Degree Collimator Research Articles

Evaluation of the ability of three commercially available dosimeters to detect systematic delivery errors in step-and-shoot IMRT plans.

There is limited data on error detectability for step-and-shoot intensity modulated radiotherapy (sIMRT) plans, despite significant work on dynamic methods. However, sIMRT treatments have an ongoing role in clinical practice. This study aimed to evaluate variations in the sensitivity of three patient-specific quality assurance (QA) devices to systematic delivery errors in sIMRT plans. Four clinical sIMRT plans (prostate and head and neck) were edited to introduce errors in: Multi-Leaf Collimator (MLC) position (increasing field size, leaf pairs offset (1-3 mm) in opposite directions; and field shift, all leaves offset (1-3 mm) in one direction); collimator rotation (1-3 degrees) and gantry rotation (0.5-2 degrees). The total dose for each plan was measured using an ArcCHECK diode array. Each field, excluding those with gantry offsets, was also measured using an Electronic Portal Imager and a MatriXX Evolution 2D ionisation chamber array. 132 plans (858 fields) were delivered, producing 572 measured dose distributions. Measured doses were compared to calculated doses for the no-error plan using Gamma analysis with 3%/3 mm, 3%/2 mm, and 2%/2 mm criteria (1716 analyses). Generally, pass rates decreased with increasing errors and/or stricter gamma criteria. Pass rate variations with detector and plan type were also observed. For a 3%/3 mm gamma criteria, none of the devices could reliably detect 1 mm MLC position errors or 1 degree collimator rotation errors. This work has highlighted the need to adapt QA based on treatment plan type and the need for detector specific assessment criteria to detect clinically significant errors.

A Dosimetric Analysis of Dorsal Nerve Root Ablation Therapy using a Virtual Cone Technique

Virtual cone radiosurgery has been established as an effective treatment for trigeminal neuralgia. This delivery technique employs multiple static-field arcs with HDMLC-defined apertures to deliver high doses to small target volumes (<1cc) with sharp dose gradients. As a replacement to a physical cone, virtual cone delivery has the advantage of reduced treatment times and superior intra-fractional motion monitoring capability, while eliminating the need for Co-60 radiosurgery units. There is currently clinical interest in employing virtual cone techniques in the ablation of dorsal nerve root ganglia as a treatment for chronic thoraco-abdominal pain. We hypothesize that a standard template for single-fraction virtual cone treatment may be established to achieve ablative 90Gy point doses to the ganglia while adhering to conventional OAR dose constraints. This would enable the first such application of virtual cone treatment outside of cranial cases. Dosimetric analysis of virtual cone plans was performed within our institution’s treatment planning system using a linear Boltzman transport equation model on a 1mm grid. 10MV flattening filter-free beams were used at a 2400 MU/minute dose rate. Calculation accuracy was assessed using orthogonal film measurements in an anatomical phantom. Film-based verification indicated that dose discrepancies relative to the calculation model were <4%. Having established the accuracy of our calculations, various arc arrangements, HDMLC-defined field sizes, and collimator angles were investigated. The optimal field arrangement involved 10 equally weighted arcs at 0, ±7.5, and ±15 degree couch angles. For left-nerve root treatment, arcs ran from 10-170 degrees, with a 60-120 degree skip arc (with mirrored arcs for right-nerve root treatment). Delivery time was 25 minutes. The four innermost HDMLC leaves formed a 10mm x 4mm aperture, with jaws at 11mm x 5mm. ±5 degree collimator angles were used in repeated arcs. Doses were normalized to a 90Gy point dose at the target center. In both a thorax phantom and a test patient plan, this arrangement produced a 60Gy isodose surface approximately 5.5mm x 7.5mm x 7.5mm along the lateral, A-P, and S-I axes, respectively. The maximum dose to the spinal cord was 4.14Gy, and the maximum dose to the esophagus was 12.08Gy, with a V12 of <0.001cc. Results of this dosimetric study indicate that ablative doses can be delivered to dorsal nerve root ganglia while respecting normal tissue constrains. Furthermore, due to the robustly conformal dose profile achieved and the elimination of inverse-planning, it is probable that this virtual cone technique can be safely employed as a standardized template treatment for most patients (subject to dose constraint verification), eliminating the need for time-consuming optimization and allow for the expedited treatment of patients with intractable pain.

