Backup Jaws Research Articles

Viability of the virtual cone technique using a fixed small multi-leaf collimator field for stereotactic radiosurgery of trigeminal neuralgia.

Dosimetric uncertainties in very small (≤1.5×1.5 cm2 ) photon fields are remarkably higher, which undermines the validity of the virtual cone (VC) technique with a diminutive and variable MLC fields. We evaluate the accuracy and reproducibility of the VC method with a very small, fixed MLC field setting, called a fixed virtual cone (fVC), for small target radiosurgery such as trigeminal neuralgia (TGN). The fVC is characterized by 0.5 cm x 0.5cm high-definition (HD) MLC field of 10MV FFF beam defined at 100cm SAD, while backup jaws are positioned at 1.5 cm x 1.5cm. A spherical dose distribution equivalent to 5mm (diameter) physical cone was generated using 10-14 non-coplanar, partial arcs. Dosimetric accuracy was validated using SRS diode (PTW 60018), SRS MapCHECK (SNC) measurements. As a quality assurance measure, 10 treatment plans (SRS) for TGN, consisting of various arc ranges at different collimator angles were analyzed using 6 MV FFF and 10 MV FFF beams, including a field-by-field study (n=130 fields). Dose outputs were compared between the Eclipse TPS and measurements (SRS MapCHECK). Moreover, dosimetric changes in the field defining fVC, prompted by a minute (±0.5-1.0mm) leaf shift, was examined among TPS, diode measurements, and Monte Carlo (MC) simulations. The beam model for fVC was validated (≤3% difference) using SRS MapCHECK based absolute dose measurements. The equivalent diameters of the 50% isodose distribution were found comparable to that of a 5mm cone. Additionally, the comparison of field output factors, dose per MU between the TPS and SRS diode measurements using the fVC field, including±1mm leaf shift, yielded average discrepancies within 5.5% and 3.5% for 6 MV FFF and 10 MV FFF beams, respectively. Overall, the fVC method is a credible alternative to the physical cone (5mm) that can be applied in routine radiosurgical treatment of TGN.

Characterization and dosimetry of photon multileaf collimated electron beams using Gafchromic film measurements

Previous studies indicated that clinical electron beam shaping involving a photon multileaf collimator (MLC) can be achieved. It is usually applied in Modulated Electron Radiation Therapy or MERT. This technique requires accurate clinical electron beam characterization before implementation in advanced treatment planning techniques. This study presents the dosimetric characterization of clinical electron beams shaped by an Elekta Synergy linear accelerator equipped with a 160 leaf Agility MLC for electron beams energies of 4, 6, 8, 10, 12, and 15 MeV. Dosimetric parameters of regular fields ranging from 1 × 1 to 20 × 20 cm2 were investigated to reflect those fields used in electron beam therapy at two source-to-surface distances (SSD) of 60 and 70 cm. Percent depth doses (PDDs), in-line, and cross-line beam profiles and output factors (OFs) were measured. Beam profiles were found to be wider compared to electron beams collimated by standard applicators. Penumbrae formed by the pMLC in cross-line direction was wider compared to penumbrae formed by the back-up jaws in in-line direction. The jaws are located below the MLC and are closer to the plane of measurement. The beams produced at 60 cm SSD also produced smaller penumbra. Beam spacing was also investigated and a clinical case was investigated for total scalp treatment in a head phantom and measured with Gafchromic film. The clinical investigation result showed good dose distribution over the therapeutic region and the treatment time is 3 times faster compared to the use of standard applicators for field greater than 5 × 5 cm2. All the measurements for beam dosimetric characterization were performed using Gafchromic EBT3 film and solid phantoms.

Sensitivity of the IQM and MatriXX detectors in megavolt photon beams.

