Volume 34, Issue 6, June 2007
Index of content:
- Therapy Scientific Session: Room M100F
- IMRT: QA
34(2007); http://dx.doi.org/10.1118/1.2761389View Description Hide Description
Purpose: To develop an independent treatment verification method which can validate radiation field delivery in real‐time throughout the treatment course. The system is designed to capture common treatmentdelivery errors, and is intended to eliminate the need for pre‐treatment dosimetricquality assurance of intensity modulated radiation therapy(IMRT) and enable the implementation of image guided adaptive radiation therapy.Method and Materials: A monitoring system, termed Integral Quality Monitor (IQM), has been developed that utilizes an area integrated energy fluence monitoring sensor (AIMS) positioned after the final beam shaping device (i.e. multileaf collimator(MLC)) and a signal prediction algorithm, IQM_Calc. The AIMS consists of a novel large area ionization chamber with a gradient oriented along the direction of the MLC motion. The measured signal from the AIMS can be compared in real‐time with the IQM_Calc predicted values. A prototype AIMS has been built with 2 mm thick Aluminum plates, an area of 22 cm × 22 cm and continuously varying electrode separation of 2 to 22 mm. The IQM_Calc uses a modified sector integration of MLC defined apertures and accounts for MLC characteristics such as: rounded leaf ends, transmission, and relative output factor. Testing of the IQM system was performed on Varian and Elekta linear accelerators. Results: Initial results for prostate IMRT fields show an average agreement of 2% between the measured IQM signals and the IQM_Calc results. For a 3 mm simulated MLC leaf positioning error, the signal of a prostate IMRT field changed by 2%. Conclusion: It is demonstrated that the prototype IQM system has the capability of verifying the accuracy of treatmentdelivery in real‐time. The system is also capable of capturing common treatment errors. The IQM system has the potential of playing an important role in the challenging QA environment of modern radiation therapy.
34(2007); http://dx.doi.org/10.1118/1.2761390View Description Hide Description
Purpose: The purpose of this work was to develop techniques for utilizing exit detector data in a helical tomotherapy system to automate the QA process. The clinical significance of this study is that an analysis of the exit detector data acquired during treatment delivery could be used to ensure that the correct delivery sequence is being administered to the patient. Method and Materials:Software applications were developed to automatically analyze uncompressed detector data. To test this software, a MLC test sequence was designed that allows each MLC leaf to be tested using the exit detector data. The goals of this test are 1.) To identify MLC problems (stuck leafs, bad valves, etc…) before failure, and 2.) Perform QA Tests on MLC parameters that affect the delivered dose. Additionally, a clinical test case was created using the treatment delivery sequences for a head & neck patient. The original treatment delivery sequence was modified and 12 known MLC errors were inserted in the MLC controller file. The procedures were delivered and the software was used to analyze the exit detector data. Results: The MLC QA Test was delivered on four occasions with two MLCs. A software application developed by the investigators was then used to analyze the exit detector data from these deliveries. The software correctly performed Latency Tests, Projection Centering Tests, and MLC Transit Time Tests. For each delivery, the tests showed that the MLCs were properly functioning. For the Head & Neck test case, the shape‐detection algorithm was able to identify 11 out of 12 known MLC errors >10 msec. Conclusion: A technique was developed for performing automated QA of the MLC and individual patient deliveries using exit detector data in a helical tomotherapy system. With the tomotherapy detector array, errors in MLC position > 10 msec have been detected.
34(2007); http://dx.doi.org/10.1118/1.2761391View Description Hide Description
Purpose: Several years since the advent of IMRT, most institutions still perform experimental plan verifications with film and ion‐chamber. In the past year of measurements of Pinnacle3IMRT plans in our clinic, most isocenter dose measurements were verified to be within 3%, but the 10% that were off by 4–7% were repeated or required MU adjustments. In this work, we trace the main sources of discrepancies by measurement and simulation. Methods: We used an independent dose calculation engine (MCKS) that performs kernel‐based superposition using Monte Carlo sampling of photons, interaction points, and corresponding monoenergetic kernels. The photon source for the simulations is a single phasespace plane scored just above the collimator and generated by an EGS/BEAMnrc code. Even though MCKS models the detailed MLC design (curved ends, tongue‐and‐groove, inter‐leaf gap, and leaf offsets) the MLC material density is fine‐tuned with the measurement of “uniform” intensity map delivered as sequence of narrow strips, which amplifies the leakage dose several fold. Results & Conclusion: Using the code and measurement, we found that a major source of error in IMRT plans from Pinnacle3 is the handling of MLC leakage. For example, if a 1.8% MLC transmission is used, then delivering a 10×10 field with ten 10×1 strips produces a dose overestimation by 12% on the central axis. This reduces to 8% if a 1.5% value is used. In typical IMRT plans, the leakage error is less dramatic, but can frequently exceed the 3% acceptance criterion. Also, no single transmission value is suitable for all possible IMRT fields because MLCscatter varies with field size and shape. Some errors also result from the use of lumped empirical square field output factors to complex IMRT segments without regard to the energy or direction of head scatterphotons.