SU-G-TeP2-04: Comprehensive Machine Isocenter Evaluation with Separation of Gantry, Collimator, and Table Variables

Purpose: To develop and demonstrate application of a method that characterizes deviation of linac x-ray beams from the centroid of the volumetric radiation isocenter as a function of gantry, collimator, and table variables. Methods: A set of Winston-Lutz ball-bearing images was used to determine the gantry radiation isocenter as the midrange of deviation values resulting from gantry and collimator rotation. Also determined were displacement of table axis from gantry isocenter and recommended table axis adjustment. The method, previously reported, has been extended to include the effect of collimator walkout by obtaining measurements with 0 and 180 degree collimator rotation for each gantry angle. Twelve images were used to characterize the volumetric isocenter for the full range of available gantry, collimator, and table rotations. Results: Three Varian True Beam, two Elekta Infinity and four Versa HD linacs at five institutions were tested using identical methodology. Varian linacs exhibited substantially less deviation due to head sag than Elekta linacs (0.4 mm vs. 1.2 mm on average). One linac from each manufacturer had additional isocenter deviation of 0.3 to 0.4 mm due to jaw instability with gantry and collimator rotation. For all linacs, the achievable isocenter tolerance was dependent on adjustment of collimator position offset, transverse position steering, and alignment of the table axis with gantry isocenter, facilitated by these test results. The pattern and magnitude of table axis wobble vs. table angle was reproducible and unique to each machine. Conclusion: This new method provides a comprehensive set of isocenter deviation values including all variables. It effectively facilitates minimization of deviation between beam center and target (ball-bearing) position. This method was used to quantify the effect of jaw instability on isocenter deviation and to identify the offending jaw. The test is suitable for incorporation into a routine machine QA program. Software development was performed by Radiological Imaging Technology, Inc.

Influence of collimator rotation on dose distribution and delivery in intensity modulated radiation therapy for parotid cancer

Purpose: To evaluate the influence of collimator rotation in IMRT planning with respect to the target coverage and dose to critical structures. In addition, the delivery efficiency of desired fluence with collimator rotation is assessed. Methods : The computed tomography (CT) datasets of 5 patients with parotid cancer were employed for this study. Dynamic IMRT plans were generated with a dose prescription of 60 Gy in 30 fractions. IMRT plans were generated with five unilateral fields using 6MV X-rays. Four different plans were generated for each patient by keeping the collimator angle at 0, 30, 60, and 90 degree. All plans were analyzed using dose volume histogram. Conformity index (CI) and heterogeneity index (HI) were calculated. The total monitor units (MU) required to deliver one fraction were noted and compared. To verify the delivery efficiency; the measured fluence on IBA I’mRT MatriXX ionization chamber array detector was compared with the TPS dose plan with 2D gamma evaluation. Results: There is not much difference in the PTV Dmax and Dmean with respect to the different collimator angles. The PTV coverage is best at collimator angle of 0 degree. A slight reduction in CI was observed with plans at other collimator angles as compared to 0 degree. The HI values were almost similar for plans with collimator angle 0, 30, and 60 degree. The plan with 90 degree collimator showed a slightly higher heterogeneity for the PTV. A slight reduction in the average Dmax to spinal cord was observed for the plan with collimator angle 30 degree as compared to other angles whereas maximum value of Dmax to spinal cord was at collimator angle 60 degree. No clinically relevant difference was observed among the plans with respect to brainstem and mandible Dmax. An increase in average of oral cavity Dmax and Dmean was observed for collimator angle 60 and 90 degree as compared to collimator angle 0 and 30 degree. Not much difference was observed with respect to Dmax and Dmean for contralateral parotid and cochlea with plans at different collimator angles. A decrease in MU required to deliver a fraction was observed for the plan with collimator angle 30 degree as compared to other angles. The plan with 90 degree collimator required maximum MU. The 2D γ index evaluation of planned and delivered fluence showed almost similar results for plans with different collimator angles. Conclusion: An individual case-specific collimator rotation may aid in achieving the desired dose distribution and relative sparing of critical structures in IMRT.

