Volume 34, Issue 6, June 2007
Index of content:
- 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 2 (Therapy): IMRT: Planning Verification and QA
34(2007); http://dx.doi.org/10.1118/1.2760345View Description Hide Description
Purpose: To report on a tool that automatically extracts the intra‐fraction motion from a 4D CT dataset and integrates the information with treatment planning which allows accurate calculation of delivered dose in each fraction and in the presence of respiratory motion. Method: For this project, we have integrated the ITK/VTK libraries with a commercial planning system, Eclipse (Varian Medical Systems, Palo Alto, CA). The 4D registration tool provided a smooth description of the respiratory motion and was used to map contours delineated in one phase to all phases of a 4D CT dataset. Resulting segmentation was saved in DICOM format and imported together with the associated datasets in Eclipse. Dose calculation and dose‐volume histograms were calculated directly for each phase of the dataset using the original beam fluences. The dose in each phase can be added or subtracted directly in Eclipse to provide the dose summation over the whole respiratory cycle or to inspect the dose differences between phases produced by motion. Computation of 4D DVHs for static, serial organ such as the spinal cord is possible directly with Eclipse. For moving parallel organs such as the lung, we provide an independent DICOM‐compliant tool to sum‐up individual dose matrixes taking into account organ motion. Results: Dose differences between the inhalation phase (used for planning) and the exhalation phase can be easily assessed using the plan evaluation tool. By summing treatment plans for each phase, comparison of the planned and delivered dose distributions can be assessed in treatment planning.Conclusion: Individual dose verification of treatment plans in presence of intra‐fraction motion is possible using available commercial treatment planning systems.
SU‐DD‐A2‐02: Analysis of Film Registration Techniques in Intensity Modulated Radiation Therapy Quality Assurance34(2007); http://dx.doi.org/10.1118/1.2760346View Description Hide Description
Purpose: The purpose of this study was to evaluate manual and fiducial‐based plan‐to‐film registration techniques, identify advantages and disadvantages of manual and fiducial‐based plan‐to‐film registration techniques, develop a new automatic plan‐to‐film registration technique using a genetic algorithm, and compare the performance of the automatic registration to the manual and fiducial‐based techniques. Method and Materials: Ten patient plans (4 Head & Neck and 6 Prostate) were selected for the IMRT plan‐to‐film registration study. For each patient, the IMRTdose measurements were obtained in both axial and coronal planes using radiographic film, resulting in 20 test films. Dosecalibration films were irradiated at the same time as the test cases, and the test cases were processed at the same time. The calculated and measured dose distributions were registered with one another using RIT113 dosimetry software. Manual registration was performed by visual selection of four points in common on the dose and film images. Fiducial‐based registration was performed using the “Template” tool in RIT113 and marks placed on the films prior to irradiation. An automatic plan‐to‐film registration genetic algorithm was created that performs image registration by optimizing the best longitudinal and rotational shifts. Results: Of the techniques, the manual registration technique was inferior because it was highly susceptible to inter‐ and intra‐user variations. The fiducial‐based technique often resulted in incorrect registrations due to incorrect fiducial placement. Of the three techniques evaluated in this study, the genetic algorithm‐based registration provided the best agreement between calculated and measured dose distributions. Conclusion: Fiducials should be used for an initial registration, and the automatic technique should be used to calcuate the spatial offset between the plan and the film. Using these techniques in combination should improve the time required to perform IMRT QA and decrease the incidence of false positives. Conflict of Interest: Research consultant for RIT.
34(2007); http://dx.doi.org/10.1118/1.2760347View Description Hide Description
Purpose: To measure and correct the two dimensional lag and ghosting effects of an a‐Si EPID for dynamic MLC delivery in IMRT verification. Method and Materials: Varians aS500 EPID was used to acquire portal images at an SID of 105 cm with 2 cm solid‐water build‐up. Each image was averaged over 8 frames acquired in ‘continuous acquisition’ mode with Varis Portal Vision's service monitor (IAS3) to construct lag and signal‐to‐MU curves. To quantify lag, images were acquired post‐irradiation by inhibiting the M‐holdoff‐In signal, for a range of MUs. The resulting lag and signal‐to‐MU curves were used to calculate cumulative lag and cumulative dose‐response curves. Both these cumulative curves were used to correct the 2‐D lag and ghosting effects of the EPIDimages.Results: We found that the 2D effects of lag and ghosting can be corrected by measuring and then applying lag and signal‐to‐MU curves. This technique provides a maximum correction of approximately 1% for dynamic head‐and‐neck IMRT deliveries. Conclusion: Two‐dimensional lag and ghosting effects of the aS500 EPID can be measured and corrected. This approach can be of particular importance for the EPIDIMRT verification of MLC dynamic deliveries.
