Prema Rassiah-Szegedi

Evaluation of alignment error due to a speed artifact in stereotactic ultrasound image guidance

Ultrasound (US) image guidance systems used in radiotherapy are typically calibrated for soft tissue applications, thus introducing errors in depth-from-transducer representation when used in media with a different speed of sound propagation (e.g. fat). This error is commonly referred to as the speed artifact. In this study we utilized a standard US phantom to demonstrate the existence of the speed artifact when using a commercial US image guidance system to image through layers of simulated body fat, and we compared the results with calculated/predicted values. A general purpose US phantom (speed of sound (SOS) = 1540 m s(-1)) was imaged on a multi-slice CT scanner at a 0.625 mm slice thickness and 0.5 mm x 0.5 mm axial pixel size. Target-simulating wires inside the phantom were contoured and later transferred to the US guidance system. Layers of various thickness (1-8 cm) of commercially manufactured fat-simulating material (SOS = 1435 m s(-1)) were placed on top of the phantom to study the depth-related alignment error. In order to demonstrate that the speed artifact is not caused by adding additional layers on top of the phantom, we repeated these measurements in an identical setup using commercially manufactured tissue-simulating material (SOS = 1540 m s(-1)) for the top layers. For the fat-simulating material used in this study, we observed the magnitude of the depth-related alignment errors resulting from the speed artifact to be 0.7 mm cm(-1) of fat imaged through. The measured alignment errors caused by the speed artifact agreed with the calculated values within one standard deviation for all of the different thicknesses of fat-simulating material studied here. We demonstrated the depth-related alignment error due to the speed artifact when using US image guidance for radiation treatment alignment and note that the presence of fat causes the target to be aliased to a depth greater than it actually is. For typical US guidance systems in use today, this will lead to delivery of the high dose region at a position slightly posterior to the intended region for a supine patient. When possible, care should be taken to avoid imaging through a thick layer of fat for larger patients in US alignments or, if unavoidable, the spatial inaccuracies introduced by the artifact should be considered by the physician during the formulation of the treatment plan.

A proto-type design of a real-tissue phantom for the validation of deformation algorithms and 4D dose calculations

The purpose of this study is to design a real-tissue phantom for use in the validation of deformation algorithms. A phantom motion controller that runs sinusoidal and non-regular patient-based breathing pattern, via a piston, was applied to porcine liver tissue. It was regulated to simulate movement ranges similar to recorded implanted liver markers from patients. 4D CT was applied to analyze deformation. The suitability of various markers in the liver and the position reproducibility of markers and of reference points were studied. The similarity of marker motion pattern in the liver phantom and in real patients was evaluated. The viability of the phantom over time and its use with electro-magnetic tracking devices were also assessed. High contrast markers, such as carbon markers, implanted in the porcine liver produced less image artifacts on CT and were well visualized compared to metallic ones. The repositionability of markers was within a measurement accuracy of +/-2 mm. Similar anatomical patient motions were reproducible up to elongations of 3 cm for a time period of at least 90 min. The phantom is compatible with electro-magnetic tracking devices and 4D CT. The phantom motion is reproducible and simulates realistic patient motion and deformation. The ability to carry out voxel-based tracking allows for the evaluation of deformation algorithms in a controlled environment with recorded patient traces. The phantom is compatible with all therapy devices clinically encountered in our department.

Individualized margins for prostate patients using a wireless localization and tracking system.

