Volume 35, Issue 6, June 2008
Index of content:
- Joint Imaging/Therapy: Scientific Session: Room 352
Measurements — Calibration and QA
35(2008); http://dx.doi.org/10.1118/1.2962943View Description Hide Description
Purpose: The purpose of this study was to evaluate the dosimetric accuracy of a helical tomotherapy system using an anthropomorphic RANDO phantom for patient‐specific QA. The results of this study will allow a benchmark to be established for dosimetric accuracy in heterogeneous materials. Method and Materials: Patient‐specific IMRT dosimetric verification was performed for 24 patients using a male RANDO Phantom. Treatment plans were created using the Hi‐Art TomoTherapy treatment planning and delivery system. The phantom was placed in the Vac‐Lok bag with radiographic film placed in a transverse plane between two of the phantom slices. The films were scanned and analyzed using both the Tomotherapy IMRT QA and the RIT dosimetry software. Horizontal profiles, vertical profiles, Gamma pass/fail analysis, and Gamma histograms were calculated for each test case. The reproducibility of the film was tested for both a prostate and lung case. Results: On average, the lung patients had 27.2% of the pixels exceeding gamma, while the prostate patients had 14.7%. For the prostate test cases, only two of the films had greater than 20% of the pixels exceeding the gamma threshold, while only four of the lung test cases were below the 20% pixel threshold. The reproducibility of film dosimetry found that the standard deviation for the number of pixels exceeding the gamma threshold was 3.9%.
Conclusion: In this study, an anthropomorphic RANDO phantom was used to evaluate the doses calculated by a helical tomotherapy system. The dosimetric agreement between calculated and measured was worse than published results in homogenous phantoms, especially for target volumes located in the lung. Further investigation will be needed to determine if these errors are due to film response at heterogeneous tissue interfaces or from actual dose calculation errors.
35(2008); http://dx.doi.org/10.1118/1.2962944View Description Hide Description
Purpose: The AAPM is currently working to provide an update/addendum to the TG‐51 dosimetry protocol. As part of this work measured and calculated kQ factors for a range of thimble‐type ionization chambers not listed in TG‐51 were obtained. The aim was to investigate the accuracy of the calculations and obtain objective evidence on the performance of these chambers for reference dosimetry.Method and Materials: The same formalism and computer programs as employed in TG‐51 were used to calculate kQ factors. As well as 0.6 cm3 reference chambers, factors were calculated for scanning and micro chambers from the major manufacturers. Measurements were made using the and Elekta Preciselinac facilities at the National Research Council of Canada. The aim was to characterize the chambers over the range of energies applicable to TG‐51 and determine whether the chamber met the requirements of a reference class instrument. Chamber settling, ion recombination and polarity were investigated and absorbed dose calibration coefficients were obtained for and 6, 10 and 25 MV photon beams. Results: As might be expected, 0.6 cm3 thimble chambers showed the most predictable performance and experimental kQ factors were obtained with a relative uncertainty of 0.1%. The performance of scanning and micro chambers was somewhat variable. Some chambers showed very good behaviour and gave reasonable agreement between measured and calculated kQ factors but others showed anomalous polarity and recombination corrections that require further investigation. For the well‐behaved chambers, agreement between measured and calculated kQ factors was within 0.4%. However, for some chambers differences of nearly 1% were seen that may be related to the recombination/polarity issues. Conclusion: Experimental and calculated kQ data have been obtained for a wide range of thimble chambers that can be used by the AAPM and clinical users in choosing suitable detectors for reference dosimetry.
35(2008); http://dx.doi.org/10.1118/1.2962945View Description Hide Description
Purpose: To develop simple, accurate, and preferably filmless Quality Assurance (QA) procedures for Helical Tomotherapy, by using the signals from the systems' built‐in MegaVolt CT‐detector (MVCT). Method and Materials: We developed three new QA procedures, all using data files imported from the Tomotherapy machines, containing 85 control signals and signals from all 640 MVCT channels, at a sample rate of 30Hz. An important role in this QA program is taken by the StepWedge, a step‐like aluminium block positioned over the couch end in the gantry.
1) StepWedge procedure: to check laser alignment, couch movement and velocity, beam energy and field width. This 5min procedure is performed with a static beam, while couch and StepWedge move 20cm into the gantry. 2) Completion procedure: to check the correctness of a completion made by Tomotherapy software after an interruption. This procedure is based on the StepWedge procedure, interrupted by the user and completed by Tomotherapy software, of which the correctness is tested. 3) MLC‐Gantry‐Synchrony procedure: to check synchronization of MLC and gantry rotation. During this procedure the couch is static while the gantry rotates 40× 20s, opening the middle two leaves at gantry angles 0°, 120° and 240°. Results: From the StepWedge procedure the position of the transversal and sagittal lasers can be checked with an accuracy of ∼0.5mm, the field width with an accuracy of ∼0.5mm, beam energy consistency with an accuracy of ∼1%, and couch movement and velocity with an accuracy of ∼0.1mm resp. ∼0.5%. The accuracy of the Completion procedure, in terms of a possible additional couch shift, is ∼0.1mm. MLC synchronization with gantry rotation can be checked with an accuracy of ∼0.1°. Conclusion: The StepWedge combined with MVCT offer a fast, accurate and filmless QA program, allowing monitoring of almost all relevant parameters of Helical Tomotherapy.