This study focused on evaluating the sensitivity of integral quality monitoring (IQM®) system and MatriXX detectors. These two detectors are recommended for radiotherapy pre-treatment quality assurance (QA). IQM is a large wedged-shaped ionisation chamber mounted to the linear accelerator (linac) head in practice. MatriXX consists of an array of ionisation chambers also attached to the linac head. In this study, the dosimetric performance and sensitivity of MatriXX and IQM detectors were evaluated using the following characteristics: reproducibility, linearity, error detection capability and three-dimensional conformal radiotherapy (3D-CRT) plans of the head and neck, thorax and pelvic regions. This study indicates that the signal responses of the large ionisation chamber device (IQM) and the small pixel array of ionisation chambers device (MatriXX) are reproducible, linear and sensitive to MLC positional errors, backup jaw positional errors and dose errors. The local percentage differences for dose errors of 1%, 2%, and 3% were, respectively, within 0.35-8.23%, 0.78-16.21%, and 1.10-24.41% for the IQM device. While for the MatriXX detector, the ranges were between 0.24-3.19, 0.57-6.43 and 0.81-12.95, respectively. Since IQM is essentially a double wedge-shaped large ionisation chamber, its reproducibility and detection capability are competitive to that of MatriXX. In addition, the sensitivity of the two QA systems increases with an increase in escalation percentage, and the signal responses are patient plan specific. The two detectors response signals have good correlations and they are accurate for pre-treatment QA. Statistically, (P<0.05) there is a significant difference between the IQM and MatriXX response to dose errors.

Spine SBRT With Halcyon™: Plan Quality, Modulation Complexity, Delivery Accuracy, and Speed.

Purpose: Spine SBRT requires treatment plans with steep dose gradients and tight limits to the cord maximal dose. A new dual-layer staggered 1-cm MLC in Halcyon™ treatment platform has improved leakage, speed, and DLG compared to 120-Millennium (0.5-cm) and High-Definition (0.25-cm) MLCs in the TrueBeam platform. Halcyon™ 2.0 with SX2 MLC modulates fluence with the upper and lower MLCs, while in Halcyon™ 1.0 with SX1 only the lower MLC modulates the fluence and the upper MLC functions as a back-up jaw. We investigated the effects of four MLC designs on plan quality for spine SBRT treatments.Methods: 15 patients previously treated at our institution were re-planned according to the NRG-BR-002 guidelines with a prescription of 3,000 cGy in 3 fractions, 6xFFF, 800 MU/min, and 3-arc VMAT technique. Planning objectives were adjusted manually by an experienced planner to generate optimal plans and kept the same for different MLCs within the same platform.Results: All treatment plans were able to achieve adequate target coverage while meeting NRG-BR002 dosimetric constraints. Planning parameters were evaluated including: conformity index, homogeneity index, gradient measure, and global point dose maximum. Delivery accuracy, modulation complexity, and delivery time were also analyzed for all MLCs.Conclusion: The Halcyon™ dual-layer MLC can generate comparable and clinically equivalent spine SBRT plans to TrueBeam plans with less rapid dose fall-off and lower conformity. MLC width leaf can impact maximum dose to organs at risk and plan quality, but does not cause limitations in achieving acceptable plans for spine SBRT treatments.

SU‐F‐T‐271: Comparing IMRT QA Pass Rates Before and After MLC Calibration

Purpose:To compare IMRT QA pass rates before and after an in‐house MLC leaf calibration procedure.Methods:The MLC leaves and backup jaws on four Elekta linear accelerators with MLCi2 heads were calibrated using the EPID‐based RIT Hancock Test as the means for evaluation. The MLCs were considered to be successfully calibrated when they could pass the Hancock Test with criteria of 1 mm jaw position tolerance, and 1 mm leaf position tolerance. IMRT QA results were collected pre‐ and postcalibration and analyzed using gamma analysis with 3%/3mm DTA criteria. AAPM TG‐119 test plans were also compared pre‐ and post‐calibration, at both 2%/2mm DTA and 3%/3mm DTA.Results:A weighted average was performed on the results for all four linear accelerators. The pre‐calibration IMRT QA pass rate was 98.3 ± 0.1%, compared with the post‐calibration pass rate of 98.5 ± 0.1%. The TG‐119 test plan results showed more of an improvement, particularly at the 2%/2mm criteria. The averaged results were 89.1% pre and 96.1% post for the C‐shape plan, 94.8% pre and 97.1% post for the multi‐target plan, 98.6% pre and 99.7% post for the prostate plan, 94.7% pre and 94.8% post for the head/neck plan.Conclusion:The patient QA results did not show statistically significant improvement at the 3%/3mm DTA criteria after the MLC calibration procedure. However, the TG‐119 test cases did show significant improvement at the 2%/2mm level.

