Sun Nuclear Corporation Research Articles

TH‐C‐BRB‐04: Dose‐Painting VMAT and Patient Dose QA: Showcase of Whole Brain Radiotherapy with Hippocampal‐Sparing and Simultaneous Boost to Brain Metastases

Purpose: Radiotherapy techniques for sparing the hippocampus during cranial irradiation are emerging. This work is to study dose‐painting with volumetric modulated arc therapy (VMAT) in the setting of oligo brain metastases and the clinically relevant dose QA. Methods: A patient having 3 brain metastases was re‐planned using 2‐arc 6 MV photons with in‐house VMAT treatment planning system (TPS). The clinical goal is to dose paint the whole brain and brain metastases to 32 Gy and 63 Gy in 15 fractions, respectively while keeping the mean dose to the hippocampus less than 6 Gy. The VMAT dose QA was carried out using ArcCHECK (AC) and 3DVH (Sun Nuclear Corp) on a Varian Trilogy LINAC. The AC Planned‐Dose‐Perturbation (ACPDP) algorithm was employed to estimate the actual patient dose distribution, at the same grid resolution as that from the TPS calculation. The patient's DICOM RT dose/plan/structures/CT images were loaded into the software. The measured doses and TPS calculated doses for relevant structures were compared. Their differences represented systematic errors induced from the combination of TPS dose calculation algorithm and beam‐delivery. Results: Clinical planning criteria were all satisfied. A 99.5% 3D matching rate for 2%/2mm DD/DTA was observed between the ACPDP reconstruction and TPS‐calculated dose. Despite the excellent agreement, the relative dose differences for patient's critical structures between measurements and TPS are: 0.8%, mean brain lesions; 0.7%, mean whole brain; 9% (<3% of prescription), mean L_hippocampus; 9% (<3% of prescription), mean R hippocampus; 0.1%, max cord; 1.8%, max chiasm; 3.7%, max brainstem; 0.5%, max L_OpticN; 1.9%, max R OpticN, 0.2%. Conclusions: For the application of dose painting VMAT planning for patients with oligo brain metastases, the patient relevant dose QA result has demonstrated the capability and the accuracy of our VMAT technique, allowing for 3‐level dose‐painting to spare critical structures with acceptable target coverage and homogeneity.

SU‐E‐T‐348: Measurement‐Guided 4D VMAT Dose Reconstruction on an Arbitrary Homogeneous Dataset

Purpose: To develop and validate a VMAT QA tool that takes as input a 2D, low‐density (∼10 mm) empirical dose map from a commercial helical diode array, and outputs a high density, volumetric, time‐resolved (4D) dose matrix on an arbitrary patient dataset. At first, the method validation is limited to a homogeneous ‘patient’. Methods: A VMAT treatment is delivered to a diode array (ArcCHECK, Sun Nuclear Corp., Melbourne, FL). 3DVH software (Sun Nuclear) derives the high‐density volumetric dose using measurement‐guided dose reconstruction (MGDR). MGDR cylindrical phantom results are then used to perturb the 3D TPS dose on the patient dataset, producing a semi‐empirical volumetric dose grid. Four‐dimensional dose reconstruction on the patient is also possible by morphing the sub‐beam instead of composite dose. For validation, TG‐119 structures and objectives were used. 3DVH and TPS cumulative point doses were compared to ion chamber in a cube water‐equivalent phantom (‘patient’). The shape of the phantom is different from the ArcCHECK and the targets were placed asymmetrically. Ion chamber dose sampled at 10Hz was compared to time‐resolved 3DVH point doses. Coronal and sagittal absolute film dose distributions in the cube were compared with 3DVH and TPS. Results: Across four TG‐119 plans, the average PTV point dose difference in the cube between 3DVH and ion chamber is 0.0±0.9%. Average film vs. 3DVH gamma analysis passing rates are 88.6, 96.1, and 99.5% for 1%/2mm, 2%/2mm, and 3%/3mm criteria, respectively. 4D MGDR was also sufficiently accurate. Conclusions: Even for a well‐commissioned TPS, comparison metrics show on average better agreement between MGDR and measurement compared to MGDR and TPS on the arbitrary‐shaped phantom (‘patient’). The method requires no more accelerator time than standard QA, while producing more clinically relevant information. Validation in a heterogeneous thoracic phantom is under way, as is ultimate application to virtual motion studies. Supported in part by a grant from Sun Nuclear Corp.

