Volume 35, Issue 6, June 2008
Index of content:
- Therapy: Scientific Session: Room 350
- Tx Planning and Delivery — New Techniques
35(2008); http://dx.doi.org/10.1118/1.2962826View Description Hide Description
Purpose: To investigate Intensity Modulated Arc Therapy plans (IMAT) with variable dose rate single arc and constant dose rate multiple arc approaches; to provide a formula for the prediction of their delivery times, and a metric to evaluate IMAT treatment plan adaptability. Method and Materials: IMAT plans were generated for localized prostate and oropharynx cancers. For each site, we generated IMAT plans for delivery with variable dose rate single arc and with constant dose rate multiple arcs. Treatment delivery times were modeled mathematically. A maximum leaf motion per unit gantry angle metric is used here to measure the complexity of leaf motions in treatment deliveries, and we proposed to use this metric to assess IMAT plan adaptability. IMAT plans were compared based on dosimetric qualities (target conformality and uniformity indices, dose‐volume indices of critical organs),delivery times, and plan adaptabilities. All IMAT plans were generated on Pinnacle 8.1v (Philips Medical Systems) for Varian linacs.Results: Overall, both IMAT approaches have comparable dosimetric plan qualities, although generating variable dose rate single arc IMAT plans required increased angular sampling to accommodate larger leaf motions, especially when there are two or more target volumes. The delivery time for the single arc is limited by both the gantry rotational speed and the maximum leaf motion between field segments, whereas, the multiple arc approach is generally limited by the gantry rotational speed alone. For the prostate plan, the delivery times are 1.2 minutes and 2 minutes respectively. In the oropharynx case, the single arc and the multiple arc deliveries were similar at approximately 2 minutes. The adaptability indices reflected the complexity of leaf motion in the single arc compared to the multiple arc approach. Conclusion: IMAT plans with variable and constant dose rate deliveries were evaluated under multiple considerations for potential clinical utilizations.
35(2008); http://dx.doi.org/10.1118/1.2962827View Description Hide Description
Purpose: Recent arc therapy techniques such as arc‐modulated radiation therapy (AMRT) developed at the University of Maryland and Varian's RapidArc™ allow variable segment‐weightings in order to expand the optimization domain. As a result, these plans may require a varying dose rate (DR) for delivery. To evaluate the necessity of DR variation in arc therapy delivery, the variable‐DR plans were translated in such a way that they can be delivered with a constant DR. Method and Materials: Four cases were selected for this study: 1 HN, 1 lung, 1 prostate and 1 brain. A single‐arc AMRT plan was generated for each case. Planning of AMRT started with optimization of ideal intensity maps with 36 equi‐spaced beams in Pinnacle followed by segmentation of the intensity maps into a deliverable AMRT MLC sequence. During leaf‐sequencing, the segment weightings are allowed to vary. In translating variable‐DR AMRT plans into constant‐DR plans, the angular spacing of the original beams were changed from equi‐spacing to spacing according to their weightings. Hence, apertures with more MUs occupy a greater angular range. To account for any field shifting in the process, a field shape correction was applied ensuring proper target coverage. Results and Conclusion: DVH comparisons show that constant‐DR plans were comparable to the corresponding variable‐DR plans in 3 of the 4 cases. Significant degradation occurred in the constant‐DR plan of the prostate case due to the large MU variations in the original variable‐DR plan, causing the beams to deviate significantly from their original positions. The estimated delivery times of the constant‐DR plans are 3 to 30 times longer than the variable‐DR plans due to large MLC shape variation within a small beam interval. It is hereby shown that DR variation is crucial to AMRT delivery in order to maintain excellent plan quality and efficient delivery time.
35(2008); http://dx.doi.org/10.1118/1.2962828View Description Hide Description
Purpose: To compare calculated and delivered dose distributions from RapidArc™ (a form of volumetric modulated arc therapy, Varian Medical Systems). RapidArc is a novel approach for delivering single arc therapy in less than 2 minutes, while achieving dose distributions comparable to current IMRT. We report the first detailed dosimetric validation for 5 different tumour sites with RapidArc plans. Method and Materials: Clinical RapidArc plans were generated with a pre‐clinical version for single cases with glioblastoma multiforme, multiple brain metastases, nasopharynx‐, oropharynx‐ and pancreas carcinoma. All five plans were delivered with a Varian Linac and measured in a solid water phantom for 5 coronal planes, 2 cm separated, using double Gafchromic®EBT films. Plans were also measured using ionisation chamber arrays (MatriXX). Measured and calculated dose distributions were compared using 2D gamma evaluation with limits of 2mm and 3.5% (of typical PTV dose in phantom). Results: All 25 film measurements showed high agreement with calculations, with a mean gamma of 0.29 and on average 1% (maximum 3%) of the film surface exceeding a gamma of 1.0. Relatively strong spatial dose modulations could be measured, within 95–107% dose range in the PTV, which were not completely predicted by calculations. This could lead to local dose changes >3% when changing a plane by 2mm. MatriXX measurements corresponded better with dose calculations than film measurements, which may be due to the limited resolution of 7.6mm of MatriXX. Conclusion: RapidArc accurately delivers the planned dose distributions. Film measurements may be preferred for dosimetric verification as more dose modulation is detectable than with ionisation array measurements. A “2.5D” gamma evaluation taking into account multiple adjacent dose planes would show better agreement for film dosimetry. Our results indicate that RapidArc can be introduced into clinical practice. Conflict of Interest: Research was a collaboration with Varian Medical Systems.
