Rotational Setup Errors Research Articles

A Treatment Planning Investigation Into the Dosimetric Effects of Systematic Prostate Patient Rotational Set-Up Errors

The purpose of this study was to investigate the potential dosimetric effects of systematic rotational setup errors on prostate patients planned according to the RTOG P-0126 protocol, and to identify rotational tolerances about either the anterior-posterior (AP) or left-right (LR) axis, under which no correction in setup is required. Eight 3-dimensional conformal radiation therapy (3D-CRT) treatment plans were included in the study, half planned to give 7020 cGy in 39 fractions (P-0126 Arm 1) and the other half planned to give 7920 cGy in 44 fractions (P-0126 Arm 2). Systematic rotations of the pelvic anatomy were simulated in a commercial treatment planning system by rotating opposing apertures in the opposite direction to the simulated anatomy rotation. Rotations were incremented in steps of 2.5° to a maximum of ±5.0° and ±10.0° about the AP and LR axis respectively. Dose distributions were evaluated with respect to the planning objectives set out in the P-0126 protocol. For patients on Arm 2 of the study, maintaining the prescribed dose to 98% of the PTV was found to be problematic for superior-end-posterior rotations beyond 5.0°. The results also show that maintaining a rectal dose less than 7500 cGy to 15% of the volume can become problematic for cases of small rectal volume and large superior-end-anterior rotations. We found that setting rotational tolerances will depend on which Arm of the protocol the patient is, and how well the initial plan meets the protocol objectives. In general, we conclude that for rotations about the AP axis, no tolerance level is required; however, cases presenting extreme rotations should be investigated as routine practice. For rotations about the LR axis, we conclude that a tolerance level for patients on Arm 2 of the protocol should be set at ±5.0°. This tolerance represents the systematic setup error which would require correction if a variation to the initial plan was deemed unacceptable.

TH-D-AUD A-06: Study of Translational and Rotational Setup Errors and Their Correction Methods for Head & Neck Patients Using Kilovoltage Cone-Beam Computed Tomography (kV CBCT)

Purpose: To investigate the magnitude of the six degree setup errors in head&neck patients (HNC) and evaluate which correction data, obtained from three degree and six degree 3D/3D registration, is more appropriate for setup correction if the setup errors are corrected by translational shifts. Method and Materials: kV CBCT images were acquired on the first day of treatment and weekly thereafter for 21 HNC patients treated with IMRT. A total of 145 CBCT image sets were acquired. The CBCT images were registered with the corresponding planning CT images using two different 3D rigid registration approaches. With Approach1 the registrations were conducted with translations alone, with Approach2 all six degrees were taken into account. The setup error with the maximum rotational error was simulated on planning CT for two patients, then the errors were corrected by applying the translational data obtained from Approach1 (Correction1) and Approach2 (Correction2), respectively. Dosimetric indices were compared for the two corrections. Results: For these 21 HNC patients, the average translational errors determined with Approach1 were 1.0±3.5, 0.8±3.5, 1.6±3.8mm and the values determined with Approach2 were 1.1±5.0, 0.4±3.8, 2.2±4.7mm in LR, AP and SI directions respectively. The average rotational errors determined by Approach2 were 0.6°±1.1°, 0.1°±1.9°, 0.3°±0.8° and the average maximum errors were 0.9°±1.6°, 0.5°±3.0°, 0.4°±1.1° around LR, AP and SI axes respectively. The PTV prescription dose coverage was 86.1% and 92.3% for patient1, 92.1% and 92.4% for patent2 with Correction1 and Correction2 respectively. Conclusion: Relatively large rotational errors were observed in HNC patients. Instinctively, it appeared that the Correction1 were more accurate than Correction2 if only translational corrections were involved. The result for patient1 showed that it may not be the case. The dosimetric impact of both corrective approaches has to be further investigated to evaluate which approach should be applied to correct the setup errors.