SU‐D‐BRCD‐02: A Novel Cylindrical 3D Water Scanner for Beam Data Collection: I. Validation of the Automatic‐Setup

To determine the accuracy and reproducibility of the automatic- setup procedure of a 3D water scanner (3D SCANNER, Sun Nuclear Corp). The procedure automatically aligns the scanner's coordinate system to the Linac coordinate system. A unique off-center detector holder extends the scan range to 630 mm, eliminating the need for scanner shifts for large-field scans. The automatic-setup procedure provides means to automatically (1) level the tank; (2) zero the detector position to water surface; (3) align the scanner's coordinate system to the Linac system; and (4) correct motor hysteresis. After automatic-setup, (1) and (2) were evaluated visually by three physicists. Step (3) was tested by moving one of the jaws to the isocenter and scans were made along the jaw edge. Angular misalignment between the scanner and the Linac would Result in a noticeably tilted profile. Step (4) was tested by scanning with the chamber moving forward and then backward five times. The accuracy of the off-center detector holder was tested by comparing scans made using the regularand the off-center detector holders. All tests were repeated three times. All tests of the water surface and horizontal chamber movementwere acceptable from the nine independent evaluations. Scans along the jawedge resulted in < 1% in profile tilt, corresponding to < 0.05 degree in angular misalignment. Same direction scans resulted in field edge differences < 0.05 mm, and < 0.2 mm when compared to scans in thereverse direction. There was no difference in accuracy when different scan speeds were used. Profiles obtained using the regular and the off-centerdetector holders were identical. The automatic-setupprocedure resulted in accurate alignment of the 3D SCANNER to the Linac. These results were user-independent. It also provides the ability to scan large fields without the need to change tank setup. This work was partially supported by a research grant from Sun Nuclear Corp., Melbourne, FL.

SU-E-T-72: Microdosimetric Signal-To-Noise Comparison of Edge Detector and Small Ion Chamber

Purpose: Small Diodes are widely used for beam data measurements, specially for stereotactic dosimetry, small field dosimetry and electron dosimetry. Hence it is important that data measured with such diodes be as accurate as possible so that data can be accurately modeled in the treatment planning systems. This study attempts to compare the signal-to-noise ratio of SNC edge detector with Scanditronix CC01, CC04 and CC13 compact ionization chambers. Methods: The signal-to-noise ratio for each of the detectors were measured on a Varian 2100C machine with 5×5 field size and80 dose rate. This was then compared to the mathematical models of the CC01, CC04, CC13 ion chambers and the Edge diode detector. Monte Carlo code PENELOPE (v 2008) was used for simulating the microdosimetric spectra in each detector for several energies. These spectra were used for estimating the detector inaccuracy of measurement at 0.05 cGy dose delivered to the detector. Results: Monte Carlo simulation and analysis confirmed that a significant portion of the measurement noise is coming from the interaction between ionizing radiation and the detector's sensitive volume as opposed to the electrometer or cable connections. This microdosimetric noise is inherent to the detector design and cannot be minimized by other than statistical methods. The magnitude of the silicon diode microdosimetric noise is estimated to be 1.08% +/− 0.05% at 0.05 cGy and for the 0.125 cm3 ion chamber is 0.28% +/− 0.05% at 0.05 cGy. Conclusions: Monte Carlo simulation and analysis show that the microdosimetric noise model is consistent with the difference in noise signal magnitude between silicon diodes and ion chambers under a radiation beam. Ion chambers also have a slightly better signal-to-noise-ratio as compared with the edge detector. Study was partially funded by Sun Nuclear Corporation, Melbourne, FL.

SU-D-213CD-04: Dosimetric Evaluation of HexaPOD as a Tool for Patient Setup Verification Accuracy.

