Index of content:
Volume 34, Issue 6, June 2007
- Therapy Moderated Poster Session: Exhibit Hall C
- Moderated Poster — Area 1 (Therapy): Brachytherapy
SU‐DD‐A1‐01: Fast Dose Calculation for Pigmented Villonodular Synovitis Treated with P‐32 Radiocolloids34(2007); http://dx.doi.org/10.1118/1.2760338View Description Hide Description
Purpose: Pigmented Villonodular Synovitis (PVNS) is a joint disease that usually afflicts the knee. It is characterized by overgrowth of the joint's lining tissue, creating friable frond‐like appendages and resulting in monoarticular joint pain, effusion, and ultimately joint damage. The goal of therapy is to treat the synovial surface, controlling the growth and sclerosing the friable vessels. “Radioisotope synovectomy” procedure with P‐32 injected into the knee joint is an excellent candidate for such treatment due to the proper half life and steep dose gradient of P‐32 beta decay. However, it is often difficult to estimate the dose distribution in the irregular‐shape joint space for the beta emission. In this work, we develop a fast and accurate Monte Carlo based dose calculation, and validate it with spherical phantoms. Method and Materials: A “dose matrix kernel” is simulated with Monte Carlo code (BEAMnrc) for single‐voxel P‐32 source, with the matrix size 15×15×15 in 1 mm voxels. After CT scan with contrast, the patient knee joint space (P‐32 region) is segmented out. Three‐dimensional dose distribution is then obtained by convolving the P‐32 region with the pre‐calculated dose kernel with consideration of the medium scaling factor for heterogeneity correction. This dose calculation method is validated using spherical phantoms of various diameters. Results: Compared with the literature, the doses calculated from our method agree with others within 1% at the sphere centers and within 5% at the boundary. For the patient case, the dose calculation shows a non‐uniform distribution over the joint surface. The average dose can differ more than 50% from an estimation using spherical geometry with equivalent volume. Conclusion: It is important to calculate P‐32 dose distribution in 3D using actual geometry. The developed method is fast and relatively accurate. Further validation is under progress using TLD and film dosimetry.
SU‐DD‐A1‐02: Acceptance Testing, Commissioning, and Initial Clinical Experience with a Commercial Electronic Brachytherapy System34(2007); http://dx.doi.org/10.1118/1.2760339View Description Hide Description
Purpose: The Axxent® Electronic Brachytherapy system (Xoft Inc.) employs a miniaturized high‐dose‐rate x‐ray source located within an x‐ray catheter. The 50 kV disposable x‐ray source is used in conjunction with a balloon applicator to deliver partial breast irradiation to breast cancer patients following lumpectomy. We report on the acceptance, commissioning, and initial clinical experience with this system. Method and Materials: In addition to the manufacturer's prescribed acceptance tests, a study was performed to address concerns about the potential electrical leakage from the encapsulated tube to the surrounding patient tissues. A system was designed that simulates human body equivalent impedance to measure the electrical leakage. Commissioning was performed in the Plato treatment planning system (Nucletron) following the recommendations of TG‐43. A study has been initiated comparing the dosimetric results for the Axxent® System (x‐ray tube) with the MammoSite® Radiation Therapy System (Ir‐192 HDR source). The study will compare target coverage and normal tissue sparing and determine the required skin bridge for each of these technologies. Our first clinical cases are expected in April 2007 and will involve ten fractions of 3.4Gy delivered twice daily over five days. A total dose of 34Gy will be prescribed 1cm from the surface of the balloon applicator. Results: Acceptance testing and commissioning of the electronic brachytherapy system has been completed. Leakage measurements were performed using a meter consisting of a commercial line powered digital voltmeter, modified with the resistor‐capacitor network suggested by the IEC60601‐1 electrical safety standard. The results showed no measurable leakage currents above the x‐ray ambient noise levels. Initial results in a retrospective comparison demonstrated that it may be possible to improve skin sparing with Xoft through optimized dwell times at multiple dwell positions. Conclusion: An electronic brachytherapy system has been implemented in our clinic. Initial clinical results will be reported.