Dosimetric impact of the 160 MLC on head and neck IMRT treatments.

The purpose of this work is to investigate if the change in plan quality with the finer leaf resolution and lower leakage of the 160 MLC would be dosimetrically significant for head and neck intensity‐modulated radiation therapy (IMRT) treatment plans. The 160 MLC consisting of 80 leaves of 0.5 cm on each bank, a leaf span of 20 cm, and leakage of less than 0.37% without additional backup jaws was compared against the 120 Millennium MLC with 60 leaves of 0.5 and 1.0 cm, a leaf span of 14.5 cm, and leakage of 2.0%. CT image sets of 16 patients previously treated for stage III and IV head and neck carcinomas were replanned on Prowess 5.0 and Eclipse 11.0 using the 160 MLC and the 120 MLC. IMRT constraints for both sets of 6 MV plans were identical and based on RTOG 0522. Dose‐volume histograms (DVHs), minimum dose, mean dose, maximum dose, and dose to 1 cc to the organ at risks (OAR) and the planning target volume, as recommended by QUANTEC 2010, were compared. Both collimators were able to achieve the target dose to the PTVs. The dose to the organs at risk (brainstem, spinal cord, parotids, and larynx) were 1%–12% (i.e., 0.5–8 Gy for a 70 Gy prescription) lower with the 160 MLC compared to the 120 MLC, depending on the proximity of the organ to the target. The large field HN plans generated with the 160 MLC were dosimetrically advantageous for critical structures, especially those located further away from the central axis, without compromising the target volume.PACS number: 87.55 D‐

Multi-institutional Feasibility Study of a Fast Patient Localization Method in Total Marrow Irradiation With Helical Tomotherapy: A Global Health Initiative by the International Consortium of Total Marrow Irradiation

To develop, characterize, and implement a fast patient localization method for total marrow irradiation. Topographic images were acquired using megavoltage computed tomography (MVCT) detector data by delivering static orthogonal beams while the couch traversed through the gantry. Geometric and detector response corrections were performed to generate a megavoltage topogram (MVtopo). We also generated kilovoltage topograms (kVtopo) from the projection data of 3-dimensional CT images to reproduce the same geometry as helical tomotherapy. The MVtopo imaging dose and the optimal image acquisition parameters were investigated. A multi-institutional phantom study was performed to verify the image registration uncertainty. Forty-five MVtopo images were acquired and analyzed with in-house image registration software. The smallest jaw size (front and backup jaws of 0) provided the best image contrast and longitudinal resolution. Couch velocity did not affect the image quality or geometric accuracy. The MVtopo dose was less than the MVCT dose. The image registration uncertainty from the multi-institutional study was within 2.8 mm. In patient localization, the differences in calculated couch shift between the registration with MVtopo-kVtopo and MVCT-kVCT images in lateral, cranial-caudal, and vertical directions were 2.2 ± 1.7 mm, 2.6 ± 1.4 mm, and 2.7 ± 1.1 mm, respectively. The imaging time in MVtopo acquisition at the couch speed of 3 cm/s was <1 minute, compared with ≥15 minutes in MVCT for all patients. Whole-body MVtopo imaging could be an effective alternative to time-consuming MVCT for total marrow irradiation patient localization.