To quantify the dosimetric accuracy of 6 degrees vs. 3 degrees of motion for patient positioning for cone-beam CT and HexaPOD Evo RT system (Elekta, Inc.) using the ArcCHECK (Sun Nuclear Corporation, software v.6.0) QA phantom. The average absolute values of our daily reported patient hexapod shift data were recorded and averaged for 18 patients. Using the CT scans of ArcCHECK, a QA plan was created in the Pinnacle (v.9.0) treatment planning system. By setting up ArcCHECK according to the QA plan and measuring the dose on Elekta Synergy, the baseline data was established. Next, an artificial shift was introduced in our phantom setup. A cone-beam CT scan of the phantom was obtained and images adjusted. The resulting translational and rotational shifts were recorded. The table was then moved according to the translational shifts only, and another measurement was taken. Next, using hexapod translational and rotational shifts, the table was moved in 6 degrees of motion, and the dose was re-measured. For 18 IMRT patients, the average translational shifts were: x= 0.53 cm, y=0.38 cm and z= 0.516 cm, and average rotational shifts were pitch=0.979°, roll=0.8° and yaw= 0.67°. The ArcCHECK phantom measured results were quantified using 3%, 3mm distance to agreement gamma analysis criteria when comparing measured to planned dose. Our measurements showed that an average 97.3% of the points agreed in plans with no shift. After the artificial shift was introduced, and cone-beam CT translational shifts were applied, 76.3% of points agreed. When both rotational and translational shifts were applied using Hexapod, an average of 93.4% of points agreed. Using hexapod for imaging and patient positioning results in a significantly more accurate dose delivery as compared to using only CBCT, as shown by our ArcCHECK QA phantom results.

MO-A-213AB-08: 2D Water Equivalent Path Length Imaging Technique for Pre-Treatment Range Verification in Proton Therapy.

To assess the potentials of a novel detector for providing a transmission image of the patient in terms of the Water Equivalent Path Length (WEPL) values and to evaluate the detector potential for real-time imaging of moving targets. The method is based on the principle that for passively scattered proton beams the WEPL of any point located in the dose plateau of a spread-out Bragg-Peak can be derived from the time dependence of the dose rate function measured at this point. A flat-panel 2D detector array (Sun Nuclear Corporation, 249 diodes, pitch ∼6 mm, 2 ms time resolution) was placed distal to a range of phantoms with varying complexities. The dose rate received by all diodes were measured as functions of time and analyzed to obtain the WEPL values. To assess the potential real-time features of this imaging technique, a Lucite cube was imaged while moving with a sinusoidal pattern with the amplitude and period comparable to a typical mobile tumor. In water tanks, millimeter accuracy in the determination of the WEPL could be achieved and the geometrical shape of wedge and sphere phantoms could be reproduced. In more complex phantoms such as a Lucite step-like compensator or Medulloblastoma patient compensators, multiple Coulomb scattering and range mixing cause a slight deterioration in the reconstruction of the WEPL. We found that tracking of a moving target in the coronal plane is potentially feasible. The technique gives less than 1 cGy of dose to patients and is therefore ideal for 'range-tuning' prior to treatment. For clinical applications, the beam for 'tuning' will need a deeper range than the prescription and once the radiological path length to the dosimeter is determined and compared with that from planning, the proton range for the actual treatment can be adjusted.

SU-E-T-285: Flatness as a Measure of Changes in Photon Energy for Megavoltage X- Ray Radiotherapy.

To compare the sensitivity of flatness based energy metrics versus an attenuation based metric (percent depth dose) as a function of energy changes from the linac's nominal clinical energy. Energy changes were accomplished by adjusting the bending magnet current (BMI) ±15% away from the value used clinically. Two metrics for flatness, relative flatness in the central 80% of the field (flatness) and average maximum dose along the diagonals normalized by central axis (CAX) dose (diagonal normalized flatness, DNF), were measured using a commercially available ion chamber array (IC profiler, Sun Nuclear, Melbourne FL). Percent depth dose was measured in water for depths of 5 and 10 cm in 3×3 and 10×10 cm̂2 field sizes. The sensitivity of percent depth dose (PDD) and flatness to energy changes were smaller than changes in DNF. For 6X the changes in PDD were from -1+ to +3% and the changes in flatness were from -5% to +2% for energy changes of ±15% from the nominal energy, for 18X the PDD changed from -0.5% to +2.5% and the flatness from -5% to +2% over this energy range. DNF changed from +12% to -8% for 6X and from +8% to -6% for 18X over a ±15% change from the nominal energy and showed a near linear correlation with energy. In addition DNF was the only metric that was found to be sensitive to both increases and reductions of energy for both the 6 and 18 MV beams. Diagonal normalizedflatness was found to be the most sensitive metric to energy changes for photon beams of 6 and 18 MV. The IC profiler allows this metric to be conveniently measured as part of routine linac quality assurance. Sun Nuclear Corporation provided the IC profiler.

