Index of content:
Volume 36, Issue 6, June 2009
- Therapy Moderated Poster Session: Exhibit Hall ‐ Area 1
- Moderated Poster ‐ IMRT I
36(2009); http://dx.doi.org/10.1118/1.3181069View Description Hide Description
Purpose: To evaluate the effect of VMAT sequencer, MLC leaf motion range and MU difference for adjacent control points in VMAT plan, and gantry interval used for dose calculation on the treatment delivery accuracy. Material and Methods: VMAT plans with leaves moving across the field in alternate open‐close pattern for every 10 degrees of rotation were created on Pinnacle SmartArc 8.1v and Ergo++ V1.7.1, where VMAT DICOM RT plans were generated with different sequencers. Various MLC motion range and MU with different ratios were assigned to the adjacent control points. The plans were delivered on Elekta Synergy. Measurements with Matrixx ion‐chamber array were analyzed. Gamma passing rate were evaluated using dose calculated by interpreting the MU and MLC shapes between control points with different gantry intervals. Results: DICOM plans transferred from SmartArc had higher gamma passing rate than those from Ergo++. Passing rate decreased as the MU ratio increased for Ergo++ because its VMAT sequencer averaged the MU from two adjacent control points. Less than 2% difference was observed between dose calculated with 1 and 2 degree gantry interval. Compared to plans with 2.5cm MLC motion, passing rate for 5.0cm plans decreased by 5–15%. Verification results degraded as MU ratio increased from 5:5 to 1:9 if dose was calculated using the gantry interval of 2 degrees or larger. Gamma pass rate decreased as the calculation interval increased from 2 to 10 degrees for VMAT plans were calculated assuming MLC leaves were stationary between control points. Conclusion: Smaller MLC leaf motion range can improve VMAT delivery accuracy. A sequencer capable of keeping original MU for each control point should be used to generate VMAT DICOM RT plan. Fine gantry interval should be used for dose calculation to show the VMAT plan that reflects the delivered dose accurately.
Research supported by Elekta.
SU‐DD‐A1‐02: Incorporating Prior Knowledge Into the Segment Optimization in Segment‐Modulated Arc Therapy36(2009); http://dx.doi.org/10.1118/1.3181070View Description Hide Description
Purpose: Recently, various stochastic algorithms have been applied to the segment modulated arc therapy (SMART) inverse planning. Most, if not all, algorithms are brute force trial‐and‐error search in nature without inclusion of a priori knowledge for segment position and shape. The purpose of this work is to provide an effective way to speed up the SMART planning process by incorporating dosimetric knowledge of the system into the segment shaping. Methods: The SMART inverse planning was performed in two steps. First, the maximum intensity map (MIM) was evaluated for each beam. The intensity of MIM is defined as the maximum intensity without violating the threshold dose of the organ‐at‐risks (OARs) located on the path of the beamlet. Based on MIM, a probabilistic map (MP) model was properly established which describes the probability for a beamlet to be open (=1) or close (=0). A rule of thumb is that a beamlet traversing sensitive OARs (which is bad) tends to have smaller chance to open and vice versa. Second, a SMART inverse planning was performed based on simulated annealing method. In each step, MP serves as a likelihood function for the trial change of a segment. With this likelihood, a bad segment tends to be rejected without spending valuable CPU‐time to calculate the dose distribution and vice versa. Results: The proposed method was tested on a prostate case. A comparison of a prostate SMART planning with and without prior knowledge indicated that the computational efficiency was increased by a factor of ∼4. Besides, it is observed better sparing of OARs is also achieved by using MIM‐guidance. Conclusion: MIM information provides useful priors for the selection of potentially good segment shape and position and greatly facilitates the search for a set of optimal segments for SMART.