Poster - Thur Eve - 64: Preliminary investigation of arc configurations for optimal sparing of normal tissue in hypofractionated stereotactic radiotherapy (HF-SRT) of multiple brain metastases using a 5mm interdigitating micro-multileaf collimator

Purpose: To evaluate normal tissue sparing in intra-cranial HF-SRT, comparing various arc configurations with the Synergy Beam Modulator (SynBM) and Agility linacs, the latter incorporating leaf interdigitation and backup jaws. Methods: Five patients with multiple brain metastases (BMs), (5 BMs (n=2), 3 BMs (n=3)) treated with HF-SRT using 25 Gy (n=2) or 30 Gy (n=3) in 5 fractions, were investigated. Clinical treatment plans used the SynBM. Each patient was retrospectively re-planned on Agility, employing three planning strategies: (A) one isocenter and dedicated arc for each BM; (B) a single isocenter, centrally placed with respect to BMs; (C) the isocenter and arc configuration used in the SynBM plan, where closely spaced (<5cm) BMs used a dedicated isocenter and arcs. Agility plans were normalized for PTV coverage and heterogeneity. Results and Conclusion: Strategy A obtained the greatest improvements over the SynBM plan, where the maximum OAR dose, and mean dose to normal brain (averaged for all patients) were reduced by 55cGy and 25cGy, respectively. Strategy B was limited by having a single isocenter, hence less jaw shielding and increased MLC leakage. The maximum OAR dose was reduced by 13cGy, however mean dose to normal brain increased by 84cGy. Strategy C reduced the maximum OAR dose and mean dose to normal brain by 32cGy and 9cGy, respectively. The results from this study indicate that, for intra-cranial HF-SRT of multiple BMs, Agility plans are equal or better than SynBM plans. Further planning is needed to investigate dose sparing using Strategy A and the SynBM.

SU‐E‐T‐33: An EPID‐Based Method for Testing Absolute Leaf Position for MLC Without Backup Jaws

Purpose:Methods in common use for MLC leaf position QA are limited to measurements relative to an arbitrary reference position. The authors previously presented an EPID‐based method for efficiently testing accuracy of leaf position relative to the mechanical isocenter for MLC with backup jaws. The purpose of this work is to extend that method to the general case of MLC without backup jaws.Methods:A pair of collimator walkout images is used to determine the location of the mechanical isocenter relative to the center of one field using a parameter called X‐offset. The method allows for shift of the imager panel to cover subsets of MLC leaves within the limited field of view of the imager. For a shifted panel position, an image of three beam strips defined by a subset of MLC leaves allows determination of the position of each leaf relative to the isocenter. The location of the isocenter is determined by applying X‐offset to an image of a single rectangular field obtained at that panel position. The method can also be used to test backup jaws instead of MLC leaves. A software tool was developed to efficiently analyze the images.Results:The software tool reports leaf position and deviation from nominal position, and provides visual displays to facilitate rapid qualitative interpretation. Test results using this method agree well with results using the previous method requiring backup jaws. Test results have been successfully used to recalibrate one model MLC (Elekta MLCi2™). Work in progress includes extension of the software tool to other MLC models, and quantification of reproducibility of the measurements.Conclusion:This work successfully demonstrates a method to efficiently and accurately measure MLC leaf position, or backup jaw position, relative to the mechanical isocenter of the collimator.

SU-E-T-129: An EPID-Based Method for Testing Accuracy of MLC and Backup Jaws

Purpose: Current methods for QA of MLC leaf position are limited to measurements relative to an arbitrary reference position. There also are no reported methods for testing accuracy of backup jaw position as recommended by AAPM Task Group 142. The purpose of this project was to develop an efficient method for testing accuracy of MLC and backup jaws relative to the mechanical isocenter. Methods: A set of four EPID images is used to determine the position of backup jaws and MLC leaves relative to the mechanical isocenter. The first pair of images are of a rectangular field that is rotated 180 degrees between the two images to locate the isocenter. The third image, consisting of three field strips defined by backup jaws, allows for determination of the position of the backup jaws across its range of travel relative to a line passing through the mechanical isocenter and parallel to the jaws. The fourth image, a set of three strips defined by the MLC, allows for determination of the position of each leaf relative to the isocenter reference line determined from the backup jaw image. A software routine was developed to efficiently analyze the images. The accuracy of the test results was validated with water phantom scans. Results: The test can be completed in less than 10 minutes. The software tool reliably and automatically reports, over a 20‐cm range of travel, jaw setback for each leaf, leaf position and jaw position deviation from nominal, and provides visual displays to facilitate rapid qualitative interpretation. The test was used to guide adjustment of MLC and backup jaw calibration of two matched linacs. Conclusion: This work demonstrates a method to efficiently and accurately measure MLC leaf and backup jaw position relative to the mechanical isocenter of the collimator.

