Volume 34, Issue 6, June 2007
Index of content:
- Therapy Scientific Session: Ballroom A
- Measurement: Calibration and QA (II)
WE‐E‐BRA‐01: Effects of Major Component Replacement in a Tomotherapy HiART System On Radiotherapy QA Measurements34(2007); http://dx.doi.org/10.1118/1.2761599View Description Hide Description
Purpose: On our Tomotherapy system several major components have been changed frequently in one year. This study investigates the possible relationship of component failure with treatment load, and the related QA issues following component replacement. Method and Materials: Tomotherapy HiART is a complex radiotherapy equipment where a linac, magnetron and all the components are mounted on a rotating slip ring gantry. It operates at a high dose rate compared to conventional linacs. The major components — target, magnetron and the linac have been changed five, three and two times, respectively during one year of use. Following major component replacement, the PDD, profiles, output and energy have been measured with ion chambers, water scanner and films to bring the system to agreement with commissioning condition. Results: The output and the energy constancy variation over the entire year were within +/− 6 % and +/− 3.5%, respectively with no definite predictor of an imminent component failure. The dose rate varied between −10% and +5% during the same period. Annual average of daily treatments was 24 with a maximum daily average of 37.3 treatments in a month. Target change interval ranged between 12 and 80 days. The mean time between major component failures was 35.6 days. Conclusion:Analysis of the data could not identify any pattern of component failure. Examination of targets indicated holes, flakiness and uneven erosion in some targets indicating unknown changes in the beam. Magnetron changes were primarily due to arcing. Target and linac change involved more alignment, beam characteristics and QA measurements and longer downtime. The hole in the target poses a beam spectrum different from the commissioned beam. Target or the linac replacement caused more downtime and QA measurements than the magnetron change.
34(2007); http://dx.doi.org/10.1118/1.2761600View Description Hide Description
Purpose: To create a motion fixture capable of testing real‐time targeted radiation therapydelivery systems. Method and Materials: The 4D Phantom is a custom positioning system with stages for accurate reproduction of 3D internal tumor motion and 1D external surrogate motion. A standard film box filled with solid water was sent through two 3D lungtissue trajectories, one from 4DCT and spirometry data and the other measured from a fiducial marker that had been bronchoscopically implanted in a canine lung. An external trigger synchronized motion with the beginning of therapy, and the 4D Phantom gated the linear accelerator via a reed relay based on the real‐time longitudinal position of treatment isocenter. The gating windows, 3mm and 4mm long, encompassed end‐expiration for each motion trajectory. A four‐field, 6MV, 3DCRT plan from a randomly chosen lungcancer patient was delivered and measured for static and for gated and ungated treatments with each motion trajectory. The distance from the edge of the film to the edge of the treatment field was used to align pairs of films for analysis in RIT. Results: The dose distributions' relative positions varied within 2–6mm. Post‐alignment comparisons between static and motion films revealed large misdosed regions. The gated films had fewer areas and smaller magnitudes of dosimetric error than did the ungated films, but hot spots of 10% error were present even in the gated films. Conclusion: The experiment revealed a systematic shift in that the dose was centered at an average phantom position rather than the planned treatment isocenter. Additionally, although gating reduces dosimetric errors due to tissue motion, they can occur during gated radiation delivery. Further work is needed to determine if static film dosimetry is sufficient for QA of gated therapy. The 4D Phantom is capable of improved real‐time targeted radiation therapy QA.