SU‐DD‐A1‐03: Dosimetric Characterization of Model CS‐1 131Cs Source by Thermoluminescence Dosimetry in Liquid Water34(2007); http://dx.doi.org/10.1118/1.2760340View Description Hide Description
Purpose: To determine the dosimetric characteristics of a recently introduced brachytherapy source by performing measurements in liquid water employing thermo‐luminescence dosimeters(TLD).Method and Materials: Small capsules containing 14 mg of lithium fluoride were constructed from capillary tubes and were supported in a water phantom by two plastic jigs. The jigs allowed the capsules to be positioned around a source in circular and spiral patterns designed to permit measurement of dose rate constant, anisotropy function, and radial dose function. The radioactive source was mounted on the tip of a thin graphite rod with its long axis either parallel or perpendicular to the plane of the TLD pattern. To assure confidence in the results, thirteen different seeds were employed, and measurements were performed multiple times. The measureddosimetric parameters were based on the AAPM Task Group 43 formalism. Results: The dose rate constant measured in liquid water was 1.08 cGy/U ± 5%, and was based on the air‐kerma strength standard established by the National Institute of Standards and Technology. Measured values for the anisotropy function F(r,θ) and the radial dose function g(r) also were determined. The results were compared with recently published values. Conclusions: It appears that this is the first time a complete set of dosimetric parameters for a brachytherapy seed has been measured in liquid water. This method avoids the uncertainty introduced by the use of water‐equivalent plastic. Key words: Brachytherapy seed, Solid water, TLD, TG‐43.
SU‐DD‐A1‐04: Monte Carlo Validation of Clinical Brachytherapy Dosimetry Under Partial Scatter Conditions for Neutron‐Emitting Sources34(2007); http://dx.doi.org/10.1118/1.2760341View Description Hide Description
Purpose: Monte Carlo (MC) models were generated in support of a clinical trial on the effectiveness of neutron‐based brachytherapy for a patient treated with a plaque containing Cf‐252 sources. Because the AAPM brachytherapydosimetry formalism does not replicate partial scatter conditions of superficial brachytherapy,MC simulations were performed to evaluate treatment time and dose distributions generated using conventional methods. Method and Materials: Clinical calculations employed the AAPM dosimetry formalism with modified parameters for the neutrondose component. MC simulations utilized MCNP5 and track length estimator tallies. Computations applied a rectilinear mesh to tabulate neutron transport, including induced photons, and primary photontransport in a 14×14×5 cm3 volume with 9 mm3 voxels. Patient surface was simulated using a 20 cm radius hemisphere of water, with a corresponding hemisphere of air. For comparison to the AAPM formalism, the air was replaced with water. An RBE of 6 converted results to cGy‐eq for the neutron component. Results were normalized to 0.1 mg Cf‐252 source strength. Results: At the 6 mm prescription depth, calculated dose rates were 186±2 and 205±2 cGy‐eq h−1 at plaque center and 24 mm offset, respectively. The central 4×4 cm2 area received 227±32 cGy‐eq h−1. For comparison, full‐scatter simulations yielded 205±2 cGy‐eq h−1 at plaque center and 244±32 cGy‐eq h−1 over a 4×4 cm2 area; although, computation time increased by a factor of 6.6. Dose ratios of full‐ (4π) to partial‐scatter (2π) environments increased from 1.07 to 1.10 as depth increased from 0.4 to 5 cm. Approximately 90% of the dose‐equivalent was due to neutrons, while neutron physical dose was 68% and 57% of the total at 0.6 and 5.0 cm depths, respectively. Conclusion:Dose can be overestimated upto 10% by assuming full‐scatter conditions for Cf‐252 plaque brachytherapy.MC simulations are recommended to validate treatment plans generated using conventional methods.
SU‐DD‐A1‐05: Turn‐On Dose and Transit Time Adjustments in Treatment Planning for the Axxent® Electronic Brachytherapy System34(2007); http://dx.doi.org/10.1118/1.2760343View Description Hide Description
Purpose: To analyze the dosimetric impact of x‐ray source turn‐on time and inter‐dwell position transit times for application of the Axxent® Electronic Brachytherapy System to APBI. Materials and Methods: At the first dwell position, the treatment timer starts after the source has ramped‐up to full operating voltage and beam current (50 kV, 300 μA), so planned dose‐delivery time does not account for a small “turn‐on” dose. Radial dose functions were calculated with MCNP5 for operating voltages from 20 to 50 kVp. Turn‐on dose was estimated by temporally averaging these distributions using the voltage and beam current ramp profiles. For subsequent dwell positions, the timer starts when the source begins moving to the next dwell position so elapsed time includes the transit time. (The source remains on during the time between dwell positions, typically 0.7 seconds for a 0.5 cm step). Dose contribution during transit was estimated using Varian BrachyVision™ by subtracting the transit time from the second and subsequent dwell positions, then adding extra dwell positions at midpoints between original positions with times equal to the transit time. Results: The composite turn‐on dose profile from Monte Carlo results was equivalent to 2 seconds of additional time at the first dwell position with source operation at 50 kVp. This corresponds to < 0.5% of a typical treatment time. Whether or not transit time is accounted for, the planned doses at prescription points 1 cm outside of a typical balloon agree to within an average of 0.1% with a standard deviation of 0.2%. Conclusions: Turn‐on dose may be approximated in treatment planning by adding 2 seconds to the first dwell time. Dose during source transit may be ignored when using a balloon applicator for APBI.