34(2007); http://dx.doi.org/10.1118/1.2760348View Description Hide Description
Purpose: To validate the accuracy of the table and gantry motion of the Tomotherapy Hi‐Art system and to measure the range of table deflection and to assess the impact on dose delivery. Method and Materials: The Tomotherapy Hi‐Art system delivers radiation dose to a target using a helical tomotherapy delivery. Because of this fact, the mechanical accuracy of the table speed and gantry rotation speed, which determine the pitch of the delivery, are extremely important. The amount of table sag relative to the treatment plan is also critical. Table speed and gantry speed were measured by attaching a marker with three reflective spheres to both the table top and gantry structure. The position of the markers was monitored with a Polaris infra‐red camera system. Measurements were taken for 4 treatments of varying pitch.
The table top sag for 0, 150, and 300 pounds weight was measured both at the end of the table and at the isocenter for different table positions. The impact of this on a delivered plan was checked by measuring a plan using film dosimetry with 0, 150, and 300 pounds of added weight. Results: Table speed was verified to be accurate within 0.5%. The average gantry rotation period was found to be within 0.02 seconds of the planned value. The maximum sag at the end of the table ranged from 1.25 to 3.35 cm. The sag at isocenter ranged from 0.5 to 1.4 cm. The results translate to a maximum deflection angle of 1.1 degrees. The resulting films showed a maximum shift in isodose curves of 1 mm in the vertical direction. Conclusion: The mechanical systems on the Tomotherapy unit are capable of producing the planned pitch values with high accuracy. The table deflection while substantial does not significantly affect the accuracy of the delivery.
34(2007); http://dx.doi.org/10.1118/1.2760349View Description Hide Description
Purpose: To investigate robustness of improved dose uncertainty model and its applicability to dose verification. Method and Materials: Previously, we developed a dose uncertainty model by introducing space‐oriented and non‐space‐oriented dose uncertainties, and a novel scheme of accounting dose accumulation history. Although the model was able to provide good estimations, its application was limited to cases with a few mm range of spatial displacement. In this study, we have improved and generalized the model by incorporating a convolution method. The new model categorizes uncertainty sources from both treatment machine and patient into eight degrees of freedom. Probability density functions are convolved with dose calculation to obtain dose uncertainty maps. For this study, input parameters were determined through 6‐year annual QA data, 64 setup measurements using an IR camera, and 105 dose measurements of seven dose levels. To examine applicability of the model to dose verification, treatment plans of four test patterns were made and delivered to film‐phantom dosimetry system. Percentage of measured points within the dose uncertainty bound (pass rate) was evaluated with confidence interval of the uncertainty to verify the robustness of the model. Results: Mechanical setup accuracies were 0.2±1.0 mm (x), 0.1±0.9 mm (y), 0.0±1.1 mm (z), 0.0±0.1° (pitch), 0.0±0.0° (roll), 0.1±0.2° (yaw), and 0.0±0.0° (collimator). The relative dose perturbation at 200 cGy was 0.4%. The uncertainty distributions were obtained with these measured model parameters and dose verification pass rates were evaluated according to the confidence interval of dose bound. The pass rates showed similar trends as theoretical statistical confidence. For test patterns, the pass rates by the uncertainty‐based test with three standard deviations were similar to those obtained in conventional γ‐test with 3% and 3 mm criteria. Conclusion: With a few simple test fields, robustness of the approach and possibility of application to dose verification were observed.
34(2007); http://dx.doi.org/10.1118/1.2760350View Description Hide Description
Purpose: To evaluate portal dosimetry using EPID as an alternative method for patient specific IMRT pre‐treatment verification. Dose profiles obtained using EPID and EDR2 film are compared and analyzed.Method and Materials: A total of 40 fields from patients' IMRT plans were retrospectively analyzed. 35 of which were planned with 23MV and 5 fields with 6MV. Hybrid plan was created using Eclipse TPS for each patient. All fields were measured using EPID and EDR2 film at gantry angle of 0°. Film was placed transversely into a solid water phantom at the depth of 10.3cm. EPIDimager was set to 105cm.