34(2007); http://dx.doi.org/10.1118/1.2761601View Description Hide Description
Purpose: To present a framework for the calculation of ionisation chamber response to arbitrary modulated fields as a convolution of responses to narrow fields and to test this approach for realistic chamber geometries in open fields. Materials and Methods: Using the EGSnrc/C++ Monte Carlo(MC) class library system, the response of a detailed model of the Exradin A12 chamber was calculated for and 6 MV pencil beams sweeping over a flat phantom. A pencil beam response kernel was collected for an irregularly spaced array of pencil positions. A procedure was developed to reconstitute chamber response in modulated fields by convolution of pencil beam response over the modulated field fluence. We tested the accuracy of this approach by comparing in open fields the relative calibration coefficients obtained for realistic chamber geometries reconstituted from pencil beam kernels with calibration coefficients obtained directly. Results:MCk Q values within 0.1% and 0.4% of TG‐51 for 6 MV and 18 MV photon beams, respectively. Pencil beam kernels showed that the response of the chamber is strongly dependent on the geometrical details of the chamber for pencil beam positions hitting areas such as chamber tip, tip of the electrode,etc. Open field reconstituted chamber response was in agreement with direct calculations of the open field response to within 0.3% for and 6 MV photon beams. Conclusions:Ionisation chamber response in modulated fields is strongly dependent on the details of the delivery. However, we can accurately account for it with our method, in which chamber response in arbitrarily modulated radiation beams can be calculated by convolving pre‐calculated response kernels over the actual fluence profile used in the delivery of modulated radiotherapy. These techniques will allow guidance of chamber‐based IMRT QA procedures by correcting chamber readings during deliveries that potentially provoke electronic disequilibrium.
WE‐E‐BRA‐04: Brachytherapy Source Power Measurements Using a Liquid Helium Active Radiometric Calorimeter34(2007); http://dx.doi.org/10.1118/1.2761602View Description Hide Description
Purpose: To characterize a liquid helium active radiometric calorimeter for contained power and emitted power measurements. The calorimeter has been used for direct measurement of low dose rate and beta emitting brachytherapy sources. Method and Materials: Measurements were performed by placing a brachytherapy source into a silver absorber whose temperature was actively controlled slightly above that of liquid helium. An electrical substitution method was used; so the difference in power required to maintain a set temperature with and without the source present was attributed to the source itself. Corrections for increased thermal mass of the system during measurement were determined empirically using nonradioactive “dummy” sources, and corrections for escaped energy were calculated using Monte Carlo MCNP5 models. Data were analyzed using a linear least squares approximation of values obtained while the source was in place and after the source was removed. The distance between the fit lines at the time the source was removed was taken as the source power. Individual , , and sources were investigated. Results: Classical variance for preliminary measurements was found to be 60nW. The correction factor for source thermal mass was 0.611μW; and for , , and , respectively, corrections for escaped energy were 1.0000, 1.0045, and 1.0182 for contained power measurements and 1.0081, 1.0101, and 1.0823 for emitted power measurements. Conclusion: Preliminary results reveal improved sensitivity of the instrument over the previous liquid nitrogen version (Rev. Sci. Instrum., 76, 2005). Small thermal drifts during measurement indicate that slight further modifications to the design may be required to anchor the absorbers fully to the actively controlled detector housing. Also, increasing the size and weight of the source insertion cap may also improve measurement repeatability. While preliminary data are promising, further measurements will improve dataset statistics for both radioactive sources and the measured correction factor.
WE‐E‐BRA‐05: Reducing the Frequency of Linac Output Check: A Statistical Model of Linac Output Fluctuation Based On a 3 Year History to Evaluate New Tolerances as a Function of Test Frequency for 12 Linear Accelerators34(2007); http://dx.doi.org/10.1118/1.2761603View Description Hide Description
Purpose: The study is based on a formalism to model the fluctuation of 12 linac outputs during a 3 year period. From a semi‐empirical statistical model and TG‐40 recommendations, a QA program is built from calculated action levels as a function of test frequency and time‐dependant probability distributions.Method and Materials: The linac output data is analyzed to fit a statistical model taking into account a systematic and a random component in the daily fluctuation. A 3 year history of daily output measurements totalizing 71 independent energies of photons and electrons is used to evaluate the parameters in each model. Action levels are calculated as a function of test frequency from obtained models based on tolerances defined in TG‐40. A confidence level of 95% is used to define the QA program such that machine output is kept within given limits, the latter being obtained by fitting tolerance functions with TG‐40 action levels. Measurement uncertainties are taken into account in the model and Gaussian statistics are used in the formalism. Results: Comparison between models and data history are in agreement with Gaussian statistics. For each linac and energy, tolerance functions are obtained from data history and new action levels are used with reduced test frequency. The tolerance of 2% recommended by TG‐40 for monthly output constancy check is reduced to values ranging from 1.2% to 1.8% for a test frequency of two months. Linaccalibration frequency is reassessed to values ranging from 4 to 12 months. Conclusion: While daily check should be kept constant to prevent unpredictable variations, the frequency of output check can be reduced using new action levels based on a program‐defined limit and confidence level. Probability distributions can be used to evaluate linaccalibration frequency based on machine stability. The model can be extended to other linac parameters.