Research sponsored by Xoft, Inc.
34(2007); http://dx.doi.org/10.1118/1.2760344View Description Hide Description
Purpose: The aim of this study is to find out if the stranded seeds improve the quality of the permanent prostate seed implant by doing retrospective dosimetric analysis for patients with localized prostate cancer and have been treated using loose or stranded Iodine‐125 seeds in 2005 and 2006 at our center. Method and Materials: The dosimetric results reconstructed from patient CT scans at the same day of seed implant (day1) and the 21st day after implant (day21) were compared between 31 patients with loose seeds and 31 patients with stranded seeds. Treatment plans were all generated in real time by a single experienced medical physicist using a single planning system before the procedure of implant. Most of the implants were performed by a single experienced radiationoncologist with transrectal ultrasoundimage and x‐ray image guidance using preloaded needles. The needles with stranded seeds were loaded in real time using a special‐designed seed loader by putting seeds and spacers in plastic sleeves which are made of PGA and Lactide materials. Results: There is some improvement on the mean value of D90 at day21 for stranded seeds (95.7% vs. 93.4%) but it is not significant (p=0.247). And no significant difference was observed on the mean values of V100 and V150 at day21 (86.9% vs. 87.0% with p=0.485 and 53.7% vs. 53.4% with p=0.465). They also have a similar histogram distribution. The mean values of D90 and V100 at day1 show that patients with stranded seeds are even little bit worse than those with loose seeds, 80.5% vs. 87.4% with p=0.025 and 79.0% vs. 82.3% with p=0.063. Conclusion: Comparing the dosimetric parameters at day21 and day1, we conclude that the quality of prostate seed implant was not improved significantly by using stranded seeds which required more resources and manpower.
- Moderated Poster — Area 1 (Therapy): IMRT: Optimization and Delivery
34(2007); http://dx.doi.org/10.1118/1.2760364View Description Hide Description
Purpose: To systematically evaluate step‐and‐shoot Intensity‐Modulated‐Radiation‐Therapy (IMRT) plans generated by Direct‐Machine‐Parameter‐Optimization (DMPO) and by Two‐Step‐Approach (TSA) using identical optimization parameters in Pinnacle 3treatment planning system. Method and Materials: Using Pinnacle 3 version7.6c, TSA plans of total eight patients with Head‐and‐Neck, Prostate and Lungcancers were generated using identical optimization parameters from clinical plans used DMPO. The dose of planned‐target‐volume (PTV) in TSA plan was scaled to closely match at prescribed dose volume in the DMPO plan. Three PTV dosimetric indices: dose‐coverage, dose‐conformity and dose‐inhomogeneity, were generated for each plan. Dosimetric comparisons were performed for organ‐at‐risk (OAR) with both “maximum‐dose‐objectives” and “dose‐volume‐based‐objectives”. Final dose recalculation using EGS4‐based in‐house Monte‐Carlo program for each plan was performed and corresponding dosimetric data were obtained. Film‐based IMRT QA was performed for three patients. Results: On average, total monitor‐units (MUs) are about 25% higher of TSA than DMPO. The averaged segment‐numbers and PTV dosimetric indices are almost identical between plans from DMPO and TSA. The maximum‐dose (defined at 0.1cc) of Head‐and‐Neck and Lung OARs with “maximum‐dose‐objectives” of TSA are, on average, ∼2.5Gy and ∼0.9Gy lower than those of DMPO, respectively. The averaged dose difference in prostate OARs with “maximum‐dose‐objectives” is small. For OARs with “dose‐volume‐based‐objectives”, there is little difference between TSA and DMPO for all sites. The Monte‐Carlo dose recalculations showed similar trends. The agreement between Pinnacle3 calculations and film measurements is 99% for all fields using 3%–3mm criteria. Conclusion: Dosimetric comparisons between DMPO and TSA IMRT plans demonstrated that using identical optimization parameters, DMPO plans have less total MUs and similar averaged segment‐number as well as almost identical PTV dosimetric index values as TSA plans. For Head‐and‐Neck and lung plans, TSA has noticeable better sparing of OARs with “maximum‐dose‐based‐objectives”, which is confirmed by Monte‐Carlo recalculations. Film QA demonstrated both TSA and DMPO plans are very accurate.