This study investigates the dosimetric benefits of designing patient‐specific margins for prostate cancer patients based on 4D localization and tracking. Ten prostate patients, each implanted with three radiofrequency transponders, were localized and tracked for 40 fractions. “Conventional margin” (CM) planning target volumes (PTV) and PTVs resulting from uniform margins of 5 mm (5M) and 7 mm (7M) were explored. Through retrospective review of each patients tracking data, an individualized margin (IM) design for each patient was determined. IMRT treatment plans with identical constraints were generated for all four margin strategies and compared. The IM plans generally created the smallest PTV volumes. For similar PTV coverage, the IM plans had a lower mean bladder (rectal) dose by an average of 3.9% (2.5%), 8.5% (5.7%) and 16.2 % (9.8%) compared to 5M, 7M and CM plans, respectively. The IM plan had the lowest gEUD value of 23.8 Gy for bladder, compared to 35.1, 28.4 and 25.7, for CM, 7M and 5M, respectively. Likewise, the IM plan had the lowest NTCP value for rectum of 0.04, compared to 0.07, 0.06 and 0.05 for CM, 7M and 5M, respectively. Individualized margins can lead to significantly reduced PTV volumes and critical structure doses, while still ensuring a minimum delivered CTV dose equal to 95% of the prescribed dose. PACS numbers: 87.53.Kn, 87.55.D

Optimization of Isocenter Location for Intensity Modulated Stereotactic Treatment of Small Intracranial Targets

PURPOSE To quantify the impact of isocenter location on treatment plan quality for intensity-modulated stereotactic treatment of small intracranial lesions. METHODS AND MATERIALS For 18 patients previously treated by stereotactic-intensity modulated radiosurgery (IMRS) or intensity-modulated radiation therapy (IMRT), a retrospective virtual planning study was conducted wherein the impact of isocenter location on plan quality was measured. Treatment indications studied included six arteriovenous malformations, six acoustic neuromas, and six intracranial metastases, ranging in volume from 0.71 to 3.21 cm(3) (mean = 2.26 cm(3)), 1.08 to 2.84 cm(3) (mean = 1.73 cm(3)), and 0.19 to 2.30 cm(3) (mean = 0.79 cm(3)), respectively. Variation of isocenter location causes the geometric grid of pencil beams into which the target is segmented for intensity-modulated treatment to be altered. The impact of this pencil-beam-grid redefinition on achievable conformity index was quantified for three collimators (Varian Millennium 120; BrainLab MM3; Nomos binary Mimic) and three treatment planning systems (TPS; Varian Eclipse v6.5; BrainLab BrainScan v5.31; Best-Nomos Corvus v6.2), resulting in the evaluation of 3,446 treatment plans. RESULTS For all patients, collimator, and TPS combinations studied, a significant variation in plan quality was observed as a function of isocenter and pencil-beam-grid relocation. Optimization of isocenter location resulted in treatment plan conformity variations as large as 109% (min = 15%, mean = 51%, max = 109%). CONCLUSION Optimization of isocenter location for IMRT/IMRS treatment of small intracranial lesions in which pencil-beam dimensions are comparable to target dimensions, can result in significant improvements in treatment plan quality.

Planning for mARC treatments with the Eclipse treatment planning system

While modulated arc (mARC) capabilities have been available on Siemens linear accelerators for almost two years now, there was, until recently, only one treatment planning system capable of planning these treatments. The Eclipse treatment planning system now offers a module that can plan for mARC treatments. The purpose of this work was to test the module to determine whether it is capable of creating clinically acceptable plans. A total of 23 plans were created for various clinical sites and all plans delivered without anomaly. The average 3%/3 mm gamma pass rate for the plans was 98.0%, with a standard deviation of 1.7%. For a total of 14 plans, an equivalent static gantry IMRT plan was also created to compare delivery time. In all but two cases, the mARC plans delivered significantly faster than the static gantry plan. We have confirmed the successful creation of mARC plans that are deliverable with high fidelity on an ARTISTE linear accelerator, thus demonstrating the successful implementation of the Eclipse mARC module. PACS numbers: 87.55.D‐, 87.55.ne, 87.57.uq,