36(2009); http://dx.doi.org/10.1118/1.3181071View Description Hide Description
Purpose: To utilize angular beam's‐eye‐view dosimetrics (BEVD) information to improve the computational efficiency and plan quality of inverse planning of aperture modulated arc therapy (AMAT). Materials & Methods: In BEVD‐guided inverse planning, the angular space spanned by a rotational arc is represented by a large number of fixed‐gantry beams with angular spacing of ∼2.5°. Each beam is assigned with an initial aperture shape determined by the beam's‐eye‐view (BEV) projection of the planning target volume (PTV) and an initial weight. Instead of setting the beam weights arbitrarily, which slows down the subsequent optimization process and may result in a sub‐optimal solution,a‐priori knowledge about the quality of the beam directions derived from a BEVD is adopted to initialize the weights. In the BEVD calculation, a higher score is assigned to directions that allow more dose to be delivered to the PTV without exceeding the dose tolerances of the organs at risk (OARs), and vice versa. Simulated annealing is then utilized to optimize the segment shapes and weights. The BEVD‐guided inverse planning is demonstrated by using a pancreatic case and the results are compared with conventional approach without BEVD guidance. Results: An a‐priori knowledge guided inverse planning scheme for AMAT is established. The inclusion of BEVD guidance significantly improves the convergence behavior of AMAT inverse planning and results in much better OAR sparing as compared to the conventional approach. Conclusions: BEVD‐guidance facilitates AMAT treatment planning and provides a comprehensive tool to maximally utilize the technical capacity of the new arc therapeutic modality.
36(2009); http://dx.doi.org/10.1118/1.3181072View Description Hide Description
Purpose: Dose painting, or heterogeneous dose prescription is desired in many situations including re‐optimization in ART to fix hot/cold spots from previous deliveries, and dose boosting based on theragnostic imaging. With minimal modification of the current TomoTherapySM planning workflow, we developed a simple scheme and studied the feasibility of dose painting in TomoTherapySMtreatment planning.Method and Materials:Optimization of TomoTherapySM treatments is driven by a DVH‐based objective function and user relies on DVHs to evaluate plan quality. Given an arbitrary prescribed dose distribution, we introduce a “complimentary‐dose”, which is the difference between a (homogeneous) “reference prescription” and the “heterogeneous prescribed distribution”. During each optimizationiteration, the “complimentary‐dose” is added back to the “calculated‐dose” for DVH‐based objective function evaluation. The DVH shows “optimized‐dose”, which is the summation of “complimentary‐dose” and “calculated‐dose”. The ideal DVH is still a vertical line through the reference point. All DVH constraints for tumors and OARs are employed as in regular optimization. A similar method could be used for re‐optimization in ART to fix previous errors as identified in a prior dose. Results: We used phantom studies to evaluate the feasibility of dose painting in TomoTherapySMtreatment planning. Various discrete and continuous prescribed dose distributions were tested. Dose profiles and effective DVHs are used to evaluate the results. For boosting discrete regions, the results show that TomoTherapy® technology is able to resolve boost regions as small as 1 cm in diameter. Concave and convex continuous prescribed dose distribution, with gradient up to 20%/cm, can be well achieved via TomoTherapySM dose painting. Conclusions: We developed and studied dose painting in TomoTherapySMtreatment planning. Phantom studies show that TomoTherapy® is an ideal modality for dose painting. Dose boosting or hot/cold spots fixing region as small as 1 cm and dose painting with gradient up to 20%/cm, is achievable.
36(2009); http://dx.doi.org/10.1118/1.3181073View Description Hide Description
Purpose: With the commercial introduction of deliverycontrol systems for Volumetric Modulated Arc Therapy (VMAT), the need has arisen for reliable and efficient techniques for performing patient specific VMAT quality assurance (QA). In this work, we have studied three patient specific QA techniques for VMAT: ion chamber with film, 2D diode array, and a 2D ion chamber array. Materials and Methods: The three QA techniques we have utilized are: (1) a stack of solid water slabs with an inserted ion chamber and film sandwiched between two slabs; (2) a 2D diode array (MapCHECK™ device inserted into a MapPHAN™ phantom); and (3) a 2D ion chamber array (MatriXX™ inserted into a MULTICube™ Phantom). Ten VMAT plans were delivered to all three QA systems on an Elekta Synergy linac.Results: With the highest spatial resolution among all three systems, film measurements can provide very good QA results when analyzed in relative mode. Absolute dose comparisons were performed for both the MapCHECK™ and MatriXX™ systems. The average passing rate in gamma analysis were 95.0% and 98.5% using 3%/3mm criteria for the above two systems, respectively. The slightly lower passing rate for MapCHECK™ QA may be attributed to the angular and dose rate dependence of the diode response. It is also observed that the MapCHECK™ QA is more sensitive to the tongue‐&‐groove effect when diodes fall between two leaves. The MatriXX™ system provides slightly higher QA passing rates. However it may be less sensitive to large dose variation within small regions due to its 7.62mm detector grid size. Conclusions: All three systems can be used for VMAT plan QA provided users are attentive about the strengths and limitations of the QA device.
