Rotational Setup Errors Research Articles

Comparison of setup error using different reference images: a phantom and lung cancer patients study

The purpose of this study was to compare setup errors obtained with kilovoltage cone-beam computed tomography (CBCT) and 2 different kinds of reference images, free-breathing 3D localization CT images (FB-CT) and the average images of 4-D localization CT images (AVG-CT) for phantom and lung cancer patients. This study also explored the correlation between the difference of translational setup errors and the gross tumor volume (GTV) motion. A respiratory phantom and 14 patients were enrolled in this study. For phantom and each patient, 3D helical CT and 4D CT images were acquired, and AVG-CT images were generated from the 4D CT. The setup errors were determined based on the image registration between the CBCT and the 2 different reference images, respectively. The data for both translational and rotational setup errors were analyzed and compared. The GTV centroid movement as well as its correlation with the translational setup error differences was also evaluated. In the phantom study, the AVG-CT method was more accurate than the FB-CT method. For patients, the translational setup errors based on FB-CT were significantly larger than those from AVG-CT in the left-right (LR), superior-inferior (SI), and anterior-posterior (AP) directions (p < 0.05). Translational setup errors differed by >1 mm in 32.6% and >2 mm in 12.9% of CBCT scans. The rotational setup errors from FB-CT were significantly different from those from AVG-CT in the LR and AP directions (p < 0.05). The correlation coefficient of the translational setup error differences and the GTV centroid movement in the LR, SI, and AP directions was 0.515 (p = 0.060), 0.902 (p < 0.001), and 0.510 (p = 0.062), respectively. For lung cancer patients, respiration may affect the on-line target position location. AVG-CT provides different reference information than FB-CT. The difference in SI direction caused by the 2 methods increases with the GTV movement. Therefore, AVG-CT should be the prefered choice of reference images.

SU-E-T-553: Evaluation of Rotational Errors in Treatment Setup of Stereotactic Body Radiotherapy (SBRT) of Lung Cancer

Purpose: The purpose is to evaluate the rotational setup errors in SBRTtreatment for lungtumors using cone beam CT(CBCT), to investigate the dosimetric impact of rotational setup error, and to determine if translational shifts can compensate the rotation. Methods: Online setup CBCT (n=38) from ten lung patients treated with SBRT were re‐evaluated offline by matching with reference CT using adjustment to 3 rotational angles (pitch, roll and yaw). To investigate the dosimetric impact, planning CTimages were rotated using the actual magnitudes obtained from offline match and the dosimetric changes were quantified by comparing the recalculated dose distribution to the original plans. Translational corrections were simulated by manual translational registration of the target volumes on the rotated images to the original CT. The correction efficacy was evaluated by comparing the recalculated plans to the original treatment plan. Results: The systematic and random setup errors in rotational angles were −0.01±0.50, − 0.57±1.08, −0.11±0.42 degrees for pitch, roll and yaw, respectively. Out of 38 acquired CBCTimages, 57.9% had rotational error >1 degree in any axis, 15.7% had error >2 degree in any axis and only 5.3% had error >3 degree in any axis of rotation. Our previous simulation study using various degrees of rotations showed that target coverage reduced significantly for rotation greater than 3 degrees. Plans recalculated using actual patient roll motions obtained from offline match showed similar reduction in tumor coverage. Translational correction resulted in a fairly good recovery of target coverage except it may result in increased dose to organs at risk (OAR). Conclusions: Rotational setup errors for lungSBRT are relatively small in magnitude and not likely to affect GTV coverage significantly. Translational corrections can be optimized to compensate for rotational setup errors; however, caution needs to be paid regarding dose increase to OAR.

SU‐E‐T‐795: Measuring Uncertainty in Dose Delivered to the Cochlea Due to Setup Error during External Beam Treatment of Patients with Cancer of the Head &amp; Neck

