Faculty Bibliography

Purpose: Gated delivery involves repeated beam holds, during which beam properties require pulses to stabilize. As a result, TG-142 recommends measuring output and energy of gated delivery systems for monthly and annual quality assurance, respectively. Detector arrays have proven effective at calculating energy changes from changes in beam profiles. This work studies the robustness of energy calculations derived from beam profiles to beam changes during ramp up for gated beam delivery.
Method(s): The Sun Nuclear IC Profiler (ICP) and Quad Wedges were used to measure output and energy of gated 6X, 6X-FFF, and 10X-FFF photon beams. ICP was compared to standard 2 ion chamber PDD20/PDD10 measurement taken at 100 cm SSD using Standard Imaging Exradin A12 and A16 chambers with PC Electrometer. A series of beam pulses consisting of 0.5, 1, 1.5, 2, 2.5, 3, 5, 10, and 50 MU bursts were delivered in 5 successive repetitions using Varian Developer mode on a TrueBeam to simulate gated delivery. Measurements were taken at 125 and 100 ms for ICP and ion chambers, respectively. PDD20/PDD10 measurements were converted to PDD10 for comparison to the ICP PDD10 results. All analysis was performed in Matlab.
Result(s): Average ICP flatness and symmetry was within 1% for all energies and within 0.5% beyond 2 MU, indicating beam stability for all burst durations. The maximum observed PDD10 energy deviations from baseline were 0.09+/- 0.05% and 0.6+/- 1.9% for the ICP and ion chamber pair, respectively. Mean ICP deviations for MU bursts <3 MU and all ICP standard deviations were an order of magnitude smaller than ion chamber.
Conclusion(s): Both techniques measured energies within the 2% TG-142 recommendation for all MU bursts. Significant beam asymmetry during ramp up was not observed and ICP provided stable energy and output measurement for very low MU delivery, validating its use for gating QA

Purpose: Radiation induced interstitial pneumonitis and late renal dysfunction are major concerns for patients undergoing total body irradiation (TBI). The purpose of this work is to evaluate the dosimetry of VMAT-based TBI plans generated using Varian Eclipse scripting.
Method(s): Three full-body CT datasets (two patients, one anthropomorphic CIRS phantom) were used. An in-house Eclipse script was developed to generate optimized field arrangements using the body contour, user origin, and couch longitudinal travel. Plans consisted of a lower-body AP/PA portion and an upper-body VMAT portion (8 full arcs with 4-isocenters). Treatment plans to 1320 cGy (165 cGy x 8fx) were generated with dose directives: [PTV V100% >=90- 95%; Total lung Dmean <900 cGy; Kidneys Dmean <1100 cGy]. All plans used 6MV photons and were calculated using the AAA algorithm. Upperbody VMAT plan dosimetry was evaluated 'in-phantom' placing 12 OSLDs in different key locations (lung, kidneys, bone, and soft tissue). Additionally, dosimetric verification was performed for the three plans using Varian portal dosimetry, PerFraction(SNC) and ArcCheck(SNC) with a global gamma criterion of 2%/2 mm.
Result(s): Planning objectives were met for the three treatment plans with the following averages: PTV V100% = 94.02%, total lung Dmean = 872.9 cGy, and kidneys Dmean = 1075.8 cGy. The dose deviation between Eclipse and the OSLDs (relative to the prescribed dose) averaged 0.98%, with each individual dose deviation within +/-4%. Dose ranged between 52.5 cGy (lung) and 187.5 cGy (bone) for OSLD measurements. The average passing rate for all 24 fields (8 per plan) was 98.0%, 99.76% and 98.6% for portal dosimetry, PerFraction and ArcCheck respectively. The lowest passing rate of any individual field was 95.4%, 99.0% and 91.8% for portal dosimetry, PerFraction and ArcCheck respectively.
Conclusion(s): Eclipse scripting can assist in creating robust multi-isocentric VMATbased TBI treatment plans to block lungs and kidneys without compromising target coverage. Dosimetric accuracy and deliverability was confirmed using in-phantom OSLD dosimetry, Varian portal dosimetry, PerFraction and Arc-Check verification