35(2008); http://dx.doi.org/10.1118/1.2962829View Description Hide Description
Purpose: To evaluate the capabilities and characteristics of a delivery system for volumetric modulated arc therapy (VMAT). Method and Materials: A linear acceleratorcontrol system capable of dynamic MLCmotion, variable gantry speed, and dose rate modulation during arc delivery was evaluated. The speed and stability of MLC and gantry motion, and the stability of the dose rate was analyzed through a series of test arcs. The transition efficiency between variable dose rates and gantry speeds, reproducibility, and delivery performance of a clinical prostate VMAT treatment were also analyzed.Deliverycharacteristics and mechanical reproducibility were evaluated by analyzing dynamic delivery log file output and dosimetric agreement was evaluated using a cylindrical phantom with two orthogonal diode arrays. Results: The VMAT control system was capable of maintaining a constant dose rate within 3–5% after a 40° stabilization distance during constant MU/degree arcs from 0.21 to 3.33 MU/deg. Dose rate fluctuations were accounted for by gantry speed regulation during delivery. Leaf motion was stable with maximum positional errors of < 1 mm as the leaves traveled to their maximum extent and changed direction. Leaf errors were independent of gantry angle. Transitions between different MU/deg during delivery were smooth, with stabilization at the new dose rate in < 5 degrees. Dosimetric analysis of a prostate VMAT delivery had a gamma index (3%/3mm) of < 1 for 98.4% of diodes receiving > 4% of the maximum dose. Conclusion: The linear acceleratorcontrol system was capable of delivering VMAT plans with constant and variable MU/deg. Small dose rate fluctuations are inherent in the system and are compensated for by dynamic changes of the gantry speed. Dynamic MLCmotion is stable and not dependent on the gantry angle. Evaluation of the system on additional clinical plans is warranted.
TH‐C‐350‐05: Performance of a Beam Tracking System for Treatment of Moving Targets with Scanned Ion Beams35(2008); http://dx.doi.org/10.1118/1.2962830View Description Hide Description
Purpose: Scanned ion beam therapy of intra‐fractionally moving targets requires motion mitigation to prevent deteriorations of the delivered doses due to interplay. A beam tracking system was developed and experiments with various detectors were performed to assess system performance. Method and Materials: Tracking of moving targets with scanned ion beams requires simultaneous lateral and longitudinal range adaptation of pencil beam positions. Lateral beam tracking is achieved directly with the beam scanning system. Longitudinal tracking is performed with a dedicated energy modulation system consisting of fast, motorized absorber wedges. In experiments with radiographic films, a range telescope, and an array of pin‐point ionization chambers in a water phantom, the precision of lateral, longitudinal, and 3D motion compensation was measured. Motion was induced laterally by a sliding table and longitudinally by ramp‐shaped absorbers. Motion detection was performed with a laser‐triangulation sensor. Experimental tracking data were compared to results of measurements with stationary detector systems. Films were analyzed with respect to geometrical shape as well as mean, minimum, and maximum response level within the target region. Ionization chamber data were analyzed for different volumes of the setup. Analysis of range telescope data focused on Bragg‐peak shapes and ranges. Results: Film analysis yielded conservation of geometrical shape and responses (mean±SD, minimum, maximum) of (0.276±0.010, 0.241, 0.288) and (0.275±0.007, 0.252, 0.284) for stationary irradiation and tracking respectively. Within the target volume, ionization chamber data with beam tracking deviated from stationary irradiation by 0.3 ± 1.5% with −2.7% minimum and 3.7% maximum deviation. The Bragg peak range for tracking deviated by less than 0.25 mm water‐equivalent from the stationary reference. Conclusion: The tracking system for scanned ion beams successfully restored dose distributions for moving targets by adapting individual Bragg peaks during irradiations in quasi real time.