Dose profiles for all fields were compared. γ index proposed by Low D.A. et al was used to quantify the disagreement between measured and planned dose distributions. Criteria used for γ index was set at 3%/3mm. To exclude the data outside of treatment fields, only dose higher than 20% of maximum dose was included for calculation. γ values were grouped with different dose ranges and averaged for both and EPID and EDR2 film. Results: Both EPID and EDR2 film data show good correlation between measured and planned dose. Average γ value was 0.63±0.03 for EPID and 0.61±0.03 for film dosimetry. It is statistically insignificant between the results from EPID and EDR2 film using 6MV and 23MV. However, a significant difference of γ index for the dose less than 40% of maximum dose (p<0.05) is observed using EPID and EDR2 film. Conclusion: Use of EPID to measure IMRT pre‐treatment QA appears a comparable result to film dosimetry. In advent of digital imaging,EPIDdosimetry would be a quick and easy alternative for IMRT planning verification.
- 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 2 (Therapy): Measurements: Calibration and QA
34(2007); http://dx.doi.org/10.1118/1.2760370View Description Hide Description
Purpose: Designing a scintillation detector for protonradiotherapy requires careful considerations. Most scintillators exhibit some energy dependence due to quenching under proton irradiation. Water equivalence, one of the main advantages of plastic scintillators for photon measurement has yet to be validated for the wide range of proton energies used for proton therapy. In this work, we studied the most important factors that would affect the performance of a scintillation detector.Method and Materials: Experimental measurements were used to study quenching effect in scintillators and the amount of the spurious light produced in the optical light guides. Monte Carlo simulations were performed with GEANT4. Proton beams between 50 and 250 MeV were simulated to study the water equivalence, the optimal size, the optimal coating of plastic and inorganic scintillators. Results: We found that the dose deposited in a plastic scintillator was within 2% of the dose deposited in a similar volume of water on the whole depth dose curve for protons with energies higher than 50 MeV. Inorganic scintillators received a dose 5 to 10% lower than the dose in a similar volume of water. The main disadvantage of plastic scintillators is the quenching effect that reduces the amount of scintillation and result in 26% and 14% underestimation at the Bragg peak of 50 MeV and 250 MeV, respectively. To avoid the averaging effect, the radius of the scintillation detector should be 0.25 mm or less. To improve the signal of such small volume detector the scintillator can be coated with a diffuse reflector. Conclusion: According to our experimental and Monte Carlo analysis of scintillation detection in the higher energy range for proton therapy, it is possible to construct an effective detector that would overcome the problems traditionally encountered in protondosimetry.
34(2007); http://dx.doi.org/10.1118/1.2760371View Description Hide Description
Purpose: To evaluate radiation detectors for the dosimetry of kilovoltage x‐ray beams. Method and Materials: The study evaluated cylindrical and parallel plate ionization chambers provided with the PTW MP3 water tank system. Kilovoltage x‐ray beams were generated by a Pantak DXT300 x‐ray unit with energies ranging from 50 to 280 kVp. The following data was measured: percentage depth dose curves in water, relative detector response both in air and in solid water with the 6 cm diameter applicator. The reference detector was an NE 2571 ionization chamber that had been calibrated in a primary standards laboratory. The EGSnrc Monte Carlo code V4.2.2 was used to calculate depth dose curves in water for comparison with measured data. The x‐ray spectrum of primary beam was determined using an analytical program, XRAYBEAM, and subsequently verified by calculation of the half value layer. Results: For the measured depth doses, the PTW 0.3 cc thimble chamber, NACP, Markus and Roos chamber were in good agreement with the Monte Carlo calculated depth dose data. The agreement was better than 3% in most cases, and a maximum deviation of 4% for the 50 kVp x‐ray beam. The M31002 pinpoint chamber gave a greater deviation, up to 9% for the 50 kVp beam. The relative detector response, as compared to the NE 2571, indicated that thimble chambers had a small variation of up to 5% over the energy range. However the relative detector response for the parallel plate chambers was large, with the deviation increasing as the x‐ray beam energy decreased, up to 70% for the Markus chamber. Similar large response changes also occurred for the pinpoint chamber. Conclusion: The parallel plate chambers (Markus, Roos, NACP) were found to be suitable for depth dose measurements in water for kilovoltage x‐ray beams.