Purpose: To understand the relationship between hearing loss and cochlea dose in patients treated for cancers of the head and neck with external beam IMRT, uncertainties in planned and delivered cochlea doses must be known. We use treatment cone beam scans to determine the positional and dosimetric uncertainty for the cochlea. Methods: Data from 6 head &amp; neck patients with 6 planning CT and 29 Cone Beam CTs taken at time of treatment are used in this study. Patients were treated 70 Gy to the primary target, and between 50–60 Gy to nodal and sub‐clinical disease using IMRT dose painting. We use 6 dimensional rigid body registration to find translational and rotational setup error between the planning CT and Cone Beam CT scans. The Planning CT is rotated and translated so that the cochlea positions match those seen in the cone beam scans and cochlea doses recalculated. Results: We found most probable value of the magnitude of mean error in cochlea position is 0.25&amp;#61617;0.02cm, but it can increase to 7mm in some patients. Uncertainty in the dose to cochlea depends on each treatment plan and relative positions of the cochlea and target volumes. Most of differences in doses to cochlea in our study are around 2% of planned dose. But for some patients this may be as large as 5–9%. Conclusions: Patient setup error introduces uncertainty in the position of the cochlea. Consequent uncertainties in dose heavily depends on the relative position of cochlea w.r.t. the target volumes and the individual treatment plan. Results from this study will be used to model the uncertainty in the dose response of hearing loss.This research is supported in part by NIH under RO1‐CA129182.

SU‐E‐J‐136: Clinical Evaluation of a Robotic 6‐Degree of Freedom Treatment Couch for Frameless Radiosurgery

Purpose: To evaluate the added value of 6 degree‐of‐freedom (DOF) patient positioning with robotic couch compared to 4DOF positioning for intracranial lesions and to estimate the immobilization characteristics of the Brainlab frameless mask, more specifically the setup errors and the intrafraction motion. Methods: Forty patients with 66 brain metastases treated with frameless stereotactic radiosurgery and 6DOF robotic couch were enrolled. Patient positioning was performed with the Brainlab ExacTrac stereoscopic x‐ray system. Positioning results were collected before and after treatment to assess patient setup error and intrafraction motion. Existing treatment plannings were loaded and simulated for 4DOF positioning and compared to the 6DOF positioning. The clinical relevance was analyzed by means of the Paddick conformity index (CI) and the ratio of prescribed isodose volume covered with 4DOF to that obtained with the 6DOF positioning. Results: Results. The mean 3D setup error before 6DOF correction was 1.91 mm (SD1.25mm). The rotational errors were larger in the longitudinal (0.23° SD0.82°) direction compared to the lateral (−0.09° SD0.72°) and vertical (−0.10° SD1.03°) ones (p&lt;0.05). The mean 3D intrafraction shift was 0.58 (SD0.42mm). The intrafractional rotational errors were comparable,0.01° (SD0.35°), 0.03° (SD0.31°), −0.03° (SD0.33°), for the vertical, longitudinal and lateral, respectively. The mean CI decreased from 0.68 (SD 0.08) (6DOF) to 0.59 (SD 0.12) (4DOF) (p&lt;0.05). A loss of prescribed isodose coverage of 5% (SD0.08) was found with the 4DOF positioning (p&lt;0.05). Half a degree for longitudinal and lateral rotations can be identified as a threshold for coverage loss. Conclusions: With a mask immobilization, patient setup error and intrafraction motions need to be evaluated and corrected for. The 6DOF patient positioning with 6DOF robotic couch to correct translational and rotational setup errors improves target positioning with respect to treatment isocenter, which is in direct relation with the clinical outcome, compared to 4DOF positioning.

Setup Accuracy of the Novalis ExacTrac 6DOF System for Frameless Radiosurgery

Stereotactic radiosurgery using frame-based positioning is a well-established technique for the treatment of benign and malignant lesions. By contrast, a new trend toward frameless systems using image-guided positioning techniques is gaining mainstream acceptance. This study was designed to measure the detection and positioning accuracy of the ExacTrac/Novalis Body (ET/NB) for rotations and to compare the accuracy of the frameless with the frame-based radiosurgery technique. A program was developed in house to rotate reference computed tomography images. The angles measured by the system were compared with the known rotations. The accuracy of ET/NB was evaluated with a head phantom with seven lead beads inserted, mounted on a treatment couch equipped with a robotic tilt module, and was measured with a digital water level and portal films. Multiple hidden target tests (HTT) were performed to measure the overall accuracy of the different positioning techniques for radiosurgery (i.e., frameless and frame-based with relocatable mask or invasive ring, respectively). The ET/NB system can detect rotational setup errors with an average accuracy of 0.09° (standard deviation [SD] 0.06°), 0.02° (SD 0.07°), and 0.06° (SD 0.14°) for longitudinal, lateral, and vertical rotations, respectively. The average positioning accuracy was 0.06° (SD 0.04°), 0.08° (SD 0.06°), and 0.08° (SD 0.07°) for longitudinal, lateral and vertical rotations, respectively. The results of the HTT showed an overall three-dimensional accuracy of 0.76 mm (SD 0.46 mm) for the frameless technique, 0.87 mm (SD 0.44 mm) for the relocatable mask, and 1.19 mm (SD 0.45 mm) for the frame-based technique. The study showed high detection accuracy and a subdegree positioning accuracy. On the basis of phantom studies, the frameless technique showed comparable accuracy to the frame-based approach.