WE-G-BRB-03: On the Detectability of Jaw Decalibrations with the DAVID System.

To evaluate if the recently described problems with the calibration of jaws at Varian linear accelerators can be detected with the DAVID system. Recently Varian Clinac field size issues resulted in a widespread medical device correction notice, stating that incorrect jaw positioning due to excessive connector resistances may result in clinically relevant deviations from the intended field size. Since field size checks via the light field, separately for each administered field segment, are not practicable, it might be helpful to know that an independent, permanently active field size surveillance system is routinely used in our department since four years. The DAVID system (PTW-Freiburg, Germany) is a translucent multiwire transmission ionisation chamber, placed in the linac accessory holder. Each detection wire of the chamber is positioned in theprojection of the midline of the associated MLC leaf pair, resulting in a signal proportional to the leaf pair aperture. High lateral resolution has been achieved by a deconvolution software (Looe et al, PMB 55, 2010, 3981- 3992). MLC decalibrations are detected by comparison between measured and reference signal profiles. The side wings of the DAVID system signal profiles are sensibly influenced by the position of the backup jaw blocks limiting the field size in the direction at right angles to the MLC travel direction. Systematic variation of the backup jaw positions for several field sizes and IMRT plans has revealed high sensitivity of the signal profile side wings with respect to jaw decalibrations. The DAVID system is able to detect, without any delay, suddenly appearing jaw decalibrations down to positioning errors less than 1mm. Use of the DAVID system provides the independent and permanent surveillance of correct field sizes including the position of the backup jaws, making checks of light-field congruence for each field segment unnecessary.

VMAT testing for an Elekta accelerator

Volumetric‐modulated arc therapy (VMAT) has been shown to be able to deliver plans equivalent to intensity‐modulated radiation therapy (IMRT) in a fraction of the treatment time. This improvement is important for patient immobilization/ localization compliance due to comfort and treatment duration, as well as patient throughput. Previous authors have suggested commissioning methods for this modality. Here, we extend the methods reported for the Varian RapidArc system (which tested individual system components) to the Elekta linear accelerator, using custom files built using the Elekta iComCAT software. We also extend the method reported for VMAT commissioning of the Elekta accelerator by verifying maximum values of parameters (gantry speed, multileaf collimator (MLC) speed, and backup jaw speed), investigating: 1) beam profiles as a function of dose rate during an arc, 2) over/under dosing due to MLC reversals, and 3) over/under dosing at changing dose rate junctions. Equations for construction of the iComCAT files are given. Results indicate that the beam profile for lower dose rates varies less than 3% from that of the maximum dose rate, with no difference during an arc. The gantry, MLC, and backup jaw maximum speed are internally consistent. The monitor unit chamber is stable over the MUs and gantry movement conditions expected. MLC movement and position during VMAT delivery are within IMRT tolerances. Dose rate, gantry speed, and MLC speed are accurately controlled. Over/under dosing at junctions of MLC reversals or dose rate changes are within clinical acceptability.PACS numbers: 87.55.de, 87.55.Qr, 87.56.bd

VMATに要求されるBackup JAWの位置精度と QA方法に関する検討

The tolerance of the Backup diaphragm (Backup JAW) setting in Elekta linac was specified as 2 mm according to the AAPM TG-142 report. However, the tolerance and the quality assurance procedure for volumetric modulated arc therapy (VMAT) was not provided. This paper describes positional accuracy and quality assurance procedure of the Backup JAWs required for VMAT. It was found that a gap-width error of the Backup JAW by a sliding window test needed to be less than 1.5 mm for prostate VMAT delivery. It was also confirmed that the gap-widths had been maintained with an error of 0.2 mm during the past one year.