A comparative study on patient specific absolute dosimetry using slab phantom, acrylic body phantom and goat head phantom

Purpose: To evaluate the influence of collimator rotation in IMRT planning with respect to the target coverage and dose to critical structures. In addition, the delivery efficiency of desired fluence with collimator rotation is assessed. Methods : The computed tomography (CT) datasets of 5 patients with parotid cancer were employed for this study. Dynamic IMRT plans were generated with a dose prescription of 60 Gy in 30 fractions. IMRT plans were generated with five unilateral fields using 6MV X-rays. Four different plans were generated for each patient by keeping the collimator angle at 0, 30, 60, and 90 degree. All plans were analyzed using dose volume histogram. Conformity index (CI) and heterogeneity index (HI) were calculated. The total monitor units (MU) required to deliver one fraction were noted and compared. To verify the delivery efficiency; the measured fluence on IBA I’mRT MatriXX ionization chamber array detector was compared with the TPS dose plan with 2D gamma evaluation. Results: There is not much difference in the PTV Dmax and Dmean with respect to the different collimator angles. The PTV coverage is best at collimator angle of 0 degree. A slight reduction in CI was observed with plans at other collimator angles as compared to 0 degree. The HI values were almost similar for plans with collimator angle 0, 30, and 60 degree. The plan with 90 degree collimator showed a slightly higher heterogeneity for the PTV. A slight reduction in the average Dmax to spinal cord was observed for the plan with collimator angle 30 degree as compared to other angles whereas maximum value of Dmax to spinal cord was at collimator angle 60 degree. No clinically relevant difference was observed among the plans with respect to brainstem and mandible Dmax. An increase in average of oral cavity Dmax and Dmean was observed for collimator angle 60 and 90 degree as compared to collimator angle 0 and 30 degree. Not much difference was observed with respect to Dmax and Dmean for contralateral parotid and cochlea with plans at different collimator angles. A decrease in MU required to deliver a fraction was observed for the plan with collimator angle 30 degree as compared to other angles. The plan with 90 degree collimator required maximum MU. The 2D γ index evaluation of planned and delivered fluence showed almost similar results for plans with different collimator angles. Conclusion: An individual case-specific collimator rotation may aid in achieving the desired dose distribution and relative sparing of critical structures in IMRT.

SU‐E‐T‐529: Study of Second Cancer Risk in the Treatment of Multiple Lung Lesion by SBRT Using Rapid Arc: Comparison Between Different Treatment Planning Parameters

Purpose:To compare the secondary cancer risk (SCR) between different plan settings (Collimator angle, number of Arcs and manual jaw opening) for Rapid Arc plan in the treatment of multiple lung lesion.Methods:Patient with multiple lung lesion was planned with single isocenter (i) single arc (CW:200‐160deg) and (ii) dual complementary arc. The plans were optimized to yield similar plan evaluation/acceptance criteria. The concept of equivalent OED (linear‐exponential, plateau, and the linear dose– response model) was used to evaluate the Second Cancer Risk (SCR). Then, the single arc plan was generated using five different collimator setting (0,30,45,90,120 deg) and the OED was analyzed. Also, plans were generated with automatic jaw setting and compared against the manual jaw setting.Results:The SCR values were normalized to the single arc plan. For dual complementary arc, the relative SCR for Rt.lung were (linear, plateau, linear‐exponential) (1.02,1.05,1.04), (0.99,1.02,1.02) for Lt.lung, (1.01,1.07,1.06) for Spinal cord, (1.18,1.1,1.09) for Esophagus, (1.3,1.19,1.16) for Heart, (1,1.01,1.02) for Lt.Breast and (1.05,1.05,1.05) for Rt.Breast. Since the single arc plan was superior in terms of SCR further analysis was carried out for various collimator angles. The low dose bath volumes 5Gy in cc for single and dual arc plans were 5468 and 5886 cc respectively. The computed OED values (linear‐exponential) for Rt.Lung were (1,0.94,0.96,1.06,0.97), (1,0.89,0.97,1.11,0.9) for Lt.Lung, (1,0.86,0.98,1.2,0.92) for Spinal cord, (1,0.87,0.99,1.15,0.91) for esophagus, (1,0.88,1.01,1.11,0.94) for heart, (1,0.83,0.97,1.20,0.86) for lt.breast and (1,0.94,0.95,1.03,1.11) for Rt.breast for collimator setting (0,30,45,90,120 deg) respectively.Conclusion:The Second cancer risk is considerably high for dual arc plan compared with single arc. In single arc plan the 30 degree collimator angle plan showed the least OED values for all critical structures, with the 90 degree collimator angle being the most. No significant difference between the manual and the automatic jaw setting plans were found.