34(2007); http://dx.doi.org/10.1118/1.2761392View Description Hide Description
Purpose: Characterize, commission and evaluate a dual plane diode matrix IMRT QA device. Method and Materials: A novel device consisting of diode matrices in two orthogonal planes inserted in a cylindrical acrylic phantom of 22cm diameter is characterized, commissioned and evaluated for radiotherapyquality assurance. The system interfaces readily with a networked computer making the whole IMRT QA process very efficient in multi accelerator and multi physicist department. . It detects charge per accelerator pulse, computes and displays measured dose distribution in 3D space. The temperature dependence of the diode is corrected. The precision, stability, pulse rate dependence, dose rate dependence, angular dependence, linear response, energy response of the system and the calculation accuracy at non detector locations are evaluated in addition to comparing multiple simple and complex iso‐dose distributions from TPS to measured distributions. The software readily analyses dose profiles in any orientations, %dose, DTA and gamma index of the entire 3D distributions. Results: The precision and the day‐to‐day reproducibility of measured data of a single field are excellent, making additional ion chamber measurement unnecessary. The measured data indicated excellent dose linearity and pulse rate independence. Comparison of simple and complex treatment plans with delivered treatment showed good agreement considering the error bars. Conclusions: The Delta 4 system is highly efficient, accurate and reproducible. The instantaneous and automatic data acquisition combined with the error analysis, report and database capability built into the system make it easy, convenient and efficient to use in a busy clinic. Conflict of Interest Statement: One of the co‐author is President and CEO of ScandiDos AB Company, which supplied Delta4 at no cost for evaluation.
TU‐D‐M100F‐05: Pre‐Treatment Verification of Large‐Field IMRT Dose Painting Plans for Head and Neck Cancer Using a Commercial QA Device34(2007); http://dx.doi.org/10.1118/1.2761393View Description Hide Description
Objective: To develop a practical procedure for routine pre‐treatment verification of large dose painting fields using a commercial IMRT QA device, MapCheck. Materials and Methods: Optimized dose painting intensity maps were applied to a 40×40×40 cm3 digital phantom. With a voxel size of 1 mm3, the dose distributions were re‐calculated for a coronal plane at a depth of 5 cm. The DMLC files were then transferred to a Varian Clinac for plan verification. We first delivered a beam with MapCheck at isocenter. The isocenter was then shifted superiorly and inferiorly by 4 cm, respectively. Two more exposures were made. The isocenter was shifted to the left and right by 4 cm, respectively. Two more exposures were made. To detect accurately the steep dose gradient regions, we doubled the effective detector surface density by shifting the isocenter by 5 mm along the X and Y axes. Two more exposures were made. Finally, we merged all these files to yield a desirable dose distribution. We used three different criteria for data analysis: percent difference, distance to agreement (DTA), and γ index. We set the percent difference threshold to 3% and the DTA to 3 mm. Results: Using these criteria, 99% ∼ 99.5% dose points passed the test for most of our IMRT dose painting plans. For a ten‐field plan, it took about 35 minutes to complete the data acquisition and post‐analysis. Conclusions: The proposed procedure not only improves the efficiency, but also enhances the accuracy of measured dose distribution in the steep dose gradient regions. MapCheck is a handy, fast, and practical tool for routine pretreatment verification of large‐field dose painting plans.
