (2) IGRT plan by copying the original plan with aligning the surgical clips, (3) online adaptive plan by tailoring the original plan to conform to the anatomy of the day, and (4) a new plan re- optimized based on the daily anatomy. RESULTS: The adaptive and re- optimization plans are in general superior than the two repositioning plans in terms of both target coverage and critical structure sparing. For example, the averages of dose volume quantities for all daily CTs are: rectum V45Gy 55.7±18.0% (one standard deviation), 57.3±17.5%, 48.2±11.8%, 42.5±9.6%; rectum V60Gy 31.8±20.3%, 34.0±16.6%, 22.6±9.7%, 16.5±7.4%; bladder V45Gy 30.0±11.9%, 39.5±24.2%, 37.6±16.8%, 36.5±16.2%; bladder V60Gy 17.4±9.2%, 25.4±18.1%, 24.7±12.7%, 23.9±12.0%; PTV V100 81.9±16.6%, 88.7±7.9%, 92.9±4.6%, and 94.6±2.4% for the above (1)-(4) plans, respectively. CONCLUSIONS: The online adaptive replanning scheme is effective to account for interfractional variations in post-operative radiotherapy of prostate bed. This work is supported partially by MCW Cancer Center Fotsch Foundation.

SU-E-J-11: Characterization of Interfractioanal Anatomical Variations in Post-Operative Radiation Therapy for Prostate Bed.

PURPOSE: To quantitatively characterize the interfractional anatomic variations in post-operative radiation therapy (RT) for prostate bed, so that appropriate strategy that can fully address these variations can be developed. METHODS: A total of 102 daily pre-treatment CT acquired using an in-room CT (CTVision, Siemens) for 10 patients treated with post-operative IG-IMRT of prostate bed. Prior to each fraction, patients were repositioned to correct for interfractional translational shifts based on the alignment of both anterior rectal wall and surgical clips between the daily CT and the planning CT. The PTV was assumed to be the volume enclosed by the 95% prescription isodose surface generated for the conventional four field box. Contours of the PTV, rectum and bladder on each daily CT were generated by populating the planning contours using an auto-segmentation tool based on deformable registration (ABAS, Elekta) with manual editing. Interfrcational variations in the volumes, shapes and positions of these contours were obtained. The displacement of the center of mass (DCOM) with respect to the isocenter was used to measure interfractional organ motion, and the maximum overlap rate (MOR) was used to measure organ deformation. RESULTS: Interfractional variations in the volumes of rectum and bladder were in the range of 50-270% (average 116±41%) and 30-180% (average 67±26%), respectively. The averages of DCOM for rectum are: - 0.35±0.46cm (lateral, varying from -1.58 to 1.23cm), -0.33±0.99cm (longitudinal, varying from -2.4 to 1.8cm), and -3.41±1.14cm (vertical, varying from -5.56 to -0.99cm). These values for bladder are: -0.20±0.50cm (lateral, varying from -1.48 to 0.61cm), 3.63±1.10cm (longitudinal, varying from -5.7 to 3.1cm), and -0.31±0.97cm (vertical, varying from -1.68 to 2.77cm). CONCLUSIONS: Large interfractional changes in organ volumes, shapes and positions are seen in post-operative RT for prostate bed. These changes cannot be accounted for by the current standard practice of IGRT repositioning. This work is supported partially by MCW Cancer Center Fotsch Foundation.

TU-E-BRA-01: A Comparison of Various Online Strategies to Account for Interfractional Variations for Pancreatic Cancer.

