Systematic Position Errors Research Articles

A four-year comprehensive analysis of random and systematic position errors in a RT centre running two linacs equipped with kV-CBCT

A four-year comprehensive analysis of random and systematic position errors in a RT centre running two linacs equipped with kV-CBCT

Azimuthal polarization filtering for accurate, precise, and robust single-molecule localization microscopy.

Many single nanoemitters such as fluorescent molecules produce dipole radiation that leads to systematic position errors in both particle tracking and super-resolution microscopy. Via vectorial diffraction equations and simulations, we show that imaging only azimuthally polarized light in the microscope naturally avoids emission from the z-component of the transition dipole moment, resulting in negligible localization errors for all emitter orientations and degrees of objective lens misfocus. Furthermore, localization accuracy is maintained even in the presence of aberrations resulting from imaging in mismatched media.

Dosimetry modeling for focal high-dose-rate prostate brachytherapy

The dosimetry of focal high-dose-rate prostate brachytherapy was assessed. Dose volume histogram parameters, robustness to source position errors, and Monte Carlo (MC) simulations were compared for whole-gland (WG), hemi-gland (HEMI), and ultra-focal (UF) treatment plans. Tumor volumes were delineated based on MRI and template biopsy results for 9 patients. WG, HEMI, and UF plans were produced assuming 19Gy single fraction monotherapy treatments. For UF plans, a 6-mm margin was applied to the visible tumor to create a focal-planning target volume (F-PTV). Systematic source position shifts of 1-4mm were applied to assess plan robustness. The dosimetric impact of steel catheters was assessed using MC simulation. Mean D90 and V100 were 20.4Gy and 97.9% for prostate in WG plans, 22.2Gy and 98.1% for hemi-prostate in HEMI plans, and 23.0Gy and 98.2% for F-PTV in UF plans. Mean urethra D10 was 20.3, 19.7, and 9.2Gy in WG, HEMI, and UF plans, respectively. Mean rectal D2cc was 12.5, 9.8, and 4.6Gy in WG, HEMI, and UF plans, respectively. Focal treatment plans were sensitive to source position errors-2mm systematic shifts reduced mean prostate D90 by 0.7%, hemi-prostate D90 by 2.6%, and F-PTV D90 by 8.3% in WG, HEMI, and UF plans, respectively. MC simulation results were similar for all plan types with most dose volume histogram parameters reduced by <2%. HEMI and UF treatments can achieve higher D90 values compared with WG treatments with reduced organ at risk dose. Focal treatments are more sensitive to systematic source position errors than WG treatments.

SU-E-T-405: Robustness of Volumetric-Modulated Arc Therapy (VMAT) Plans to Systematic MLC Positional Errors

Purpose: To evaluate the dosimetric impact of systematic MLC positional errors (PEs) on the quality of volumetric-modulated arc therapy (VMAT) plans. Methods: Five patients with head-and-neck cancer (HN) and five patients with prostate cancer were randomly chosen for this study. The clinically approved VMAT plans were designed with 2–4 coplanar arc beams with none-zero collimator angles in the Pinnacle planning system. The systematic MLC PEs of 0.5, 1.0, and 2.0 mm on both MLC banks were introduced into the original VMAT plans using an in-house program, and recalculated with the same planned Monitor Units in the Pinnacle system. For each patient, the original VMAT plans and plans with MLC PEs were evaluated according to the dose-volume histogram information and Gamma index analysis. Results: For one primary target, the ratio of V100 in the plans with 0.5, 1.0, and 2.0 mm MLC PEs to those in the clinical plans was 98.8 ± 2.2%, 97.9 ± 2.1%, 90.1 ± 9.0% for HN cases and 99.5 ± 3.2%, 98.9 ± 1.0%, 97.0 ± 2.5% for prostate cases. For all OARs, the relative difference of Dmean in all plans was less than 1.5%. With 2mm/2% criteria for Gamma analysis, the passing rates were 99.0 ± 1.5% for HN cases and 99.7 ± 0.3% for prostate cases between the planar doses from the original plans and the plans with 1.0 mm MLC errors. The corresponding Gamma passing rates dropped to 88.9 ± 5.3% for HN cases and 83.4 ± 3.2% for prostate cases when comparing planar doses from the original plans and the plans with 2.0 mm MLC errors. Conclusion: For VMAT plans, systematic MLC PEs up to 1.0 mm did not affect the plan quality in term of target coverage, OAR sparing, and Gamma analysis with 2mm/2% criteria.