Effect of the natural radioactivity concentrations and 226Ra/238U disequilibrium on cancer diseases in Penang, Malaysia

Natural radioactivity measurements and assessment of radiological hazards in soil and sand samples obtained from Penang, Malaysia were carried out using the Exploranium GR-135 Plus “Identifier” Radioisotope Identification Device and high-resolution High Purity Germanium (HPGe) detector system. The activity concentrations of 238U, 226Ra, 232Th, and 40K were found to be 184±11, 396±22, 165±14, and 835±28Bqkg−1 respectively, and the external gamma dose rate is 315±44nGyh−1 for soil samples. For sand samples, the activity concentrations of 238U, 226Ra, 232Th and 40K were 31±8, 62±16, 36±6, and 369±17Bqkg−1, respectively, and the external gamma dose rate is 66±12nGyh−1. To assess the radiological hazard of radioactivity present in the samples, the radium equivalent activity, annual effective dose, annual gonadal dose equivalent, external hazard, and internal indices were calculated.The Raeq values of soil samples were higher than the limit of 370Bqkg−1, which is equivalent to a gamma dose of 1.5mSvyr−1, whereas the Raeq for sand samples was lower than 370Bqkg−1. The calculated concentrations by HPGe spectroscopy were compared with the measured concentrations detected by a GR-135 spectrometer. The calculated and measured gamma dose rates had an ideal correlation coefficient R of 0.72. The gamma dose rates in Penang increased with the average annual age-standardized rates (ASR) for all cancers between 1994 and 2010. The effects of the pH value of soil and sand samples on natural radionuclides concentrations were investigated. The high concentration of 226Ra/238U ratio disequilibrium (226Ra/238U of 1.76–2.33) was observed in the sampling sites. Moreover, a portable continuous radon monitor (SNC, model 1029, Sun Nuclear Corporation) was used to measure the radon concentration of the soil surface. The radon concentrations were found to vary from 7 to 50Bqm−3. A positive correlation was observed between the radon and radium concentrations in samples measured by the SNC continuous radon monitor and HPGe detector.

Dosimetric evaluation of radiation dose rate effect in respiratory gated intensity modulated radiation therapy.

Purpose:To investigate the dosimetric accuracy of the sliding window gated IMRT compared with the static treatment, using varying dose rates.Materials and methods:This study measured changes in output and diode array response with changing dose rate, verified the precision of the motion table, and measured changes in dose distribution accuracy with film and diodes at two depths with changing dose rate. During 4DCT (4 Dimensional Computed Tomography), the patient’s respiratory signals and target motion were recorded and imported to the XY4D simulation table of SUN NUCLEAR Corporation to simulate the patient’s respiration and tumour motion. A single field of each sliding window IMRT plan with 30º wedge and one for lung cancer were used in this study. Three irradiating conditions, static and moving target with and without gating, were applied to both plans.Results:The standard deviations of output, with the dose rates changing from 300–600 MU/min, were 0.065 cGy and 0.169 cGy for the ionisation chamber and diode, respectively. The verification of the motion table shows very good precision with 9.98 ± 0.02 cm (true value = 10.0 cm). The measurements by MapCheck show the gamma index of the planned absolute dose distribution in static and moving targets with gating, resulting in more than 96% passing for all dose rates. The absolute dose distribution measured by film for the static target was agreeable with the value of moving target with gating.Conclusion:The sliding window gated IMRT technique is able to deliver an accurate dose to a moving target with the dose rate of 300–600 MU/min that is suitable for clinical treatment.

Evaluating dosimetric accuracy of flattening filter free compensator-based IMRT: Measurements with diode arrays