Purpose: The purpose of this study is to quantify interfractional dosimetric variations in radiotherapy of oropharyngeal cancer and investigate the application of statistical process control (SPC) to determine significantly deviated fractions for management.
Method(s): Thirteen oropharyngeal cancer patients treated by IMRT or VMAT with daily CBCT were retrospectively reviewed. CBCT images of every other fraction were imported to the software Velocity and registered to planning CT using the 6DOF couch shifts generated during patient setup. Using Velocity Adaptive Monitoring module, the setup-corrected CBCT was matched to planning CT using a deformable registration. The module also generated dose volume histograms (DVHs) at each CBCT from planning doses for the deformed plan structure sets. Volumes and dose metrics at each fraction were calculated and rated with plan values to evaluate interfractional dosimetric variations using a SPC framework. T-tests between plan and fraction volumes were performed to find statistically insignificant fractions. Average, upper and lower process capacity limits (UCL, LCL) of each dose metric were derived from these fractions using conventional SPC guidelines.
Result(s): GTV and OAR volumes in first 13 fractions had no significantly changes from the plan, subsequently reduced by 10% to treatment completion, except oral cavity. There were 3%-4% increases in parotid mean doses, but no significant differences in dose metrics of GTVand other OARs. The changes were organ and patient dependent. Control charts for various dose metrics were generated to assess the metrics for individual patient. The occurrences of one or several dose metrics out of the control limits warrant immediate investigation of the fraction.
Conclusion(s): Daily CBCT could be used to monitor dosimetric variations of targets and OARs resulting from volume changes and tissue deformation in oropharyngeal cancer radiotherapy. Treatment review with guidance of a SPC tool may enable objectively and consistently identify significantly deviated fractions

Purpose: This study was to investigate the setup accuracy for automatic rotational corrections using 6 degree of freedom (6DoF) couch and online cone beam computerized tomography (CBCT).
Method(s): A commercial phantom (BAT phantom) with tissue and air equivalent materials was scanned with CT Sim at 1.25 mm plane spacing and 0.98 mm pixel size. A 3D plan was generated in Eclipse for the spherical target of 3 cm diameter located at the center of the phantom. Several lowdensity structures were also outlined as organ at risks (OARs) surrounding the target. The phantom was aligned to the treatment position on the 6DoF couch of a Varian TrueBeam with CBCTverification for both target and OARs outlined before varied rotational errors were introduced in pitch and roll using a titled supporting platform and verified by a calibrated digital level. Following CBCT imaging, the setup corrections were calculated online using the 3D-to-3D auto-matching function on Truebeam and compared against the actual rotational error. We analyzed the deviations at different levels of error.
Result(s): The auto-matching on TrueBeam appeared of good quality even for low contrast and small structures. Deviations of the rotational corrections calculated with 6DoF from the measured are summarized in Tables I-III. The max difference in rotation is 0.7degree for both pitch and roll. However, it also has been observed that there were deviations in every degree of both translational and rotational freedom, which may lead to over-correction in clinic
Conclusion(s): 6 DoF couch and auto-matching offers the convenience for automatic corrections of patient setup in conjunction with CBCT imaging. This preliminary study using a phantom revealed the uncertainties of rotational correction by 6DoF without a deformation of the image. Further study is warranted to establish the criteria for the clinical applications of the auto correction with 6DoF and for the necessary physics QA

Purpose: For the Cyberknife (Accuray Inc. Sunnyvale CA) stereotactic radiosurgery and stereotactic body radiotherapy treatment unit, a series of collimating cones are used with diameters ranging from 5 mm to 60 mm. The measurement of output factors (OFs) in these small radiation fields is challenging due to limitations in detector size. Currently multiple detectors are used for the measurement of output factors, but if not water equivalent, Monte Carlo corrections are required. The Exradin W2 (Standard Imaging Inc., Middleton, WI) plastic scintillation device (PSD) has an active volume of 0.0008 cm³ and is water equivalent. This work reports OFs and scanning data measured by the W2 PSD and a diode in Cyberknife.
Method(s): A Cyberknife M6 and a W2 PSD in a MP3 water phantom (PTW, Freiberg, Germany) were used to take TPR, profile, and OF measurements. The W2 comes with a specialized electrometer, MAX SD, used to measure the Cerenkov light ratio using the manufacturer's recommendation. Profiles, tissue phantom ratios (TPR), and OF values were taken using the clinically relevant (5, 7.5 and 10) mm cones, with OFs normalized to the 60 mm cone and scanning measurements normalized to maximum dose. All results were compared with PTW 60018 diode measurements taken during commissioning.
Result(s): The W2 scintillator required the slowest scan speed (0.1 mm/s) in order to eliminate noise, and TPR data was smoothed using a moving average filter. Comparison between diode and W2 profiles and TPR data showed good agreement. Monte Carlo adjusted diode OFs and W2 measured OFs were within 2.6%.
Conclusion(s): This work demonstrated the feasibility of using a water equivalent, small volume commercial W2 PSD in Cyberknife. Scanning data was near equivalent, but showed significant noise and required smoothing. W2 measured OFs agreed with MC-corrected diode measurements taken during commissioning