TU‐FF‐A1‐04: Dosimetric Effect of Setup Errors On NPC IMRT Treatments

Purpose: To investigate the resulting dosimetry error, change in TCP and NTCP from random and systematic setup errors for IMRT treatment of NPC. Method and Materials: The IMRT plans of 16 NPC patients treated with SIB technique were analyzed. Random setup errors were simulated by convolving the fluence of each field with Gaussian distributions of σ equal to 2 or 3 mm. Leaf motion calculation was then performed on the convolved fluence and the corresponding dose calculated. Translational setup errors were simulated by translating the isocenter in the x, y and z direction by 2 and 3 mm respectively. Patient pitch, yaw and roll of up to 3° were simulated by collimator, couch and gantry rotations. The dose distributions were computed after the change. For the original plan and altered plans, DVHs were generated for GTV and the following: L/R optic nerves, brainstem and spinal cord. Dosimetric indices, such as EUD, TCP and NTCP were computed and compared. Results: TCP is not significantly affected by setup errors. For 3 mm random error, the average decrease in TCP was 1.1%. For systematic errors, the average decrease in TCP was <1%. Average change in NTCP for brainstem and spinal cord was found to be with ±1% for both random and systematic setup errors. However, systematic setup errors results in a substantial increase in NTCP for the optic nerves, especially when isocenter is shifted craniocaudally. The average increase of NTCP from 3 mm shift is 22.2%. Patient yaw also resulted in the greatest increase in NTCP amongst rotational setup errors. Conclusion: Our results indicate that both random and systematic setup errors have little effect on TCP. Change in NTCP for brainstem and spinal cord were also negligible for our cases. Nevertheless, the optic nerves are very sensitive to systematic setup errors.

Dosimetric Effects of Setup Uncertainties on Breast Treatment Delivery

This study aimed to assess the dosimetric impact of setup errors during the delivery of radiotherapy to the breast, and use this information to make recommendations on intervention tolerances for portal imaging of breast treatments. Translational and rotational setup errors were simulated for 10 recent breast patients using an Oncentra MasterPlan treatment planning system. The effect of these errors on the breast and tumor bed target volumes receiving 95% and 107% of the prescribed dose were assessed. For the majority of patients, shifts of up to 10 mm or a 4° patient rotation about the cranio-caudal axis had no significant effect on the dose distribution. Changes in dosimetry were more likely if the reference plan contained large hot or cold spots. For a typical patient, it is estimated that a shift of 5 mm in any one direction, or a 2° patient rotation would not cause more than a 5% change in the target volume receiving between 95% and 107% of the prescribed dose. If combinations of errors occur, greater dosimetric changes would be expected. It is concluded that individual patient shifts of up to 5 mm or rotations about the cranio-caudal axis of 2° or less are unlikely to affect dose-volume histogram parameters by an amount judged as clinically significant. Setup errors exceeding these values may cause large dosimetric changes for some patients, particularly those with larger hot or cold regions in the dose distribution, and intervention is therefore recommended.

Linac-based on-board imaging feasibility and the dosimetric consequences of head roll in head-and-neck IMRT plans

Kilovoltage imaging systems on linear accelerators are used for patient localization in many clinics. The purpose of this work is to assess on-board imaging (OBI) detection of systematic setup errors and in particular, the dosimetric consequences of undetected head roll in head-and-neck intensity modulated radiation therapy (IMRT) plans when using these systems. The system used in this study was the Trilogy linear accelerator and associated software (Varian Medical Systems, Palo Alto, CA). Accuracy of OBI localization was evaluated using an anthropomorphic head phantom. The head phantom is rigidly attached to a specially designed positioning device with 5° of freedom, 3 translational and 2 rotational in the axial and coronal planes. Simulated setup errors were 3° and 5° rotations in the axial plane and displacements of 5 mm in the left-right, anterior-posterior, and superior-inferior directions. The coordinates set by the positioning device were compared with the coordinates obtained as measured by using the image matching tools of paired 2-dimensional (2D) orthogonal image matching, and 3D cone-beam computed tomography (CT) volume matching. In addition, 6 physician-approved IMRT plans of nasopharynx and tonsil carcinoma were recalculated to evaluate the impact of undetected 3° and 5° head roll. Application of cone-beam CT (CBCT) for patient localization was superior to 2D matching techniques for detecting rotational setup errors. The use of CBCT allowed the determination of translational errors to within 0.5 mm, whereas kV planar was within 1 to 2 mm. Head roll in the axial plane was not easily detected with orthogonal image sets. Compared to the IMRT plans with no head roll, dose-volume histogram analysis demonstrated an average increase in the maximal spinal cord dose of 3.1% and 6.4% for 3° and 5° angles of rotation, respectively. Dose to the contralateral parotid was unchanged with 3° roll and increased by 2.7% with 5° roll. The results of this study show that volumetric setup verification using CBCT can improve bony anatomy setup detection to millimeter accuracy, and is a reliable method to detect head roll. However, the magnitude of possible dose errors due to undetected head roll suggests that CBCT does not need to be performed on a daily basis but rather weekly or bi-weekly to ensure fidelity of the head position with the immobilization system.