Dosimetric consequences of rotational setup errors with direct simulation in a treatment planning system for fractionated stereotactic radiotherapy

The purpose was to determine dose‐delivery errors resulting from systematic rotational setup errors for fractionated stereotactic radiotherapy using direct simulation in a treatment planning system. Ten patients with brain tumors who received intensity‐modulated radiotherapy had dose distributions re‐evaluated to assess the impact of systematic rotational setup errors. The dosimetric effect of rotational setup errors was simulated by rotating images and contours using a 3 by 3 rotational matrix. Combined rotational errors of ± 1°,± 3°,± 5° and ± 7° and residual translation errors of 1 mm along each axis were simulated. Dosimetric effects of the rotated images were evaluated by recomputing dose distributions and compared with the original plan. The mean volume of CTV that received the prescription dose decreased from 99.3%± 0.5% (original) to 98.6%± 1.6% (± 1°), 97.0%± 2.0% (± 3°), 93.1%± 4.6% (± 5°), and 87.8%± 14.2% (± 7°). Minimal changes in the cold and hot spots were seen in the CTV. In general, the increase in the volumes of the organs at risk (OARs) receiving the tolerance doses was small and did not exceed the tolerance, except for cases where the OARs were in close proximity to the PTV. For intracranial tumors treated with IMRT with a CTV‐to‐PTV margin of 3 mm, rotational setup errors of 3° or less didn't decrease the CTV coverage to less than 95% in most cases. However, for large targets with irregular or elliptical shapes, the target coverage decreased significantly as rotational errors of 5° or more were present. Our results indicate that setup margins are warranted even in the absence of translational setup errors to account for rotational setup errors. Rotational setup errors should be evaluated carefully for clinical cases involving large tumor sizes and for targets with elliptical or irregular shape, as well as when isocenter is away from the center of the PTV or OARs are in close proximity to the target volumes.PACS number: 87.53.Bn

SU‐GG‐J‐159: Dosimetric Consequences of Rotational Setup Errors Using Direct Simulation in a Treatment Planning System for Fractionated Stereotactic Radiotherapy (SRT)

Purpose: To determine dose‐delivery errors resulting from systematic rotational setup errors for fractionated stereotactic radiotherapy (SRT) using direct simulation in a treatment planning system (TPS). Methods and materials: Ten patients who received IMRT of brain tumors were studied. Simulation of systematic rotation was done by applying a 3×3 rotation matrix to the original images and contours. Combined rotational errors of ±1°, ±3°, ±5° and ±7° and residual translation errors of 1 mm along each axis were simulated. The rotated images were re‐imported into the TPS and the dose effects were evaluated by re‐computing the dose distributions. Target volumes receiving ≥ 100% (V100Rx), 93% (V93Rx), and 110% (V110Rx) of the prescription dose level and the volumes of the 6 organs at risk (OARs) receiving more than the tolerance dose levels were evaluated. Results: The accuracy of the image rotation algorithm was validated using the image registration package within the TPS and was found to be within 0.5 mm/0.2° in translation/rotation transformation, respectively, and the volumes of the rotated contours are within 2% of the original contours. A systematic reduction in CTV coverage was observed, with the mean V100Rx of the CTV to be 99.3±0.5% (original), 98.4±1.2% (±1°), 95.8±3.0% (±3°), 90.0±7.3% (±5°), and 81.4±13.1% (±7°). In most cases, OAR doses fell within the tolerance levels. However, for two cases where D0.1cc (dose to 0.1 cm3) of the brain stem received close to the dose limit in the original plan, even a ±1° rotations had made D0.1cc &gt; 55Gy, which exceeded our dose limit. Conclusions: We conclude that for rotations less than ±3°, no action level is required; however, cases presenting extreme rotations and high doses in the brain stem should be investigated as routine practice. This procedure allows easy evaluation of dosimetric consequences when systematic rotational errors are present in patient setup.