PURPOSE: To indentify effective methods to address the large interfractional variations for pancreas irradiation, we compared various used/proposed online strategies. METHODS: The daily CTs acquired using a respiration-gated in-room CT for 9 pancreatic cancer patients treated with IGRT (i.e., online repositioning based on rigid-body alignment) were analyzed. The contours of the pancreas and duodenum on each daily CT set were generated by populating those from the planning CT using a deformable registration tool (ABAS, Elekta) with manual editing. PTV was generated with 3 mm margin. Nine online strategies were considered: 1) IGRT with 0 mm additional margin (AM), 2) IGRT with 2mm AM, 3) IGRT with 5mm AM, 4) IGRT with plan renormalized to maintain 95% PTV coverage, 5) Full scale reoptimization, 6) Reoptimization starting from the original plan, 7) Segment Aperture Morphing (SAM) from the original plan based on PTV shape change 8) SAM plus Segment Weight Optimization (SWO), 9) Reoptimization starting from the SAM plan. One way ANOVA (analysis of variance) was applied to plan qualities for the 9 strategies to assess statistical significance in difference. RESULTS: The standard IGRT strategies (1-3) resulted in either inadequate PTV coverage or higher duodenum doses. Margin expansion along is not efficient to account for the changes. Full-scale reoptimization resulted in the best plan but requiring delineation of several structures. Reoptimization on top of available plan (strategies 6 and 9) was considerably faster. SAM strategy (7) is the fastest online replanning, as it requires only one structure (target) delineation, and it's plan quality was comparable to that for the full-scale reoptimization. CONCLUSION: Online replanning strategies can lead to either reduced duodenum dose or improved target coverage as compared to the current practice of IGRT. The SAM-based online replanning is comparable to the full scale reoptimization and is efficient for practical use.

Treatment planning dosimetric parameters for a (90)Y coil source used in intravascular brachytherapy.

BACKGROUND: (90)Y coil sources have been used in animal and clinical trials for treatment of restenosis in intravascular brachytherapy (IVBT). This study aims to determine the American Association of Physicists in Medicine (AAPM) Task Group-60 (TG-60) dosimetric quantities in regions surrounding the balloon wall for use in treatment planning computer systems. METHODS: The Monte Carlo method was used to determine the dose distribution, using MCNP4B2 code. The coil source was modeled by a hollow cylinder of 2.9 cm length centered in a balloon (2.5 mm diameter) filled with carbon dioxide (CO(2)) at 5 atm. Scoring voxels consisted of contiguous annular disk shells with 0.1 mm spacing in the radial direction and 0.2 mm spacing in the longitudinal direction. The scoring region ranges from the center of the source to 1.0 cm in the longitudinal direction, and from 0.13 to 1 cm in the radial direction. In the plane containing the source axis, the Monte Carlo-generated doses in rectilinear coordinates are converted to polar coordinates. RESULTS: The dose rate of the source is provided in both Cartesian and polar coordinates. The dose rate constant [D(r(0),theta(0))], anisotropy function [F(r,theta)], and radial dose function [g(r)] were generated from these values and listed in tabular format. At shallow angles and longer distances from the source center, large values of the anisotropy function resulted, deviating two orders of magnitude from unity. CONCLUSIONS The doses given to the intima, media, and adventitia are very crucial quantities in IVBT. The calculated TG-60 dosimetric quantities, used commonly in conventional brachytherapy applications, provide a means for the user to determine the three-dimensional dose surrounding the balloon catheter. These parameters can be used in future treatment planning system for IVBT. We also discuss the need to develop a new formalism specific to longer sources used in IVBT.

Rapid method for deblurring spiral MR images.

A method for fast off-resonance frequency deblurring of spiral MR images is presented. The method utilizes image-space deconvolution. The off-resonance phase is approximated as a separable quadratic function to allow rapid one-dimensional deconvolution with a small compromise in accuracy. The method is used to deblur an MR angiographic image to illustrate its effectiveness.

Effects of interleaf order for spiral MRI of dynamic processes.

The effects of the temporal order in which spiral interleaves are collected are discussed, in the context of artifacts from moving or changing objects. Simulations and in vivo experiments demonstrate the properties of four different ordering methods. Specific applications discussed include cardiac and interventional magnetic resonance imaging, as well as inflow and contrast-enhanced magnetic resonance angiography.