Research Treatment Planning System Research Articles

SU‐E‐T‐334: Clinical Implementation of Gating and Dose Verification with Scanned Ion Beams at HIT

Purpose: Medical physics aspects of gated irradiation to moving tumors using scanned ion beams. Methods: At our institute, more than 700 patients have been treated with proton and carbon ion beams since its start. The first patient has now been treated with the application of gating. For this purpose, the Anzai respiratory motion detection and gating system was interfaced with the treatment control system such as to control spill pausing. The patient presented with a liver tumor moving with peak‐to‐peak amplitude of ∼1cm due to breathing. Treatment planning was performed using the Siemens Syngo RT‐Planning system on a 3DCT dataset. The gating window was determined from 4DCT. Plan verification was performed in a motion phantom, consisting of a PMMA block equipped with a 3D set of 24 pinpoint chambers mounted on the Quasar moving platform. The breathing motion perpendicular to the beam was simulated based on the trajectory obtained from the Anzai system during 4DCT acquisition. Dose was measured at three conditions: static phantom, moving phantom with continuous beam delivery, and moving phantom with gated irradiation. The statically delivered dose distribution was used as a reference. Further, a 4D dose calculation was performed on the 4DCT dataset using a research treatment planning system. Results: The relative rms deviations in the homogenous dose region were ∼18% without gating, and ∼6% with gating. The 4D dose calculation showed a decrease in target coverage by ∼20% and ∼6%, respectively, for the 95% isodose line. Conclusions: Gating of a scanned carbon ion beam was applied for treatment of a moving liver tumor and a workflow was developed to verify the dose to be actually delivered. With respect to continuous dose delivery, the target dose improvement achieved due to gating is considered to be significant for this patient.

TH‐C‐BRC‐09: Tracking Moving Tumors with a Scanned Carbon Beam: Robustness to Changing Target Motion Characteristics and Tracking Uncertainties

Purpose: Beam tracking of moving tumors with scanned carbon ion radiotherapy achieves uniform target dose by adapting carbon pencil beams to follow moving tissues using real‐time lateral magnetic deflection and range modulation. The purpose of the current study was to establish the robustness of target dose coverage that can be achieved in the presence of changing target motion characteristics and uncertainties in the motion tracking system. Methods: A 4D research treatment planning system code for ion therapy was extended to compute the resulting target dose coverage when various random and systematic uncertainties were introduced to ideal tracking dose simulations. Treatment plans were prepared using the in‐ house code for a hypothetical sphere target volume with sinusoidal motion in a water phantom. Uncertainties were simulated by shifting pencil beam coordinates and modifying target motion signals. To simulate imperfect tracking, we considered variations to the following: (1) respiratory period, (2) starting respiratory phase, (3) target position drift, (4) random tracking offsets, (5) range tracking wedge acceleration, and (6) external respiratory motion signal phase delay. We also considered imperfect tracking with combinations of varying target motion characteristics and tracking uncertainties. A total of 13713 dose distributions were computed. For all cases, mean, standard deviation, minimum, and maximum of target dose were analyzed. Results: For perfect tracking of the moving sphere, mean and standard deviation of target dose were 98.2 and 0.8 % of the prescription dose, respectively. Of the variations considered, systematic drifts of target position of up to 5 mm had the most significant effect on target coverage ((6.6 ± 20.3) % decrease), and range tracking wedge accelerations above 1.0 g had the least effect on target dose ((0.0 ±1.1) %). Conclusions: The results provide insight into beam tracking precision requirements and the need to monitor and mitigate changes in patient motion.Work supported by research grant from the Deutscher Akademischer Austausch Dienst, Bonn, Germany.

Advanced treatment planning methods for efficient radiation therapy with laser accelerated proton and ion beams