SU-GG-T-11: Dosimetric Impacts of Rotational Setup Errors on Head &amp; Neck IMRT Treatments

Purpose: To investigate the dosimetric impact of the residual rotational setup errors on head&neck (HNC) IMRT with conventional clinical three translational setup corrections, and to evaluate the necessity of the full six degree (6D) setup correction. Methods and Materials: A total of 66 kV CBCTimage sets were acquired on the first day of treatment and weekly thereafter for 10 HNC patients. The CBCTimages were registered with the planning CTimages using two 3D rigid registration methods. Method1 determines the translational errors only and method2 determines 6D errors. To simulate the clinical setup correction, the 6D setup errors were simulated in the treatment planning system and then were corrected by using the corresponding translational data determined by method1. For each patient, dose distributions for 6 to 7 fractions with various setup uncertainties were generated, and plan sum was created to simulate the total dose distribution through an entire course and was compared with the original treatment plan. The first phase plan with prescription about 50 Gy was used in this study. Results: Without rotational corrections and with only translational corrections, for PTV, the plan sum showed that the average Vp (prescription dose coverage) was decreased by 5.9%, D95 (dose that covers 95% of target) was decreased 1.8 Gy. For CTV, though the Vp could be decreased by 8% for treatments on selected days, the plan sum showed that the average Vp was decreased by 1.7% and D98 was decreased 0.1 Gy. The average dose increase to spinal cord was 0.7 Gy. Conclusions: With conventional translational setup correction, the overall CTV prescription dose coverage decreases insignificantly because of the 5mm margin from CTV to PTV. The dose increase to spinal cord is not significant. So for HNC IMRT treatment with target margin, the six degree setup correction is not necessary.

Preliminary Studies for a CBCT Imaging Protocol for Offline Organ Motion Analysis: Registration Software Validation and CTDI Measurements

Cone-beam X-ray volumetric imaging in the treatment room, allows online correction of set-up errors and offline assessment of residual set-up errors and organ motion. In this study the registration algorithm of the X-ray volume imaging software (XVI, Elekta, Crawley, United Kingdom), which manages a commercial cone-beam computed tomography (CBCT)–based positioning system, has been tested using a homemade and an anthropomorphic phantom to: (1) assess its performance in detecting known translational and rotational set-up errors and (2) transfer the transformation matrix of its registrations into a commercial treatment planning system (TPS) for offline organ motion analysis. Furthermore, CBCT dose index has been measured for a particular site (prostate: 120 kV, 1028.8 mAs, approximately 640 frames) using a standard Perspex cylindrical body phantom (diameter 32 cm, length 15 cm) and a 10-cm-long pencil ionization chamber. We have found that known displacements were correctly calculated by the registration software to within 1.3 mm and 0.4°. For the anthropomorphic phantom, only translational displacements have been considered. Both studies have shown errors within the intrinsic uncertainty of our system for translational displacements (estimated as 0.87 mm) and rotational displacements (estimated as 0.22°). The resulting table translations proposed by the system to correct the displacements were also checked with portal images and found to place the isocenter of the plan on the linac isocenter within an error of 1 mm, which is the dimension of the spherical lead marker inserted at the center of the homemade phantom. The registration matrix translated into the TPS image fusion module correctly reproduced the alignment between planning CT scans and CBCT scans. Finally, measurements on the CBCT dose index indicate that CBCT acquisition delivers less dose than conventional CT scans and electronic portal imaging device portals. The registration software was found to be accurate, and its registration matrix can be easily translated into the TPS and a low dose is delivered to the patient during image acquisition. These results can help in designing imaging protocols for offline evaluations.

Evaluation of rotational set-up errors in patients with thoracic neoplasms

Objective To assess the rotational set-up errors in patients with thoracic neoplasms. Methods 224 kilovohage cone-beam computed tomography (KVCBCT) scans from 20 thoracic tumor pa-tients were evaluated retrospectively. All these patients were involved in the research of Evaluation of the residual set-up error for online kilovohage cone-beam CT guided thoracic tumor radiation. Rotational set-up errors, including pitch, roll and yaw, were calculated by 'aligning the KVCBCT with the planning CT, using the semi-automatic alignment method. Results The average rotational set-up errors were -0.28°±1.52°, 0.21°± 0.91° and 0.27°± 0. 78° in the left-fight, superior-inferior and anterior-posterior axis, respective-ly. The maximal rotational errors of pitch, roll and yaw were 3.5°, 2.7° and 2.2°, respectively. After cor-rection for translational set-up errors, no statistically significant changes in rotational error were observed. Conclusions The rotational set-up errors in patients with thoracic neoplasms were all small in magnitude. Rotational errors may not change after the correction for translational set-up errors alone, which should be e-valuated in a larger sample future. Key words: Thoracic neoplasms/radiotherapy; Rotational set-up error; Tomography,X-ray computed,kilovohage cone-beam