Research sponsored by Elekta Corporation.
36(2009); http://dx.doi.org/10.1118/1.3181074View Description Hide Description
Purpose: To compare RapidArc (Varian Medical Systems) intensity modulated arc therapy and dynamic multileaf collimated IMRT (DMLC) delivery accuracy using patient specific quality assurance measurements. Method and Materials: We selected 6 cases (head and neck, para‐aortic lymph nodes, and 4 prostates) that were treated using both DMLC and RapidArc. The DMLC and RapidArc plans were equivalent with respect to target and critical structure dosimetry. Absolute dose measurements were made with an ionization chamber in a custom acrylic phantom, which was positioned such that the chamber was located in a high‐dose, low‐gradient region of the dose distribution. Film was used to obtain the dose distribution in a coronal plane at the level of the ionization chamber. A 2D ionization chamber array (Matrixx, IBA Dosimetry) was used to obtain a second measurement in the same coronal plane. The gamma index was calculated for both the film and 2D array measurements using the criteria of 3%/3mm. Results: The mean difference between the ionization chamber measurements and the calculations was 1.0% for both RapidArc and DMLC. The ranges were 0.6% to 1.6% and 0.0% to 2.3%, respectively. The mean fraction of pixels with gamma > 1 for the film was 4.3% (range 0.8% to 6.9%) for RapidArc and 10.4% (range 5.4% to 17.3%) for DMLC. For the 2D array the mean fraction of pixels with gamma > 1 was 0.5% (range 0.0% to 1.1%) for RapidArc and 1.6% (range 0.5% to 4.1%) for DMLC. Conclusion: The results demonstrate that RapidArc delivery is at least as accurate as that for DMLC. The mean fraction of pixels with gamma >1 and the range of fractions for both film and the 2D chamber array suggests that RapidArc may be modestly more accurate than DMLC.
Conflict of Interest: Research sponsored by Varian Medical Systems.
- Moderated Poster ‐ IMRT II
36(2009); http://dx.doi.org/10.1118/1.3182247View Description Hide Description
Purpose: A main concern about the IMRT dose validation tool using Monte Carlo(MC) simulation and R&V system/Dynalog file is the potential inconsistency between the actual leaf‐end positions and those recorded by the Dynalog file. The present study investigates an accurate, fast and independent method to validate the accuracy of the dynalog files using aSi‐EPID images.Materials and Methods: A computer program was developed to detect the MLC segmented field edges in EPIDimages (1024×768 pixels, pixel size: 0.392mm). Standard reference MLC segmented fields were designed and leaf‐end positions were measured accurately. EPIDimages for these reference MLC fields were recorded and the leaf‐end positions were calculated as the locations where the image intensity is 50% of the maximum. Small corrections were made to minimize the effect of scatteredphotons (background). Daily EPIDimages of the same MLC segmented fields were compared to the original images and to check the accuracy of the Dynalog files. The patient‐specific Dynalog files were used for MC based patient‐specific treatment verification. Results: Both the MLC and EPID were calibrated to produce accurate and consistent leaf‐end positions. The results showed that the EPID‐extracted leaf‐end positions were within ±0.167mm of their actual positions, while the average RMS leaf‐end deviation was 1.73mm. Differences between daily EPID and Dynalog leaf‐end positions were established and to be monitored on the long term. Random small leaf position variations have negligible effect on the patient dose distribution but a 1mm systematic leaf‐end‐position error could result in a 3% change in the delivered dose. Conclusion: A daily QA tool is developed to check the accuracy of the Dynalog file and MLC leaf‐end positions as part of the comprehensive IMRT QA procedure. This ensures the accuracy of the MC based patient‐specific IMRT dose verification using the information recorded in the R&V system/Dynalog files.