Pre-Treatment Dose Verification of Imrt Using Gafchromic Ebt3 Film and 2DArray

Purpose: The present work deals with pre-treatment dosimetric verification of 10 prostate intensity-modulated radiotherapy (IMRT) plans obtained by Gafchromic EBT3 film and 2D-Array seven29 to establish quality control protocol for the delivery of prostate IMRT in our clinic. Methods: 10 prostate patients were irradiated according to IMRT techniques with 6 MV photon beams produced by a Varian DHX linear accelerator. Dose plans were computed using Eclipse 8.9 (version8.9, Varian, Palo Alto, CA, United States) treatment planning system (TPS). Pre-treatment dosimetric verification was carried out on field-perfield and total IMRT plan basis measuring 2D dose distributions in RW3 solid water phantom (PTW-Freiburg, Germany). For 10 patient plans, the dose distribution was re-calculated with the phantom CT scan and delivered to the phantom with the original and 0 degrees gantry, collimator and table angles. Thus IMRT quality control (QC) plans were generated. Dose distribution measurements were measured with Gafchromic EBT3 film and 2D-Array seven29 (PTW-Freiburg, Germany) two dimensional ionization chamber system. Verification measurements of total IMRT plans and each individual beams were compared with expected dose maps, and differences were evaluated by Verisoft program (PTW-Freiburg, Germany). To provide comparisons of multidimensional dose distributions, dose comparison tools such as gamma dose distribution, distance- to- agreement (DTA) and dose difference (DD) have been developed. The gamma dose distribution tool was used in our study. Three different gamma criteria of dose difference (DD) and distance to agreement (DTA) (3%/3 mm, 4%/4 mm and 5%/5 mm DD / DTA) are selected. These criteria are evaluated while suppressing the dose of 5% from dose distribution. Criterion validity accepted as section with gamma value less than or equal to 1 (γ ≤ 1) to be 90%. Results: In the comparison of dose distributions obtained from TPS and the results of PTW 2D-Array seven29 and Gafchromic EBT3 film dosimetry systems, it was showed that the compatibility of both methods were above 90% with respect to 3% DD and 3mm DTA criteria. PTW 2D-Array seven29 results compared to the results of the film was closer to the TPS data and this difference was statistically meaningful (p 0.05). Conclusions: The measurements performed with PTW 2D-Array seven29 were closer to the TPS data than film measurements and it takes less time during clinical use, so it could be preferred for routine use. The present results suggest the gamma criteria of 3%/3 mm as the most suitable criteria for prostate IMRT quality assurance.

SU-E-T-706: Effect of Gantry and Collimator Positional Errors On Volumetric Modulated Arc Therapy(VMAT)

Purpose: Gantry and Collimator positional errors occur during VMAT delivery. The purpose of this work is to understand the impact of such errors on VMAT dose distribution for selected head and neck cases. Methods: Five head and neck single arc (360 degree) VMAT treatment plans were created. Two types of systematic errors were simulated for an error magnitude of 1 degree on gantry and Collimator rotations respectively. For gantry, the systematic errors were, 1)Gantry angle is shifted in the same direction of VMAT arc by one degree 2)Gantry angle is shifted in the opposite direction of VMAT arc by one degree. In case of collimator, the systematic errors of 1 degree in the collimator angle in both positive and negative directions were simulated.The gantry and collimator angles were shifted for all the control points of VMAT arc. The dose was calculated and DVH was computed. Results: Conformity index (CI), D2, D98 and Dmean for PTV did not show significant variations for gantry and collimator errors in both positive and negative directions for all the 5 patients. For a positive gantry error, the average decrease in Dmean to the left parotid was found to be 80cGy and the same found to be increased for right parotid by 60cGy. However for both the directions of collimator error, the average increase in D0.01 for spinal cord was 100cGy. For a positive collimator error, the average increase in Dmean for left parotid was 90cGy and the same was decreased by 40cGy for right parotid. For a negative collimator error, the right parotid dose was increased and left parotid dose was decreased. Conclusion: Effect of one degree collimator error was found to be more significant compared to one degree Gantry error since collimator error increased the maximum dose to spinal cord and the mean dose to the parotids.