Risk and Intervention Assessment for Rotational and Translational Setup Errors on Spinal Cord Dose for Stereotactic Body Radiotherapy (SBRT) of Paraspinal Tumors: An Automated Planning Study

Translational image-guided setup corrections for SBRT of paraspinal tumors ensure geometrical accuracy at isocenter but do not address rotational errors. The objectives of this work are to assess the impact of combined residual rotational and translational errors on spinal cord dose and determine factors that predict cord dose sensitivity to these errors. Nine-field IMRT plans (18 Gy/1 fraction) were generated retrospectively for 16 spinal metastases patients with the objective of maximizing target coverage while keeping the dose to 0.1cc of the cord to 10Gy (Dcord). The maximum dose to 0.1cc of the volume consisting of the cord + 2mm margin (DPRV2) was recorded. To assess the impact of rotational error on Dcord, the treatment planning system was used in batch mode to recalculate each plan after introducing simulated combinations of rotational errors around the isocenter (0-12° around X, Y and Z-axis). These simulations were repeated after adding 1 mm residual translational errors along both X and Y-axis such that the cord was brought closer to the high dose region. Rotations resulting in the actual cord dose equal to DPRV2 were deemed significant. Rotation combinations were reduced to a single equivalent rotation (θeq) around an arbitrary axis using eigenvector formalism, and the minimum θeq for which Dcord = DPRV2 was compiled for each patient. Two predictive factors for Dcord sensitivity to rotational errors were evaluated using regression analysis. 3422 rotational and translational error combinations were evaluated. Without any translational shift, rotations where θeq = 2.6°-10.2° were required for Dcord to reach DPRV2 (12.74-14.98 Gy). In combination with a 1 mm residual translational shift, θeq decreased to 1.4°-7.7°. The strongest predictor for cord dose sensitivity to rotational errors was the maximum distance between the isocenter and the center of the cord cross-section on the most inferior or superior CT slices with 95% isodose (diso-cord). A linear model based on the first 8 patient plans using diso-cord and the dose gradient at the target-cord interface was able to predict θeq within ± 0.9° (standard deviation) for the next 8 patients. Simulations of combined setup errors in paraspinal SBRT plans showed limited cord dose sensitivity to rotational errors and demonstrated the importance of minimizing residual translational errors. A linear model predicting cord dose sensitivity to rotational errors was designed and will be used to evaluate intervention strategies such as the six-degree of freedom couch.

TH-D-303A-07: Dosimetric Impact of Rotational Setup Error in Stereotactic Body Frame Radiation Therapy (SBRT)

Purpose:SBRT uses specialized fixation devices to achieve high dose gradient with minimal margin, however setup errors cannot be totally eliminated. The dosimetric impact of rotational setup error is investigated in the context if translational shifts can compensate for it. Methods and Materials: To simulate patient rotational setup errors, CTimages of a phantom and actual clinical SBRT cases (liver: n=3, lung: n=3) were rotated around the body center using an in‐house image processing toolkit. The dosimetric impact of uncorrected roll was quantified by comparing the recalculated dose distribution to the original plan. Manual translational registration was then performed to match the target volumes on the rotated images to the original datasets to simulate translational correction. The original plan was then recalculated using the corrected CT datasets and the resultant dose distributions compared to the original ones. Results: The simulated rotation of phantom results in reduced dose‐volume coverage of CTV and PTV. While minimal for rotations <3 degrees, CTV coverage decreased sharply for larger rotation. For all clinical cases, a 3‐degree rotation resulted in less than 5% reduction in the CTV volume covered by 90% prescribed dose (CTV90). A larger rotation led to significant dose reduction in tumor targets as well as dose changes to critical organs (OAR). Manual translational registration resulted in good recovery of both CTV90 and PTV80. However, increased dose to OAR was observed in some cases where the selection of beam angle was close to the OAR. Conclusion: It is concluded that uncorrected rotational setup error larger than 3 degree could cause significant dose changes to tumor targets and OAR. Image guided translational correction can compensate for rotational setup errors. Despite the improvement of CTV matching, caution needs to be paid regarding dose increase to OAR when large rotation angle is corrected.