SU-FF-J-128: Characterization of Liver Motion Based On Implanted Markers

Purpose: The purpose of this work is to evaluate the use of implanted markers to characterize motion and deformation of livertumors and tissue.Method and Materials: We retrospectively analyzed 4DCT data sets of livercancer patients treated with stereotactic body radiation therapy at our center. All of these patients had either gold‐ or carbon‐markers (Civco medical solutions, IA) implanted in the surrounding tissue of the lesion. Using a Varian RPM system, ten phase‐CT‐image sets were acquired for treatment planning and prior to every treatment. Two references points, one in bony anatomy and one external were used for analysis. The movements of markers with reference to each other and the reference‐locations for each phase as well as the shift of the geometric center of the implanted markers were studied. The intra session and inter session motion range and geometric deformation was investigated. Results: The average motion‐range of the three markers for these patients differed in location, direction and magnitude for each day as well as from day to day. All motion vectors of these markers show a predominance in SI motion, but deviated for some degrees from each other. The spread of motion vectors additionally indicates a shift of the centroid of the virtual marker triangle for each phase, showing the deformation of tissue and the change in position in LR and AP direction. The greatest difference in motion‐range between the treatment days for one marker was 10.4 mm for one marker, which was located centrally the liver.Conclusion: Motion of liver and livertumors can be visualized with implanted markers and is similar to previous publications, comparing organ outlines/edges for motion deduction. Markers additionally provide information on different rates of motion at point locations and deformation within the region of interest in the organ, helping deformation algorithm validation.

Dosimetric impact of the 160 MLC on head and neck IMRT treatments.

The purpose of this work is to investigate if the change in plan quality with the finer leaf resolution and lower leakage of the 160 MLC would be dosimetrically significant for head and neck intensity‐modulated radiation therapy (IMRT) treatment plans. The 160 MLC consisting of 80 leaves of 0.5 cm on each bank, a leaf span of 20 cm, and leakage of less than 0.37% without additional backup jaws was compared against the 120 Millennium MLC with 60 leaves of 0.5 and 1.0 cm, a leaf span of 14.5 cm, and leakage of 2.0%. CT image sets of 16 patients previously treated for stage III and IV head and neck carcinomas were replanned on Prowess 5.0 and Eclipse 11.0 using the 160 MLC and the 120 MLC. IMRT constraints for both sets of 6 MV plans were identical and based on RTOG 0522. Dose‐volume histograms (DVHs), minimum dose, mean dose, maximum dose, and dose to 1 cc to the organ at risks (OAR) and the planning target volume, as recommended by QUANTEC 2010, were compared. Both collimators were able to achieve the target dose to the PTVs. The dose to the organs at risk (brainstem, spinal cord, parotids, and larynx) were 1%–12% (i.e., 0.5–8 Gy for a 70 Gy prescription) lower with the 160 MLC compared to the 120 MLC, depending on the proximity of the organ to the target. The large field HN plans generated with the 160 MLC were dosimetrically advantageous for critical structures, especially those located further away from the central axis, without compromising the target volume. PACS number: 87.55 D‐

SU‐E‐U‐08: Presentation of a New Intrafractional Prostate Monitoring Method with Ultrasound Image Guidance During Radiotherapy Treatment

Purpose: Clinical presentation of a new intrafractional ultrasound image guidance (USIG) prostate monitoring/tracking method. Methods: Clarity ultrasound system has recently released a new feature to monitor/track intrafractional prostate motion using a trans‐perineal image acquisition position. After initial localization and image guidance correction, the ultrasound probe remains fixed in the sagittal‐plane, trans‐perineal imaging position via a couch‐mounted arm‐support system. The software then enters a live monitoring mode, where the scanning ultrasound probe continuously acquires fanned sagittal images for tracking of during‐treatment prostate position. Compared to other tracking technologies (e.g. RF implanted beacon tracking, or real‐time fluoro monitoring), the ultrasound system has the unique advantages of live 3D image display, without invasive procedure or imaging radiation dose. We present our initial experience using this monitoring feature for 8 patients and 218 treatment sessions. Results: The monitoring software functioned as expected during treatment with consistent reporting of prostate deviation from isocenter location, and notification of instances where positional error exceeded our tolerance of 3 mm for 5 second. Comparisons of prostate tracking data streams obtained from Clarity USIG with data streams obtained in our clinic from Calypso RF tracking of a similar patient population show similarity of recorded motion information for both methods. Average during‐treatment prostate motion for both tracking methods was on the order of 1–2 mm in 3 cardinal directions, with roughly 5% of instances where motion exceeded 3 mm, 5 second tolerance level. When Monitoring reported that tolerance was exceeded we confirmed this by performing redundant, static imaging Alignment, with confirmation that target position had, indeed, changed as reported. Conclusion: The newly released trans‐perineal ultrasound Monitoring approach was confirmed to function well clinically, and to report accurate intrafractional monitoring data. Acquired motion streams were consistent with our Calypso monitoring experience obtained on a similar patient population in our own clinic.