Validation of a new control system for Elekta accelerators facilitating continuously variable dose rate

Elekta accelerators controlled by the current clinically used accelerator control system, Desktop 7.01 (D7), uses binned variable dose rate (BVDR) for volumetric modulated arc therapy (VMAT). The next version of the treatment control system (Integrity) supports continuously variable dose rate (CVDR) as well as BVDR. Using CVDR opposed to BVDR for VMAT has the potential of reducing the treatment time but may lead to lower dosimetric accuracy due to faster moving accelerator parts. Using D7 and a test version of Integrity, differences in ability to control the accelerator, treatment efficiency, and dosimetric accuracy between the two systems were investigated. Single parameter tests were designed to expose differences in the way the two systems control the movements of the accelerator. In these tests, either the jaws, multi leaf collimators (MLCs), or gantry moved at constant speed while the dose rate was changed in discrete steps. The positional errors of the moving component and dose rate were recorded using the control systems with a sampling frequency of 4 Hz. The clinical applicability of Integrity was tested using 15 clinically used VMAT plans (5 prostate, 5 H&N, and 5 lung) generated by the SmartArc algorithm in PINNACLE. The treatment time was measured from beam-on to beam-off and the accuracy of the dose delivery was assessed by comparing DELTA4 measurements and PINNACLE calculated doses using gamma evaluation. The single parameter tests showed that Integrity had an improved feedback between gantry motion and dose rate at the slight expense of MLC control compared to D7. The single parameter test did not reveal any significant differences in the control of either jaws or backup jaws between the two systems. These differences in gantry and MLC control together with the use of CVDR gives a smoother Integrity VMAT delivery compared to D7 with less abrupt changes in accelerator motion. Gamma evaluation (2% of 2 Gy and 2 mm) of the calculated doses and DELTA4 measured doses corrected for systematic errors showed an average pass rate of more than 97.8% for both D7, Integrity BVDR, and Integrity CVDR deliveries. Direct comparisons between the measured doses using strict gamma criteria of 0.5% and 0.5 mm showed excellent agreement between D7 and Integrity delivered doses with average pass rates above 95.7%. Finally, the Integrity control system resulted in a significant 35% (55 +/- 13 s) reduction in treatment time, on average. Single parameter tests showed that the two control systems differed in their feedbac loops between MLC, gantry, and dose rate. These differences made the VMAT deliveries more smooth using the new Integrity treatment control system, compared to the current Desktop 7.01. Together with the use of CVDR, which results in less abrupt changes in dose rate, this further increases the smoothness of the delivery. The use of CVDR for VMAT with the Integrity desktop results in a significant reduction in treatment time compared to BVDR with an average reduction of 35%. This decrease in delivery time was achieved without compromising the dosimetric accuracy.

SU‐D‐BRA‐02: Penumbra Measurement for a Flattening‐Filter‐Free Artiste Linear Accelerator

Purpose: The primary benefit of removing the flattening filter of a linear accelerator is to increase the dose rate by a factor of 3 to 6. Additionally, change in scatter conditions in the head of the accelerator may also decrease the penumbra, which could lead to better target conformality. The purpose of this work is to evaluate this effect on a Siemens Artiste linear accelerator. Methods: Profiles of 10 ×10 cm2 flat and flattening‐filter‐free (FFF) fields were measured in water using a scanning diode with 0.3 mm steps. The penumbra created by both the 160‐leaf multileaf collimator (MLC) and the backup jaw (Y‐jaw) was measured. The profile of the FFF beam was renormalized so that for both flat and FFF beams, the penumbra was defined as the distance between the points with 20% and 80% of the central flat beam dose. The penumbra was measured every 5 cm along the major axes and around the central axis at four depths from 1.5 to 20 cm. Results: The average penumbra defined by the MLC and Y‐jaw was 4.9 mm and 5.9 mm, respectively for the flat beam, and 4.9 mm and 5.2 mm for the FFF beam. The flat beam penumbra increased with depth from 4.6 to 9.2 mm and 5.5 to 10.1 mm for the MLC and Y‐jaw respectively. The corresponding increase for the FFF beam penumbra was 4.6 to 9.2 mm and 4.6 to 8.8 mm. The average paired difference was &lt;0.1 mm for the MLC and 0.8 mm for the Y‐jaw. Conclusion: The MLC penumbra was not affected by removing the flattening filter, while the decreased scattering in the FFF beam reduced the Y‐jaw penumbra by 0.8 mm. Removing the flattening filter does not clinically significantly improve the way the MLC can conform to the target.This research was partially supported by Siemens Medical Solutions.