34(2007); http://dx.doi.org/10.1118/1.2761394View Description Hide Description
Purpose: The use of gating in the delivery of radiation therapy (RT) has been shown as an effective means of treatingtumors susceptible to motion. However, due to electronic and mechanical limitations of the radiationdelivery equipment, the precision of gated IMRT is susceptible to equipment process time delays. The purpose of this study is to investigate the temporal accuracies of gated RT using a simple and inexpensive method that doesn't require complex or fast sampling equipment to measure response times. Method and Materials: Using a Varian Trilogy treatmentdelivery system together with a Varian RPM respiratory gating system, the response times of both normal RT and step‐and‐shoot (SS) IMRT gated deliveries are investigated. Radiation dosage profiles, recorded using radiographic films, are digitalized to allow extraction of temporal data. In addition the same response time technique is used with an electronic portal imagining device (EPIDs). Results: Static to dynamic comparison of specific MLC patterns is used to extract the gating response time. It is found that the time, from when an object enters into the specified phase to when the radiation beam is actually turned on, is approximately 0.15 s. This time error is found to be systematic in nature and is approximately the same for both normal and SS‐IMRT gated deliveries.Conclusion: A general and simple procedure has been developed that will allow the response time of a LINAC based gating systems to be made. The ∼150 ms systematic timing error found to occur in the Trilogy/RPM system can be corrected by modification of the gating software provided if the patients breathing is regular and periodic.
34(2007); http://dx.doi.org/10.1118/1.2761395View Description Hide Description
Purpose: To evaluate a Web‐based Quality Assurance tool for maintaining the clinical integrity of the Tomotherapy machine. Method and Materials: This tool takes advantage of the output of the two distinct ion chamber systems on the Tomotherapy machine. One is the standard sealed ion chamber that resides in the head of a machine near the primary collimators and is used to calculate Monitor Units. The other is a set of pressurized Xenon detectors that are used for imaging. Both systems monitor ionizations independently and are rigidly attached so that their geometry is constant regardless of the orientation of the beam on the gantry which makes possible testing for both static and rotational modes. The collected data can be reviewed via a Web page and a downloadable Report in PDF format is available within seconds after collection of the data. Results: As an example, one performs a 200 second rotational delivery with the couch removed from the beam. The data are analyzed in both an integrated and pulse‐by pulse methodology to determine consistency between the two chamber systems, changes in output and changes in energy. The collected information is evaluated relative to a standard delivery created during machine commissioning. Comparison to standard ion chamber measurements in phantom under static conditions indicate that the analyses are consistent for calibration and more sensitive to energy changes than percentage depth dose measurements made in phantom. Additional information about the machine's performance is identified easily. Conclusion: New technology offers an opportunity to perform standard Quality Assurance tests in a new manner. The simple test described here is more sensitive and performed more quickly than traditional methodology. Other tests are possible. Conflict of Interest: Three authors are employees of Tomotherapy, Inc.
34(2007); http://dx.doi.org/10.1118/1.2761396View Description Hide Description
Purpose: To develop a comprehensive protocol and software tool for the quality assurance of the HiArt Tomotherapy MLC system. Method and Materials: Seven MLC test patterns were designed and an in‐house software called “Tomo MLC QA” was developed to generate the sinogram for the test patterns and analyzed the test results. The seven test patterns are: box in box, modified checkerboard, leakage test, IEC‐X gradient, IEC‐Y gradient, complex field A, and complex field B. Kodak EDR2 ReadyPak films were used for all the measurements with our test patterns. Results: The films from the measurements were analyzed with the “Tomo MLC QA” tool. Pass and Fail criteria were specified for each test according to published QA data. Most tests passed the set criteria, although some failures were detected and corrected. Conclusion: In this study, seven test patterns were designed as part of a QA toolkit for the MLC of a helical tomotherapy unit. A software platform called “Tomo MLC QA” was developed to facilitate the analysis of the seven tomotherapy MLC QA tests. The software can be easily adapted to any treatment center with a helical tomotherapy unit to perform customized tomotherapy MLC QA or to standardize testing. Additional test patterns can be developed based on the “Tomo MLC QA” platform. The seven designed test patterns for tomotherapy MLC QA are not aimed to test all characteristics of the tomotherapy MLC. However, the patterns allow the users to test some of the mechanical and dosimetric properties of the tomotherapy MLC. The analysis results based on the seven test patterns from our institution were reported. All tests passed at the inhouse set criteria, indicating that our unit's MLC is operating according to specifications. The analysis revealed some minor problems regarding beam angle position that require further investigation.