SU‐EE‐A3‐02: Imaging Guided Frameless Stereotactic RadioSurgery Using CBCT 6D Image Registration and 6D Couch On Novalis Tx™ System

Purpose: To investigate localization accuracy for image‐guided frameless stereotactic‐radiosurgery (IG‐SRS) using CBCT 6D image registration and 6D‐couch on Novalis Tx system. Method and Materials: Novalis Tx system is a powerful radiosurgery system featuring BrainLAB 6D‐couch and Varian CBCT. In this work, we performed phantom and patient studies to investigate IG‐SRS with CBCT 6D guidance. In phantom studies, a head phantom was positioned on couch, scanned with CBCT, and matched with planning CT using 6D image registration. Matching results were used to test accuracy of CBCT 6D/6D match for correcting translational and rotational setup errors. In patient studies, 10 intracranial SRS cases were randomly selected. Patients were initially positioned with BrainLAB mask system with target‐positioning‐overlays (TAPO). Patients were then scanned with CBCT, and positions were adjusted based on CBCT. Discrepancies between CBCT 3D/3D match and 6D/6D match were quantitatively analyzed. Results: Phantom experiments showed that translational discrepancies in root‐mean‐square (RMS) were 0.39, 0.23, and 0.36mm along lateral, longitudinal, and vertical directions. Rotational discrepancies in RMS were 0.38°, 0.19°, and 0.22° in pitch, roll, yaw, respectively. In retrospective patient studies, setup displacements measured in RMS between BrainLAB TAPO and CBCT 6D/6D match were upto 2.5mm translationally and 1.0° rotationally. If positioning was corrected using CBCT 3D/3D match, translational discrepancies in RMS were &lt;0.5mm except 0.71mm in vertical, and rotational displacements in RMS were 0.67°, 0.88° and 0.22° in pitch, roll, yaw, as compared with CBCT 6D/6D match. CBCT guided 6D correction was performed in two cases to achieve necessary positioning accuracy. After 6D adjustments, translational and rotational displacements of upto 4mm and 0.9° were measured in CBCT 6D/6D match were fully corrected using 6D‐couch. Conclusion: CBCT 6D/6D match offers unprecedented view to verify patient positioning in both translations and rotations. Combined with 6D‐couch, translational and rotational setup errors can be effectively minimized.

SU‐FF‐J‐40: A Simple Method for Dosimetric Assessment in H&amp;N Radiotherapy with Rotational Setup Errors

Purpose: Rotational setup error for H&amp;N patients is detected using cone‐beam CT. Fusion with 3D image can determine a sequence of three Euler angles for the best matching with the planned patient position. Currently, correction for rotational errors is not available except with the use of a 6‐degree‐of‐freedom treatment couch. However, it is possible to compensate an arbitrary rigid‐body rotation by just a simple couch rotation in addition to gantry and collimator angle changes. We introduce a formulation for compensating a rotational error with respect to a given planned beam position for adaptive treatment or easy dosimetric evaluation for no correction. Method: For each planned beam position, the beam axis in patient coordinates upon rotational error is calculated, pointing out of the beam source plane. A couch rotation is first applied to move the planned beam axis back to the source plane. The adapted gantry and collimator angles are then calculated to align with the planned beam axis and radiation portal. The inverse transformation determines the effective beam position in planning CT for dosimetric evaluation with no correction. Results: Our method was validated by phantom experiments. Light fields were compared for various beam positions before and after phantom rotations. The delivered dose distribution for two patients with 2∼3 degrees of rotations were calculated and compared with the planned distribution, showing marginal deterioration with 1.5% target dose reduction and 2% parotid dose increase. Conclusions: A simple “theoretical” solution is available for handling rotational setup error when rigid‐body approximation applies. Its practical application is to evaluate the dosimetry for no correction, and our data showed that rotational error was less of a concern compared to as large as over 5% dose variation with patient anatomy change due to weight loss or tumor shrinkage.

