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PURPOSE: The purpose of this study was to investigate influence of different scanning speeds on measurements of photon beam flatness and symmetry. METHODS: Commissioning and quality assurance of linear accelerators require extensive beam measurements. To increase efficacy, we evaluated flatness, symmetry and penumbra of 6MV photon beam using the Varian-TrueBeamTM system. Scanning speeds were 0.3, 0.5, 0.75, 1, 1.5, and 2.5cm/s. Measurements were performed in water phantom (BluePhantom2 , IBA-Dosimetry) at depths of maximum dose, 5,10, and 20cm, for 10×10 cm field size. For each scanning speed and depth, measurements were repeated five times to give results sufficient statistical significance, in both crossline and inline directions. Beam flatness was calculated using variation over mean (80%), whereas symmetry was calculated using point difference quotient (IEC) algorithm. After filed scanning chamber (Wellhofer) was fully stopped, system was paused for stabilization time of 15s to avoid buildup of ripples. RESULTS: It was noticed for all measurements that minimum and maximum flatness and symmetry were recorded when scanning speeds were 0.3cm and 2.5cm, respectively. For depth of maximum dose, maximum flatness and symmetry were 0.82% and 100.58% (crossplane), and 0.94% and 100.96% (inplane). The average was 0.76% and 100.38% (SD 0.04 and 0.12) for crossplane; 0.89% and 100.87% (SD 0.04 and 0.06) for inplane measurements. As the scanning depth increased, flatness and symmetry increased, but SD for all measurements was within the same range (0.04-0.07 and 0.04-0.12). The maximum absolute difference for flatness and symmetry for maximum and minimum speed were 0.16% and 0.34%.However, for scanning speeds from 0.5-1cm/s, results were almost identical with maximum SD 0.03 for both flatness and symmetry. Use of different scanning speeds did not influence penumbra; SD was 0 for all measurements. CONCLUSIONS: This study reveals small influence of scanning speed within predefined range. Consequently, difference in measurements does not have clinical significance.
RESUMO
PURPOSE: It is well established that using image guidance for prostate motion allows reduction of margin, dose escalation, decreased toxicity and recently improved outcomes. However, current methods only account for translational motion, not rotational variations. The purpose of this study is to assess whether rotations in anatomy lead to significant changes in the delivered dose for prostate patients. METHODS: Under an IRB approved protocol, 11 consecutive patients underwent prostate IMRT using IGRT with implanted metal-oxide semiconductor field-effect transistors (MOSFETs); the Dose Verification System (DVS) manufactured by Sicel Technologies. Two dosimeters were implanted per patient. From conebeam CT (CBCT) registration, corrections were applied to all translational errors. For rotations larger than 3 degrees, patient were repositioned and realigned to attempt to correct the rotation. Both translational and rotational errors based on the CBCT were documented. The daily DVS readings were compared to CBCT rotations about each axis (pitch, roll and yaw) and the root-mean square (RMS) rotation. RESULTS: 372 CBCT images were acquired. The correlation between rotation and DVS measurement was analyzed using linear regression. The R2 value for pitch was 0.059 and 0.144 for each dosimeter, respectively. For roll, the R2 values were 0.049 and 0.001. For yaw, the values were <0.001. For the RMS rotation, R2 was 0.034 and 0.038. As it could confound results, the angular dependence of the dosimeters was measured during commissioning and found that it was approximately 0.5% for 5 degree rotations. CONCLUSIONS: We did not find any significant correlation between prostate rotation around any axis and discrepancy in DVS reading. These results show that rotations seen clinically do not have a substantial effect on the dose delivered to the prostate. Further studies will attempt to determine at what angle rotations begin to affect the dose distribution, if at all.
RESUMO
PURPOSE: The purpose of this study was to present a novel method for real- time tumor tracking using a commercially available robotic treatment couch, and to evaluate tumor tracking accuracy. METHODS: Commercially available robotic couches are capable of positioning patients with high level of accuracy; however, currently there is no provision for compensating tumor motion using these systems. Elekta's existing commercial couch (PreciseTM Table) was used without changing its design. To establish the real-time couch motion for tracking, a novel control system was developed and implemented. The tabletop could be moved in horizontal plane (laterally and longitudinally) using two Maxon-24V motors with gearbox combination. Vertical motion was obtained using robust 70V-Rockwell Automation motor. For vertical motor position sensing, we used Model 755A-Accu- Coder encoder. Two Baumer-ITD_01_4mm shaft encoders were used for the lateral and longitudinal motions of the couch. Motors were connected to the Advance Motion Controls (AMC) amplifiers: for the vertical motion, motor AMC-20A20-INV amplifier was used, and two AMC-Z6A8 amplifiers were applied for the lateral and longitudinal couch motions. The Galil DMC-4133 controller was connected to standard PC computer using USB port. The system had two independent power supplies: Galil PSR-12- 24-12A, 24vdc power supply with diodes for controller and 24vdc motors and amplifiers, and Galil-PS300W72 72vdc power supply for vertical motion. Control algorithms were developed for position and velocity adjustment. RESULTS: The system was tested for real-time tracking in the range of 50mm in all 3 directions (superior-inferior, lateral, anterior- posterior). Accuracies were 0.15, 0.20, and 0.18mm, respectively. Repeatability of the desired motion was within ± 0.2mm. CONCLUSIONS: Experimental results of couch tracking show feasibility of real-time tumor tracking with high level of accuracy (within sub-millimeter range). This tracking technique potentially offers a simple and effective method to minimize healthy tissues irradiation.Acknowledgement: Study supported by Elekta,Ltd. Study supported by Elekta, Ltd.