SU‐E‐T‐467: The Trick in MLC Calibration to Correct Underdose Region in IMRT Delivery

Purpose: By verifying the consistency between the planning dose and the delivered dose for each segment, we figure out the cause of the low dose regions inside the treatment field which only occur in verification images, but not in the treatment plan. Methods: The treatment planning system is ADAC Pinnacle version 9.0 with SmartArc. The IMRT plan is transferred to IMPAC oncology management system version 8.3 to drive the LINACs. Instead of taking an overall two or three dimensional dose distribution, we take the fluence maps for each beam by using Kodak XV films on two ELEKTA Precise LINACs and one ELEKTA Synergy LINAC. Results: Low dose regions are discovered in only Synergy LINAC and always in gun‐to‐target (G‐T) direction. By comparing the fluence maps between treatment plan and verification film segment by segment, we find these low dose regions result from the jaw position, i.e. the MLC directional backup jaw is too close to cause these unplanned underdoses in the treatment volume. We recheck the light‐field and the radiation field again; there is coincidence between the light‐field and the radiation field and just on the correct position. According to the statute, each radiation field edge is to be aligned with the displayed diaphragm within ±1 mm and each light‐field edge is to be aligned with the radiation field edge within ±1 mm. We recalibrate the MLC directional backup jaws and MLCs position to 0.5 mm larger than display and recheck the fluence maps, the low dose regions disappear eventually Conclusions: When doing the radiation field size calibration, one should calibrate the MLC moving directional radiation field size 0.5 mm larger than display each side on ELEKTA machine to prevent the unanticipated low dose regions inside the tumor in IMRT delivery.

SU‐GG‐T‐198: A Pitfall in IMRT Verification: The Jaw Position

Purpose: By verifying the consistency between the planning dose and the delivered dose for each segment, we try to figure out the cause of the high dose regions outside the treatment field which only shown in measured dose, but not in the treatment plan. Method and Materials: Our treatment planning system is ADAC Pinnacle version 8.0m. The IMRT plan is transferred to IMP AC oncology management system version 8.3 to drive the LINACs. Instead of taking an overall two dimensional dose distribution, we took the fluence maps for each beam by using Kodak XV films on two ELEKTA Precise LINACs and one ELEKTA Synergy. Results: High dose regions are obviously in only one Precise LINAC and always perpendicular to MLC leakages. By comparing the fluence maps between treatment plan and verification film segment by segment, we find it occurs on the MLC 0.8 cm gap which is designed to prevent one side MLCs to collide with the opposite MLCs. The gap should be blocked by the primary jaw and MLC directional backup jaw. However, because of the machine limitation, the primary jaws are not allowed to cross the central line, the gap is simply blocked by the MLC directional backup jaw only (3cm tungsten) occasionally and slight transmission doses are generated (about 15%). If the backup jaw goes to the wrong position and does not block the MLC gap, a high dose region occurs during dose delivery, but not shown in plan. Conclusion: When the backup jaw position is not compatible with treatment plan, the unanticipated high dose region emerges not only outside of the field but also inside the field. Even if one checks the jaw position by any open field, this situation sometimes occurs. The best way is to recalibrate the jaw mechanical parameters and recheck the incorrect segment.