MO‐EE‐A1‐02: Dose Escalation by Target‐Tracking Treatments Planned by 4D Direct Aperture Optimization: A Proof of Principle36(2009); http://dx.doi.org/10.1118/1.3182248View Description Hide Description
Purpose: To evaluate a novel 4D direct aperture optimization system capable of planning a 4D‐tracking delivery while ensuring compliance with the delivery constraints. The planning system was tested by deriving the change in tumor‐control probability (TCP) by dose escalation with a fixed mean lung dose (MLD) in a digital phantom dataset. Method and Materials: The treatment fields were divided into phases matched to phases of a 4D‐patient model. Compliance with delivery constraints was ensured by restricting equipment configurations within phases and changes between phases to deliverable settings. The optimization cost was evaluated by comparing the dose distribution summed over the breathing cycle with a prescription. The potential clinical benefit of the system was evaluated with a dose escalation study on a 4D digital phantom. The acceptable MLD was derived from a reference static plan with 64Gy prescribed to the target with motion‐encompassing margins. The TCP was optimized for the fixed MLD, where the TCP was evaluated at 12 months using the method of Martel et al. (1999). Results: With a fixed MLD (13Gy) the 4D‐tracking treatment achieved a mean target dose of 79.6Gy as compared to 63.8Gy for a static plan and 73.9Gy for a plan optimized on the 4D patient but with no motion of the delivery equipment (denoted 4D‐static plan). The resultant TCP increased from 49.6% for the reference plan to 75.4% for the 4D‐tracking plan compared to 45.6% for the static plan and 66.5% for the 4D‐static plan. Conclusion: A treatment‐planning system capable of optimizing target‐tracking treatments complying with delivery constraints has been developed and tested. The tracking deliveries produced have been evaluated by the allowed dose escalation for a simple example case, where a substantial improvement in TCP was observed. This finding shows that the technique is very promising and worthy of further study.
36(2009); http://dx.doi.org/10.1118/1.3182249View Description Hide Description
Purpose: 4D CT is increasingly used to define the ITV, the envelope of the CTV as it moves during breathing, and PTV (ITV + set up margin). IMRT plans are designed to provide uniform dose to the PTV. We propose a planning method to design true 4D IMRT plans in which the PTVs of the individual phases of the 4D CT as well as the conventional PTV may receive non‐uniform doses, but the cumulative doses to the PTVs of individual phases, computed using deformable image registration (DIR), are uniform. Methods: The non‐uniform dose prescription for conventional PTV was obtained by solving linear equations by requiring motion‐convolved 4D dose to be uniform to the PTV for the end exhale phase (PTV50%) and by constraining maximum inhomogeneity to be 30%. A plug‐in code to Pinnacle was developed to perform the IMRT optimization based on this non‐uniform PTV dose prescription. The 4D dose was obtained by summing the mapped doses from individual phases of the 4D CT using DIR. This 4D dose distribution was compared with that of the ITV method. The robustness of 4D plans over the course of radiotherapy was evaluated by computing the sum of the 4D dose distributions for each weekly CT mapped to the planning 4D CT using DIR. Results: The 4D dose distribution provided additional lung sparing by 5% for V5, V10, V20 and V30 compared to the use of the ITV method. The dose volume histograms of PTV50%, CTV, lung, spinal cord, and heart for the cumulative dose over the course of IMRT were similar to those for 4D dose at the time of original planning. Conclusion: The proposed 4D planning method may increase lung sparing compared to the ITV method used commonly in the clinics and is robust against inter fractional set‐up and anatomy changes.
36(2009); http://dx.doi.org/10.1118/1.3182250View Description Hide Description
Introduction In IMRTtreatment planning, multi‐leaf collimator(MLC) with a maximum leaf spread constraint is used to deliver the prescribed intensity maps (IMs). However, the maximum leaf spread of an MLC may require splitting a large IM into several overlapping sub‐IMs that are each delivered separately. Existing approaches usually require fixed sizes of sub‐IMs. We developed a method producing sub‐IMs of flexible sizes subject to the maximum leaf spread, which may improve the delivery efficiency. In this work, we propose to optimally split an IM into sub‐IMs while minimizing the total complexity of the sub‐IMs. Method and Material The complexity measure of an intensity map we use is the total sum of positive gradients of all rows. We solve the optimal field splitting problem using dynamic programming. Our algorithm also balances minimum beam‐on times of the resulting sub‐IMs. We evaluated the performance of our algorithm by implementing it on clinical IMs obtained from the Department of Radiation Oncology, University of Iowa and comparing it to ommercial software derived solutions. 14 IMs from pelvic treatment plans on 2 patients were used for splitting resulting in 3 sub‐IMs (3‐splitting) algorithm experiments and 20 IMs from head & neck treatment plans from 3 patients were used for 2‐splitting algorithm experiments. We replaced the field splitting method in Pinnacle with our method and the results were compared. Results For 3‐splitting cases, the number of segments was reduced by 12.5%, and the number of MU's was improved by 30.0%, along with 144‐sec reduction of beam delivery time per fraction. For 2‐splitting, the number of segments was reduced by 5.3%, and the number of MU's was improved by 27.6%. The performance for 3‐splitting cases was better than for 2‐splitting. Conclusion We have developed an optimal field splitting method which appears to outperform the commercial software Pinnacle.