SU‐E‐T‐307: Susceptibility of SBRT to Delivery Uncertainties Using RapidArc Or Dynamic IMRT

Purpose:To determine if SBRT is more susceptible to delivery uncertainties when using either RapidArc or dynamic IMRT.Methods:Systematic or random uncertainties in MLC leaf positions, gantry position and dose rate were incorporated into RapidArc (RA) and sliding‐window IMRT (SW) treatment plans for retrospective SBRT lung and liver cases, which also included the use of FFF or SRS energies and therefore higher dose rates. Initial plan comparisons included homogeneity and conformity indices. Dose distributions and dose‐value histograms of original and modified plans were compared using Gamma analysis and dose indices of planned treatment volumes (PTV) and organs at risk (OAR).Results:SBRT lung PTVs using RA were more susceptible to systematic MLC delivery errors than SW (∼6%/mm vs. ∼4%/mm), in contrast to a previous study examining hyperfractionated prostate and head‐and‐neck treatments; esophagus and cord Dmin and Dmax values changed proportionately for both techniques; RA plans were superior globally but not locally (PTV) using Gamma analyses. Uniformly distributed random variations of 2 mm for active MLC leaves had a negligible effect on all PTV dose distributions (Dmin, Dmax &lt; 0.4%). PTV coverage was affected equally when incorporating systematic gantry errors.Conclusion:RapidArc deliveries were found to be more sensitive to systematic errors in MLC leaf positions, in contrast to a previous study on hyperfractionated treatments. However, this is reasonable as both studies correlate average segmental field size or mean leaf gap with adverse dosimetric effects due to such uncertainties; mean leaf gaps for either technique in this study were similar. Gantry angle variations had similar effects on both techniques.

SU‐E‐T‐425: Spherical Dose Distributions for Radiosurgery Using a Standardized MLC Plan

Purpose:To investigate a standardized MLC treatment plan to generate small spherical dose distributions.Methods:The static virtual cone plan comprised six table positions with clockwise and counterclockwise arcs having collimator angles 45 and 135 degrees, respectively, at each position. The central two leaves of a 2.5 mm leaf width MLC were set to a constant gap. Control points were weighted proportional to the sine of the gantry angle. Plans were created for the 10 MV flattening‐filter‐free beam of a TrueBeam STx (Varian Medical Systems) with gaps of 1, 1.5, 2, and 3 mm and were delivered to a phantom containing radiochromic film. Dose was calculated using the Eclipse AAA (Varian Medical Systems). A dynamic plan in which the table and gantry moved simultaneously with 1.5 mm gap was also created and delivered using the TrueBeam developer mode.Results:The full‐width‐half‐max (FWHM) varied with leaf gap, ranging from 5.2 to 6.2 mm. Calculated FWHM was smaller than measured by 0.7 mm for the 1 mm gap and ≤ 0.4 mm for the larger gaps. The measured‐to‐calculated dose ratio was 0.93, 0.96, 1.01, and 0.99 for 1 mm, 1.5 mm, 2 mm, and 3 mm gaps, respectively. The dynamic results were the same as the static. The position deviations between the phantom target position and the center of the dose distribution were &lt; 0.4 mm.Conclusion:The virtual cone can deliver spherical dose distributions suitable for radio surgery of small targets such as the trigeminal nerve. The Eclipse AAA accurately calculates the expected dose, particularly for leaf gap ≥ 1.5 mm. The measured dose distribution is slightly larger than the calculation, which is likely due to systematic leaf position error, isocenter variation due to gantry sag and table eccentricity, and inaccuracy in MLC leaf end modeling.