Set-up errors in patients undergoing image guided radiation treatment. Relationship to body mass index and weight loss

Background. The purpose of this study was to quantify the set-up errors of patient positioning during IGRT and to correlate set-up errors to patient-specific factors such as weight, height, BMI, and weight loss. Patients and methods. Thirty four consecutively treated head-and-neck cancer patients (H&N) and 20 lung cancer patients were investigated. Patients were positioned using customized immobilization devices consisting of vacuum cushions and thermoplastic shells. Treatment was given on an Elekta Synergy accelerator. Cone-beam acquisitions were obtained according to a standardized Action Limit protocol and compared to pre-treatment CT images. The average 3D deviation from three initial cone beam scans was compared to deviations at the 10th and 20th treatment session and correlated by linear regression analysis to height, weight, and BMI, and in H&N to weight loss as expressed by the relative weight change over time. Results. The SD of the translational and rotational random set-up errors during the first three sessions for H&N were 0.9mm (Left-Right), 1.1mm (Anterior-Posterior), 0.7mm (Cranio-Caudal) and 0.7° (LR-axis), 0.5° (AP-axis), and 0.7° (CC-axis). The equivalent data for lung cancer patients were 1.1mm (LR), 1.1mm (AP), 1.5 mm (CC) and 0.5° (LR-axis), 0.6° (AP-axis), and 0.4° (CC-axis). The median BMI for H&N and lung was 25.8 (17.6–39.7) and 23.7 (17.4–38.8), respectively. The median weekly weight change for H&N was −0.3% (−2.0 to 1.1%). With H&N and lung cancer analyzed separately, no statistically significant correlation was observed between set-up errors and height, weight, BMI, or weight change during treatment, irrespectively whether the 3D deviations from the initial three cone beam scans or scans from the 10th or 20th treatment sessions were used. Conclusion. This IGRT study did not support the hypothesis that set-up errors during radiotherapy are correlated to patient height, weight, BMI, or weight loss.

Study of Rotational Setup Errors and Their Dosimetric Impacts on Head and Neck IMRT Treatments Using Kilovoltage Cone-Beam Computed Tomography (kV CBCT)

With the on-board kV CBCT available, translational and rotational (six degree) setup errors can be obtained through a 3D/3D registration between the planning CT and CBCT images. The purpose of this study is to determine the magnitude of the rotational setup errors in head & neck patients and investigate the dosimetric impact of the residual rotational setup errors on head & neck IMRT to evaluate whether the full six degree setup correction is necessary.

Detection of treatment setup errors between two CT scans for patients with head and neck cancer

Accuracy of treatment setup for head and neck patients undergoing intensity-modulated radiation therapy is of paramount importance. The conventional method using orthogonal portal images can only detect translational setup errors while the most frequent setup errors for head and neck patients could be rotational errors. With the rapid development of image-guided radiotherapy, three-dimensional images are readily acquired and can be used to detect both translational and rotational setup errors. The purpose of this study is to determine the significance of rotational variations between two planning CT scans acquired for each of eight head and neck patients, who experienced substantial weight loss or tumor shrinkage. To this end, using a rigid body assumption, we developed an in-house computer program that utilizes matrix transformations to align point bony landmarks with an incremental best-fit routine. The program returns the quantified translational and rotational shifts needed to align the scans of each patient. The program was tested using a phantom for a set of known translational and rotational shifts. For comparison, a commercial treatment planning system was used to register the two CT scans and estimate the translational errors for these patients. For the eight patients, we found that the average magnitudes and standard deviations of the rotational shifts about the transverse, anterior-posterior, and longitudinal axes were 1.7 +/- 2.3 degrees, 0.8 +/- 0.7 degrees, and 1.8 +/- 1.1 degrees, respectively. The average magnitudes and standard deviations of the translational shifts were 2.5 +/- 2.6 mm, 2.9 +/- 2.8 mm, 2.7 +/- 1.7 mm while the differences detected between our program and the CT-CT fusion method were 1.8 +/- 1.3 mm, 3.3 +/- 5.4 mm, and 3.0 +/- 3.4 mm in the left-right, anterior-posterior, and superior-inferior directions, respectively. A trend of larger rotational errors resulting in larger translational differences between the two methods was observed. In conclusion, conventional methods used for verifying patient positioning may misinterpret rotational shifts as translational shifts, and our study demonstrated that rotational errors may be significant in the treatment of head and neck cancer.