WE‐E‐BRA‐06: Imaging Linear Accelerator Startup Using a High‐Speed Scintillation Based Electronic Portal Imaging Device (Hi‐EPID)34(2007); http://dx.doi.org/10.1118/1.2761604View Description Hide Description
Purpose: Using the high‐speed scintillation based electronic portal imaging device (Hi‐EPID), to analyze the characteristics of the fluence delivered from a commercial clinical linear accelerator (Synergy® S, Elekta). To study the fluence output at short time scales. Method and Materials: The Hi‐EPID consists of a high speed digital camera coupled with a fast terbium‐doped gadolinium‐oxy‐sulfide (Gd2O2S:Tb) scintillator. The camera can record every single radiation pulse out of the accelerator, at up to 500 fps. Both open field and multileaf collimator(MLC) based intensity modulated radiation therapy(IMRT) fields were evaluated. The startup time of linac delivery was recorded. Results: Fluence delivery irregularities were observed with the Hi‐EPID device, within 250 ms. For the open field delivery, the fluence increased with time until it reached a plateau, in approximately 100 to 250 ms, and then remained constant. However, in the case of a six segment IMRT field delivery, the fluence variation differed between segments. For some segments, the fluence increased in the beginning and then reached a plateau. For others, the fluence rose and fell sharply, followed by a slower rise to the plateau. The rapid fluence variation before a plateau was as high as 60% of the maximum. A field size dependence of fluence was observed. An analysis of field shapes showed no unplanned motion of MLC was during step and shoot delivery, as has been reported for other clinical linac.Conclusion: The Hi‐EPID is capable of imaging the fluence out of a linear accelerator with high temporal resolution. The startup characteristics of the accelerator can be studied. For short time IMRT segments, fluence and dose rate varied over a significant portion of the segment. As the dose rate fluctuates during startup, its effect on IMRT delivery can be studied.
This work was supported by NCI grant R01‐CA‐100636.
WE‐E‐BRA‐07: Experimental Determination of the LiF:Mg,Ti TLD Energy Response Correction Factor for 125I Brachytherapy Sources34(2007); http://dx.doi.org/10.1118/1.2761605View Description Hide Description
Purpose: To further explore the energy response correction factor for LiF:Mg,Ti thermoluminescent dosimeters for use in experiments with low dose rate brachytherapy sources. Method and Materials: Two distinct experiments were performed to determine the value of the energy response correction factor, E(r), as defined in TG‐43U1, for seeds. The University of Wisconsin Variable Aperture Free Air Chamber was used to determine the air kerma strengths of Best Medical 2301 seeds. A 3×3 array of TLD‐100 chips sealed in a thin plastic bag was attached to the center of the aperture using Kapton tape. was used to irradiate a set of comparison TLDs to a dose to water equivalent to the dose to water given by . Monte Carlo simulations were used in both cases to calculate the dose to water from the measured air kerma. The two experiments had different annealing techniques: one using a standard anneal of one hour at 400°C followed by 24 hours at 80°C, and the other using a standard anneal at the beginning of the sorting process and then only the 80°C anneal between cycles. Results: E(r) was determined to be 1.514 ± 0.019 using the 80°C technique and 1.549 ± 0.025 using the standard technique. These values are significantly higher than the currently applied value of 1.41, but not unexpected. TG‐43U1 noted that the work of Davis et al. also observed an anomalous over‐response relative to theoretical predictions. It is speculated that the differences between these two experimental values may be the result of the annealing techniques and TL readout characteristics, which will be further investigated. Conclusion: It appears that LiF:Mg,Ti TLDs are indeed intrinsically non‐linear in their response to low energy photons. The solid state processes that cause this effect are not taken into account by Monte Carlo.