SU‐E‐T‐375: Evaluation of the Ability of Three Commercially Available Dosimeters to Detect Systematic Delivery Errors in IMRT Plans

Purpose: To evaluate the sensitivity of three patient‐specific IMRT QA devices to systematic delivery errors. Methods: Four clinical step‐and‐shoot IMRT plans were selected, two simple prostate, one medium complexity head and neck (H&amp;N) and one complex H&amp;N. Twelve plan variations with errors introduced were created: Multi‐Leaf Collimator (MLC) positional errors (all leaf pairs shifted in the same direction (1, 2, 3 mm) and opposite direction (1, 2, 3 mm)); collimator rotation offsets (1, 2, 3 degrees) and gantry rotation offsets (0.5, 1, 2 degrees). The total dose for each plan was measured using an ArcCHECK helical diode array. Each field for each plan, excluding those with gantry offsets, was measured using an Electronic Portal Imager and a MatriXX Evolution 2D ionisation chamber array. The measured dose was compared to the calculated dose for the no error plan using Gamma analysis with 3%/3mm, 3%/2mm, and 2%/2mm criteria. Results: The ability of the different devices to detect errors was similar. In most cases pass rates reduced with increasing errors and/or stricter gamma criteria as expected. The ArcCHECK demonstrated sensitivity to 2 degree gantry errors, with a reduction in pass rate of 6–14% between the 1 and 2 degree errors in all plans for all gamma criteria. Even for the 2%/2mm criteria, no device detected a 3 degree collimator error in either prostate plan, however collimator errors were detected for the H&amp;N plans which had elongated fields. For all criteria, there were different scenarios for each detector where MLC position errors of 3 mm were not detected. Conclusion: None of the devices could reliably detect MLC position or collimator rotation errors that were not within recommended linear accelerator monthly QA tolerances. There were no consistent differences between the ability of the devices to detect systematic delivery errors. Financial support has been provided by the Saudi Arabian Cultural Mission towards this work.

SU-E-T-133: Isocenter Measurements with the Winston-Lutz Test: Impact on Treatment Planning.

To assure high accuracy in radiation delivery during linear accelerator based stereotactic radiosurgery procedures, a Winston-Lutz test is typically performed to measure the effective isocenter wander over the wide range of gantry and couch angles employed. Ideally, this deviation is lower than 1-1.5 mm depending on institutional tolerances. Automation of this test with portal imaging allows a large amount of data to be taken and processed. This study explores the effect of various collimation techniques on beam-center stability, and addresses one technique of repositioning in response. Winston-Lutz shots were obtained using a Trilogy linear accelerator, BrainLab Winston-Lutz pointer, and electronic portal imaging from a full range of gantry and couch angles. The test was repeated using MLCs, a 10mm stereotactic cone from BrainLab, and jaws for beam collimation. All images were processed using an in-house MATLAB-based program which calculates the distance between the tungsten ball and aperture center with accuracy within ∼0.1mm. Ideal shifts were calculated by minimizing this variation from gantry rotation alone, and performed manually. The in-plane gantry sag was found to be on the order of ∼0.8mm, independent of collimation technique or setup. The MLC carriage had a ∼0.3mm cross-plane displacement to the inferior from the cone center, negated by a 90 degree collimator rotation. The total gantry misalignment was greatest near gantry 180 (∼1.5mm), varying by collimation method. Optimizing to minimize gantry error reduced errors to less than 0.7mm but increased the effect of couch rotation. Isocenter evaluation is dependent on collimation method used, and should be conducted with the same technique as delivery. It may be useful to consider the varying delivery accuracy from different couch/gantry angles during treatment planning. Repositioning methods after couch rotation may affect ideal laser positions.

SU-E-T-88: Evaluating Gantry Sag on Linear Accelerators and Introducing an MLC-Based Compensation Strategy.