Laser plasma acceleration can potentially replace large and expensive cyclotrons or synchrotrons for radiotherapy with protons and ions. On the way toward a clinical implementation, various challenges such as the maximum obtainable energy still remain to be solved. In any case, laser accelerated particles exhibit differences compared to particles from conventional accelerators. They typically have a wide energy spread and the beam is extremely pulsed (i.e., quantized) due to the pulsed nature of the employed lasers. The energy spread leads to depth dose curves that do not show a pristine Bragg peak but a wide high dose area, making precise radiotherapy impossible without an additional energy selection system. Problems with the beam quantization include the limited repetition rate and the number of accelerated particles per laser shot. This number might be too low, which requires a high repetition rate, or it might be too high, which requires an additional fluence selection system to reduce the number of particles. Trying to use laser accelerated particles in a conventional way such as spot scanning leads to long treatment times and a high amount of secondary radiation produced when blocking unwanted particles. The authors present methods of beam delivery and treatment planning that are specifically adapted to laser accelerated particles. In general, it is not necessary to fully utilize the energy selection system to create monoenergetic beams for the whole treatment plan. Instead, within wide parts of the target volume, beams with broader energy spectra can be used to simultaneously cover multiple axially adjacent spots of a conventional dose delivery grid as applied in intensity modulated particle therapy. If one laser shot produces too many particles, they can be distributed over a wider area with the help of a scattering foil and a multileaf collimator to cover multiple lateral spot positions at the same time. These methods are called axial and lateral clustering and reduce the number of particles that have to be blocked in the beam delivery system. Furthermore, the optimization routine can be adjusted to reduce the number of dose spots and laser shots. The authors implemented these methods into a research treatment planning system for laser accelerated particles. The authors' proposed methods can decrease the amount of secondary radiation produced when blocking particles with wrong energies or when reducing the total number of particles from one laser shot. Additionally, caused by the efficient use of the beam, the treatment time is reduced considerably. Both improvements can be achieved without extensively changing the quality of the treatment plan since conventional intensity modulated particle therapy usually includes a certain amount of unused degrees of freedom which can be used to adapt to laser specific properties. The advanced beam delivery and treatment planning methods reduce the need to have a perfect laser-based accelerator reproducing the properties of conventional accelerators that might not be possible without increasing treatment time and secondary radiation to the patient. The authors show how some of the differences to conventional beams can be overcome and efficiently used for radiation treatment.

SU‐GG‐J‐108: Validation Study of a Software Tool for Consensus Analysis of Experts' Contours for Generating Atlases of Radiotherapy Target and Normal Structures

Purpose: To analyze and validate the ability of a newly developed software tool to estimate and analyze consensus contours from manually delineated structures by panel of expert radiation oncologists for building reference atlases for radiotherapy and dosimetry practitioners. Method and Materials: Several statistical methods and a graphical user interface were developed as an integral part of an in‐house research treatment planning system. For evaluation purposes, we used three breast cancer CT scans from the RTOG Breast Cancer Atlas Project labeled A, B, and C. All participating physicians contoured specific structures before and after the consensus panel met without the software aid. Differences in the contours were analyzed qualitatively and quantitatively using metrics based on apparent agreement, kappa statistics, and STAPLE estimates by the developed consensus software tool. Estimates of the consensus contours were analyzed for the different structures before consensus and compared with the experts' manual consensus contour. Dice similarity coefficient (DSC) was used for comparative evaluation between the software and the experts' results. Results: Dice similarity calculations for all pre‐consensus STAPLE estimates and the final consensus panel structures reached 0.80 or greater for the heart, left breast, right breast, case A lumpectomy, case B lumpectomy, and the chest wall. The heart, case A lumpectomy, left breast, and right breast all exhibited a DSC greater than 0.90. The highest Dice coefficients were seen in the comparison of the STAPLE estimated left breast contours, which almost reached 0.96 at 100% confidence level. DSCs were in the range of .70 to .79 for the supraclavicular and internal mammary nodes. Conclusion: Using the consensus software tool incorporating several statistical methods allowed excellent ability to create contours similar to the ones generated by expert physicians and provided metrics for the confidence levels of agreement.

Feasibility of small animal cranial irradiation with the microRT system

To develop and validate methods for small-animal CNS radiotherapy using the microRT system. A custom head immobilizer was designed and built to integrate with a pre-existing microRT animal couch. The Delrin couch-immobilizer assembly, compatible with multiple imaging modalities (CT, microCT, microMR, microPET, microSPECT, optical), was first imaged via CT in order to verify the safety and reproducibility of the immobilization method. Once verified, the subject animals were CT-scanned while positioned within the couch-immobilizer assembly for treatment planning purposes. The resultant images were then imported into CERR, an in-house-developed research treatment planning system, and registered to the microRTP treatment planning space using rigid registration. The targeted brain was then contoured and conformal radiotherapy plans were constructed for two separate studies: (1) a whole-brain irradiation comprised of two lateral beams at the 90 degree and 270 degree microRT treatment positions and (2) a hemispheric (left-brain) irradiation comprised of a single A-P vertex beam at the 0 degree microRT treatment position. During treatment, subject animals (n=48) were positioned to the CERR-generated treatment coordinates using the three-axis microRT motor positioning system and were irradiated using a clinical Ir-192 high-dose-rate remote after-loading system. The radiation treatment course consisted of 5 Gy fractions, 3 days per week. 90% of the subjects received a total dose of 30 Gy and 10% received a dose of 60 Gy. Image analysis verified the safety and reproducibility of the immobilizer. CT scans generated from repeated reloading and repositioning of the same subject animal in the couch-immobilizer assembly were fused to a baseline CT. The resultant analysis revealed a 0.09 mm average, center-of-mass translocation and negligible volumetric error in the contoured, murine brain. The experimental use of the head immobilizer added 0.1 mm to microRT spatial uncertainty along each axis. Overall, the total spatial uncertainty for the prescribed treatments was +/-0.3 mm in all three axes, a 0.2 mm functional improvement over the original version of microRT. Subject tolerance was good, with minimal observed side effects and a low procedure-induced mortality rate. Throughput was high, with average treatment times of 7.72 and 3.13 min/animal for the whole-brain and hemispheric plans, respectively (dependent on source strength). The method described exhibits conformality more in line with the size differential between human and animal patients than provided by previous prevalent approaches. Using pretreatment imaging and microRT-specific treatment planning, our method can deliver an accurate, conformal dose distribution to the targeted murine brain (or a subregion of the brain) while minimizing excess dose to the surrounding tissue. Thus, preclinical animal studies assessing the radiotherapeutic response of both normal and malignant CNS tissue to complex dose distributions, which closer resemble human-type radiotherapy, are better enabled. The procedural and mechanistic framework for this method logically provides for future adaptation into other murine target organs or regions.