SU-FF-J-80: Evaluation of Rotational and Translational Setup Variations for Brain Tumor Patients with Mask for Immobilization

Purpose: Rotational setup errors for braintumor patients undergoing external beam radiotherapy with a thermoplastic mask system were not well investigated and often ignored during the planning and treatment process. In this study, both translational and rotational setup variations are evaluated for braintumor patients undergoing external beam radiotherapy, using daily helical tomotherapy megavoltage computed tomography (MVCT) scans. Method and Materials: 15 primary braintumor patients who finished fractionated helical tomotherapy treatment were retrospectively selected. The number of fractions ranged from 29 to 34. Prior to each treatment fraction, an MVCT scan was performed over the brain volume. The MVCT images were registered with the planning CTimages based on bony structure using rigid translations in three dimensions and rotations in pitch, roll, and yaw. Results: The standard deviations (SD) for translations ranged 0.78 to 2.32 mm, 1.11 to 2.65 mm, and 0.52 to 6.65 mm in the RL, SI, and AP directions, respectively. The SD's ranged 0.28° to 0.83°, 0.35° to 1.04°, and 0.41° to 2.41° in pitch, roll, and yaw, respectively. 20.2%, 20.9%, and 38.9% of all the daily setups had more than 1° setup errors in pitch, roll, and yaw, respectively; and 58.4% had more than 1° errors in at least one rotational directions. 2.2%, 3.1%, and 20.9% had more than 2° setup errors in pitch, roll, and yaw, respectively, and 24.3% had more than 2° setup errors in at least one rotational directions. Residual translational setup errors in the absence of rotational setup corrections were also evaluated. Conclusion: Rotational setup variations can be significant for braintumor patients immobilized with masks, and setup corrections without rotations could lead to significant overall setup errors. Margins for treatment planning should take rotational setup variations into account. Daily image‐guided setup can be used to position the target volume.

Influence of Rotations on Dose Distributions in Spinal Stereotactic Body Radiotherapy (SBRT)

To evaluate the impact of rotational setup errors on dose distribution in spinal stereotactic body radiotherapy (SBRT). Thirty-nine cone beam computed tomography (CBCT) scans from 16 SBRT treatment courses were analyzed. Alignment (including rotation) to the treatment planning computed tomography was performed, followed by translational alignment that reproduced the actual positioning. The planned fluence was then applied to determine the delivered dose to the targets and organs at risk. The mean planning target volume (PTV) was 71.01 mL (SD +/- 60.05; range, 22.62-250.65 mL). Prescribed dose (to the 62-82% isodose) was 14-30 Gy in one to six fractions. The average rotational displacements were 0.38 +/- 1.21, 1.12 +/- 1.82, and -0.51 +/- 2.0 degrees with maximal rotations of -4.29, 5.76, and -6.64 degrees along the x (pitch), y (yaw), and z (roll) axes, respectively. PTV coverage changed by an average of -0.07 Gy (SD +/- 0.20 Gy) between the rotated and the original plan, representing 0.92% of prescription dose (SD +/- 2.65%). For the spinal cord, planned with 2-mm expansion to create a planning organ at risk volume (PRV), the difference in minimum dose to the upper 10% of the PRV volume was 0.03 +/- 0.3 Gy (maximum, 0.9 Gy). Other organs at risk saw insignificant changes in dose. PRV expansion generally assures safe treatment delivery in the face of typically encountered rotations. Given the variability of delivered dose within this expansion for certain cases, caution should be taken to properly interpret doses to the cord when considering clinical dose limits.

Rotation of the Sacrum During Bellyboard Pelvic Radiotherapy

Patients with cervical, uterine, and rectal carcinomas are usually treated in the prone position using the bellyboard positioning device. Specific and uncomfortable prone position gives rise to uncertainties in the daily set-up of patients during the treatment. During investigation of translational movements, rotational movements of the pelvis are observed and investigated. The film portal imaging is used to discover patient positioning errors during treatment. We defined the rotational set-up errors by angle deviations of the sacrum. Thirty-six patients were included in the study; 15 patients were followed during the whole treatment and 21 during the first 5 consecutive treatment days. The image acquisitions were completed in 84%. Systematic and random positioning errors were analyzed in 725 images. Approximately half of the patients had adjusted to the bellyboard in the first few fractions, with sacrum angles remaining the same for the rest of the treatment. The other half had drifts of the sacrum angle during the whole treatment. The rotation of the sacrum during treatment ranged up to 14°, causing the usual set-up verification and correction procedure to result in errors up to 15 mm. Rotational movements of the patient pelvis during bellyboard pelvis radiotherapy can introduce considerable patient position error.

