Robert J. Staton

Dosimetric effects of rotational output variation and x-ray target degradation on helical tomotherapy plans

In this study, two potential sources of IMRT delivery error have been identified for helical tomotherapy delivery using the HiART system (TomoTherapy, Inc., Madison, WI): Rotational output variation and target degradation. The HiArt system is known to have output variation, typically about +/- 2%, due to the absence of a dose servo system. On the HiArt system, x-ray target replacement is required approximately every 10-12 months due to target degradation. Near the end of target life, the target thins and causes a decrease in the beam energy and a softening of the beam profile at the lateral edges of the beam. The purpose of this study is to evaluate the dosimetric effects of rotational output variation and target degradation by modeling their effects and incorporating them into recalculated treatment plans for three clinical scenarios: Head and neck, partial breast, and prostate. Models were created to emulate both potential sources of error. For output variation, a model was created using a sine function to match the amplitude (+/- 2%), frequency, and phase of the measured rotational output variation data. A second model with a hypothetical variation of +/- 7% was also created to represent the largest variation that could exist without violating the allowable dose window in the delivery system. A measured beam profile near the end of target life was used to create a modified beam profile model for the target degradation. These models were then incorporated into the treatment plan by modifying the leaf opening times in the delivery sinogram. A new beam model was also created to mimic the change in beam energy seen near the end of target life. The plans were then calculated using a research version of the PLANNED ADAPTIVE treatment planning software from TomoTherapy, Inc. Three plans were evaluated in this study: Head and neck, partial breast, and prostate. The D50 of organs at risk, the D95 for planning target volumes (PTVs), and the local dose difference were used to evaluate the changes in the modified treatment plans. Dosimetric effects from rotational variation were found to be low (less than 1%) for a typical variation of +/- 2%. Even using a variation of +/- 7%, DVH values and dose distributions were altered by less than 2% for all scenarios. The dosimetric effects of target degradation were found to be slightly more significant. For a model using data taken just before target failure, dosimetric differences of 2%-4% were observed in the recalculated plans when compared to the original plans. The largest effects (up to 4.5%) were observed for PTVs that were located at deeper depths as seen in the prostate plan. Overall, the recalculated plans show that the dosimetric effects of rotational variation and target degradation are on the order of 1%-4% for helical tomotherapy on the HiART system and do not pose a risk for significant deviations from the original treatment plan.

A GPU based high‐resolution multilevel biomechanical head and neck model for validating deformable image registration

PURPOSE Validating the usage of deformable image registration (dir) for daily patient positioning is critical for adaptive radiotherapy (RT) applications pertaining to head and neck (HN) radiotherapy. The authors present a methodology for generating biomechanically realistic ground-truth data for validating dir algorithms for HN anatomy by (a) developing a high-resolution deformable biomechanical HN model from a planning CT, (b) simulating deformations for a range of interfraction posture changes and physiological regression, and (c) generating subsequent CT images representing the deformed anatomy. METHODS The biomechanical model was developed using HN kVCT datasets and the corresponding structure contours. The voxels inside a given 3D contour boundary were clustered using a graphics processing unit (GPU) based algorithm that accounted for inconsistencies and gaps in the boundary to form a volumetric structure. While the bony anatomy was modeled as rigid body, the muscle and soft tissue structures were modeled as mass-spring-damper models with elastic material properties that corresponded to the underlying contoured anatomies. Within a given muscle structure, the voxels were classified using a uniform grid and a normalized mass was assigned to each voxel based on its Hounsfield number. The soft tissue deformation for a given skeletal actuation was performed using an implicit Euler integration with each iteration split into two substeps: one for the muscle structures and the other for the remaining soft tissues. Posture changes were simulated by articulating the skeletal structure and enabling the soft structures to deform accordingly. Physiological changes representing tumor regression were simulated by reducing the target volume and enabling the surrounding soft structures to deform accordingly. Finally, the authors also discuss a new approach to generate kVCT images representing the deformed anatomy that accounts for gaps and antialiasing artifacts that may be caused by the biomechanical deformation process. Accuracy and stability of the model response were validated using ground-truth simulations representing soft tissue behavior under local and global deformations. Numerical accuracy of the HN deformations was analyzed by applying nonrigid skeletal transformations acquired from interfraction kVCT images to the models skeletal structures and comparing the subsequent soft tissue deformations of the model with the clinical anatomy. RESULTS The GPU based framework enabled the model deformation to be performed at 60 frames/s, facilitating simulations of posture changes and physiological regressions at interactive speeds. The soft tissue response was accurate with a R(2) value of >0.98 when compared to ground-truth global and local force deformation analysis. The deformation of the HN anatomy by the model agreed with the clinically observed deformations with an average correlation coefficient of 0.956. For a clinically relevant range of posture and physiological changes, the model deformations stabilized with an uncertainty of less than 0.01 mm. CONCLUSIONS Documenting dose delivery for HN radiotherapy is essential accounting for posture and physiological changes. The biomechanical model discussed in this paper was able to deform in real-time, allowing interactive simulations and visualization of such changes. The model would allow patient specific validations of the dir method and has the potential to be a significant aid in adaptive radiotherapy techniques.