SU‐FF‐T‐20: A Method to Achieve Uniform Dose Distributions in Small Animal Subcutaneous Tumors

Purpose: In the current radiotherapy paradigm, a major treatment planning goal is target dose uniformity. However, a boost dose to as large a fraction as possible of the target volume (partial target boosting) may improve local control without increasing the risk of normal tissue injury. We are investigating the feasibility of using 6 MV photon beam from Varian 2100 Linac to deliver controlled, non‐uniform dose distributions to subcutaneous small animal tumors. The focus here is to achieve dose uniformity as the first step. Methods: A superficial structure was created near the thigh of a healthy mouse to simulate a subcutaneous tumor. It had a maximum dimension of 9mm and density equal to water. The fast Monte Carlo code, Dose Planning Method, (DPM) was coupled to our research treatment planning system, CERR (Computational Environment for Radiotherapy Research), to conduct the planning study. The mouse was placed on the in‐house microRT system. Different boluses and beam arrangements were tested. Results: By computing and evaluating the different plans using 2, 4, and 7mm boluses, it was found that 4mm bolus was enough to provide buildup. Two slightly‐angled (35° and 240°) or parallel‐opposed (60° and 240°) beams can achieve a tumor dose with 3% RMS, the minimum and maximum dose as 0.89Gy and 1.05Gy respectively, for the mean dose 1Gy. This suggested that the uniform dose distributions can be delivered. Conclusions: 6MV photon beam is capable of delivering uniform dose on a superficial tumor of 9mm. Thus it is achievable to deliver a non‐uniform dose on the tumor, provided accurate positioning and beam collimation, to facilitate the investigation of tumor response to heterogeneous dose. The microRT hardware and CERR planning system are a useful platform for this study. Conflict of Interests: This research was partially supported by a grant from Varian Medical Systems, Inc.

TH‐C‐230A‐08: A Prototype Rotational Immobilization System for a Proposed Static‐Gantry MicroRT Device with Tomographic Capabilities

Purpose: Proposed small animal irradiation devices are typically isocentric gantry systems. Multi‐axis gantry systems require significant resources to develop and maintain. A static gantry system could provide the functionality of a multi‐axis gantry via subject rotation if subject motion could be minimized. In this study, we evaluated internal organ motion of mice within a prototype immobilization device during rotation. Method and Materials: Mice were anesthetized and immobilized in a prototype device designed to rigidly support animal positioning during rotation along the cranio‐caudal axis. The head and tail of the mouse were allowed to extend beyond the immobilization device as an internal control. Validation of internal and external immobilization was assessed using computed tomography imaging at multiple rotation angles. CT images were co‐registered (translated/rotated) using our research treatment planning system (CERR) into a common reference frame. Internal organ motion was assessed qualitatively by examination of internal anatomy in overlaid multiplane CT images. Quantitative evaluation of organ motion was assessed by delineating organ structures in CERR and comparing relative organ volumes and centers of mass. Results: CT imaging demonstrated minimal exterior contour changes (<1mm) in the immobilized regions during rotation. Un‐immobilized regions demonstrated the expected gravitational positional changes. Internal organs demonstrated sub‐millimeter changes in organ centers of mass (heart and lung) and small (<5 mm3) changes in volume during rotation. These variations were similar to the differences when the same CT was re‐contoured multiple times by the same operator and likely represent intra‐observer contouring variations. Conclusion: A microRT device with a stationary irradiator, collimator, tomographic imaging system, and rotating subject would reduce the overall cost and complexity of the unit. This study demonstrates rotational immobilization of small animals is feasible.This work supported in part by NIH R21 CA108677 and by a grant Varian, Inc.