Astrometric calibration of the Xuyi Schmidt Telescope Photometric Survey of the Galactic Anti-center (XSTPS-GAC)

We present astrometric calibration of the Xuyi Schmidt Telescope Photometric Survey of the Galactic Anti-center (XSTPS-GAC). XSTPS-GAC is the photometric part of the Digital Sky Survey of the Galactic Anti-center (DSS-GAC), which is a photometric and spectroscopic sky survey, in combination with LAMOST. In order to select an astrometric reference catalog, we made comparisons between the four widely used astrometric catalogs, GSC2.3, USNO-B1.0, UCAC3 and PPMXL. PPMXL shows relatively small systematic errors in positions and more homogeneous proper motion distributions toward the Galactic Anti-center (GAC), and was selected as the reference catalog. Based on the high quality and bright reference stars that were picked out from PPMXL, we performed a 4th-order polynomial fitting in image units, to construct the transformation relation between coordinates used by XSTPS-GAC and standard coordinates, and to simultaneously correct the image distortions in the CCD. Then we applied the derived relation to all sources to obtain their mean celestial coordinates based on the International Celestial Reference System. For bright point sources with r < 17.0 mag, the accuracy of astrometric calibration could reach about 80 mas for each of the g, r, i bands, with systematic errors being less than 10 mas. But for the faint sources at the brightness limit of the survey, which was r ∼ 19.0 mag, the accuracy can still reach 200 mas. After combining all observations, the final weighted average coordinates could reach an accuracy of less than 70 mas for bright stars. For faint stars, the rms residuals of weighted coordinates decrease to ∼ 110 mas. The final combined XSTPS-GAC coordinates show a good consistency with the Sloan Digital Sky Survey.

RADIO-OPTICAL REFERENCE FRAME LINK USING THE U.S. NAVAL OBSERVATORY ASTROGRAPH AND DEEP CCD IMAGING

Between 1997 and 2004 several observing runs were conducted mainly with the CTIO 0.9 m to image ICRF counterparts (mostly QSOs) in order to determine accurate optical positions. Contemporary to these deep CCD images the same fields were observed with the US Naval Observatory (USNO) astrograph in the same bandpass. They provide accurate positions on the Hipparcos/Tycho-2 system for stars in the 10 to 16 magnitude range used as reference stars for the deep CCD imaging data. Here we present final optical position results of 413 sources based on reference stars obtained by dedicated astrograph observations which were reduced following 2 different procedures. These optical positions are compared to radio VLBI positions. The current optical system is not perfectly aligned to the ICRF radio system with rigid body rotation angles of 3 to 5 mas (= 3 sigma level) found between them for all 3 axes. Furthermore, statistically, the optical minus radio position differences are found to exceed the total, combined, known errors in the observations. Systematic errors in the optical reference star positions as well as physical offsets between the centers of optical and radio emissions are both identified as likely causes. A detrimental, astrophysical, random noise (DARN) component is postulated to be on about the 10 mas level. If confirmed by future observations, this could severely limit the Gaia to ICRF reference frame alignment accuracy to an error of about 0.5 mas per coordinate axis with the current number of sources envisioned to provide the link. A list of 36 ICRF sources without the detection of an optical counterpart to a limiting magnitude of about R=22 is provided as well.