Precision of Image-Guided Radiotherapy (IGRT) in Six Degrees of Freedom and Limitations in Clinical Practice

To evaluate the precision of image-guided radiotherapy (IGRT) using cone-beam computed tomography (CB-CT) for volume imaging and a robotic couch for correcting setup errors in six degrees of freedom. 47 consecutive patients with 372 fractions were classified according to whether a patient fixation device was used (pat(fix): n = 28) or not (pat(non-fix): n = 19). Prior to treatment a CB-CT was acquired and translational and rotational setup errors were corrected online without an action level using a robotic couch (HexaPOD). A second CB-CT was acquired after the correction process and after treatment in 134 and 238 fractions, respectively. In 17 fractions (4.6%) rotational errors > 3 degrees exceeded the motion range of the HexaPOD. Errors (3D vector) after the correction process were significantly smaller for pat(fix) compared to pat(non-fix) (p < 0.001): 0.9 mm +/- 0.5 mm and 1.6 mm +/- 0.8 mm, respectively. For pat(non-fix) the correction of rotational errors resulted in displacements of the patients on the angled couch of 0.6 mm/1 degree. Intrafractional motion further decreased precision in pat(non-fix) but not in pat(fix). Very high precision in cranial and extracranial treatment of immobilized patients was demonstrated. Without application of adequate immobilization the correction of rotational errors and intrafractional patient motion significantly decreased the accuracy of the online correction protocol.

SU-FF-J-44: Accuracy of Automatic Couch Corrections with On-Line Volumetric Imaging

Purpose: To assess the accuracy of patient positioning using on-line image-guidance and automatic couch correction. Method and Materials: During device commissioning, automated movements of the robotic couch were assessed using a Rando phantom and a Polaris optical navigation system giving sub-millimeter accuracy at approximately 18 Hz. Clinically, after assessment of a volumetric scan, a couch correction is applied if the displacement falls outside a pre-defined action-level of 3mm in any translational plane or greater than 5° of rotation. If the setup was outside of tolerance, the remote automatic couch was used to adjust patient position. Verification scans were acquired after the shift and the residual error analysis was evaluated retrospectively. The data from 34 patients over a one month period is included in this study, representing a total of 135 CBCT volumetric images. Results: The Rando phantom's residual error over 10 trials for a 10mm movement in the L/R, S/I, and A/P directions was 0.16±0.48mm, 0.32±0.30mm, and 0.11±0.12mm. The mean residual error for 34 patients in the L/R, S/I and A/P directions was 0.00±0.11cm, 0.01±0.10cm, and −0.03±0.08cm respectively. The rotational setup errors about the L/R, S/I and A/P axes were 0.67±1.33°, −0.47±1.30°, and −0.32±1.31°. The maximum measurement in the translational L/R, S/I and A/P directions was 0.28cm, 0.29cm, and 0.22cm, while the minimum values were −0.30cm, −0.29cm, and −0.27cm. The maximum rotational residual was 4.3°, 3.0°, and 3.1° about the L/R, S/I and A/P axes, while the minimum values were −2.2°, −3.9°, and −3.8°. Rotational errors are still apparent, as the remote couch does not correct for this. Conclusions: The remote automatic couch movement is reliable and accurate for adjusting the patient position. This gives the confidence that we can treat the intended target on a daily basis, quickly and efficiently. Research sponsored by Elekta.