SU‐EE‐A1‐02: Experimental Evaluation and Verification of the Deliverability Aspects of IMRT Beams Optimized with Adaptive Diffusion Smoothing34(2007); http://dx.doi.org/10.1118/1.2760365View Description Hide Description
Purpose: To experimentally determine the impact of adaptive diffusion smoothing (ADS) on the delivery accuracy and efficiency of IMRT fields. Method and Materials:IMRToptimization was performed on several cases with and without the use of an ADS penalty applied within the objective function. The ADS penalty is based on diffusion principles and promotes smoothing in beam areas that are not essential to meeting the cost function objectives. Previous studies have shown that the use of the ADS penalty results in IMRT plans that are dosimetrically equivalent, less complex, and require fewer MU to deliver compared to standard IMRT. All plans were sequenced and delivered via step‐and‐shoot delivery. Film and ion‐chamber dosimetry were performed, and the total MU, delivery time, and differences between convolution/superposition calculations and film measurements for standard and ADS IMRT beams were evaluated. Results: Measurements verified that IMRT plans optimized using the ADS penalty were less likely to exhibit small regions of disagreement due to factors such as tongue‐and‐groove compared to standard IMRT plans. In particular, the in‐field agreement between calculations and measurements for the ADS plans was superior to the more modulated standard IMRT plans. The use of ADS resulted in the area outside a +/− 5 cGy criteria between calculations and film measurements decreasing from 3.7 to 1.8 % in a head/neck example and from 10.8 to 6.7 % in a prostate example. In addition, the total MU for SMLC delivery was reduced by 20 to 45 % in all cases with no loss in plan quality according to the DVHs and dose metrics. Conclusion: The use of the ADS penalty inside an inverse IMRT plan objective function reduces beam complexity without sacrificing dosimetric quality and results in significantly more efficient and accurate delivery of IMRT fields.
Supported in part by NIH grant P01‐CA59827.
34(2007); http://dx.doi.org/10.1118/1.2760366View Description Hide Description
Purpose: Water equivalent pathlength (WEL) variations due to respiration can change the penetration of a charged particlebeam, and result in beam overshoot to critical organs or undershoot to the tumor. We have analyzed range fluctuations by analyzing four‐dimensional CT (4DCT) data and quantitatively assessing potential beam overshoot. Methods and Material: 4DCT images were acquired with a multi‐slice CT scanner. The maximum intensity volume (MIV) was calculated by temporal maximum intensity projection (MIP) processing. Two targets were designed for charged particlebeam therapy. The first target volume calculates the MIV over the entire respiratory cycle (ITV‐Rx), while the second target volume is the MIV corresponding to gated radiotherapy (over a 30% phase window around exhale). These targets were used to calculate boli that were then applied to the 4DCT data to estimate beam penetration. Analysis metrics include range fluctuation, overshoot volume, both as a function of gantry angle. We compared WEL fluctuations observed in treating the ITV Vs gated treatment in 11 lung patients. WEL fluctuation and beam overshoot into normal lung are displayed over a beams‐eye view display. Results: WEL fluctuations were less than 29.8 mm‐WEL and 12.0 mm‐WEL for ITV‐Rx and gated‐Rx, respectively for all patients. Gated‐Rx reduced beam overshoot volume by approximately a factor of four compared to ITV treatment. Such range fluctuations can affect the efficacy of treatment, and result in excessive dose to a distal critical organ.Conclusions: Time varying WEL range fluctuationanalysis provides information useful to determine appropriate patient specific treatment parameters in the charged particleradiotherapy. This analysis can also be useful for optimizing planning and delivery.
SU‐EE‐A1‐04: Comparison of Real‐Time Tracking & 4D Inverse Planning for Managing Patient Respiratory Motion34(2007); http://dx.doi.org/10.1118/1.2760367View Description Hide Description
Purpose: Real‐time tracking and 4D‐planning at the mean target position have been two potential methodologies to manage respiratory target motion. In this study, we evaluated each method based on dose‐volume criteria of organs in lungcancerradiotherapy.Method and Materials: Four patients with respiratory target excursions 1.5cm to 3.0cm were included. Each patient had 4D‐CT scans at 10 breathing phases. Deformable organ registration was applied to obtain subvolume displacement mapping for each phase of CTimage. First, an idealized real‐time tracking technique was evaluated assuming perfect estimation of target motion and beam tracking. Inverse planning was performed on each breathing phase CTimage without using margins for target and normal structures. Treatment dose was accumulated from does of each breathing phase. Secondly, a 4D‐inverse planning was performed on the mean 4D‐CT image using the corresponding pdf of respiratory motion created from the 4D‐CTs. Same beams and prescription dose (70Gy), as well as same objective and constraints, were applied in the planning optimization, and the cumulative dose was constructed accordingly. DVH and EUD in the GTV, lung,heart and cord were used for the evaluation. Results: Cumulative doses in target are similar for both techniques. The beam intensity modulation in the 4D inverse planning is much higher than the one in the real‐time tracking, but it can be delivered using beam compensator. Lung,heart and cord DVHs are similar with the corresponding EUDs, 4.6±2.2Gy, 8.3±4.6Gy and 11±4.35Gy for the tracking technique, and 5.3±2.3Gy, 8.8±5.1Gy and 11.9±5.0Gy for the 4D inverse planning. Conclusion: Treatment technique with the 4D‐inverse planning and online mean target position control is clinically practical. Compared to the idealized real‐time tracking, 4D‐inverse planning achieves slightly degraded, but similar. However, this degradation could be vanished when practical tracking error was considered.