Gantry sag is one of the well-known sources of mechanical imperfections that compromise the spatial accuracy of radiation dose delivery. This study aims to quantify the gantry sag on multiple linacs and to investigate a multiple leaf collimator (MLC)-base strategy to compensate for gantry sag. We used the Winston-Lutz method to measure the gantry sag on three Varian linacs. A ball-bearing phantom was imaged with a square radiation field during gantry rotation. The images were analyzed to derive the radiation isocenter and subsequently the gantry sag, that is, the superior-inferior wobble of the radiation field center from the radiation isocenter as a function of gantry angle. Compensation for gantry sag was attempted by offsetting the MLC leaves at 90-degree collimator angle. The amount of offset was the opposite of measured gantry sag, which was gantry angle-specific. Gantry sag was reproducible within a six-month period. On the three linacs, the maximum gantry sag was found to vary from 0.7 mm to 1.0 mm, depending on the linac and the collimator angle. The radiation field center moved inferiorly, or away from the gantry, when the gantry was rotated from 0 to 180 degrees. Comparison of gantry sag at 0- and 90-degree collimator angles showed that the uncertainty in MLC leaf positions did not increase the gantry sag. Instead, gantry sag was caused primarily by nonideal gantry rotation. After the MLC compensation was applied, the maximum gantry sag was reduced to less than 0.2 mm. The results indicated that gantry sag on a linac can be quantitatively measured with sub-millimeter precision, using a simple ball-bearing phantom and the electronic portal imaging device. Reduction of gantry sag is feasible by applying a gantry angle-specific correction to MLC leaf positions at 90 degree collimator angle.

SU-E-T-24: Development and Implementation of an Automated Algorithm to Determine Radiation Isocenter, Radiation vs. Light Field Coincidence, and Analyze Strip Tests.

The aim of this study was to develop a phantom and analysis software that could be used to quickly and accurately determine the location of radiation isocenter using the Electronic Portal Imaging Device (EPID). The phantom could then be used as a static reference point for performing other tests including: radiation vs. light field coincidence, MLC and Jaw strip tests, and Varian Optical Guidance Platform (OGP) calibration. The solution proposed uses a collimator setting of 10×10 cm to acquire EPID images of the new phantom constructed from LEGO® blocks. Images from a number of gantry and collimator angles are analyzed by the software to determine the position of the jaws and center of the phantom in each image. The distance between a chosen jaw and the phantom center is then compared to the same distance measured after a 180 degree collimator rotation to determine if the phantom is centered in the dimension being investigated. The accuracy of the algorithm's measurements were verified by independent measurement to be approximately equal to the detector's pitch. Light versus radiation field as well as MLC and Jaw strip tests are performed using measurements based on the phantom center once located at the radiation isocenter. Reproducibility tests show that the algorithm's results were objectively repeatable. Additionally, the phantom and software are completely independent of linac vendor and this study presents results from two major linac manufacturers. An OGP calibration array was also integrated into the phantom to allow calibration of the OGP while the phantom is positioned at radiation isocenter to reduce setup uncertainty contained in the calibration. This solution offers a quick, objective method to perform isocenter localization as well as laser alignment, OGP calibration, and other tests on a monthly basis.

SU‐C‐211‐03: Using OVH and IMRT Plan Data to Automate VMAT Planning: A Head‐And‐Neck Study

Purpose: Although VMAT is widely used, VMAT planning faces challenges similar to those of IMRT planning, particularly with respect to planning efficiency and plan quality. The study is to investigate whether VMAT planning efficiency and quality can be improved through an overlap volume histogram (OVH)‐driven, automated planning application using an IMRT database. Methods: Based on comparable dosimetric results between planner‐generated head‐and‐neck VMAT and IMRT plans, an in‐house developed OVH‐driven, automated planning application containing a database of 182 prior clinical head‐and‐neck IMRT plans manually created by dosimetrists is built into Pinnacle SmartArc to automate VMAT planning on an Elekta Infinity Linac. The only required inputs to the application are the contours of the CTVs and OARs delineated on a patient's planning CT. All the planning tasks are fully automatically executed by the application without any user intervention. Double arcs, 2 degree sampling frequency of gantry, 180 s/arc maximum delivery time, 45 degree collimator angle and 0.25 cm/deg constrain leaf motion are used in VMAT planning. VMAT plans of 4 oropharynx, 4 nasopharynx and 4 larynx patients are fully automatically generated and compared with corresponding clinical IMRT plans in the database. Efficiency is evaluated by the number of optimization rounds required for a complete plan. Results: All VMAT plans are automatically generated in two optimization rounds, while the average optimization rounds in clinical IMRT oropharynx, larynx and nasopharynx planning are 28, 36 and 67 respectively. In VMAT plans, significant dose reductions (p&lt;0.05) to the cord+4mm (D0.1cc: 3.7 Gy), brainstem (DO.1cc: 4.9 Gy), brachial plexus (D0.1cc: 1.6 Gy), larynx (V(50 Gy): 5.3%) and inner ear (mean dose: 4.4 Gy) with a slight degradation in low‐level PTV coverage (V95: 0.3%; p=0.25) are observed. Conclusions: IMRT‐data‐ driven VMAT planning is an efficient and effective way to automate VMAT planning.