Kilo-Voltage Cone-Beam Computed Tomography Setup Measurements for Lung Cancer Patients; First Clinical Results and Comparison With Electronic Portal-Imaging Device

Kilovoltage cone-beam computed tomography (CBCT) has been developed to provide accurate soft-tissue and bony setup information. We evaluated clinical CBCT setup data and compared CBCT measurements with electronic portal imaging device (EPID) images for lung cancer patients. The setup error for CBCT scans at the treatment unit relative to the planning CT was measured for 62 patients (524 scans). For 19 of these patients (172 scans) portal images were also made. The mean, systematic setup error (Sigma), and random setup error (sigma) were calculated for the CBCT and the EPID. The differences between CBCT and EPID and the rotational setup error derived from the CBCT were also evaluated. An offline shrinking action level correction protocol, based on the CBCT measurements, was used to reduce systematic setup errors and the impact of this protocol was evaluated. The CBCT setup errors were significantly larger than the EPID setup errors for the cranial-caudal and anterior-posterior directions (p < 0.05). The mean overall setup errors after correction measured with the CBCT were 0.2 mm (Sigma = 1.6 mm, sigma = 2.9 mm) in the left-right, -0.8 mm (Sigma = 1.7 mm, sigma = 4.0 mm) in cranial-caudal and 0.0 mm (Sigma = 1.5 mm, sigma = 2.0 mm) in the anterior-posterior direction. Using our correction protocol only 2 patients had mean setup errors larger than 5 mm, without this correction protocol 51% of the patients would have had a setup error larger than 5 mm. Use of CBCT scans provided more accurate information concerning the setup of lung cancer patients than did portal imaging.

2057: Repositioning Accuracy of Two Different Mask Systems: A Comparison by True 3D-3D Matching With Cone-Beam CT

2057: Repositioning Accuracy of Two Different Mask Systems: A Comparison by True 3D-3D Matching With Cone-Beam CT

Dosimetric effects of patient rotational setup errors on prostate IMRT treatments

The purpose of this work is to determine dose delivery errors that could result from systematic rotational setup errors (ΔΦ) for prostate cancer patients treated with three-phase sequential boost IMRT. In order to implement this, different rotational setup errors around three Cartesian axes were simulated for five prostate patients and dosimetric indices, such as dose–volume histogram (DVH), tumour control probability (TCP), normal tissue complication probability (NTCP) and equivalent uniform dose (EUD), were employed to evaluate the corresponding dosimetric influences. Rotational setup errors were simulated by adjusting the gantry, collimator and horizontal couch angles of treatment beams and the dosimetric effects were evaluated by recomputing the dose distributions in the treatment planning system. Our results indicated that, for prostate cancer treatment with the three-phase sequential boost IMRT technique, the rotational setup errors do not have significant dosimetric impacts on the cumulative plan. Even in the worst-case scenario with ΔΦ = 3°, the prostate EUD varied within 1.5% and TCP decreased about 1%. For seminal vesicle, slightly larger influences were observed. However, EUD and TCP changes were still within 2%. The influence on sensitive structures, such as rectum and bladder, is also negligible. This study demonstrates that the rotational setup error degrades the dosimetric coverage of target volume in prostate cancer treatment to a certain degree. However, the degradation was not significant for the three-phase sequential boost prostate IMRT technique and for the margin sizes used in our institution.

Pitch, roll, and yaw variations in patient positioning

To use pretreatment megavoltage-computed tomography (MVCT) scans to evaluate positioning variations in pitch, roll, and yaw for patients treated with helical tomotherapy. Twenty prostate and 15 head-and-neck cancer patients were selected. Pretreatment MVCT scans were performed before every treatment fraction and automatically registered to planning kilovoltage CT (KVCT) scans by bony landmarks. Image registration data were used to adjust patient setups before treatment. Corrections for pitch, roll, and yaw were recorded after bone registration, and data from fractions 1-5 and 16-20 were used to analyze mean rotational corrections. For prostate patients, the means and standard deviations (in degrees) for pitch, roll, and yaw corrections were -0.60 +/- 1.42, 0.66 +/- 1.22, and -0.33 +/- 0.83. In head-and-neck patients, the means and standard deviations (in degrees) were -0.24 +/- 1.19, -0.12 +/- 1.53, and 0.25 +/- 1.42 for pitch, roll, and yaw, respectively. No significant difference in rotational variations was observed between Weeks 1 and 4 of treatment. Head-and-neck patients had significantly smaller pitch variation, but significantly larger yaw variation, than prostate patients. No difference was found in roll corrections between the two groups. Overall, 96.6% of the rotational corrections were less than 4 degrees. The initial rotational setup errors for prostate and head-and-neck patients were all small in magnitude, statistically significant, but did not vary considerably during the course of radiotherapy. The data are relevant to couch hardware design for correcting rotational setup variations. There should be no theoretical difference between these data and data collected using cone beam KVCT on conventional linacs.