34(2007); http://dx.doi.org/10.1118/1.2760368View Description Hide Description
Purpose: Interplay between organ motion and leaf motion has been shown to generally have a small dosimetric impact for most clinical IMRT treatments. However, it has also been shown that for some MLC sequences there can be large daily variations in the delivered dose, depending on details of the patient motion or number of fractions. This study investigates guidelines for dynamic MLC sequences that will keep daily dose variations within 10%. Materials and Methods: Dose distributions for a range of MLC separations (0.2 – 5.0cm) and displacements between adjacent MLCs (0 – 1.5cm) were exported from Eclipse to purpose‐written software which simulated the dose distribution moving across a moving target. Target motion parallel and perpendicular to the MLC motion was investigated for a range of amplitudes (0.5 – 4.0cm), periods (1.5 – 10s), and MLC speeds (0.1 – 3.0 cm/s). Target motion was modeled as sin6. MLC sequences were identified which kept dose variations within 10% compared to the dose delivered with no motion. Results were confirmed experimentally by measuring the dose delivered to MOSFETs in a moving phantom for a range of MLC sequences. Results: The maximum allowable MLC speed when target motion is parallel to the MLC motion can be conservatively summarized as a simple function of target amplitude and MLC separation. When the target motion is perpendicular to MLC motion the maximum allowable MLC speed can be described as a function of MLC slit width and the displacement of adjacent MLCs. The guidelines were successfully applied to two‐dimensional motion. Rules were less restrictive for periods<4s, indicating that it may be useful to monitor or control patient breathing. Conclusion: Some MLC sequences should be avoided. The use of simple guidelines when treating moving targets using dynamic IMRT can reduce the possibility of large variations in delivered dose.
SU‐EE‐A1‐06: Helical Tomotherapy Planning for Left‐Sided Breast Cancer Patients with Positive Lymph Nodes: Compared to Conventional Multi‐Port‐Breast Technique34(2007); http://dx.doi.org/10.1118/1.2760369View Description Hide Description
Purpose: The objective of this study was to evaluate the feasibility of using helical tomotherapy for left‐sided breast cancer patients with involved lymph nodes. Method and Materials: Four left‐sided breast cancer patients treated using conventional multi‐port‐breast technique were retrospectively planned on Tomotherapy planning system. PTVs including chest‐wall/breast, supraclavicular, axillary and internal‐ mammary lymphnodes were contoured. Optimized treatment plans were generated on Tomotherapy TPS using 25mm field‐width with pitch of 0.42. The modulation factors varied from 1.5–2.6. All plans had a prescription of 50.4Gy to 93% and 46.9Gy to 98% of the PTV. Directional blocking was used on the right side to limit the dose to the contra‐lateral‐breast and lung. The optimization goals for planning were to protect the heart and lungs from receiving excessive doses. Resulting plans were compared against a conventional multi‐port breast technique. Lung toxicities using the Lymann‐Kutcher‐Burman model were estimated for tomotherapy plans. The parameters used for these calculations are TD50%=30.8Gy, slope(m)=0.37 and the exponent(a)=1. Results: Tomotherapy increased the minimum dose to the PTV (D99% = 44.6Gy for tomotherapy versus 30.5Gy for 3D) while improving the homogeneity index (HI = 1.16 for tomotherapy and 1.52 for 3D). The mean V20Gy for the left lung decreased from 32.6% (3D) to 16.4% (tomotherapy) while keeping the mean right lung dose well under 4Gy. However, the mean V5Gy volume increased from 26.4% (3D) to 42.6% (tomotherapy). The mean V35Gy for the heart decreased from 6.5%–2.5%, while the mean heart dose increased from 9.5Gy–11.3Gy for conventional and tomotherapy, respectively. The estimated NTCP for lung range from 1.4% to 2.4% for tomotherapy plans. Conclusion: Tomotherapy plans have better conformity and dose homogeneity than the 3D‐ plans. Tomotherapy provided improved sparing for the heart and lungs.Conflict of Interest: This work supported in part by Tomotherapy, Inc.