TH‐D‐352‐04: Dose — Position Verification of 4D Radiotherapy Using the RADPOS System in a Deformable Lung Phantom35(2008); http://dx.doi.org/10.1118/1.2962946View Description Hide Description
Purpose: To evaluate a novel 4D dosimetry system (RADPOS) in conjunction with a deformable lung phantom as a quality assurance tool for 4D radiotherapy.Method and Materials: RADPOS probes, consisting of a MOSFET dosimeter combined with an electromagnetic positioning sensor were placed inside the deformable lung phantom: one detector inside and the other outside the tumour, inside the lung portion of the phantom. CT scans were taken with the phantom in three breathing phases, end of inhale (EOI), middle of inhale (MOI), and end of exhale (EOE). The detector position inside the phantom was read with the RADPOS software and compared to the position determined from the CT data. A three‐field treatment plan was created using as “planning dataset” the CTimage of the phantom in the EOE phase, with the two detectors at the positions described above. The breathing cycle was divided into two states, EOE and EOI. The same treatment was delivered twice, with the phantom in the EOE and in the EOI phases. RADPOS measured doses during both irradiations were compared to the treatment plan calculated values. Results: The detector displacements measured by the RADPOS system were within 1.4 mm, −0.1 mm, and 1.5 mm of measurements from the CT registration for movement between the EOE and EOI, EOI and MOI, and MOI and EOE phases, respectively. There was no trend in the differences in RADPOS‐measured and the calculated doses for individual beams and breathing states, with a maximum deviation of 3.52 cGy (2.2% of total dose) for the detector inside the tumour and 4.66 cGy (4.2% of the total dose) for the detector in the lung tissue. Conclusion: Our results indicate that RADPOS combined with deformable lung phantom can be a useful tool for quality assurance in 4D treatment delivery.
Project supported by a HTX‐OCE‐IRAP grant.
TH‐D‐352‐05: Optically Stimulated Luminescence (OSL) Dosimeters Can Be Used for Remote Dosimetry Services35(2008); http://dx.doi.org/10.1118/1.2962948View Description Hide Description
Purpose: To evaluate aluminum‐oxide optically stimulated luminescence(OSL)dosimeters as a potential alternative to thermoluminescent dosimeters(TLDs) for remote dosimetry services provided by the Radiological Physics Center (RPC) at the University of Texas M. D. Anderson Cancer Center. Method and Materials:OSLdosimeters were placed equidistant (< 1 cm) from the center of a 20 cm × 20 cm Solid Water™ (SW) phantom which provided backscatter and build‐up. OSLdosimeters were also irradiated in an acrylic mini‐phantom based on the RPC's mailable TLD system mini‐phantom. For modality‐dependence measurements, dosimeters were irradiated to doses of either 100 or 300 cGy with 6 or 15 MV photons or 8 or 15 MeV electrons. All other irradiations were performed with a Co‐60 unit. A Landauer microStar™ reader was used to measure the dosimeter responses. Results: The calculated percent standard deviation of the reproducibility readings was less than 1.4% for doses of 50 cGy and 300 cGy, and less than 0.9% for a dose of 1000 cGy. The measured dose response was linear at doses less than 600 cGy, and independent of modality. Field‐size output factors measured with OSLdosimeters agreed with those measured with an ion chamber within 1.5%. Heat, cold and humidity had no effect on the dosimeters, but exposure to light significantly decreased their response. Measurements of fading demonstrated that a 4% loss of signal occurs over the first ten days after irradiation, after which the response changes less than 1% up to 90 days. The dosimeters lost 0.2% of signal with each successive reading. Conclusion: The precision of OSLdosimeters is comparable to that provided by TLDs used for remote dosimetry and warrants further investigation. Conflict of Interest: This work was supported in part by Landauer Corporation and by PHS grant CA10953 from the NCI, DHHS.
TH‐D‐352‐06: An Overview of Comprehensive Proton Machine Quality Assurance at the University of Texas M.D. Anderson Cancer Center35(2008); http://dx.doi.org/10.1118/1.2962949View Description Hide Description
Purpose: Since the publication of ICRU‐59, there has not been any dedicated report constitute to proton therapy machine quality assurance (QA). We present a comprehensive machine QA program that is implemented at the proton therapy center in Houston (PTC‐H). Method and Materials: PTC‐H uses a Hitachi proton therapy machine for treatment of cancer patients. The machine consists of a synchrotron accelerator that delivers proton beams in range of 70–250 MeV to three gantries and one fixed beam rooms. The majority of the QA procedures are based on the AAPM TG‐40. Specific tests include mechanical, beam quality, dose delivery system, imaging system tests, safety interlock checks and information flow to the Electronic Medical Record. Frequency of these tests depends on many factors such as safety requirements, availability of the treatment rooms, state regulations, consistency, and reproducibility. Results: Analysis of our results for many tests for a period of one year will be presented. The dose outputs remained stable within 1%, Spread Out Bragg Peaks widths and proton ranges were consistently within 1‐mm, the flatness/symmetry remained stable within 2%, gantry/treatment couch isocentricity were found to be within 1°, and mechanical limits were within 1‐mm. Periodic issues with information flow have been discovered. Conclusion: We have developed a comprehensive machine QA program at PTC‐H. The QA program was gradually and cautiously developed at the beginning with numerous frequencies. It is hoped that by sharing our experiences in developing such a QA program, we will provide an insight to upcoming new facilities for proton therapy to establish their program.