36(2009); http://dx.doi.org/10.1118/1.3182251View Description Hide Description
Purpose:Intensity modulated radiation therapy has been a very popular and effective treatment technique for the treatment of prostate, head & neck and liver etc. Meanwhile, another innovative treatment technique, intensity modulated arc therapy, was developed to complement some drawbacks of IMRT like long treatment time and low MU efficiency. Since the IMAT completes the treatment just within one or two rotations, it is not easy to get optimized leaf sequences in a deterministic way. In this study, we tried to get optimized IMAT treatment plan by genetic algorithm. Method and Materials: First, the start/end positions of MLC leaves at each rotation angle with 10° interval were selected as optimization variables and encoded into genetic chromosomes. They experience genetic operations such as generation, selection, crossover, mutation and reproduction and the most optimized solution remains in the end of iteration. The constraint of maximum leaf speed was included in these operations. And the fitness of each population was evaluated by DVH volume constraint based objective function. IMAT dose distribution was calculated as a weighted sum of MLC shape at each angle and related Dij matrix similar to IMRTdose calculation. The algorithm was implemented in our treatment planning system and the dose distributions and DVHs of single and double gantry rotation cases were compared. Results: IMAT plan gave comparable results with conventional IMRT even with single gantry rotation and there was not significant improvement in double gantry rotations. Genetic algorithm required about 3,000 generations to reach optimized value due to its stochastic nature. Conclusion: It was possible to optimize IMAT plan with genetic algorithm and the results are optimized MLC leaf sequences readily deliverable in general linear accelerators. It can be an efficient method to solve IMAT optimization problem despite of relative slow convergence.
36(2009); http://dx.doi.org/10.1118/1.3182252View Description Hide Description
Purpose: An adaptive re‐planning framework was used to account for tissue deformation during RT treatment courses. The framework is based on a multivariate control concept called model predictive control (MPC). A variety of control rules and optimization schemes were investigated using a prioritization‐based optimization method. Performance of the MPC control was evaluated with conventional evaluation metrics such as DVH, coverage, and homogeneity as well as model‐based plan evaluation metrics. Method and Materials: The Head/Neck IMRT treatment process was simulated using mid‐course CT scans, tracking treatment doses to tissues at the voxel level. Off‐line adaptation was performed with the changing patient model, updating the state of the process along with the dose‐to‐date information. Constrained optimizations were performed for re‐planning as well as for pre‐treatment planning. Multivariate control schemes including maximum constraint violation were utilized for re‐planning. Results: With the adaptation of the treatment plan to the patient‐specific anatomical changes, the predicted outputs could be efficiently controlled improving the robustness to those changes. Although the results show that the general trend in the predicted outputs follow the tissue volume changes, the clinical decision to re‐plan should be made by assessing the total predicted treatment output expectations. For cases investigated, a limited number (3∼4) of re‐plans were sufficient to control high priority outputs while the use of a tighter tolerance increased the number of re‐plans. Re‐plans were triggered mostly by the target coverage criteria. The studied course simulations indicate that significant changes are shown early in treatment. Therefore, frequent updates of the patient models may be necessary and critical to plan adaptation. Conclusion: The proposed multivariate control framework provided an intuitive method of articulating clinicians' decisions and priorities for re‐planning. In addition, the use of prioritization in re‐optimization may allow more efficient re‐planning which requires fewer planning resources.