Compensator-based IMRT coupled with the high dose rate flattening filter free (FFF) beams offers an intriguing possibility of delivering an intensity modulated radiation field in just a few seconds. As a first step, the authors evaluate the dosimetric accuracy of the treatment planning system (TPS) FFF beam model with compensators. A 6 MV FFF beam from a TrueBeam accelerator (Varian Medical Systems, Palo Alto CA) was modeled in PINNACLE TPS (v. 9.0, Philips Radiation Oncology, Fitchburg WI). Flat brass slabs from 0.3 to 7 cm thick and an 18° brass wedge were used to adjust the beam model. A 2D (MAPCHECK) and 3D (ARCCHECK) diode arrays (Sun Nuclear Corp, Melbourne FL), were investigated for use with the compensator FFF beams. Corrections for diode sensitivity caused by the spectral changes in the beam were introduced. Four compensator plans based on the AAPM TG-119 report were developed. A composite ion chamber measurement, beam by beam MAPCHECK measurements, and a composite ARCCHECK measurement were performed. The array results were analyzed with the same thresholds as in TG-119 report-3%/3 mm with global dose normalization-as well as with the more stringent combinations of the gamma analysis criteria. The FFF beam shows a greater variation of the effective attenuation coefficient with brass thickness due to the prevalence of the low energy photons compared to the conventional 6X beam. As a result, a compromise had to be made while trying to achieve dose agreement for a combination of field sizes, brass thicknesses, and measurement depths (≥5 cm in water). An agreement of measured and calculated dose to within 1% was observed for brass thicknesses up to 2 cm. For the 3 cm slab, an error of up to 2.8% was noted for the field sizes above 10 × 10 cm(2), and up to 3.8% for the 5 × 5 cm(2) field. Both diode arrays exhibit a substantial sensitivity drop as the compensator thickness increases, reaching 10% for a 7 cm brass slab. A simple correction based on the brass thickness along the ray was introduced to counteract this effect. Pooled for five profiles, the average ratio of uncorrected and corrected MAPCHECK to ion chamber readings are 0.966 and 1.008, respectively. With the proper correction, all MAPCHECK measurement to calculation comparisons exhibit 100% γ(3%/3 mm) passing rates with global dose-error normalization. For the TG-119-type plans, the average γ(2%/2 mm) passing rate with local normalization is 94% (range 87.8%-98.3%). The lower ARCCHECK γ-analysis passing rates (corrected for diode sensitivity) are predictable based on the observed PDD discrepancies. However, with the 3%/3 mm thresholds and global normalization, the average γ-analysis passing rate is 96.4% (range 89.9%-100%). MAPCHECK analysis demonstrates high passing rates with the stringent γ(2%/2 mm) and local normalization criteria combination. The geometry of the ARCCHECK array creates a stress test for the FFF TPS model because of the shallow depth of the entrance diodes and large air cavity. Hence, the ARCCHECK γ-analysis passing rates are lower than with the MAPCHECK, while still on par with TG-119.

Radon Measurement Quality, How Accurate is Reasonable?

Until 2006, continuous radon monitoring devices, CR, could either be calibrated by reference to known quantities or by internal adjustments and or alignments. In 2007, a policy was advanced by the National Radon Safety Board and the National Environmental Health Association mandating internal adjustment and or alignment. Further, calibrations could only be performed by radon chamber persons authorized by the specific device manufacturer, which was a process that was impossible for many chamber operators to achieve. The paper serves to examine the technical validity for routine internal adjustments to Honeywell and Sun Nuclear (Sun Nuclear Corporation, 425A Pineda Court, Melbourne, FL 32940-7508) devices in contrast to the clear market controlling advantages of the policy. The purpose for making radon measurements is to assess risk. Comparing the uncertainties associated with risk to counting uncertainties of Honeywell and Sun Nuclear CR devices, less than 1% of model 1027 devices would have a calibration error exceeding 25%, and those devices, at this Radon Measurement Proficiency limit, would produce results that were more precise and accurate than the radon risk uncertainty. This was true for CR devices that have not been internally adjusted nor corrected in any way. It was concluded that internal adjustment or alignment better supported business principles than science.

The ArcCHECK diode array for dosimetric verification of HybridArc

The aim of this work is to evaluate dosimetric accuracy of a new treatment modality, HybridArc, in iPlan RT Dose 4.5 (BrainLAB, Feldkirchen, Germany) using a four-dimensional diode array (ArcCHECK, Sun Nuclear Corporation, Melbourne, USA). HybridArc is able to enhance dynamic conformal arcs with inversely planned elements. HybridArc plans for various sites (intracranial and extracranial) were constructed and after that these plans were recalculated for the ArcCHECK diode array with Monte Carlo (MC) and Pencil Beam (PB) dose algorithms in iPlan RT Dose. All measurements of these HybridArc plans were performed with 6 MV photon beams of a Novalis accelerator (BrainLAB, Feldkirchen, Germany) using the ArcCHECK device without and with an insert containing an ionization chamber. Comparison of the absolute dose distributions measured and calculated in iPlan RT Dose with the MC algorithm at the cylinder of the ArcCHECK diode array for HybridArc plans gives good agreement, even for the 2% dose difference and 2 mm distance-to-agreement criteria. The PB calculations significantly differ from the ArcCHECK measurements so that the MC algorithm is found to be superior to the PB algorithm in the calculation of the HybridArc plans. One of the drawbacks of the PB calculations in iPlan RT Dose is the too large arc step size of 10°. Use of a finer angular resolution may improve the PB results significantly.