TU‐D‐M100F‐09: Breathing Motion‐Induced Dose Delivery Error Evaluations as Applied to Tomotherapy Dose Delivery34(2007); http://dx.doi.org/10.1118/1.2761397View Description Hide Description
Purpose: To develop a method for evaluating breathing motion‐induced dose delivery errors in Tomotherapy dose delivery. Methods and Materials: Dosimetric inaccuracy can result from breathing‐induced tumor motion in Tomotherapy treatment delivery. Patient breathing motion patterns were simulated using quantitative spirometry‐measured patient tidal volumes and converting the tidal volume to tumor motion by varying the ratio of tumor motion to tidal volume for 34 patients. Simulations of Tomotherapy deliveries were conducted modifying the previously published techniques by using measured beam profiles instead of step‐function fluences, and couch speeds typical of Tomotherapy treatments. Radiochromic film and our in‐house 4D phantom were used to verify the algorithm. Results: As expected, the breathing motion blurred the dose distributions, but slow drifts in the average tissue position caused detectable dose errors. The dose errors were expectedly largest with the smallest (1.0cm) field size, and could be >10% for motion amplitudes comparable to the field size. As the field width increased relative to the motion amplitude, and as couch‐speed (pitch) decreased, the error also decreased, and as such these settings may be preferable for patient treatments. These slow drifts occurred over time periods that were coincident with the amount of time required for the field to pass a stationary point. Measurements agreed with the simulation. Conclusions: Previous breathing motion studies did not use real patient breathing patterns and therefore did not consider the impact of slow, relatively small drifts in those patterns. The drifts change direction during the breathing measurement, causing dose errors that are both positive and negative. While the individual dose fraction errors can be >10%, they are unlikely to occur in the same place each day, so the average dose is likely to be consistent with earlier studies. Conflict of Interest: This work supported in part by a grant from Tomotherapy, Inc.
- IMRT: Commercial Systems and Clinical Applications
34(2007); http://dx.doi.org/10.1118/1.2761519View Description Hide Description
Purpose: This work compares out‐of‐field secondary doses and associated risk of a fatal secondary malignancy from a Hi‐Art Tomotherapy machine and a conventional gantry based accelerator for an adult IMRT prostate and a pediatric cranio‐spinal treatment.Method and Materials: A conventional 3D and tomotherapy IMRT cranio‐spinal treatment plan were developed to deliver the same prescription to a pediatric anthropomorphic Rando phantom using a Philips Pinnacle and Hi‐Art Tomotherapy planning system, respectively. Similarly, an adult IMRT prostate treatment plan, using the Pinnacle and tomotherapy planning systems was developed to deliver the same prescription with the same constraints to an adult Rando phantom. The target and organs at risk (OAR) were contoured. TLD were located within each of the OARs selected. Each phantom was irradiated three times per plan. The out‐of‐field organTLDdoses for the gantry based delivery and tomotherapy treatments were compared. For each organ site, an average dose was determined and organ weighted linear non‐threshold dose response model risk factors were used to estimate the risk of a secondary fatal malignancy for each treatment.Results:Doses calculated from the adult TLD data were lower for all organs when treated with the Tomotherapy plan and the overall risk was lower. The pediatric TLDdose findings were mixed between the 3D and tomotherapy treatment, however because of the higher integral dose with the Tomotherapy, the overall risk is higher for the Tomotherapy treatment.Conclusion: The risk of a secondary fatal cancer was lower for the Hi‐Art Tomotherapy adult prostate treatment than the gantry based IMRTtreatment due to due to the lower integral out‐of‐field secondary radiation doses. The risk for the pediatric case appears higher for the Hi‐Art Tomotherapy treatment than the 3D conformal cranio‐spinal treatment. Work supported by PHS grant CA10953 awarded by NCI.