Dosimetric Changes Resulting From Patient Rotational Setup Errors in Proton Therapy Prostate Plans

To evaluate the dose changes to the target and critical structures from rotational setup errors in prostate cancer patients treated with proton therapy. A total of 70 plans were analyzed for 10 patients treated with parallel-opposed proton beams to a dose of 7,600 (60)Co-cGy-equivalent (CcGE) in 200 CcGE fractions to the clinical target volume (i.e., prostate and proximal seminal vesicles). Rotational setup errors of +3 degrees , -3 degrees , +5 degrees , and -5 degrees (to simulate pelvic tilt) were generated by adjusting the gantry. Horizontal couch shifts of +3 degrees and -3 degrees (to simulate longitudinal setup variability) were also generated. Verification plans were recomputed, keeping the same treatment parameters as the control. All changes shown are for 38 fractions. The mean clinical target volume dose was 7,780 CcGE. The mean change in the clinical target volume dose in the worse case scenario for all shifts was 2 CcGE (absolute range in worst case scenario, 7,729-7,848 CcGE). The mean changes in the critical organ dose in the worst case scenario was 6 CcGE (bladder), 18 CcGE (rectum), 36 CcGE (anterior rectal wall), and 141 CcGE (femoral heads) for all plans. In general, the percentage of change in the worse case scenario for all shifts to the critical structures was <5%. Deviations in the absolute percentage of volume of organ receiving 45 and 70 Gy for the bladder and rectum were <2% for all plans. Patient rotational movements of 3 degrees and 5 degrees and horizontal couch shifts of 3 degrees in prostate proton planning did not confer clinically significant dose changes to the target volumes or critical structures.

Evaluation of Dose Changes to Target and Critical Structures from Rotational Setup Errors (RSE) in Prostate Cancer Patients Treated with Proton Therapy

Proton therapy has proven effective in the treatment of prostate cancer. Unlike conventional X-rays, protons are highly sensitive to changes in tissue density along their path, which can cause a shift in the particle range and Bragg peak. Changes in patient positioning during treatment may significantly alter dose to structures of interest (SOI). While dose deviations with systematic rotational set-up errors (RSE) have been studied with IMRT prostate plans, to our knowledge this question has not yet been evaluated in the context of proton therapy prostate plans.

Online Kilovoltage Cone-beam CT Guided Lung Cancer Radiation: Initial Clinical Experiences from Shanghai

To introduce the initial clinical experiences of online Kilovoltage Cone-beam CT (KVCBCT) guided lung cancer radiation in our department. From March 2007 to January 2008, 30 patients with pathologically confirmed lung cancer were included in this study. After patients were positioned using their skin marks, KVCBCT scans were performed before treatment and aligned to planning CT scans using the self-developed semi-automatic alignment method. Treatment table were repositioned according to the alignment results. A second KVCBCT scan was acquired immediately after the table correction. A total of 308 KVCBCT images from 154 fractions were analyzed. The reproducibility of the semi-automatic image alignment method, workflow of the online correction, translational and rotational setup error in three dimensional conditions and residual setup error after online correction were evaluated. The reproducibility of the semi-automatic alignment method was evaluated. The alignment difference was smaller in intraobserver evaluation than in interobserver. The reproducibility of different physicians was better than the physician and radiation therapist in anterior-posterior (AP) and left-right (LR) directions. In superior-inferior (SI) direction, the percentage of alignment difference larger than 3 mm between different physicians, the physician and radiation therapist were both near 15%. The online correction protocol adds 7.6 min to the total treatment time and the workflow proved to be feasible in clinical practice. The total systematic error (∑) before online correction was 3.5 mm, 3.5 mm and 3.0 mm, the overall random error (σ) was 2.7 mm, 2.9 mm and 3.0 mm in the LR, SI, and AP direction, respectively. After online correction, the total systematic error (∑) was 0.7 mm, 0.7 mm and 0.6 mm, the overall random error (σ) was 1.2 mm, 1.5 mm and 1.3 mm in three directions. The average rotational setup error (± 1 SD) was -0.3 ± 1.5°, 0.2 ± 0.9 ° and 0.3 ± 0.8 ° in pitch, roll and yaw, respectively. According to the formalisms suggested by van Herk et al, 3mm can apply to the margin of clinical target volume (CTV) to PTV, when using the online correction. The KVCBCT guided radiation system can be applied successfully in lung cancer patients with reproducible image alignment method. With the help of the system, we have an opportunity of new realization for patient setup error, including both translational and rotational. We can improve patient setup accuracy and reduce the CTV to PTV margin from 11 mm to 3 mm with limited workload.