Evaluation of overall setup accuracy and adequate setup margins in pelvic image-guided radiotherapy: Comparison of the male and female patients

We evaluated adequate setup margins for the radiotherapy (RT) of pelvic tumors based on overall position errors of bony landmarks. We also estimated the difference in setup accuracy between the male and female patients. Finally, we compared the patient rotation for 2 immobilization devices. The study cohort included consecutive 64 male and 64 female patients. Altogether, 1794 orthogonal setup images were analyzed. Observer-related deviation in image matching and the effect of patient rotation were explicitly determined. Overall systematic and random errors were calculated in 3 orthogonal directions. Anisotropic setup margins were evaluated based on residual errors after weekly image guidance. The van Herk formula was used to calculate the margins. Overall, 100 patients were immobilized with a house-made device. The patient rotation was compared against 28 patients immobilized with CIVCO's Kneefix and Feetfix. We found that the usually applied isotropic setup margin of 8mm covered all the uncertainties related to patient setup for most RT treatments of the pelvis. However, margins of even 10.3mm were needed for the female patients with very large pelvic target volumes centered either in the symphysis or in the sacrum containing both of these structures. This was because the effect of rotation (p ≤ 0.02) and the observer variation in image matching (p ≤ 0.04) were significantly larger for the female patients than for the male patients. Even with daily image guidance, the required margins remained larger for the women. Patient rotations were largest about the lateral axes. The difference between the required margins was only 1mm for the 2 immobilization devices. The largest component of overall systematic position error came from patient rotation. This emphasizes the need for rotation correction. Overall, larger position errors and setup margins were observed for the female patients with pelvic cancer than for the male patients.

Identical Quality Assurance for Volumetric Modulated Arc Therapy in Elekta and Varian Machines.

Volumetric modulated arc therapy (VMAT) has been adopted by many clinics for its higher delivery efficiency compared to conventional intensity modulated radiotherapy techniques. Currently, the quality assurance (QA) has remained a challenge in that no identical tests are available for accelerators from different vendors. This study is the first attempt to design identical QA tests for the VMAT technique for Varian and Elekta machines. Identical procedures testing MLC positions and movements, dose rate variations, and gantry positions and movement were created for both machines. These included picket fence (PF), dose rate vs. gantry speed (DRGS) and MLC speed vs. dose rate (MLCDR) tests. Deliverable plans for these tests were made with in-house software that was deliverable for linacs from two vendors (Elekta Synergy and Varian Trilogy). The electronic portal imaging device (EPID) was used for these tests. An automated analysis method was established and software was created to quantitatively evaluate the result. The systematic gap position and width errors from PF tests were within 0.5 mm. We evaluated the detectability of this program for introduced errors down to 0.2 mm. Linear relationships existed between the introduced errors and measured errors. In the DRGS test, 99.8% and 100.0% of the intensity deviations from expected profiles were less than 3% for the Synergy and Trilogy, respectively. For the MLCDR, the intensity deviations from expected profiles less than 3% were 100.0% for Synergy and 98.1% for Trilogy. Identical test series were created and implemented for VMAT accelerators from two vendors. Test results were reported from both accelerators. Comparable results were obtained from both vendors, enabling uniform criteria to be established for VMAT quality assurance.

Potential position errors using fiducial markers for gated image guided radiotherapy

Background. Fiducials can be used as surrogate for target position during radiotherapy. However, fiducial motion could lead to potential position errors when using fiducials in four-dimensional computed tomography (4DCT) treatment planning and for gated image guided radiotherapy (IGRT). Material and methods. One gold marker (GM) and 5, 10 and 15 mm nickel-titanium (NiTi) stents were inserted in a moving phantom for the purpose of fiducial detection in 4DCT and gated IGRT. Fiducial position errors in 4DCT and BrainLAB's gated IGRT were defined as residuals between fiducial detection and the actual physical position at the instance of image acquisition. Results. Fiducials position errors correlate to speed, fiducial type and orientation during 4DCT acquisition. Lower detection accuracy was measured for the 5 mm NiTi-stent relative to the 10 and 15 mm NiTi stents and GM. Fiducials with orientation 45° relative to the scan direction showed a lower detection accuracy relative to parallel and perpendicular orientations. The standard deviation of position errors in 4DCT were up to 2.2 mm with a maximum deviation of 4.0 mm. Using BrainLAB's gated IGRT the fiducials were detected with a standard deviation of 0.6 mm and a maximum deviation of 1.9 mm. For gated IGRT no correlation to fiducial speed was found. Conclusions. Clinical use of fiducials in combination with treatment planning on mid-ventilation CT phase for moving target should include margins up to 5.5 mm due to potential systematic position errors.

