Purpose: To evaluate a novel reference chamber (Stealth Chamber by IBA) through experimental data and Monte Carlo simulations for 6 and 15 MV photon energies. Methods: Monte Carlo simulations in a water phantom for field sizes ranging from 3×3 and 25×25 cm 2 were performed for both energies with and without the Monte Carlo model of the Stealth Chamber in the beam path, and compared to commissioning beam data. Percent depth doses (PDDs), profiles, and gamma analysis of the simulations were performed along with an energy spectrum analysis of the phase-space files generated during the simulation. Experimental data were acquired in water with IBA three-dimensional (3D) blue phantom in a set-up identical to the one used in the Monte Carlo simulations. PDD comparisons for fields ranging from 1×1 to 25×25 cm 2 were performed for photon energies. Profile comparison for fields ranging from 1×1 to 25×25 cm 2 were executed for the depths of dmax, 5, 10 and 20 cm. Criteria of 1%, 1 mm to compare PDDs and profiles were used. Transmission measurements with the Stealth Chamber and a Matrixx detector from IBA were investigated. Measurements for 6 and 15 MV with fields ranging from 3×3 to 25×25 cm 2 dimensions were acquired in an open field with and without the Stealth Chamber in the path of the beam. Profiles and gamma analysis with a 1%, 1 mm gamma analysis criterion were performed. Results: Monte Carlo simulations of the PDDs and profiles demonstrate the agreement between both simulations. Furthermore, the gamma analysis (1%, 1 mm) result of the comparison of both planes has 100% of the points passing the criteria. The spectral distribution analysis of the phase spaces for an open field with and without the chamber reveals the agreement between both simulations. Experimental measurements of PDDs and profiles have been conducted and reveal the comparability of relative dosimetric data acquired with the Stealth Chamber and our gold standard the CC13 chamber. Transmission data measured with an ion chamber array (Matrixx) showed the small attenuation caused by the use of the Stealth Chamber. Conclusion: Simulations and experimental results from this investigation indicate the benefits associated with chamber positioning and time expended during the acquisition of the relative measurements of PDDs and profiles for the beam commissioning of photon beams when the Stealth Chamber is used as a reference chamber to perform these tasks. The results demonstrate that relative profiles and PDDs scanned with the Stealth Chamber in place are consistent with those made using a CC13 chamber within a 1% and 1 mm criterion.
Purpose: To evaluate the use of MobiusFX as a pre‐treatment verification IMRT QA tool and compare it with a commercial 4D detector array for VMAT plan QA. Methods: 15 VMAT plan QA of different treatment sites were delivered and measured by traditional means with the 4D detector array ArcCheck (Sun Nuclear corporation) and at the same time measurement in linac treatment logs (Varian Dynalogs files) were analyzed from the same delivery with MobiusFX software (Mobius Medical Systems). VMAT plan QAs created in Eclipse treatment planning system (Varian) in a TrueBeam linac machine (Varian) were delivered and analyzed with the gamma analysis routine from SNPA software (Sun Nuclear corporation). Results: Comparable results in terms of the gamma analysis with 99.06% average gamma passing with 3%,3mm passing rate is observed in the comparison among MobiusFX, ArcCheck measurements, and the Treatment Planning System dose calculated. When going to a stricter criterion (1%,1mm) larger discrepancies are observed in different regions of the measurements with an average gamma of 66.24% between MobiusFX and ArcCheck. Conclusion: This work indicates the potential for using MobiusFX as a routine pre‐treatment patient specific IMRT method for quality assurance purposes and its advantages as a phantom‐less method which reduce the time for IMRT QA measurement. MobiusFX is capable of produce similar results of those by traditional methods used for patient specific pre‐treatment verification VMAT QA. Even the gamma results comparing to the TPS are similar the analysis of both methods show that the errors being identified by each method are found in different regions. Traditional methods like ArcCheck are sensitive to setup errors and dose difference errors coming from the linac output. On the other hand linac log files analysis record different errors in the VMAT QA associated with the MLCs and gantry motion that by traditional methods cannot be detected.
Purpose: To evaluate the performance of the stealth chamber by IBA under large field conditions. Methods: Measurement of PDDs and profiles for fields of 10×10, 15×15, 20×20 and 25×25 cm2 were acquired in a blue phantom2 scanning system for 6 and 15 MV beams in a Varian TrueBeam linear accelerator. The stealth chamber has a field size limit of 25×25 cm2. Measurements with two different detector set were performed. First a cc13 chamber as field and reference chamber was acquired. And with a cc13 chamber as field detector and the stealth chamber as the reference chamber. PDDs for the fields mentioned above were conducted and profiles with the same fields at depth of dmax, 5, 10 and 20 cm were acquired for comparison. Results: Examination of the PDDs and profile comparison with a criterion of 1% and 1 mm agreement was performed. Very good agreement between the cc13 and stealth chamber set was observed within the chosen criterion. Conclusion: This work indicates the comparable results obtained with the use of the stealth chamber and cc13 as the reference chamber in relative dosimetry measurements.
Purpose: A narrow‐field, low energy, stereotactic radiosurgery (SRS) device (the “IRay”) designed specifically for the treatment of wet age‐related macular degeneration is being evaluated in a European clinical trial. Because of precise targeting requirements for ophthalmological SRS, a new QA method has been designed to automate the evaluation of system alignment, targeting, and kerma consistency. This poster describes that method. Method and Materials: The IRay delivers up to 24 Gy in three lOOkVp, 4mm‐diameter beams that converge on the retina with a source‐axis distance of 150mm. The beams are delivered inferiorly in a fashion that avoids the patients lens and optic nerve, requiring submillimeter targeting precision. The eye is stabilized with a suction‐enabled lens assembly (the “I‐Guide”) that has retroreflective fiducials used by the imaging system for targeting and eye motion assessment. A self‐test fixture has been designed that incorporates similar fiducials for automatic alignment, a scintillant strip for direct imaging of the X‐ray beam, and an ion chamber for kerma measurements. Results and Discussion Using the retroreflective fiducials, the IRay is able to repeatably and accurately target the fixture. This is critical not only for targeting assessment, but also for accurate kerma measurements which are difficult in a narrow field with a finite‐sized detector. Once targeted, software automatically evaluates and enforces several critical parameters including X‐ray to targeting laser coaxiality (< 100 microns), robotic motion, imaging system alignment (better than 100 microns), X‐ray targeting (within 300 microns), and air kerma consistency (within 3%). Other system tests are performed, including lighting, communication, and dose‐timer checks. Conclusion: Performing comprehensive QA on a narrow‐field, low‐energy SRS unit requiring submillimeter precision is non‐trivial. An automated method of evaluating alignment, targeting, and kerma consistency allows regular (even daily) assessment to provide confidence that all systems are performing within specification. Research sponsored by Oraya Therapeutics.
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