SU‐E‐T‐154: Evaluation and Improvement of the ArcCHECK 3D Dosimeter

Purpose: To address important questions regarding calibration and use of the ArcCHECK (Sun Nuclear Corp) detector resulting from its hollow shape and cylindrical array geometry. The goals are to investigate how accurate different treatment planning algorithms are with the phantom containing a large air‐density inhomogeneity; to establish a method of calibrating the detector against an isotropic, energy‐independent dosimeter; and to develop a robust virtual inclinometer capable of providing uninterrupted beam angle information during arc deliveries. Methods: A PMMA shell accommodating an ion chamber at the diode position was designed to replicate the dimensions of the ArcCHECK phantom. The ratio of entrance to exit dose was measured and compared to calculations by five sophisticated commercial algorithms. Field size dependence of the diode response was measured at different locations in the phantom. A novel virtual inclinometer algorithm was designed based on the coincidence of dose grid images projected with attenuation on a common plane. A formalism of reading to dose conversion accounting for the field size and detector position dependence was developed. A practical method of measuring the positional dependence factors against an ion chamber in the actual phantom geometry was designed.Results: Philips Pinnacle CCC, CMS XiO Superposition and Monaco algorithms disagreed with the ion chamber by no more than 1.5%. XiO Convolution and Varian Eclipse AAA were off by &gt;11%. Field size correction is position‐dependent and varies between the hollow and plugged phantom configurations. Virtual inclinometer is robust and deviates no more than 0.5±0.7° from the Dynalog record. An average disagreement is 0.5% between the diode and ion chamber dose measurements in a wide field Conclusions: Using hollow ArcCHECK phantom configuration with Eclipse AAA is not recommended. A robust positional correction based on the novel virtual inclinometer is feasible. Diode “reading to dose” formalism must include field size dependence.This work was supported in part by Sun Nuclear Corp.

SU-E-T-405: Evaluation of IMRT QA Using 3DVH, a 3D Patient Dose and Verification Analysis Software

Purpose: 3DVH (Sun Nuclear Corporation) is a software that uses individual field QA data to obtain a three‐dimensional dose distribution in the patient. The purpose of this poster is to evaluate 3DVH as an IMRT QA tool; mainly to present our validation results, the TPS model improvements they lead to and preliminary passing rate results on IMRT plans analysed using 3DVH. Methods: The validation was performed using simulated Mapcheck (Sun Nuclear Corporation) QA results obtained from the TPS. The accuracy of 3DVH in calculating errors was verified by introducing errors in the plans and comparing these plans with the dose 3DVH predicted. IMRT plans were verified using the 3DVH software and the results were analysed by looking at DVH values. The 3DVH results were compared to Mapcheck QA results. Results: The validation shows that 3DVH can predict errors within 3%, with discrepancies tending to increase as the induced errors become greater. The retrospective analysis of IMRT plans resulted in a TPS model modification as it brought to light the occasional under dosing of the PTVs. The rounded leaf tip model was modified to decrease the field size for fields defined by the MLC only. This change to the model was validated using picket fence measurements. The IMRT plans were recalculated using the new model; as a result, the 3DVH passing rate for a 3%/3mm gamma between the expected dose and the 3DVH dose increased from 93% to 98% on average. Further analysis of IMRT plans has shown that 3DVH is more sensitive to dose discrepancies than individual field QA. Conclusions: 3DVH is a useful tool for IMRT QA which supplements the planar dose information collected using 2D arrays or film. The analysis of IMRT plans has shown that 3DVH can detect dose differences that are not detected by individual field analysis alone.