WE‐C‐M100F‐02: Simultaneous Irradiation of Prone Breast and Regional Lymph Nodes Using Helical Tomotherapy34(2007); http://dx.doi.org/10.1118/1.2761520View Description Hide Description
Purpose: We investigate the capability of helical tomotherapy to simultaneously irradiate the involved breast and regional lymph‐node groups for breast‐cancer patients simulated in prone position. Method and Materials: We generated helical tomotherapy treatment plans for ten patients (five left breast, five right), each simulated with the involved breast suspended downward. Our target structures were the ipsilateral breast and the supraclavicular, axillary, and internal mammary chain nodes. Each target was to receive 45 Gy ± 5%. The region‐at‐risk (RAR) structures considered were the ipsilateral and contralateral lungs, contralateral breast, heart, spinal cord, esophagus, and thyroid. Results: The requirement for V45 ⩾ 95% was met for all target structures. The minimum point dose objective of 42.8 Gy was achieved for all lymph‐node structures, but for the ipsilateral breast the minimum dose on average was slightly less. The maximum point dose objective of 47.3 Gy was not achieved for any target structure; on average, it was 49.5 Gy. Among the RAR structures, contralateral breast V5 was kept below 2%. For left‐breast patients, the DVH goals for the heart were met; however, for right‐breast patients, although the aim was for the heart to receive zero dose, V10 was typically 0.4%. The other RAR DVH objectives that were not met were those for the ipsilateral lung V5 (the goal was 60% or less; 67% was achieved on average) and the esophagus V5 (the goal was 30% or less; the average was 41%). Conclusions: Helical tomotherapy can simultaneously cover the breast and regional lymph nodes with uniformity comparable to that achieved conventionally. The high ipsilateral‐lung V5 may be the primary limitation for clinical application, since there are reports that respiratory complications may correlate with the volume of lung irradiated to relatively low doses. All other RAR doses can be kept to clinically acceptable levels.
WE‐C‐M100F‐03: Underestimation Of Low‐Dose Radiation in Conventional Treatment Planning of Intensity‐Modulated Radiation Therapy34(2007); http://dx.doi.org/10.1118/1.2761521View Description Hide Description
Purpose: In radiation therapy of thoracic cancers (e.g. lungcancers), a high volume of low‐dose radiation may lead to severe and fatal pulmonary complications. The purpose of this work is to investigate dosimetric accuracy in low‐dose regions from commercial treatment planning systems (TPS) and consequent clinical implications for using IMRT in treatinglung and thoracic cancers.Methods and Materials: We retrospectively reviewed thoracic‐cancer patients treated with IMRT at our institution and for whom high‐grade lung complications occurred. These IMRT plans were evaluated and were recomputed using a Monte‐Carlo‐based (MC)treatment planning system (TPS) that explicitly accounts for modeling of the machine head and MLCs.Dose calculations from 2 commercial TPSs (Pinnacle and Corvus systems) were compared with those obtained from the MC system and measurements. Various factors that could contribute to the difference in IMRTdose calculations were analyzed, including tissue heterogeneity effect, MLCmodeling, and beam modeling.Results: Significant dosimetric errors (∼25%) were found in the low‐dose regions below 5 Gy in the commercial TPSs. Accuracy of dose calculations in the high‐dose tumor regions was acceptable (< 5%). In regions < 5 Gy of IMRT plans, MLC transmission, leakage, and scattering were found to be important contributors to the dose. These factors were also spatially variant and field‐size dependent. Without explicit modeling of the MLCs, severe underestimation of the low‐dose volume could occur in commercial TPS. The degree of low‐dose error was particularly greater in IMRT plans with larger target volumes and higher degrees of beam modulation. Conclusions:Treatment planners should be aware of potentially significant underestimation of low doses in IMRTtreatment plans for thoracic and other cancer sites. More accurate modeling of MLCs and low‐dose calculation may be achieved by using a Monte‐Carlo‐based planning system.
34(2007); http://dx.doi.org/10.1118/1.2761523View Description Hide Description
Purpose: To develop an accurate MRI‐based treatment planning method by assigning MR images the desired electron density information through the use of diagnostic CT and a deformable registration model. Method: MR and CTimages are registered using deformable or rigid registration method depending on the disease site or the patient positioning when acquiring the CT data. Based on the voxel‐to‐voxel correspondence established by the registration, the electron density distribution of the CTimages (this can be a diagnostic CT with patient positioned arbitrarily) is mapped onto the MRI and plan is computed based on the modified MR images, which take advantage of the useful features of CT and simulation MRI. To evaluate the accuracy of the MRI‐based dose calculation, six clinical cases, including three brain and three head‐and‐neck cases are studied. For each case, three IMRT plans are computed with the same beam configuration but different image basis: CT,MRI with mapped electron density, and MRI with voxel values replaced by that of water. The dose distributions from the three types of calculations are compared. Results: In all cases, the MRI with mapped electron density yielded very close dose values as compared with the CT‐based calculation. The maximum discrepancy between the two is found to be less than 2.0% in the high dose region. Compared to the MRI with mapped CT electron density, MRI with uniform water voxel value yielded quite different dose distributions in all regions, and the dose discrepancy can be as large as 3–5% for both brain and H&N cases. Conclusion: The MR images with mapped CT electron density yield very similar treatment plan compared with the CT‐based calculations. The approach has potential to eliminate simulation CT by planning a patient treatment with only simulation MRI and diagnostic CT data.