Interfractional Position Variation of Pancreatic Tumors Quantified Using Intratumoral Fiducial Markers and Daily Cone Beam Computed Tomography

The aim of this study was to quantify interfractional pancreatic position variation using fiducial markers visible on daily cone beam computed tomography (CBCT) scans. In addition, we analyzed possible migration of the markers to investigate their suitability for tumor localization. For 13 pancreatic cancer patients with implanted Visicoil markers, CBCT scans were obtained before 17 to 25 fractions (300 CBCTs in total). Image registration with the reference CT was used to determine the displacement of the 2 to 3 markers relative to bony anatomy and to each other. We analyzed the distance between marker pairs as a function of time to identify marker registration error (SD of linear fit residuals) and possible marker migration. For each patient, we determined the mean displacement of markers relative to the reference CT (systematic position error) and the spread in displacements (random position error). From this, we calculated the group systematic error, Σ, and group random error, σ. Marker pair distances showed slight trends with time (range, -0.14 to 0.14 mm/day), possibly due to tissue deformation, but no shifts that would indicate marker migration. The mean SD of the fit residuals was 0.8 mm. We found large interfractional position variations, with for 116 of 300 (39%) fractions a 3-dimensional vector displacement of >10 mm. The spread in displacement varied significantly (P<.01) between patients, from a vector range of 9.1 mm to one of 24.6 mm. For the patient group, Σ was 3.8, 6.6, and 3.5 mm; and σ was 3.6, 4.7 and 2.5 mm, in left-right, superior-inferior, and anterior-posterior directions, respectively. We found large systematic displacements of the fiducial markers relative to bony anatomy, in addition to wide distributions of displacement. These results for interfractional position variation confirm the potential benefit of using fiducial markers rather than bony anatomy for daily online position verification for pancreatic cancer patients.

SU‐E‐T‐665: Investigation of Uniform Planning Margin for Intensity Modulated Proton Therapy of Pediatric Brain Tumors

Purpose: Determine if a uniform planning target volume (PTV) margin can be used for intensity modulated proton therapy (IMPT) of centrally located pediatric brain tumors and determine the most appropriate value. Methods: Treatment planning data from three pediatric patients with centrally located brain tumors were used in this study. For each patient, two IMPT plan types, a 3 field multi‐field‐optimized (MFO) and a 3 field single‐field‐optimized (SFO), and a 5 field IMRT plan were created for 7 uniform PTV margins that ranged from 0 to 10mm. The IMPT plans did not use robust optimization. For all plan types, the target coverage was held constant (98% of CTV received 54Gy and 95% of PTV received &gt;53Gy) and normal tissue doses were minimized. Four error types, each with numerous variations, were simulated and their effect were recorded. The four error types were: systematic plan position error, systematic field position error, systematic range error, and random position error. Results: Regarding normal tissues, there was no notable difference between MFO and SFO, both were substantially better than IMRT, e.g. mean cochlea dose was 14Gy for IMRT with 3mm margin and 5Gy for the IMPT plans. In general, smaller margins gave better normal tissue sparring. For both IMPT plan types and IMRT, PTV margins less than 3mm lead to target coverage deficits under the various errors. For position errors, the IMPT and IMRT plans gave similar results as a function of margin size. For density and systematic field errors, the SFO plans maintained target coverage better than MFO. Conclusions: A uniform PTV margin is not suitable for MFO planning. However, a uniform PTV margin of 3mm is appropriate for SFO planning when considering the above uncertainties (excluding large errors). At 3mm, which is also required for IMRT, the SFO plans had superior normal tissue sparing. Varian Medical Systems