Supported in part by CA‐P01‐CA59827
- Moderated Poster ‐ Measurements I
SU‐EE‐A1‐01: Development of a System to Measure Neutron Microdosimetry Spectra in a Mixed Proton‐Neutron Field36(2009); http://dx.doi.org/10.1118/1.3181093View Description Hide Description
Purpose: To develop a miniature tissue‐equivalent proportional counter (TEPC), timing‐coincidence veto, and pulse‐height analysis system for the purpose of measuring neutronMicrodosimetry spectra in a proton beam. Method and Materials: TEPC's were constructed with active volume geometry consisting of a 2.5mm right circular cylinder and employing a 10μm diameter stainless steel wire as the detector anode centered on the axis of an A150 tissue‐equivalent plastic cylinder with 2mm thick wall. Stainless steel field tubes define the active volume which is filled with propane based tissue‐equivalent gas to 168torr simulating a tissue density volume with 2μm diameter. Two fully depleted transmission‐type silicon detectors with diameter of 31.6mm will be operated in timing coincidence with the proportional counter as a charged particle veto system. Detector pulses are digitized in a 60MS/s sampling ADC and the data acquisition software is written in the LabVIEW graphical programming language which acquires and writes pulse waveforms to disk where pulse heights and timing information are extracted for further analysis.Results: A TEPC was tested in a mixed field produced in a proton treatment room with the proton beam incident on a closed tungsten MLC. The timing and energy resolutions for the TEPCs can be estimated from alpha spectra taken with a FWT LET‐1/2 detector attached to the data acquisition system. Timing and lineal energy resolution for the TEPCs are estimated to be 200ns and >15keV/μm, respectively. Conclusion: Preliminary testing shows that the TEPC and silicon detector timing‐coincidence veto system is a viable method for extracting neutronMicrodosimetry spectra from the mixed fields present in a proton therapy treatment room. This work was supported by the US Army Medical Research and Materiel Command under Contract Agreement No. DAMD17‐W81XWH‐07‐2‐0121. Opinions, interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the US Army.
36(2009); http://dx.doi.org/10.1118/1.3181094View Description Hide Description
Purpose: The goal was to develop a scintillating fiber dosimeter and verify its suitability in assessing the dose deposited during brachytherapy HDR treatments. As a first step toward in vivo dosimetry, it has been decided to conduct a full process, from planning to treatment, using a prostate phantom. Method and Materials: The scintillating fiber dosimetrysystem is composed of a RGB photodiode (MCS3AT) detecting light emitted from a scintillator (BCF‐60) coupled to a plastic optical fiber. Fiber is aligned on the photodiode, and its output is then amplified and acquired through a National Instruments acquisition card. Signal processing, including Cerenkov and fluorescence light subtraction, is done and dose deposited during treatment is obtained through integration. A prostate phantom (CIRS ♯053) has been used to conduct the study. An ultrasound‐guided catheter insertion procedure was performed by a radiation oncologist at our institution. A total of 13 catheters were inserted, one of those being used for the dosimeter. Following a CT scan, the prostate and urethra were contoured and treatment planning was performed according to our prostate clinical protocol on a PLATO workstation (Nucletron BV, Netherlands). The plan was delivered using a microSelectron V2 192Ir afterloader. Results: We measured dose rate as function of time. It varies with distance between the dosimeter and the dwell‐positions. Dose rate integration, once the contamination component is subtracted, determines the total dose deposited during treatment. Measurements were within 2% of the dose predicted by the planning system.Conclusion: Scintillating fiber dosimeter provides an accurate method to assess dose deposited to targets and organs at risk during HDR brachytherapy.Dose rates measured allow QA of treatment delivery for each catheter and total delivered dose. Measurements have been done in a prostate phantom, but there is a potential for a variety of clinical sites.
SU‐EE‐A1‐03: The Neutron Dose Evaluation and Shielding at the Maze Entrance of the Varian Clinac 2300EX36(2009); http://dx.doi.org/10.1118/1.3181095View Description Hide Description
Purpose: After the installation of a Varian Clinac 2300EX accelerator, neutron and photon doses at the outer maze entrance were measured and compared with several empirical calculations. The measurements were taken before and after borated polyethylene (BPE) boards were installed on the maze wall as neutron absorption material. Method and Materials: The accelerator delivered an 18 MV photon beam at 600 cGy/min dose rate. An NRC NP‐2 type neutron REM‐meter and a Ludlum 14C survey meter with a 44‐6 type of probe were used to measure the neutron dose and photon dose, respectively. The measurements were taken at the center axis of the maze, 0.8 m above the floor, 0.3 m away from the maze door. Results: With the gantry head tilted close to the inner maze entrance with closed jaws, both neutron dose and photon dose reach their maximum. The measurement data were compared with empirical calculation results obtained by Kersey's method, modified Kersey's method and a newly proposed method by R.C. Falcão el al. The estimation from Kersey's method is about 2 – 4 times of the measurement (Ratio ≈ 2.4–3.8). Falcão's method has estimation about 22 times of the measurement (Ratio ≈ 21.9). The modified Kersey's method has the best prediction of the dose (Ratio ≈ 1.0). The McGinley's photon dose equation gives estimation about 80% of the measurement. After applying borated polyethylene boards as lining on maze wall, the neutron dose and the photon dose at maze entrance were decreased by 41% and 58%, respectively. Conclusion: This work indicates that Kersey's method overestimates the neutron dose by about 2 to 4 times for this study. Falcão's method largely overestimates the neutron dose. The modified Kersey's method is recommended to be used in neutron shielding calculation for the 2300EX accelerator.