34(2007); http://dx.doi.org/10.1118/1.2761524View Description Hide Description
Purposes: To review the current clinical outcome of lungcancer patients treated with IMRT; to develop a treatment planning methodology and guideline for using IMRT in treatinglungcancers.Methods:Lungcancer patients treated with IMRT were reviewed with respect to clinical outcome and treatment plans used. Major toxicity including radiation pneumonitis and esophagitis were analyzed. In the process of IMRT inverse planning, methodologies of selecting appropriate beam angles and planning objectives were developed to limit the spread of beam angles and low‐dose radiation in protecting normal lung and other critical structures. Important IMRT planning parameters and their guidelines were derived, e.g. on the number of beams, beam angles, degree of intensity modulation, and DVH constraints for normal tissues.Results: We reviewed 68 locally advanced lungcancer cases treated with IMRT and with adequate clinical followup time. The rate of grade ⩾ 3 TRP at 12 months was reduced to 8% in IMRT patients in contrast to 32% in 3D‐CRT patients treated historically, despite larger GTV sizes in the IMRT cohort. Mean lungdose and the volume of low‐dose radiation to healthy lung may be important factors affecting the risk of radiation pneumonitis. In achieving quality IMRT plans, 6 beams were used in average, with the optimal beam angles being most frequently along the anterior‐posterior direction of the thorax; the number of MLC segments per beam was confined to be less than 15 with the total MU per fraction approximately 3 times of the prescribed tumordose. Conclusions: With proper considerations given to treatment planning methodology and other technical issues involved, IMRT may be an effective and safe modality in radiation therapy of locally advanced lungcancers.
34(2007); http://dx.doi.org/10.1118/1.2761525View Description Hide Description
With increased longevity, more prostate cancer patients with hip prosthesis are anticipated. Prosthetic devices have high atomic numbers (Z) and produce dose perturbation that is dependent on Z, beam energy, and depth. TG‐63 provided recommendations for high‐Z prosthetic devices in 3D conformal therapy. In IMRT, the dosimetric implication of the high‐Z prosthesis remains uncertain which is investigated in this collaborative study with 10 different treatment planning systems (TPS). Planning target volume (PTV), and the organs at risk (OAR) namely, the bladder, the rectum and a bilateral titanium hip‐prosthesis were contoured on a CT data set of a patient and sent to each collaborator with proper guidelines for beam arrangements, energy and dose volume constraints for planning. Due to significant streaking artifacts in the CT data, users were encouraged to use their own method to correct for redistribution of CT numbers, and assign the appropriate electron densities. Since dose perturbation is significant for low energy and less sensitive with multiple fields, equally distributed 7‐fields were planned. Beam energy was also studied for comparison. One common constraint, 95% PTV must receive at least 95% of dose was strictly followed by each planner. A variety of dose algorithms were used in different TPS, such as pencil beam, superposition, and convolution. Although the results of some planning systems are closer to each other, in general, there is a wide variation in dose distribution in PTV and the OARs, as well as the minimum, the maximum and the median doses which are commonly used in plan evaluation. The variation in MU and the number of segments also vary significantly. High energy beam provided slightly better but not significant dose distribution. Ranking of TPS cannot be established based on a single clinical case. A well‐controlled phantom study is planned to validate the merit of each TPS.