Clinical, Dosimetric, and Location-Related Factors to Predict Local Control in Non-Small Cell Lung Cancer

Clinical, Dosimetric, and Location-Related Factors to Predict Local Control in Non-Small Cell Lung Cancer

SU-FF-T-360: Software Tools for 4-D and Adaptive Treatment Planning Data Visualization and Manipulation (CERR Version 3)

Purpose: Open‐source tools are needed to facilitate the use of multiple imaging datasets for adaptive and 4‐D treatment planning. Commercial systems do not yet provide effective solutions for reviewing, manipulating, and comparing multiple scan datasets taken at different times of with different imaging modalities. Our research treatment planning system, CERR (“computational environment for radiotherapy research,” pronounced ‘sir’), provides a convenient and powerful basis for constructing adaptive and 4‐D treatment planning tools. Method and Materials: The flexible Matlab‐based system CERR was modified and extended. A fast algorithm for image registration was developed and integrated with CERR. Results: We have added support in CERR for: (1) multiple patient image sets, which can be combinations of CT, MRI, or PET scans, and corresponding anatomical structure datasets, (2) rapid image registration (by hand as well as by using an automated method), (3) visualization tools appropriate to review anatomic structure changes with different scan sets, and (4) linked‐storage of multiple scan sets, eliminating memory limitations. In addition, many smaller features have been added to CERR, such as interactive profile plots of dose and/or image values. This version of CERR also provides support for IMRT treatment planning simulations. Local data storage in a sub‐directory or network scan data storage we integrated. Flexible reporting tools allow for structures defined on one dataset to be combined with dose distributions computed for another scan, and resulting dose‐volume‐histograms can be easily derived. The latest release can be downloaded from radium.wustl.edu/cerr. Conclusion: CERR version 3 provides foundational tools for research in adaptive and 4‐ D treatment planning. This framework provides a powerful basis for experimenting with deformable imaging methods, as well as other adaptive radiotherapy challenges, such as re‐optimization research.

SU‐FF‐T‐127: A Monte Carlo‐Based IMRT Plan Re‐Calculator

Purpose: IMRT QA currently is currently a time consuming activity. We hypothesize that IMRT QA labor would significantly decrease if a software tool were available which conveniently re‐checked IMRT treatment plans by independently re‐computing the expected dose distribution based on the leaf‐instructions generated by the treatment planning system. Such a system should have the capability to make detailed comparisons with the original treatment plan. Method and Materials: Our research treatment planning system CERR (Computational Environment for Radiotherapy Research) was modified and extended to include the capability of recomputing dose based on leaf‐sequences received via DICOM. Three dose computation algorithms have been implemented: (1) a simple pencil beam model which corrects for changes in central axis attenuation, but includes realistic scatter tails, (2) the Monte Carlo code VMC++, and (3) the open source Monte Carlo code DPM (dose planning method). GUI tools were developed to allow for side‐by‐side dose comparisons and comparative profile dose plots, in addition to DVH comparisons. Initial tests were performed using data generated via the Varian Helios planning system. Comparisons were made beam‐by‐beam in a simplified QA geometry as well as for total dose. Results: Initial results indicate reasonable agreement between the Helios planning system dose distributions and the pencil beam method. Differences with Monte Carlo results were greater, but energy spectral effects have not yet been added to the model. Conclusion: CERR provides a powerful and convenient environment to develop an IMRT plan re‐calculator. Initial results indicate the basic correctness of data being used for the dose recalculation. We expect to fully develop this system as a helpful tool for IMRT QA. Conflict of Interest: This research was supported by a grant from Sun Nuclear, Inc.

Investigation of using a power function as a cost function in inverse planning optimization

The purpose of this paper is to investigate the use of a power function as a cost function in inverse planning optimization. The cost function for each structure is implemented as an exponential power function of the deviation between the resultant dose and prescribed or constrained dose. The total cost function for all structures is a summation of the cost function of every structure. When the exponents of all terms in the cost function are set to 2, the cost function becomes a classical quadratic cost function. An independent optimization module was developed and interfaced with a research treatment planning system from the University of North Carolina for dose calculation and display of results. Three clinical cases were tested for this study with various exponents set for tumor targets and sensitive structures. Treatment plans with these exponent settings were compared, using dose volume histograms. The results of our study demonstrated that using an exponent higher than 2 in the cost function for the target achieved better dose homogeneity than using an exponent of 2. An exponent higher than 2 for serial sensitive structures can effectively reduce the maximum dose. Varying the exponent from 2 to 4 resulted in the most effective changes in dose volume histograms while the change from 4 to 8 is less drastic, indicating a situation of saturation. In conclusion, using a power function with exponent greater than 2 as a cost function can effectively achieve homogeneous dose inside the target and/or minimize maximum dose to the critical structures.