Magnitude and clinical relevance of translational and rotational patient setup errors: A cone-beam CT study

To establish volume imaging using an on-board cone-beam CT (CB-CT) scanner for evaluation of three-dimensional patient setup errors. The data from 24 patients were included in this study, and the setup errors using 209 CB-CT studies and 148 electronic portal images were analyzed and compared. The effect of rotational errors alone, translational errors alone, and combined rotational and translational errors on target coverage and sparing of organs at risk was investigated. Translational setup errors using the CB-CT scanner and an electronic portal imaging device differed <1 mm in 70.7% and <2 mm in 93.2% of the measurements. Rotational errors >2 degrees were recorded in 3.7% of pelvic tumors, 26.4% of thoracic tumors, and 12.4% of head-and-neck tumors; the corresponding maximal rotational errors were 5 degrees , 8 degrees , and 6 degrees . No correlation between the magnitude of translational and rotational setup errors was observed. For patients with elongated target volumes and sharp dose gradients to adjacent organs at risk, both translational and rotational errors resulted in considerably decreased target coverage and highly increased doses to the organs at risk compared with the initial treatment plan. The CB-CT scanner has been successfully established for the evaluation of patient setup errors, and its feasibility in day-to-day clinical practice has been demonstrated. Our results have indicated that rotational errors are of clinical significance for selected patients receiving high-precision radiotherapy.

SU‐FF‐T‐186: Dosimetric Effect of Patient Rotational Setup Errors On Prostate IMRT Plans

Purpose: To determine dose delivery errors that could result from systematic and random rotational setup errors for prostate patients treated with IMRT. Methods and Materials: Five prostate cancer patients treated with IMRT technique had their dose distributions re‐evaluated to assess the impact of systematic and random rotational setup errors. The IMRT treatments were delivered in three phases: 54 Gy to 95% of PTV1 (prostate and seminal vesicle plus a 10mm margin), 16.2 Gy to 95% of PTV2 (prostate plus a 5mm margin), and 5.4Gy to 95% of PTV3 (which was PTV2 excluding rectum). Rotational setup error can be equivalently simulated with matrix transformed equations by using the gantry, collimator and couch angles of treatment beams. Systematic rotational setup error ΔΦ was simulated around each of the three Cartesian axes (denoted as ΔΦLR, ΔΦAP, ΔΦSI) respectively. The dosimetric effect of random rotational setup error (Δσ) was also simulated with normal probability distribution. Various dosimetric parameters for the targets and surrounding normal organs were evaluated with respect to the IMRT plan used for treatment. Results: The cumulative dosimetric deviations of target (prostate) were within 1% when ΔΦ and Δσ were up to ±5° and 3° respectively. For OARs, when ΔΦLR=ΔΦAP=ΔΦSI=5° was simulated, the V40Gy and V70Gy of rectum deviated by 5.8±3.6% and 6.1±11.8% respectively, the V40Gy and V65Gy of bladder deviated by −1.0±2.9% and −1.9±4.4%. When ΔσLR=ΔσAP=ΔσSI=3° was simulated, the V40Gy and V70Gy of rectum deviated by −0.4±0.4% and −2.9±3.2%, the V40Gy and V65Gy of bladder deviated by 1.6±1.5% and 0.6±1.5%. Conclusions: For three phases sequential boost prostate IMRT technique, the rotational setup errors do not have a significant dosimetric impact on the cumulative target coverage when ΔΦ and Δσ were up to ±5° and 3° respectively. However, reduction of rotational setup errors can decrease doses to surrounding normal structures.