- Moderated Poster — Area 1 (Therapy): Monte Carlo Dose Calculation
34(2007); http://dx.doi.org/10.1118/1.2761449View Description Hide Description
Purpose: To evaluate a new algorithm to unfold the energy spectrum of a large field electron beam from a central axis depth dose curve. Methods and Materials: As previously reported unfolding of the spectrum of electron beams is best done (1) based on large open field depth dose measurements, (2) incorporating the air space between source and phantom in the simulation of the response matrix and (3) using an inverse geometry with a pencil beam, large scoring voxels. Here an algorithm by Chvetskov based on a special form of the general Tichonov regularization function has been tested. The response matrix was created through EGSsnrc Monte Carlo Simulations of pencil beam depth doses in 0.1 MeV steps and including the air between source and phantom. The depth dose curve to be unfolded was also created through a Monte Carlo simulation which was in excellent agreement with the measurement on a clinical machine. The advantage of using a Monte Carlo based depth dose as the input is that the spectrum is precisely known. In addition it was possible to subtract the head photon component from the depth dose data and perform the algorithms on the electron only data. For later practical implementation, this approach presents a challenge, which however can be met. The Chvetskov algorithm used for the unfolding contains one smoothing parameter the impact of which was explored. For comparison, a generic set of depth dose curves was also used in the response matrix. Results and conclusions: The algorithm identifies the main energy component well. The smoothing factor impacts the waviness of the curve substantially. Optimizing this factor will be an important task. The comparison with the generic response functions shows that the use of specific response functions,including the air improves the unfolding.
Support from NIH R01 CA104777‐01A2.
34(2007); http://dx.doi.org/10.1118/1.2761450View Description Hide Description
Purpose: A fluence benchmark for clinical electron beams with application to validation of the electron beam model of a commercial radiotherapytreatment planning system Method and Materials: A fluence benchmark for the 6–21 MeV electron beams of a clinical linear accelerator was determined for 40×40 large field. Measurements were done on a clinical accelerator that is not used to treat patients, allowing us to disassemble the accelerator and directly measure critical geometrical details and to operate the accelerator outside the normal operating range to help ascertain source and geometry details.Therefore,source and geometry in the simulation was adjusted within more restrictive limits than previous studies to match simulation results to measured central axis depth dose curves, cross‐plane and in‐plane profiles at the depth of maximum dose, dmax, and in the bremsstrahlung tail. Parallel to this process, a series of measurements were made on the same linac to commission a commercial planning system. The fluence calculated with the commercial beam model was validated with the fluence benchmark. Results: Agreement between measured and simulated depth‐dose curves upstream of the fall‐off region and depth‐ionization curves in and beyond the fall‐off region was excellent: within 1%/0.5 mm. The dose profiles matched within 1% in the high‐dose regions of the electron field and within 2 mm at the field edge. The current benchmarks approach the goal of within 1% in fluence, given 100% on the central axis, and 2 mm in position. The electron energy distributions of the 6 electron beams calculated with the commercial beam model closely matched the benchmark. Conclusion: A high accuracy, highly detailed fluence benchmark for clinical electron beams is developed and proved to be useful for validation of the electron beam model of a commercial planning system to the accuracy required for dose calculation in radiotherapy.
Support from NIH‐R01‐CA104777‐01A2.
34(2007); http://dx.doi.org/10.1118/1.2761451View Description Hide Description
Purpose: To investigate gamma‐ray emission image during the treatment of proton therapy as a possible method to verify dose delivering, such as dose value and its distribution Method and Materials: In this study, MCNPX 2.5 was used to simulate a simplified broad beam proton therapy treatment system, in which two broadening scatters were used to broad the initial mono‐energy 200 MeV beam. The beam was broadened from initial radius of ∼5 mm to that of ∼40 mm. In this simulation, protons,neutrons, electrons, and photons have been transported. In front of water phantom there is a water telescope to adjust the range of the broadened bream of proton before it ejects in to a water phantom. The images of emitted gamma‐ray were obtained on the two image planes which were 20 cm from the surface of the phantom and were parallel and perpendicular to the beam respectively. The gamma‐ray lines emission from annihilation of positrons and inelastic scatter of protons with oxygen nuclei were investigated. Additionally the image of gamma‐ray line emission from neutron capture was obtained also Results: Association between the gamma‐ray emission images and delivered dose was found.The intensity of image was related to the energy and flux of the beam. The longitude distribution of intensity of image related to the energy spectrum of ejected beam. For a beam with a given energy spectrum, the image intensity is proportional to the flux of beam. The image of gamma‐ray from neutron is closely related to the secondary dose that was mainly attributed to the neutrons generated in the treatment nozzle Conclusion: The simulation shows that the image of gamma‐ray emission is closely related to the energy and flux of proton beam. The projected 2‐D distribution of delivered dose could be estimated based of the observed emission images.