34(2007); http://dx.doi.org/10.1118/1.2761527View Description Hide Description
Purpose: To determine the feasibility of solid modulatorIMRT (SM‐IMRT) for a patient with bilateral orbital lymphoma and to compare this technique with historically used methods. Method and Materials: After immobilization and CT simulation, a CTV was contoured consisting of the entire orbital volume minus the globes. A 3 mm margin was added to generate a PTV. A dose of 34 Gy in 2 Gy fractions was prescribed to the PTV. Dose constraints were placed on the lacrimal glands, lenses, optic chiasm, and pituitary. Treatment plans were constructed for SM‐IMRT, wedged pair fields, and en face photon fields. Due to acute symptoms, treatment commenced with en face photons while planning continued. A fourth plan consisting of mixed SM‐IMRT/en face photons was constructed. Results: Acceptable target coverage was obtained with all four plans. The SM‐IMRT plan offered significant sparing to the lenses, optic chiasm, and pituitary. The mixed SM‐IMRT/en face photon plan (13 of 17 fractions IMRT) also offered significant sparing, though not at the level of the pure SM‐IMRT plan. The maximum dose objective of 30 Gy for the lacrimal glands was almost met with the SM‐IMRT plan (10% over 30 Gy). The wedged pair plan produced the most homogeneous target dose, though only because there was no attempt to sculpt dose to avoid critical structures as with SM‐IMRT. SM‐IMRT was chosen for further patient treatment.Quality assurance tests on the modulators showed excellent agreement between planning system calculations and measured dose, validating the data transfer and modulatormilling process. Conclusion: SM‐IMRT is a feasible treatment modality for bilateral orbital lymphoma. The technique offers the ability to spare critical structures that is impossible with traditional techniques. Dose can be modeled accurately for the solid modulators. A drawback of this or any IMRT technique is the longer treatment plan preparation time.
WE‐C‐M100F‐08: Use of Adaptive Filtering and Relative Intensity Limits in Deliverable IMRT Optimization to Improve Delivery Efficiency34(2007); http://dx.doi.org/10.1118/1.2761528View Description Hide Description
Purpose: To investigate the effect of adaptive filtering, relative intensity limits, and fluence matrix resolution in deliverable IMRT optimization on MU efficiency and plan quality for dynamic MLC delivery. Method and Materials: An adaptive filtering method that filters the optimized intensities as needed in combination with different relative intensity limits were used to reduce MUs needed to deliver the beams. The adaptive filtering parameters were adjusted so that it achieved the optimal results. A deliverable‐based optimization that incorporates beam‐delivery constraints directly into the IMRT optimization process was used. H&N and prostate cases were used to evaluate the quality and complexity of each plan. Each case was optimized with and without adaptive filtering and in combination with a series of relative intensity limits for fluence matrix resolution of 1.5 mm to 6 mm. The IMRT plans were evaluated in terms of dose‐volume statistics , MUs, and the complexity of fluence matrix resolution. Results: Up to 47.5% reductions in MUs were achieved using adaptive filtering in combination with relative intensity limits. When the adaptive filtering is on, the use of a relative intensity limit of 1.5 further reduced the total MUs by 6.3% and 15.9% for a fluence matrix resolution of 3‐ and 6mm respectively. Adaptive filtering in combination with relative intensity limits was able to reduce the total MUs without significantly changing the prostate and nodal PTV coverages. The changes in critical structure doses were negligible. The similar results have been obtained for the H&N case included in the study. Conclusion: The adaptive filtering was able to significantly reduce total MUs without compromising the plan quality for deliverable IMRT optimization process. The use of relative intensity limits in combination with adaptive filtering did not have a significant effect on MU efficiency as compared to the use of adaptive filtering alone.
WE‐C‐M100F‐09: Dosimetric Comparison of Linac‐IMRT and Helical Tomotherapy (HT) for Head and Neck Cancer34(2007); http://dx.doi.org/10.1118/1.2761529View Description Hide Description
Purpose: To perform a dosimetric comparison between Linac and Helical Tomotherapy‐based IMRT on head and neck (H&N) patients treated similarly and to evaluate any potential clinical consequences of dosimetric differences. Method and Materials: This is a retrospective study of 23 H&N cancer patients treated on HT. All patients were planned both on the HT and the Pinnacle planning system. The prescribed dose was 66 Gy at 2.2 Gy per fraction to the PTV. The dosimetric parameters used for comparison were: R95 = the ratio of the average dose to 95% of the PTV to the prescribed dose; Rc = the ratio of the PTV coverage to PTV volume, where the former was defined as the volume enclosed by the 66 Gy isodose surface; Biologically equivalent doses (BED) to organs at risk (OAR) and PTV dose homogeneity were also studied. The tolerance range or TR (standard deviation/PTV average dose) was used as a surrogate for PTV dose homogeneity evaluation. Results: R95 results indicated that both IMRT techniques produced comparable conformal plans. Rc values showed that HT plans generally provided better tumor coverage. TR results suggest that PTV dose homogeneity was better for HT plans. Finally, the average OAR BEDs showed a trend of better normal tissue sparing with the HT plans. The exception was for the spinal cord, in which the maximum BED using HT was slightly lower than the maximum BED using linac‐IMRT. Conclusion: This study suggests that HT plans had in general better dosimetric characteristics, especially regarding tumor coverage, PTV dose homogeneity and normal tissue sparing in physician approved plans. Dose reductions to OAR may not yield any clinical differences in outcome in virtue of the delivered OAR doses which are well below normal tissue tolerance.