Research sponsored in part by Siemens Healthcare, Particle Therapy.
35(2008); http://dx.doi.org/10.1118/1.2962831View Description Hide Description
Purpose: In recent years there has been an explosion of research work concerning the topic of charged ion acceleration using high‐power lasers. The maximum particle energy and the shape of the distribution function are the two main parameters influencing the potential utilization of the new technology in radiation therapy. Low energy transfer efficiency from the laser pulse to protons (∼ 5%) is one of the detrimental factors limiting the production of particle beams in the therapeuticenergy range. In this study we show that the energy transfer efficiency can be significantly increased through a process of splitting the single interaction stage (conventional interaction design) into multiple sub‐stages. Method and Materials: 2D Particle in Cell simulation code and 3D theoretical model were used to simulate interaction of laser sub‐pulses with multiple targets in a sequential manner. The thin layer of protons is initially located only at the back surface of the first target. The remaining targets are devoid of any contaminant hydrogen‐rich materials.Results: It was shown that in a three‐stage setting, there is ∼ 60% increase in the energy efficiency of the laser accelerator as compared to a single interaction scheme. At the same time according to the results of our 3D model, it should be possible to increase the energy efficiency by more than 100% for a six‐stage interaction setting without using more powerful lasers. Conclusion: The splitting of a single interaction site into multiple stages is an effective way of reducing an irreversible component in the energy exchange process between the laser and protons. As a result more laser energy is transformed into proton kinetic energy, thus increasing effectiveness of the “pump”.
TH‐C‐350‐07: Impacts of Dose Distribution Variations in Proton Therapy According to Gastro‐Intestinal Tract Air Filling and Breathing35(2008); http://dx.doi.org/10.1118/1.2962832View Description Hide Description
Purpose: For upper abdominal cancers, the simulation CT represents a snapshot of the possible air distribution and radiological thickness of a patient, both of which impact protondose distribution. The purpose of this dosimetric study is to analyze the effect of gastro‐intestinal tract air filling and breathing on the dosedelivered to a superior abdomen target using passive scattering proton therapy.Method and Materials: We used free‐breathing CT (FBCT) and 4D‐CT data sets showing the same 3‐cm pancreatic tumor. Air distribution was reasonable. We reproduced a situation where proton beam parameters would be calculated on a normal breathing patient with gas (no air override: O−FBCT), and we analyzed the dosimetric impacts if air was replaced by stools (air override: O+FBCT) during treatments and vice‐versa. Target coverage for 50.4 CGE in 28 fractions and dose to critical structures were evaluated for different air‐filling and breathing‐phase scenarios. Four‐field plans including 3 incidences going through some gas were compared to 2‐posterior‐field plans. Results: For the 4‐field plans, beam parameter calculations from the O+FBCT resulted in adequate target coverage when the same beam parameters were applied to the O−FBCT. The converse situation resulted in the need for an additional 1‐cm SOBP width to achieve adequate coverage for the antero‐posterior and right‐lateral beams. The 4‐field plans provided insufficient distal target coverage to account for planning uncertainties in the expiration phase, a problem avoided with the 2‐field plan. Conclusion: Our choice of planning margins on O+FBCT for upper abdominal cancerproton therapy is safe for target coverage, even if air filling variations occur during treatments. Planning on an O−FBCT, using a passive scattering technique, results in less conformal plans. The use of posterior incidences for upper abdominal cancers prevents uncertainties, especially in specific clinical situations like ileus or breathing disorders.
TH‐C‐350‐08: Dosimetric Verification of Modulated Electron Radiotherapy Delivery Using Photon Multileaf Collimator35(2008); http://dx.doi.org/10.1118/1.2962833View Description Hide Description
Purpose: To investigate the dose accuracy of modulated electron radiotherapy (MERT) delivered using the photonmultileaf collimator (pMLC) on a Siemens Primus accelerator. Method and Materials: A Monte Carlo based inverse treatment planning system was developed for the 3D treatment planning process. Phase space data of 6, 9, 12 and 15 MeV electron beams were accurately commissioned and used as the input source for Monte Carlodose calculations. Treatment planning was performed based on a 3D CT data of a “breast phantom” which mimics a breast cancer patient. SSD was chosen 60 cm in the planning based on the previous investigation. Rigorous film and ion chamberdosimetry was carefully established for the MERT plan verification using the breast phantom and a solid water phantom. The MERT plan verification was done by comparing isodose distributions, dose profiles and point doses with those obtained from the Monte Carlo plan calculations. Results: The plan was delivered with 22 segments with both energy and intensity modulated. The relative isodose distributions and dose profiles between film measurements and calculations agreed each other within 1%/1mm. Absolute doses given by the ion chamber measurements in the solid water phantom showed differences from the Monte Carlodose calculations by 1.7%, 0.5%, 1.6% and 1.5% for 6, 9, 12 and 15 MeV energy component of the plan, respectively while overall measured absolute dose accuracy is 1.37%. In addition, the dose alteration caused by the film in the lung region was identified and confirmed by recalculating the treatment plan with detailed geometry that includes both the film (thickness and density) and the breast phantom. Conclusion: Our in‐house developed Monte Carlotreatment planning system is capable of performing accurate dose calculation and treatment optimization for MERT, and the pMLC has a great potential to deliver MERT treatment plans accurately and efficiently.