35(2008); http://dx.doi.org/10.1118/1.2962950View Description Hide Description
Purpose: A helical tomotherapy accelerator has been commissioned last year in Switzerland at Lausanne University Hospital (CHUV). Despite the fact that more than 150 such instruments have already been sold around the world, this technique presents a dosimetric challenge and there is no internationally accepted protocol for the reference dose yet. The goal of the present study is to investigate different alternatives to have an independent method to determine the dose reference of the accelerator. Method and Materials: Several dosimetric techniques with various metrological traceabilities were tested in a number of phantoms in static and helical modes. The first measurements were performed with the A1SL ionization chamber, which is delivered by the vendor as a reference instrument: it is traceable to the American national metrology institute (NIST) in absorbed dose to water in a Co‐60 beam quality through a graphitecalorimeter. In Switzerland, each radiotherapy department is directly traceable to the national standard (METAS) in absorbed dose to water through a watercalorimeter. A NE 2611A ionization chamber calibrated by METAS was therefore used to determine the reference dose as well. In order to have another fully independent way of measurement, the reference dose was also determined by mean of alanine dosimeters provided by the British national laboratory (NPL) and calibrated in absorbed dose to water through a graphitecalorimeter. Finally, in order to take into account one of the chamber that is widely used in the clinical practice, the reference dose was also measured using a Farmer‐type instrument (NE 2571). Conclusion: Compared to our standard (NE 2611A), the A1SL, alanine and Farmer chamber (NE 2571) showed differences of 1.2 %, −0.4 % and −1.7 % respectively. These values are within the measurements uncertainty of the different methods and can be partially explained by the design of the chambers.
TH‐D‐352‐08: Radiation Treatment Machine Mechanical Quality Assurance Check with Megavoltage Portal Imaging Device35(2008); http://dx.doi.org/10.1118/1.2962951View Description Hide Description
Purpose:Treatment machine mechanical check is an important procedure in quality assurance (QA). Linac machines are commonly equipped with electronic portal imaging device(EPID).EPID can be used to perform mechanical QA more accurately and easily. We developed a method to use EPID to measure gantry, collimator and table isocenter runout. Method and Materials: Measurements were performed on an Elekta's ASSEX machine that equips with an a‐Si EPID and kilo‐voltage cone beam CT. A ball‐bearing phantom was position in the radiation isocenter following KV‐MV isocenter calibration procedure. Portal images were taken at various gantry, collimator and table angles. Matlab algorithms were developed to analyze the gantry, collimator and table isocenter positions. Results: Gantry, collimator and table isocenter positions were measured quantitatively with accuracy better than 0.1mm. Results clearly show the discrepancies of the three isocenters. Conclusion:EPID is a useful tool in LINAC mechanic QA. EPID isocenter measurements are more accurate and convenient than traditional approaches.
TH‐D‐352‐09: A High Precision, High Throughput Fixture for Routine Spatial Characterization of the Xoft Axxent™ Miniature X‐Ray Source35(2008); http://dx.doi.org/10.1118/1.2962952View Description Hide Description
Purpose: Xoft has developed a sophisticated fixture for testing X‐ray sources intended for use in the Axxent™ electronic brachytherapy system. This manufacturing test fixture (MTF) simultaneously measures azimuthal and polar angular distributions as well as depth‐dose. Total measurement time for each source is approximately ten minutes in the standard configuration. The measurementhardware and accompanying data acquisition and computer software form a complete system that has passed a formal Validation consisting of Installation, Operation and Performance Qualifications. Method and Materials:Measurementhardware consists of a s ingle PTW ionization chamber and nine solid state detectors which are cross‐calibrated to the ionization chamber in a regular maintenance procedure. Components underwent precision spatial measurements at the time of assembly, and custom gauges are used periodically to determine positions to 200 μm precision. The system was designed to be very accurate in positioning measurement devices with respect to the source. Furthermore the measurement techniques minimize the effect of residual misalignment through reliance on relative readings where possible and through correction coefficients determined via consistency checks. Results: Repeatability, Reproducibility and operator variability of measurements was determined to be 0.1%, 0.4% and 0.5%(1 sigma) respectively in Performance Qualification. Absolute accuracy is estimated through a detailed error budget analysis to be 1.5% for azimuthal and depth‐dose measurements and 4.3% for polar measurements.Conclusion: The MTF provides accurate measurements of critical source parameters with high throughput and negligible operator‐based variation. Although it has been designed for the Xoft x‐ray source, with the use of adapters it could be used to characterize radioactive seeds as well.