Volumetric Modulated Arc Therapy Treatment Protocol for Hypo-fractionated Stereotactic Body Radiotherapy for Localized Prostate Cancer

table rotations, collimator angle and photon beam energy have been investigated. Results: VMAT plans with ±45 degree collimator rotations required on average 38% less monitor units compared to plans with no collimator rotations and 20% less monitor units than plans with ±22.5 degree rotations. Plans with ±45 degree collimator rotations provided more homogeneous dose distribution with an average of 6% less maximal dose. VMAT plans with two arcs provided improved conformality and homogeneity with an average of 3% lower maximal dose compared to single arc plans. However, increasingthe number of arcsto threedid not provideany significantimprovement. Introductionof±5 and±10degree table rotations between arcs did not result in any significant dosimetric improvements. The selection of photon beam energy between 6MV and 10MV did not provide any notable dosimetric differences either. Conclusions: Thefinalselectionoftheprotocolincludes twocoplanar10MV RapidArcs with±45degreecollimatorrotations.This prospective clinical trial is designed to deliver 5 fractions of 8 Gy with the prescription dose covering the 95% of the planning target volume. The beam-on time for eachof the coplanar arcs is approximately 2 minutes.For the purpose ofintrafraction motionmanagement, two Rapid Arcs are further split to 4 half arcs with approximately one minute treatment time for each segment. The patient is stereoscopicallyimagedandrepositionedifnecessaryusingExacTracpriortothedeliveryofeacharc.Additionally,CBCTisobtained prior to eachfractionforthe purposesof confirmationofExacTracpositioningand volumetric evaluation of bladderand rectalfilling.

TU-D-ValA-01: Role of Collimator Angle Optimization in Intensity Modulated Radiation Therapy

Purpose:IMRT optimization involves several treatment parameters producing a complex, unstable and computationally challenging problem during its search of an optimal plan in a reasonable time. Most parameters have been studied in IMRT optimization except the collimator angle, which is investigated in this study. Method and Materials: Five head‐and‐neck and five prostate cases are selected. The head ‐and‐neck and prostate PTVs range between 79.6–441 cm3 and 86.2–250 cm3 respectively, whereas the OAR volumes vary from 11.0–46.3 cm3 and 41.1–312 cm3 respectively. The patients are treated with five or seven fields equally distributed, 0‐ degree collimator angle, using the 1 cm leaf MLC from a Siemens Primus accelerator based on the plan generated using the Nucletron Oncentra treatment planning system. While dose‐volume constraints are kept the same as in the patient's initial treatment plan, collimator angles are varied systematically (0–90 degrees) and a new treatment plan is optimized for each collimator angle. The number of beamlets, monitor units (MU) and DVHs for each collimator angle are compared. Results: The variation of the total number of beamlets with collimator angle follows the shape of a parabola and peaks at 45° collimator angle for all patients. However, the MUs appear to be relatively independent of the collimator angle. The PTV dose coverage statistics for each patient are relatively independent of the collimator angle. Similar observations are noted for all the OARs, except for the small structures for which differences could be observed in the DVHs between the different collimator angles. Conclusions:Collimator angle does not play a significant role in IMRT optimization, as long as the PTV overage is adequate. This provides an additional freedom to choose from 0–90 degree of the collimator angle for long fields without compromising the coverage with limited MLC range or treatment time.