36(2009); http://dx.doi.org/10.1118/1.3181096View Description Hide Description
Purpose:Radiation treatment machine QA can be time consuming and oftentimes not accurate. The EPID on linacs can possibly be utilized for machine QA and calibration. In this study, we have developed methods to perform QA using EPIDimages.Method and Materials: A modified ball bearing phantom was constructed and set up according to the room laser center. Portal images were taken at different collimator, gantry, and table angles on an Elekta Synergy LINAC equiped with an a‐Si EPID. Portal images of several jaw and MLC control points were taken also. All images were analyzed using custom developed software in Matlab to calculate collimator/table angle, collimator/table/gantry runout, and jaw/MLC positions. Multiple CBCTimages were also taken to verify KV‐MV coincidence and table correction accuracy. Results: Accurate quantitative results for mechanical, MLC, jaw and CBCT QA can be obtained using the developed method. Conclusions: We have developed automatic treatment machine QA methods using EPIDimages. This QA procedure is faster and more accurate than traditional QA methods.
SU‐EE‐A1‐05: Dosimetric Evalualtion of a Commercial Compensator for Spatially Fractionated Radiation Therapy36(2009); http://dx.doi.org/10.1118/1.3181097View Description Hide Description
Purpose:Spatially fractionated GRID therapy is utilized to treat large tumors by irradiating the volume through isolated small openings. The technique has shown high efficacy for bulky tumors, but it is used at relatively few facilities. In this study, we exploit the use of a prototype brass GRID, which could make GRID therapy more attractive to physicians and more widely available to patients. Method and Materials: A prototype GRID compensator was constructed by milling divergent holes of 1 cm diameter at the isocenter in a cube of brass. The GRID block is manufactured so that it can irradiate a maximum field size of 25×25cm2. Measurements for the characterization of the dosimetric properties of the GRID were performed using a Varian 23Ex linac, in a solid water phantom at 100cm SSD, for both 6MV and 18MV. Radiographic films were placed perpendicular to the beam axis to obtain lateral profiles, and parallel to the beam axis to find the percent depth doses (PDDs). A pinpoint ion chamber under the central hole was used to find the output factors. Results: The profiles have an obvious peak and valley pattern, with transmissions under the solid portion of the block of approximately 15% for 6MV and 30% for 18MV. The PDDs are less penetrating than their open‐field counterparts, for both 6MV and 18MV when compared against the 10×10cm2 open field. Conclusion: This prototype brass GRID compensator is a viable alternative to the cerrobend compensators or MLC‐based fields currently in use. Its ease of creation and use give it decided advantages. We believe this compensator can be put to clinical use, and will allow more centers to offer GRID therapy to their patients.
Research sponsored by .decimal corporation.
36(2009); http://dx.doi.org/10.1118/1.3181098View Description Hide Description
Purpose: The focus of this study is to develop a methodology using statistical process control (SPC) that provides effective quality control of mechanical parameters in external beam treatment delivery. Materials and Methods: A four‐field box clinical treatment technique was used as the evaluation tool. Parameters tested were gantry angle, table height, table lateral, and collimatorfield size. The dosimeter, GafChromic EBT film, was placed vertically in a pelvic phantom in the transverse plane at isocenter. All films were digitized and evaluated using a Vidar film scanner and RIT, Inc. software. Dose distributions of accurate treatment delivery were measured and compared to treatment delivery with a single mechanical parameter deviated at 0.5, 1.0 and 1.5 times the TG‐40 parameter specification. Each distribution was sampled in the penumbra region using three sampling sets (SS). Each SS consisted of different (size and/or position) regions of interest (ROI). Set‐A and B were 1.5cm × 1.5cm while Set‐C was 0.8cm × 1.0cm. Set‐A and C were centered in the same locations along each beam entry 7cm from isocenter. Set‐B was positioned at the corners of the dose box in the distribution. Results: Two process behavior charts (PBC) were prepared for each SS, evaluating the mean dose and mean dose range. PBC for Set‐A were unable to distinguish between accurate and inaccurate distributions. Set‐B detected all mechanical parameters deviated at 1.0 and 1.5 times the TG‐40 specification but also indicated four false‐positives. These PBC were also effectively unable to distinguish between the accurate and inaccurate distributions. Set‐C PBC detected all mechanical parameters deviated at the 1.0 and 1.5 level and reliably indicated the inaccurate distributions. Conclusions: Small and therefore more homogeneous, strategically located ROI can be effective in SPC to detect inaccurately delivered treatments. Quality control of linac mechanical parameters can be performed using SPC‐based methodology.