WE‐C‐M100F‐10: Evaluation of the Beam Segmentation Algorithm in the KonRad IMRT Treatment Planning System34(2007); http://dx.doi.org/10.1118/1.2761530View Description Hide Description
Purpose: Reducing the treatment time for IMRT patients is highly desirable. For step‐and‐shoot IMRT plans, the delivery time of a treatment fraction is determined by the number of beam segments, and to a lesser extent by the total number of monitor units (MUs). The objective of this work was to study the segmentation efficiency of the new clinical Siemens KonRad inverse treatment planning system (TPS) and compare it to the CMS XiO TPS. Method and Materials: For head and neck, liver and prostate cancer patients, step‐and‐shoot IMRT plans were designed using both CMS XiO and Siemens KonRad. Number, direction and energy of beams were the same in both systems. The plans were optimized to achieve the same clinical objectives concerning dose to the target volume and to the relevant organs‐at‐risk. The number of intensity levels were minimized until the clinical objectives could not be achieved anymore. DVHs, EUD, mean‐ and maximum‐doses were compared, as well as the number of beam segments and MUs. Finally the beams of each plan were delivered individually on a MapCheck device to verify the agreement between calculations and measurements to be less than 3%–3 mm distance‐to‐agreement. Results: Plans optimized with KonRad resulted in fewer segments and lower number of MUs and therefore reduced delivery time, while achieving similar dose distributions. CMS XiO plans exhibited a slightly steeper dose fall‐off outside the target volumes, however the difference was not clinically significant. DVHs to organs‐at‐risk were comparable. All calculated dose distributions passed the 3%–3 mm dose verification check. Conclusion: Both commercial systems produce clinical similar plans. The sequencer of the KonRad system is more efficient, reducing the number of beam segments and therefore delivery time on average by more than 20%.
- Tx Planning and Delivery: New Techniques and Systems
34(2007); http://dx.doi.org/10.1118/1.2761679View Description Hide Description
Purpose: To study the temporal evolution of the target's charge and its effect on the acceleration of protons. To optimize the proton energy by designing new target geometry, in which the protons experience the full acceleration potential created by the laser pulse. Method and Materials: The process of proton acceleration is studied by means of fully relativistic two‐dimensional PIC simulations (2D3V PIC) with initial conditions similar to a realistic experimental situation, where a relativistically intense (I0=1.92×1021W/cm2, a=30) ultrashort (∼40fs) infrared (λ = 800nm) laser pulse interacts with a Cu target of thickness 400∼nm at normal incidence. Results:Protons at rest on the back surface of the target were initially studied and their dynamics was analyzed. It was determined, that in this geometry the protons experience acceleration in only 25% of the peak of the electric field. The conditions were subsequently optimized by assuming a moving proton bunch incident on the target. A novel two‐stage acceleration scheme was proposed (chain acceleration) and a total energy gain of 50% was estimated under optimum conditions. The implications for proton therapy are discussed. Conclusion: It is shown, that in a conventional proton acceleration geometry using lasers, the acceleration conditions are far from optimum due to the fact that the protons are expelled from the target before the maximum charge build up is reached and as a result experience reduced acceleration potential. We proposed and analyzed a new acceleration scheme employing a double target, in which the protons accelerated in the first stage are properly timed with the laser pulse that ionizes the second target. Under such conditions the energy gain is substantial, opening up the way for proton acceleration above 100MeV using the today's state of laser technology.