WE-E-141-06: Identical Quality Assurance for Volumetric Modulated Arc Therapy in Elekte and Varian Machines

Purpose: Volumetric modulated arc therapy (VMAT) has been adopted by many clinics for its higher delivery efficiency than other conventional intensity modulated radiotherapy techniques. Currently, no identical quality assurance (QA) tests are available for different vendors. This study is the first attempt to design identical QA tests for the VMAT technique for Varian and Elekta machines. Methods: Procedures testing MLC positions and movements, dose rate variations, gantry positions and movements were created. These include picker fence (PF), dose rate vs. gantry speed (DRGS) and MLC speed vs. dose rate (MLCDR) tests. Deliverable plans for these tests were made with in‐house software that was deliverable for linacs from two vendors (Linac1 for Elekta and Linac2 for Varian). The electronic portal imaging device (EPID) was used to detect these tests. An automated analyses method was established and software coded to quantitatively evaluate the Result. Results: The systematic gap position and width errors from PF tests were within 0.5mm. We evaluated the detectability of this program for introduced errors down to 0.2mm. Linear relationships existed between the introduced errors and measured errors. In the DRGS test, 99.8% and 100.0% of the intensity deviations from expected profiles were less than 3% for the Linac1 and Linac2, respectively. For the MLCDR, the intensity deviations from expected profiles less than 3% were 100.0% for Linac1 and 98.1% for Linac2. Conclusion: Identical test series were created and implemented for VMAT accelerators from two vendors. Test results were reported from both accelerators. Comparable results were obtained from both vendors, and enabling uniform criteria to be established for VMAT quality assurance.

Effect of MLC leaf position, collimator rotation angle, and gantry rotation angle errors on intensity-modulated radiotherapy plans for nasopharyngeal carcinoma

The purpose of this study was to investigate the effect of multileaf collimator (MLC) leaf position, collimator rotation angle, and accelerator gantry rotation angle errors on intensity-modulated radiotherapy plans for nasopharyngeal carcinoma. To compare dosimetric differences between the simulating plans and the clinical plans with evaluation parameters, 6 patients with nasopharyngeal carcinoma were selected for simulation of systematic and random MLC leaf position errors, collimator rotation angle errors, and accelerator gantry rotation angle errors. There was a high sensitivity to dose distribution for systematic MLC leaf position errors in response to field size. When the systematic MLC position errors were 0.5, 1, and 2mm, respectively, the maximum values of the mean dose deviation, observed in parotid glands, were 4.63%, 8.69%, and 18.32%, respectively. The dosimetric effect was comparatively small for systematic MLC shift errors. For random MLC errors up to 2mm and collimator and gantry rotation angle errors up to 0.5°, the dosimetric effect was negligible. We suggest that quality control be regularly conducted for MLC leaves, so as to ensure that systematic MLC leaf position errors are within 0.5mm. Because the dosimetric effect of 0.5° collimator and gantry rotation angle errors is negligible, it can be concluded that setting a proper threshold for allowed errors of collimator and gantry rotation angle may increase treatment efficacy and reduce treatment time.