- Moderated Poster ‐ Measurements II
36(2009); http://dx.doi.org/10.1118/1.3182277View Description Hide Description
Purpose: Intensity modulated rotational arc technique requires verification of leaf positions, gantry angle and dose rate in the entire arc. This study shows how to achieve this with a detailed verification of Varian RapidArc using a fluoroscopic electronic portal imaging device(EPID).Materials and Methods: Three Rapid Arc plans (prostate 1, whole pelvis 1, and head and neck 1) are delivered on a Triology linac (Varian Medical Systems, CA). During delivery, approximately 600 fluoroscopic portal images are acquired (∼8 images/s) per arc with a PV‐aS1000 EPID, without use of secondary phantoms or blocks. Each leaf position of each gantry angle is calculated from the acquired EPIDimages offline. Gantry angle information of each portal image is acquired from the dynalog file generated during beam delivery. Leaf positions from the dynalog file are compared to scheduled positions from the DICOM RT plan file. Results: Online EPIDimage acquisition of Rapid Arc delivery is prompt, involving extension of the EPID system and beam delivery time. The measurement error depends on the displacement of EPID system relatively to the center of rotation, which is only 1mm–1.5 mm. Offline analyses show the accuracy of leaf positions for static leaf and gantry field are better than 1 mm. More than 98.5% of leaf sequences exhibit less than 3mm deviations, 83 % show 2mm and 56% for 1mm. Conclusions: Position of each leaf of each gantry angle for Rapid Arc delivery is verified within 1 mm accuracy with fluoroscopic portal images. Use of fluoroscopic EPIDimages can be considered as a practical QA tool for the verification of the Rapid Arc delivery.
This study is partially supported by Varian Medical Systems.
36(2009); http://dx.doi.org/10.1118/1.3182278View Description Hide Description
Purpose: To develop calibration procedures for a novel 4D diode array for IMRT as well as arc therapy quality assurance (QA). Method and Materials: A novel 4D diode array (ArcCHECK) was designed for rotational therapy QA. Its cylindrical and isotropic design presents a consistent detectorimage in beam's eye view for all gantry angles. Signals were acquired every 50 ms, allowing for real‐time per‐beam QA as well as measuring composite dose distributions. An efficient calibration procedure was developed to obtain diode sensitivity and directional response dependence with one full gantry rotation. In this process, each diode was in turn irradiated under the same beam condition on beam central axis. The directional response was obtained using TPS‐calculated beam profiles, which were validated in rectangular geometry. A real time algorithm to derive beam angle based on beam edge back projections was developed to apply the corresponding directional response factors. Its clinical application for IMRT QA was demonstrated with 17 IMRT head and neck beams delivered with planned beam angles. Measured dose distribution was compared to TPS calculation in three dimensions with %diff and distance‐to‐agreement analysis. Its feasibility for VMAT QA was investigated with a prostate conformal arc plan. Results: For the IMRT beams, average passing rate was 94.3%±2.1% and 99.6%±0.8% with 1%/2mm and 1%/3mm, respectively. For the conformal arc plan, the passing rate was 97.7% with 1%/2mm and 100% with 1%/3mm. Conclusion: An efficient calibration procedure was developed to obtain both the diode sensitivity and directional response dependence. Real‐time gantry angles were derived accurately based on beam edge back projections, which were used to apply directional response factors. Excellent agreement with TPS calculation was achieved for both IMRT and VMAT plans. ArcCHECK is an efficient and valuable tool for both IMRT and VMAT QA.
Research sponsored by Sun Nuclear Corporation.