SU-FF-J-56: Detecting Treatment Setup Errors for Head and Neck IMRT Patients with Multiple CT Scans

Purpose: Conventional methods determine treatment setup errors in patients undergoing intensity-modulated radiotherapy (IMRT) by using orthogonal portal images that only detect translational errors. This study utilizes image-guided radiotherapy to develop and test an algorithm for detecting both translational and rotational setup errors in head and neck patients with multiple CT scans. Method and Materials: To confirm the accuracy of the proposed algorithm to detect translational shifts, it was tested using a constructed phantom with four embedded metal marks. Upon verification, ten head and neck patients with two temporally-independent CT scans were selected. For each patient, the translational and rotational errors were calculated by aligning selected bony landmarks from the two scans using matrix transformation with an incremental guess-check method. For comparison, the translational errors were also obtained by co-registering the two scans using the same landmarks with a commercial treatment planning system and comparing the isocenter of the second CT with that of the first CT. The contribution of the rotation to the translational error represents the difference between the translational shifts as computed by the algorithm and the conventional method. Results: The results of this study showed that the average magnitude of the rotational errors about the transverse, anterior-posterior, and longitudinal axes were 2.3° ± 2.2°, 1.3° ± 0.9°, 2.1° ± 1.6°, respectively. The average contribution of this rotation to the translational error in the left-right, anterior-posterior, and longitudinal directions were 3.1 ± 3.2, 2.0 ± 4.4, 4.4 ± 7.3 mm, respectively. In general, the greater the rotational error in the setup, the greater the subsequent translational difference between the methods. Conclusion: This study found that rotational errors do contribute significantly to the treatment setup error for head and neck IMRT patients. The proposed algorithm can detect these rotations and, consequently, more accurately determine the translations setup error.

SU-FF-J-35: A Comparison Between BAT and Tomotherapy MVCT On Inter-Fractional Prostate Localization

Purpose: To compare the clinical utilities of two image‐guided target localization systems in fractionated external beam radiotherapy: BAT™ ultra‐sound system and TomoTherapy™ mega‐voltage computed tomography (MVCT) system Method and Materials: Daily prostate setup verification data is obtained for 40 patients undergoing prostate external beam radiotherapy at University of California — Davis cancer center, using the ultra‐sound based target localization system (BAT™, North American Scientific, Chatsworth, CA). This set of data contains translation (superior‐inferior, lateral and anterior‐posterior) setup errors. At the University of Arkansas for Medical Sciences, 12 prostate patients undergoing helical tomotherapy (TomoTherapy Inc., Madison, WI) are helped with onboard MVCT to guide inter‐fractional prostate setups. The obtained MVCT data contain both translation and rotational (pitch, yaw and roll) setup errors. Statistics analysis using Microsoft Excel is done for both data sets.Results: BAT™ and MVCT reported similar magnitude of inter‐fractional prostate setup errors in this study. This result agrees with another direct daily CT and BAT™ comparison study reported by Lattanzi et al. (1999). However, our data show a larger range of distribution due to different groups of patients at different institutions being used in this study (more detailed data analysis result and discussion is reported with the attached supporting document). Conclusion: Both BAT™ and Tomotherapy MVCT can be applied to image‐guided inter‐fractional prostate setup procedures. These techniques are principally more accurate than conventional skin tattoo based setups in clinical practice.

A novel method to correct for pitch and yaw patient setup errors in helical tomotherapy

An accurate means of determining and correcting for daily patient setup errors is important to the cancer outcome in radiotherapy. While many tools have been developed to detect setup errors, difficulty may arise in accurately adjusting the patient to account for the rotational error components. A novel, automated method to correct for rotational patient setup errors in helical tomotherapy is proposed for a treatment couch that is restricted to motion along translational axes. In tomotherapy, only a narrow superior/inferior section of the target receives a dose at any instant, thus rotations in the sagittal and coronal planes may be approximately corrected for by very slow continuous couch motion in a direction perpendicular to the scanning direction. Results from proof-of-principle tests indicate that the method improves the accuracy of treatment delivery, especially for long and narrow targets. Rotational corrections about an axis perpendicular to the transverse plane continue to be implemented easily in tomotherapy by adjustment of the initial gantry angle.

Reduction of set-up error and intra-factional motion in head and neck intensity modulated radiotherapy using a real-time tumor tracking system with a gold-marker implanted mouthpiece

Reduction of set-up error and intra-factional motion in head and neck intensity modulated radiotherapy using a real-time tumor tracking system with a gold-marker implanted mouthpiece