A comparison of the dosimetric effects of intrafraction motion on step-and-shoot, compensator, and helical tomotherapy-based IMRT

Intrafraction motion during intensity‐modulated radiation therapy can cause differences between the planned and delivered patient dose. The magnitude of these differences is dependent on a number of variables, including the treatment modality. This study was designed to compare the relative susceptibility of plans generated with three different treatment modalities to intrafraction motion. The dosimetric effects of motion were calculated using computational algorithms for seven lung tumor patients. Three delivery techniques — MLC‐based step‐and‐shoot (SNS), beam attenuating compensators, and helical tomotherapy (HT) — were investigated. In total 840 motion‐encoded dose‐volume histograms (DVHs) were calculated for various combinations of CTV margins and sinusoidal CTV motion including CTV offsets. DVH‐based metrics (e.g., D95% and D05%) were used to score plan degradations. For all three modalities, dosimetric degradations were typically smaller than 3% if the CTV displacement was smaller than the CTV margin. For larger displacements, technique and direction‐specific sensitivities existed. While the HT plans show similar D95% degradations for motion in the SI and AP directions, SNS and compensator plans showed larger D95% degradations for motion in the SI direction than for motion in the AP direction. When averaged over all motion/margin combinations, compensator plans resulted in 0.9% and 0.6% smaller D95% reductions compared to SNS and HT plans, respectively. These differences were statistically significant. No statistically significant differences in D95% degradations were found between SNS and HT for data averaged over all margin and motion track combinations. For CTV motion that is larger than the CTV margin, the dosimetric impact on the CTV varies with treatment technique and the motion direction. For the cases presented here, the effect of motion on CTV dosimetry was statistically smaller for compensator deliveries than SNS and HT, likely due to the absence of the interplay effect which is present for the more dynamic treatment deliveries. The differences between modalities were, however, small and might not be clinically significant. As expected, margins that envelop the CTV motion provide dosimetric protection against motion for all three modalities. PACS numbers: 87.53.Jw, 87.55.dk, 87.55.de

Megavoltage Computed Tomography Image-based Low-dose Rate Intracavitary Brachytherapy Planning for Cervical Carcinoma

Initial results of megavoltage computed tomography (MVCT) brachytherapy treatment planning are presented, using a commercially available helical tomotherapy treatment unit and standard low dose rate (LDR) brachytherapy applicators used for treatment of cervical carcinoma. The accuracy of MVCT imaging techniques, and dosimetric accuracy of the CT based plans were tested with in-house and commercially-available phantoms. Three dimensional (3D) dose distributions were computed and compared to the two dimensional (2D) dosimetry results. Minimal doses received by the 2 cm3 of bladder and rectum receiving the highest doses (DB2cc and DR2cc, respectively) were computed from dose-volume histograms and compared to the doses computed for the standard ICRU bladder and rectal reference dose points. Phantom test objects in MVCT image sets were localized with sub-millimetric accuracy, and the accuracy of the MVCT-based dose calculation was verified. Fifteen brachytherapy insertions were also analyzed. The ICRU rectal point dose did not differ significantly from DR2cc (p=0.749, mean difference was 24 cGy ± 283 cGy). The ICRU bladder point dose was significantly lower than the DB2cc (p=0.024, mean difference was 291 cGy ± 444 cGy). The median volumes of bladder and rectum receiving at least the corresponding ICRU reference point dose were 6.1 cm3 and 2.0 cm3, respectively. Our initial experience in using MVCT imaging for clinical LDR gynecological brachytherapy indicates that the MVCT images are of sufficient quality for use in 3D, MVCT-based dose planning.