Planning Target Volume Margin Evaluation and Critical Structure Sparing for Rectal Cancer Patients Treated Prone on a Bellyboard

AimsTo calculate a planning target volume (PTV) margin that would account for inter-fractional systematic and random clinical target volume positional errors for patients treated prone on a recently available couch top bellyboard and to evaluate potential critical structure dose reduction using intensity-modulated radiotherapy (IMRT) techniques. Materials and methodsTwenty-four patients (12 men and 12 women) were included in this study, all treated on a commercial bellyboard. Cone beam computed tomography (CBCT) data were acquired once every five fractions for a total of five images per patient. A three-dimensional–three-dimensional bony anatomy auto-match was carried out off-line and the residual difference in position used as a surrogate for clinical target volume inter-fractional positional errors. Systematic (Σ) and random (σ) variations were evaluated and used in PTVmargin=1.96Σ+0.7σ. The influence of intra-fractional positional errors was evaluated in the margin analysis by introducing published values. Critical structure sparing, as a function of PTVmargin size, was investigated through the evaluation of three-dimensional conformal radiation therapy (3DCRT) and IMRT treatment plans developed using the margin derived from this work, the American Society for Radiation Oncology Contouring Atlas and the Radiation Therapy Oncology Group 0822 trial specifications. ResultsThe PTVmargin that accounts for only the inter-fractional positional errors was calculated to be (anterior–posterior (AP), superior–inferior (SI), left–right (LR))=(5.2mm, 3.1mm, 2.8mm). If we assumed a combined intra-fractional motion up to 3.0mm then the required PTVmargin increased to (AP, SI, LR)=(7.0mm, 5.0mm, 5.0mm). Treatment plan evaluation showed that the bellyboard provides excellent small bowel sparing regardless of planning technique. In most cases, IMRT reduced the average femoral head, bladder and small bowel dose by 20, 15 and 40% with respect to 3DCRT planning. ConclusionA PTVmargin expansion of (AP, SI, LR)=(7.0mm, 5.0mm, 5.0mm) is required to account for all positional uncertainties. The use of a bellyboard with IMRT provides better critical structure sparing when compared with a bellyboard with 3DCRT.

Direct impact analysis of multi-leaf collimator leaf position errors on dose distributions in volumetric modulated arc therapy: a pass rate calculation between measured planar doses with and without the position errors**This work was presented in part at the 52nd Annual Meeting of the American Society for Therapeutic Radiology and Oncology in San Diego on 1 November 2010.

We propose a new method for analyzing the direct impact of multi-leaf collimator (MLC) leaf position errors on dose distributions in volumetric modulated arc therapy (VMAT). The technique makes use of the following processes. Systematic leaf position errors are generated by directly changing a leaf offset in a linac controller; dose distributions are measured by a two-dimensional diode array; pass rates of the dose difference between measured planar doses with and without the position errors are calculated as a function of the leaf position error. Three different treatment planning systems (TPSs) were employed to create VMAT plans for five prostate cancer cases and the pass rates were compared between the TPSs under various leaf position errors. The impact of the leaf position errors on dose distributions depended upon the final optimization result from each TPS, which was explained by the correlation between the dose error and the average leaf gap width. The presented method determines leaf position tolerances for VMAT delivery for each TPS, which may facilitate establishing a VMAT quality assurance program in a radiotherapy facility.