An MLC calibration method using a detector array

The authors have developed a quantitative calibration method for a multileaf collimator (MLC) which measures individual leaf positions relative to the MLC backup jaw on an Elekta Synergy linear accelerator. The method utilizes a commercially available two-axis detector array (Profiler 2; Sun Nuclear Corporation, Melbourne, FL). To calibrate the MLC bank, its backup jaw is positioned at the central axis and the opposing jaw is retracted to create a half-beam configuration. The position of the backup jaws field edge is then measured with the array to obtain what is termed the radiation defined reference line. The positions of the individual leaf ends relative to this reference line are then inferred by the detector response in the leaf end penumbra. Iteratively adjusting and remeasuring the leaf end positions to within specifications completes the calibration. Using the backup jaw as a reference for the leaf end positions is based on three assumptions: (1) The leading edge of an MLC leaf bank is parallel to its backup jaw's leading edge, (2) the backup jaw position is reproducible, and (3) the measured radiation field edge created by each leaf end is representative of that leaf's position. Data from an electronic portal imaging device (EPID) were used in a similar analysis to check the results obtained with the array. The relative leaf end positions measured with the array differed from those measured with the EPID by an average of 0.11+/-0.09 mm per leaf. The maximum leaf positional change measured with the Profiler 2 over a 3 month period was 0.51 mm. A leaf positional accuracy of +/-0.4 mm is easily attainable through the iterative calibration process. The method requires an average of 40 min to measure both leaf banks. This work demonstrates that the Profiler 2 is an effective tool for efficient and quantitative MLC quality assurance and calibration.

Multileaf Collimator Characteristics and Reliability Requirements for IMRT Elekta System

Understanding the characteristics of a multileaf collimator (MLC) system, modeling MLC in a treatment planning system, and maintaining the mechanical accuracy of the linear accelerator gantry head system are important factors in the safe implementation of an intensity-modulated radiotherapy program. We review the characteristics of an Elekta MLC system, discuss the necessary MLC modeling parameters for a treatment planning system, and provide a novel method to establish an MLC leaf position quality assurance program. To perform quality assurance on 40 pairs of individual MLC leaves is a time-consuming and difficult task. In this report, an effective routine MLC quality assurance method based on the field edge of a backup jaw as referenced in conjunction with a diode array as a radiation detector system is discussed. The sensitivity of this test for determining the relative leaf positions was observed to be better than 0.1 mm. The Elekta MLC leaf position accuracy measured with this system has been better than 0.3 mm.

SU-EE-A2-04: Comparison of a MLC Calibration Method Using a Diode Array and Electronic Portal Imaging Device

Purpose: Current methods of MLC calibration and quality assurance include the use of graph paper, scanning water tanks, ion chambers, film and EPIDs; these methods can be both laborious and/or non-quantitative. We propose a convenient and quantitative method for calibrating and performing routine quality assurance for MLCs. Method and Materials: The Profiler 2 (Sun Nuclear Corp., Melbourne, FL), a two dimensional diode array, was visually aligned to the backup jaw and irradiated with an Elekta linac. The backup jaw served as a radiation reference line and was used to determine the positional diode offsets. The MLC was stepped over the Profiler 2 and the 50% position for each diode was determined. Leaf offsets were calculated by offsetting each diodes 50% position by the 50% position of the diode corresponding to leaf pair 20 and subtracting the corresponding positional diode offset. This method was repeated with an EPID for comparison purposes. Results: Linear dose changes of approximately 15%/mm and 13.5%/mm were observed in the penumbras of the backup jaw and MLC, respectively. Leaf positions measured by the EPID and Profiler 2 differed on average by ± 0.12 mm per leaf. Leaf reproducibility of 0.36 mm or better was observed and minimal reproducibility changes were seen with the backup jaw. Conclusion: This work demonstrates the potential of the Profiler 2 as a tool for calibrating and performing quantitative QA for MLC leaf positioning. The Profiler 2 methods are fast, taking an average of 20 minutes to perform and accurately represent leaf positions and reproducibility. Conflict of Interest: This work was supported in part by SBIR Contract No. HHSN261200522014C, ADB Contract No. N43-CM-52214, the University of Florida and Sun Nuclear Corporation.