TU‐FF‐A1‐04: Benchmarking a Flexible Monte Carlo (MC) Tool Based On the Dose Planning Method (DPM) for Use in Evaluating IMRT Treatment Planning Systems34(2007); http://dx.doi.org/10.1118/1.2761452View Description Hide Description
Purpose: To benchmark a flexible Monte Carlo(MC) tool based on the Dose Planning Method (DPM) for use in evaluating Intensity Modulated Radiation Therapy(IMRT)treatment planning systems. Method and Materials: A dose calculation tool based on a flexible machine model using the Dose Planning Method (DPM),a “fast” Monte Carlo((MC)computer code, is being developed. Initial benchmark testing included a simple 10cm × 10cm multileaf collimator(MLC)diamond shaped pattern, a 3D conformal lung plan with the MLCs fully retracted, and an IMRTlung plan. Irradiations were performed using a 6MV photon beam from a Varian linear accelerator. Measurements were made in slab and anthropomorphic phantom geometries using thermoluminescent detectors(TLDs) and radiochromic film. The DPM calculation was then compared to measurements and also the calculation from the Pinnacle treatment planning system. Results: Profile comparisons from the MLCdiamond pattern irradiation showed good agreement in the penumbra region where MLC inter and intra leaf transmission effects were present. The point dose comparisons between the DPM calculation and measurement of the tumor for the 3D conformal and IMRTlung plans where within 2%. For the heart and spinal cord, the calculation for the 3D conformal and IMRTlung plans where within 7.5% of measurement, except in the conformal plan where the calculated dose point to the heart was positioned in a steep dose gradient and was 25% lower than measurement. Dose profiles through the center of the tumor showed good agreement in the PTV region, penumbra, and low doselung regions. Conclusion: This work demonstrates the feasibility of a source model based the DPM computer code to calculate dose distributions as part of the quality assurance program for clinical trials. Conflict of Interest: This work supported by PHS CA010953, CA081647, and CA085181 awarded by NCI, DHHS.
34(2007); http://dx.doi.org/10.1118/1.2761453View Description Hide Description
Purpose: An ionization chamber is often used for measuring dose distributions to validate a photon beam dose calculation algorithm for radiation treatment planning. The presence of an ionization chamber can cause dose perturbations at the point of interest especially in low‐density media. This study investigates the magnitude of this type of perturbation as a function of photon beam energy and field size in a low‐density lung medium. Method and Materials: The Monte Carlo codes BEAMnrc/DOSXYZnrc are used to simulate 6–18 MV photon beams and to calculate dose distributions in a heterogeneous phantom. We benchmarked Monte Carlodose calculations against measurements in a lung phantom using a MOSFET detector. The dose to a point of interest in a lung medium is calculated with and without an ionization chamber in order to study the perturbation due to the chamber. The Monte Carlo simulation is also used to validate the Varian Eclipse Anisotropic Analytical Algorithm (AAA). Results: The results show dose increases of up to 6% and 12% due to the presence of an RK ionization chamber at the point of measurement inside lung medium for a small 3×3 cm2 field for 6 and 18 MV incident phantom beams, respectively. However this dose perturbation becomes negligible when beam field size increases to 10×10 cm2. The results of Monte Carlo calculation show that AAA is accurate in predicting dose distributions in lung and at lung‐tissue interface for 6 MV beam. This result contradicts the conclusion by Van Esch et al (Med. Phys. vol.33, pp.4130–48, 2006). Our finding of chamber perturbations explains the discrepancies between their measurements and calculations using AAA. Conclusion:Ionization chambers are not suitable for measuring dose in low‐density medium due to perturbation.
34(2007); http://dx.doi.org/10.1118/1.2761454View Description Hide Description
Purpose: The aim of this study is to investigate the feasibility of developing treatment plans without a flattening filter for serial tomotherapy treatments using Varian 600C linac. Since the flattening‐filter reduces the dose rate, the removal of the flattening filter may decrease treatment time significantly by the reduction of MUs and the number of arcs per treatment. Method and Materials: A Monte Carlo, PEREGRINE® 1.6b, which is interfaced with Corvus 5.0 (NOMOS, PA) and integrates the serial tomotherapy (MIMiC) delivery device, was first commissioned for the Varian 600C with the flattening filter. The flattening‐filter was then removed from the phase‐space file. Several plans were generated on with and without the flattening filter in place for the MIMiC. The number of monitor units and plan quality were compared. Results: Results show that there is a significant decrease in MUs by a ratio of 1.7 to 2 for IMRT plans developed without the flattening‐filter. Since the maximum deliverable dose rate is 10 MU /deg for arc therapy on the Varian 600C, the number of arcs required to deliver the same dose without the flattening‐filter is also halved. The DVHs for the target and critical structures are similar for plans generated with and without the flattening‐filter since the optimizer employed for the generation of both the plans is the same. Conclusion: The removal of the flattening‐filter for MIMiC based plans reduces the number of MUs and treatment arcs thus, reducing treatment delivery time without affecting the plan quality. This is especially beneficial when large doses per fraction (SRS or SBRT) are employed and several arcs on the same index have to be delivered to achieve the desired prescribed dose.