Understanding the impact of RapidArc therapy delivery errors for prostate cancer

The purpose of this study is to simulate random and systematic RapidArc delivery errors for external beam prostate radiotherapy plans in order to determine the dose sensitivity for each error type. Ten prostate plans were created with a single 360° arc. The DICOM files for these treatment plans were then imported into an in‐house computer program that introduced delivery errors. Random and systematic gantry position (0.25°, 0.5°, 1°), monitor unit (MU) (1.25%, 2.5%, 5%), and multileaf collimator (MLC) position (0.5, 1, 2 mm) errors were introduced. The MLC errors were either random or one of three types of systematic errors, where the MLC banks moved in the same (MLC gaps remain unchanged) or opposing directions (increasing or decreasing the MLC gaps). The generalized equivalent uniform dose (gEUD) was calculated for the original plan and all treatment plans with errors introduced. The dose sensitivity for the cohort was calculated using linear regression for the gantry position, MU, and MLC position errors. Because there was a large amount of variability for systematic MLC position errors, the dose sensitivity of each plan was calculated and correlated with plan MU, mean MLC gap, and the percentage of MLC leaf gaps less than 1 and 2 cm for each individual plan. We found that random and systematic gantry position errors were relatively insignificant (< 0.1% gEUD change) for gantry errors up to 1°. Random MU errors were also insignificant, and systematic MU increases caused a systematic increase in gEUD. For MLC position errors, random MLC errors were relatively insignificant up to 2 mm as had been determined in previous IMRT studies. Systematic MLC shift errors caused a decrease of approximately −1% in the gEUD per mm. For systematic MLC gap open errors, the dose sensitivity was 8.2%/mm and for MLC gap close errors the dose sensitivity was −7.2%/mm. There was a large variability for MLC gap open/close errors for the ten RapidArc plans which correlated strongly with MU, mean gap width, and percentage of MLC gaps less than 1 or 2 cm. This study evaluates the magnitude of various simulated RapidArc delivery errors by calculating gEUED on various prostate plans.PACS numbers: 87.55.x, 87.55.D, 87.55.de, 87.55.dk

Dosimetric impact of systematic MLC positional errors on step and shoot IMRT for prostate cancer: a planning study

The positional accuracy of multileaf collimators (MLC) is crucial in ensuring precise delivery of intensity-modulated radiotherapy (IMRT). The aim of this planning study was to investigate the dosimetric impact of systematic MLC positional errors on step and shoot IMRT of prostate cancer. A total of 12 perturbations of MLC leaf banks were introduced to six prostate IMRT treatment plans to simulate MLC systematic positional errors. Dose volume histograms (DVHs) were generated for the extraction of dose endpoint parameters. Plans were evaluated in terms of changes to the defined endpoint dose parameters, conformity index (CI) and healthy tissue avoidance (HTA) to planning target volume (PTV), rectum and bladder. Negative perturbations of MLC had been found to produce greater changes to endpoint dose parameters than positive perturbations of MLC (p<0.01). Negative and positive asynchronised MLC perturbations of -1mm resulted in median changes in D(95) of -1.2 and 0.9% respectively. Negative and positive synchronised MLC perturbations of 1mm in one direction resulted in median changes in D(95) of -2.3 and 1.8% respectively. Doses to rectum were generally more sensitive to systematic MLC errors compared to bladder (p<0.01). Negative and positive synchronised MLC perturbations of 1mm in one direction resulted in median changes in endpoint dose parameters of rectum and bladder from 1.0 to 2.5%. Maximum reduction of -4.4 and -7.3% were recorded for conformity index (CI) and healthy tissue avoidance (HTA) respectively due to synchronised MLC perturbation of 1mm. MLC errors resulted in dosimetric changes in IMRT plans for prostate.

Adaptation of catch-up saccades during the initiation of smooth pursuit eye movements

Reduction of retinal speed and alignment of the line of sight are believed to be the respective primary functions of smooth pursuit and saccadic eye movements. As the eye muscles strength can change in the short-term, continuous adjustments of motor signals are required to achieve constant accuracy. While adaptation of saccade amplitude to systematic position errors has been extensively studied, we know less about the adaptive response to position errors during smooth pursuit initiation, when target motion has to be taken into account to program saccades, and when position errors at the saccade endpoint could also be corrected by increasing pursuit velocity. To study short-term adaptation (250 adaptation trials) of tracking eye movements, we introduced a position error during the first catch-up saccade made during the initiation of smooth pursuit-in a ramp-step-ramp paradigm. The target position was either shifted in the direction of the horizontally moving target (forward step), against it (backward step) or orthogonally to it (vertical step). Results indicate adaptation of catch-up saccade amplitude to back and forward steps. With vertical steps, saccades became oblique, by an inflexion of the early or late saccade trajectory. With a similar time course, post-saccadic pursuit velocity was increased in the step direction, adding further evidence that under some conditions pursuit and saccades can act synergistically to reduce position errors.