Dosimetry validation of treatment room shielding design

Intensity-modulated radiation therapy (IMRT) can lead to an increase in leakage radiation. The total number of monitor units (MUs) for IMRT is typically 2-5 times that for conventional treatments [the ratio of the two is used to derive the effective modulation scaling factor (MSFeff)]. Shielding calculations for IMRT can be done by applying the MSFeff to measured exposures under conservative conditions (standard beam setup 40 cm x 40 cm field, 45 degrees collimator angle) to account for the increased leakage. In this work, we verified this approach for two existing vaults housing a Siemens Primart 6 MV linac and a Varian 21Ex 10 MV linac. We measured the cumulative exposures at various locations around the vaults for typical IMRT cases and for the standard beam setup using the same MUs. For the standard beam setup, the IMRT gantry angles and eight equally spaced angles were used. Estimations of weekly exposures for IMRT were carried out using exposure rates measured under standard beam setup and the MSFeff averaged over 20 treatment cases. The accumulated exposures under realistic IMRT conditions were 30%-50% lower than the estimated values using equally spaced gantry angles except for two locations where the real IMRT leakage was higher than the estimated value by approximately 10%. Measurements using the same gantry angles yielded similar results. Our results indicate that it is adequate to use the MSFeff and previously measured exposures to estimate the leakage increase due to IMRT for an existing vault. Different approaches should be followed when considering primary or secondary barriers since the standard beam setup is overestimating the exposures behind primary barriers compared to IMRT. In such cases, a 10 cm x 10 cm field can be used for more accurate shielding evaluation.

Verification of multileaf collimator leaf positions using an electronic portal imaging device.

An automated method is presented for determining individual leaf positions of the Siemens dual focus multileaf collimator (MLC) using the Siemens BEAMVIEW(PLUS) electronic portal imaging device (EPID). Leaf positions are computed with an error of 0.6 mm at one standard deviation (sigma) using separate computations of pixel dimensions, image distortion, and radiation center. The pixel dimensions are calculated by superimposing the film image of a graticule with the corresponding EPID image. A spatial correction is used to compensate for the optical distortions of the EPID, reducing the mean distortion from 3.5 pixels (uncorrected) per localized x-ray marker to 2 pixels (1 mm) for a rigid rotation and 1 pixel for a third degree polynomial warp. A correction for a nonuniform dosimetric response across the field of view of the EPID images is not necessary due to the sharp intensity gradients across leaf edges. The radiation center, calculated from the average of the geometric centers of a square field at 0 degrees and 180 degrees collimator angles, is independent of graticule placement error. Its measured location on the EPID image was stable to within 1 pixel based on 3 weeks of repeated extensions/retractions of the EPID. The MLC leaf positions determined from the EPID images agreed to within a pixel of the corresponding values measured using film and ionization chamber. Several edge detection algorithms were tested: contour, Sobel, Roberts, Prewitt, Laplace, morphological, and Canny. These agreed with each other to within < or = 1.2 pixels for the in-air EPID images. Using a test pattern, individual MLC leaves were found to be typically within 1 mm of the corresponding record-and-verify values, with a maximum difference of 1.8 mm, and standard deviations of <0.3 mm in the daily reproducibility. This method presents a fast, automatic, and accurate alternative to using film or a light field for the verification and calibration of the MLC.

Leaf position error during conformal dynamic arc and intensity modulated arc treatments.

Conformal dynamic arc (CD-ARC) and intensity modulated arc treatments (IMAT) are both treatment modalities where the multileaf collimator (MLC) can change leaf position dynamically during gantry rotation. These treatment techniques can be used to generate complex isodose distributions, similar to those used in fix-gantry intensity modulation. However, a beam-hold delay cannot be used during CD-ARC or IMAT treatments to reduce spatial error. Consequently, a certain amount of leaf position error will have to be accepted in order to make the treatment deliverable. Measurements of leaf position accuracy were taken with leaf velocities ranging from 0.3 to 3.0 cm/s. The average and maximum leaf position errors were measured, and a least-squares linear regression analysis was performed on the measured data to determine the MLC velocity error coefficient. The average position errors range from 0.03 to 0.21 cm, with the largest deviations occurring at the maximum achievable leaf velocity (3.0 cm/s). The measured MLC velocity error coefficient was 0.0674 s for a collimator rotation of 0 degrees and 0.0681 s for a collimator rotation of 90 degrees. The distribution in leaf position error between the 0 degrees and 90 degrees collimator rotations was within statistical uncertainty. A simple formula was developed based on these results for estimating the velocity-dependent dosimetric error. Using this technique, a dosimetric error index for plan evaluation can be calculated from the treatment time and the dynamic MLC leaf controller file.