WE‐E‐M100F‐04: Commissioning and Validation of a Beam Model for Calculating Megavoltage CT Dose From Imaging with a Helical Tomotherapy Unit

Purpose: To commission and validate an MVCT beam model that allows for the calculation of dose received by patients due to megavoltage imaging on a helical tomotherapy unit (Tomotherapy, Inc., Madison, WI). Method and Materials: Percent depth dose and profile data were collected in order to commission a new MVCT beam model. The fluence output for the beam model was adjusted to match the measured dose in phantom. The model was then verified through a series of absorbed dose measurements in three phantoms (20‐cm cylindrical phantom, CIRS anthropomorphic phantom, and 30‐cm “cheese” phantom). The multiple scan average dose was recorded for all three phantoms with various changes to CTcollimator pitch and ion chamber location (central versus peripheral points). Results: The delivered doses and the computed doses were on average within 1.5% for all three phantoms, when the ion chamber was centrally located; and within 3.5%, when the chamber was located on the peripheral edge of the phantoms. The measured dose in the anthropomorphic phantom was 2.3 cGy with a pitch of 1.0 (4 mm couch movement per gantry rotation), 1.4 cGy with a pitch 2.0, and 0.90 cGy with a pitch of 3.0, these matched within 1% to the calculated dose. The computed versus measured dose was also within 1% when calculating dose in different tissue densities (lung and bone). Conclusion: This study has shown that with the development of a new MVCT beam model, dose delivered from MVCT imaging can be calculated. Validation measurements, in phantom, have verified that the computed dose can be reported to within 1.5% of the measured dose. The rationale for implementing this MVCT beam model is to provide a future method for calculating patient‐specific MVCT dose. Conflict of Interest: Co‐authors are either funded by a research grant or employed by TomoTherapy, Inc.

SU‐D‐WAB‐07: Documenting Dose Delivery for Head & Neck Radiotherapy Using a GPU Based Framework

PURPOSE To compute the delivered dose for head and neck (H&N) radiotherapy sccounting for the patient setup variations as well as physiologic changes during the entire treatment course. METHODS The real-time dose delivery documentation framework consisted of three components, a GPU based multi-resolution optical flow based 3D registration framework, a GPU based local-jocabian computation, and a GPU based dose accumulation framework. In addition, for validation purposes, a GPU based 3D gamma statistics was also employed. We evaluated the framework with ten H&N patients treated with simultaneous integrated boost on a TomoTherapy unit (Accuray Inc.). For every patient, a daily pre-treatment megavoltage CT (MVCT) and weekly kilovoltage CT (KVCT) were acquired. The ability to account for different head and neck postures can be illustrated by observing the internal tissue deformations. The local-Jacobian determinant for the PTVs and the critical structures were used to quantify anatomical volume changes for each week. The GPU-accelerated dose accumulation determined the delivered mean/minimum/maximum dose, equivalent uniform dose (EUD), dose volume histograms (DVHs), as well as 3D gamma statistics comparing the delivered dose to the plan. RESULTS The ratios (and the standard deviations) between the delivered averaged minimum doses and the planned dose, were 0.73±0.14, 0.66±0.28 and 0.78±0.19 for PTV1 (the initial target volume), PTV2 (regions of involved nodes) and PTV3 (elective nodal areas) respectively. The framework achieved a run-time of 30 sec for registering one KVCT with a planning KVCT, 0.1 sec for the forward dose accumulation and 10 sec for the 3D gamma statistical analyses making the system real-time and an effective tool towards adaptive radiotherapy. CONCLUSION The framework addresses the combined challenge of deforming the 3D anatomy using for treatment planning using the daily 3D image data and computing the delivered dose on the deformed planning 3D anatomy.

SU‐FF‐T‐175: Dosimetric Effect of Rotational Variation On Helical Tomotherapy Treatment Plans

Purpose: Helical tomotherapy is subject to rotational output variation during treatment delivery. The purpose of this study is to evaluate the dosimetric effects of the rotational variation using recalculated treatment plans which include a rotational variation.Method and Materials: In order to assess the rotational output variation, the monitor chamber signal was extracted from delivered treatment data archives and analyzed. The rotational variation was then modeled and incorporated back into the treatment plan sinogram by adjusting the leaf opening times for each projection. The modified treatment plans were then calculated using a research version of the planned adaptive treatment planningsoftware from TomoTherapy, Inc. Two treatment plans, one head and neck and one prostate, were evaluated in this study. Treatment plans were recalculated with an observed rotational variation of ±2 % and a hypothetical variation of ±10 % for comparison to the original calculated treatment plans. Results: The rotational variation from delivered treatments was found to follow a sinusoidal shape with a period equal to one gantry rotation and a magnitude of about ±2% on average. For a rotational variation of ±2%, the D95 for the target volumes and the D50 for critical structures were found to be different from the original plan by less than 0.26% for the recalculated treatment plans. For a hypothetical rotational variation of ±10%, the difference from the original was less than 0.5% for the D95 for target volumes and 1.4% for the D50 of critical structures. Conclusion: The rotational variation for delivered helical tomotherapy treatments was found to be on the order of ±2%. For this observed rotational variation, a negligible dosimetric effect on calculated helical tomotherapy treatment plans was found. Conflict of Interest: Our group holds a research grant from TomoTherapy, Inc.