- Moderated Poster — Area 1 (Therapy): Tx Planning Delivery and Modeling
TU‐EE‐A1‐01: Validation of a Prototype Deterministic Solver for Photon Beam Dose Calculations On Acquired CT Data in the Presence of Narrow Beams and Heterogeneities34(2007); http://dx.doi.org/10.1118/1.2761416View Description Hide Description
Purpose: To evaluate a deterministic method for solving the neutral and charged particletransportequations in cases where electron disequilibrium is significant. Methods and Materials: A prototype deterministic solver, Acuros®, has been developed and validated for photon beam radiotherapy. Acuros is based on the Attila® radiation transport code, which solves the differential form of the linear Boltzmann transport equation for neutral particles, and the linear Boltzmann‐Fokker‐Plank transportequation for charged particles. The angular and energy dependent photon and electron flux is solved at every spatial unknown in the computational domain, and quantities such as dose‐to‐medium (DM) are obtained by multiplying the energy dependent particle flux by the energy dependent energy deposition cross section for the associated image pixel material. Comparisons were made with the Monte Carlo code EGSnrc (DOSXYZnrc) for a head‐and‐neck case having eight 1.5×1.5 cm2photon beams with a 6 MV photon spectrum. The Acuros calculation used 123,000 elements. Since four spatial unknowns were solved in each element,this equated to 492,000 spatial degrees of freedom.The DOSXYZnrc calculation used 2.5×2.5×2.5 mm3 voxels. Results: Results were compared at 95,183 image pixels where dose > 5% of Dmax. 99.02% of pixels satisfied the 3%/3mm criteria (94,251 out of 95,183 pixels). Computational times for the Acuros and DOSXYZnrc calculations were approximately 19.6 CPU minutes (2.2 GHz Opteron processor)and 2,890 CPU minutes (0.4% avg. uncertainty in voxels > Dmax/2), respectively. Through a rewritten, radiotherapy specific solver, Acuros computational times were further reduced to 7.45 CPU minutes without affecting accuracy. When only doses in regions greater than 20% of Dmax are of interest, computational times are further reduced to 3.08 CPU minutes. Conclusions: A clinically viable combination of dose calculation speed and accuracy has been achieved using a radiotherapy specific deterministic solver.
Research funded in part by NIH grant 1R43 CA105806‐01A1.
TU‐EE‐A1‐02: Whole Procedure Accuracy of Gamma Knife Radiosurgery of Large Tumors Via Multiple Isocenter Delivery34(2007); http://dx.doi.org/10.1118/1.2761417View Description Hide Description
Purpose: Geometric accuracy of single‐isocenter Gamma Knife delivery has been well established to within 0.3 mm. The question frequently asked is the accuracy of the full‐procedure Gamma Knife treatments, particularly important for large tumors requiring multiple isocenter deliveries. In this study, we performed direct measurements to determine such accuracy. Method and Materials: An anthropomorphic head phantom with a central spherical target of 3.0 cm in diameter was employed for measurements. We placed the stereotactic frame on the phantom and followed identical imaging and treatment protocols for treatment planning and beam delivery. CT scans of varying slice thicknesses of 1 mm, 2 mm and 3 mm were acquired and used for treatment planning calculations. The phantom was irradiated using 16 isocenters delivered with Gamma Knife automatic patient positioning system (APS). Radiochromic films aligned with the center of the target were exposed. The isodose measurements were compared with the 3D dose calculations from the treatment planning system (Leksell GammaPlan Wizard 4C). Results: The overall agreements were found to be approximately 2 mm between the measurements and dose calculations. The agreement was slightly better (approximately 1.5 mm) for the peripheral isodose line (e.g., 50% of the maximum dose) but deteriorated significantly (2–4 mm) for low (e.g., 30% of the maximum dose) or high isodose lines (e.g., 80% of the maximum dose). The 2‐mm dose accuracy showed no dependence on the slice thickness of the CT studies. The 2mm overall accuracy is closely matched the dose grid box resolution of 1.8 mm that was needed to encompass the full target volume for calculations. Conclusion: Overall accuracy of Gamma Knife irradiation of a large target was found to be approximately 2 mm, regardless the slice thickness of acquired images. The dose grid resolution was likely the dominant factor contributing to this result.