Initial experience and clinical comparison of two image guidance methods for SBRT treatment: 4DCT versus respiratory-triggered imaging

For Stereotactic Body Radiation Therapy (SBRT) treatment of lung and liver, we quantified the differences between two image guidance methods: 4DCT and ExacTrac respiratory‐triggered imaging. Five different patients with five liver lesions and one lung lesion for a total of 19 SBRT delivered fractions were studied. For the 4DCT method, a manual registration process was used between the 4DCT image sets from initial simulation and treatment day to determine the required daily image‐guided corrections. We also used the ExacTrac respiratory‐triggered imaging capability to verify the target positioning, and calculated the differences in image guidance shifts between these two methods. The mean (standard deviation) of the observed differences in image‐guided shifts between 4DCT and ExacTrac respiratory‐triggered image guidance was left/right (L/R)=0.4(2.0)mm, anterior/posterior (A/P)=1.4(1.7) mm, superior/inferior (S/I)=2.2(2.0) mm, with no difference larger than 5.0 mm in any given direction for any individual case. The largest error occurred in the S/I direction, with a mean of 2.2 mm for the six lesions. This seems reasonable, because respiratory motion and the resulting imaging uncertainties are most pronounced in this S/I direction. Image guidance shifts derived from ExacTrac triggered imaging at two extreme breathing phases (i.e., full exhale vs. full inhale), agreed well (less than 2.0 mm) with each other. In summary, two very promising image guidance methods of 4DCT and ExacTrac respiratory‐triggered imaging were presented and the image guidance shifts were comparable for the patients evaluated in this study. PACS number: 87.55.ne

Assessing Four-dimensional Radiotherapy Planning and Respiratory Motion-induced Dose Difference Based on Biologically Effective Uniform Dose

Four-dimensional (4D) radiotherapy is considered as a feasible and ideal solution to accommodate intra-fractional respiratory motion during conformal radiation therapy. With explicit inclusion of the temporal changes in anatomy during the imaging, planning, and delivery of radiotherapy, 4D treatment planning in principle provides better dose conformity. However, the clinical benefits of developing 4D treatment plans in terms of tumor control rate and normal tissue complication probability as compared to other treatment plans based on CT images of a fixed respiratory phase remains mostly unproven. The aim of our study is to comprehensively evaluate 4D treatment planning for nine lung tumor cases with both physical and biological measures using biologically effective uniform dose (D̿) together with complication-free tumor control probability, P+. Based on the examined lung cancer patients and PTV margin applied, we found similar but not identical curves of DVH, and slightly different mean doses in tumor (up to 1.5%) and normal tissue in all cases when comparing 4D, P0%, and P50% plans. When it comes to biological evaluations, we did not observe definitively PTV size dependence in P+ among these nine lung cancer patients with various sizes of PTV. Moreover, it is not necessary that 4D plans would have better target coverage or higher P+ as compared to a fixed phase IMRT plan. However, on the contrary to significant deviations in P+ (up to 14.7%) observed if delivering the IMRT plan made at end-inhalation incorrectly at end-exhalation phase, we estimated the overall P+, PB, and PI for 4D composite plans that have accounted for intra-fractional respiratory motion.