TH‐C‐350‐09: 4D Treatment Planning and Dosimetric Validation of Helical Tomotherapy Treatments of Moving Tumors35(2008); http://dx.doi.org/10.1118/1.2962834View Description Hide Description
Purpose: The objectives of this study are to describe the 4D treatment planning process for moving targets and to evaluate the dosimetric consequences caused by target motion during tomotherapy delivery. Method and Materials: A lungcancer patient was CT scanned using our standard 4D‐CT protocols on Phillips brilliance 64 slice CT scanner. A maximum intensity projection CT dataset was used for treatment planning.Tumor motion trajectory and amplitude was determined from the inhale and exhale phases of the 4DCT. A three‐dimensional motion pattern was created by scaling the breathing form to an average magnitude of the target motion. The three‐dimensional trajectory was programmed into the Washington University 4D Phantom to simulate target motion. Eight EDR2 films were loaded into a phantom for dosimetric verification. Measurements were performed on a static phantom and then repeated on the oscillating phantom. Experiments were repeated for their reproducibility. Films were analyzed using Hi‐Art TPS. Results: To quantify the dose errors, gamma analysis was performed. In static phantom study, >97% of the points passed with a gamma criteria of 3% and 3mm. When the same criterion was applied for the oscillating phantom, 15–25% of the points failed. When the gamma threshold was increased to 8% and the gamma analysis window was tailored to be within the CTV, >98% of the points passed indicating that the dose errors are of the order of 5% for majority of the points. In order to find the maximum dose errors, the tolerance was increased until 100% of points passing the criteria. Conclusion: 4D treatment planning process for helical tomotherapy is discussed. Gamma analysis revealed that, the dose errors are of the order of 5% for majority of the points and the maximum dose error within the CTV was 11%.
This work is supported in part by Tomotherapy Inc.
TH‐C‐350‐10: Improving Accuracy of Electron Density Measurement in the Presence of Metallic Implants Using Orthovoltage Computed Tomography35(2008); http://dx.doi.org/10.1118/1.2962835View Description Hide Description
Purpose: To evaluate the improvement in electron density measurement and metal artifact reduction using orthovoltage computed tomography (OVCT) imaging compared with conventional kilovoltage CT (KVCT). Method and Materials: We constructed a bench‐top CTimaging system with adjustable x‐ray tube voltage up to 320 kVp. A commercial tissue‐characterization phantom loaded with various tissue substitute inserts and two metal inserts — Titanium (Ti) and Aluminum(Al) were scanned using 125 kVp (KVCT) and 320 kVp (OVCT) x‐rays, respectively. The metal artifacts are evaluated in two ways — visually in the constructed axial CTimage and by the deviation of estimated electron density from true electron density for uniform materials outside the metal. The stoichiometric calibration curves were obtained for both KVCT and OVCT imaging system by following the Schneider method. Results: The metal streak artifacts are seen to be reduced significantly in OVCT image than in KVCT image, especially in the area near Ti insert. The deviation of estimated electron density for the materials outside the metal are also reduced significantly by using OVCT than by using KVCT: from 42% (maximum) and 18% (root‐mean‐square) to 12% and 2.0% for heavy artifact area, and from 12% (MAX) and 3.0% (RMS) to 6.3% and 1.4% for light artifact area, respectively. The relationships between CT numbers (Hounsfield Unit) and relative electron densities (to water) are more predictable for both tissue substitutes and real biological tissues using OVCT than using KVCT. Unlike KVCT, the calibration curve for OVCT is insensitive to the tissue substitutes selected for direct electron density calibration.Conclusion: OVCT may be a good option for high precision radiotherapytreatment planning, especially for patients with metal implants and especially for charged particle therapy, such as proton therapy. Conflict of Interest: Collaborated with Varian Medical Systems.