SU-E-T-60: Efficient and Accurate Treatment Planning System Commissioning for the Enhanced Dynamic Wedge

Purpose: The goal is to demonstrate mathematically and experimentally an efficient Enhanced Dynamic Wedge (EDW) treatment planning system (TPS) commissioning using the PINNACE TPS; only open and 60degree EDW data are required for beam modeling. Methods: A mathematical relationship is given in the Varian user manual in which the MU of a field consisting of both open and 60degree EDW beams can be converted to the MU for a different angle EDW field. TomoDose (Sun Nuclear Corp) was used to measure EDW profiles. The open-field PDDs, profiles and output factors(OFs) were measured using an ion chamber in water and EDW profiles were measured using TomoDose in solid water at 5 depths for all field sizes (FSs)up to 30×(20,10)cm2 for 6/16MV.Only open and 60degree EDW PDDs, profiles and OFs were imported to the TPS for the EDW commissioning. Measured and calculated PDDs, profiles and wedge factors (WFs) for the other 6 EDW angles were compared. Results: The time needed to measure the commissioning data (open PDDs and profiles and 60degree EDW profiles) was approximately 3hrs. To validate replacement of the EDW PDDs with the open-field PDDs, maximum differences between the open and EDW PDDs were 0.6%.For the majority of the EDWs and FS geometries, differences between measured and computed PDDs were within 2%; for cross profiles, differences between measured (TomoDose and ion chamber) and computed values were<1%within the field,<2%outside the field except for 30×(20,10)cm2and<3%/3mm in the penumbra region. The WF differences were<1% for all FS and EDW angles. Conclusions: TomoDose measurements for the EDW agreed well with the ion chamber data. This commissioning procedure greatly reduces routine quality assurance workload and time. Our results demonstrate theoretically and practically the superiority of the diode array device in the commissioning and only the 60degree EDW beam is commissioned in the TPS instead of all 7 EDW beams.

SU-E-T-547: ArcCHECK Diode Array for Dosimetric Verification of HybridArc

Purpose: HybridArc is a new treatment modality that combines dynamic conformal arcs and intensity modulated beams in a single inverse‐optimized plan. The aim of this work is to evaluate the dosimetric accuracy of HybridArc, in iPlan RT Dose 4.5 (BrainLAB) using a cylindrical diode array (ArcCHECK, Sun Nuclear Corporation). Methods: HybridArc plans for various sites (intracranial and extracranial) were constructed. After that the dose distributions were recalculated for the cylindrical ArcCHECK diode array with Monte Carlo(MC) and Pencil Beam (PB) dose algorithms in iPlan. Measurements of these dose distributions were performed with the 6 MV photon beams of a Novalis accelerator (BrainLAB) using the ArcCHECK device without and with an insert containing an ionization chamber. The measurements were evaluated using gamma criteria of various degrees of stringency (dose difference/distance to agreement: 2%/2mm, 3%/3mm and 5%/3mm).Results: For the HybridArc plans studied here, good agreement between the ArcCHECK measurements and the MC calculations is observed (pass rates of 95%, 99% and 100% for 2%/2mm, 3%/3mm and 5%/3mm, respectively). The results are similar to the quality assurance measurements that are routinely carried out at our institution for IMRT plans, which are calculated with the PB algorithm. However, agreement between HybridArc PB calculations and measurements is less satisfactory (pass rates of 69%, 85% and 91% for 2%/2mm, 3%/3mm and 5%/3mm, respectively). This is probably due to the limited angular resolution of the PB dose calculation (10 degree discretization) Conclusions: MC dose calculations and ArcCHECK measurements of HybridArc plans showed good agreement, even when evaluated with the most stringent 2%/2mm gamma criterion. PB calculations significantly differ from the ArcCHECK measurements so that the MC algorithm is found to be superior to the PB algorithm in calculation of the HybridArc plans. However, use of a finer angular resolution may improve the PB results significantly.

SU-E-T-112: 18 MV X-Ray Beam Commissioning with the IC Profiler: A Fast 3D Relative Dosimetry Technique Using An Ion Chamber Array in a Solid Tissue-Equivalent Phantom

Purpose: To devise a technique of 3D relative dosimetry using an ion chamber array with solid phantom to significantly reduce overall measurement time needed for photon beam modeling. Methods: Measurement setup of the array was determined by comparison of acquired profiles in the array and in a water tank. Software was developed to queue measurements in step‐and‐shoot sequences and process the resulting IC array data. Beam profiles and PDDs necessary for beam modeling were acquired with the IC array for 18 MV. A model was created with the IC array data using the Pinnacle TPS and compared to a model created from water tank measurements. Both models were first compared by measuring differences between profiles and PDDs. Secondly, 3D dose distributions predicted by both models were compared using a gamma metric for a pair of treatment plans. Results: The IC array model was found to match the water tank model for most profiles. Due to fixed chamber depth in the IC array, PDDs near the surface differed in the IC array model from the water tank model. For a prostate case treatment plan, comparison of 3D dose distributions from both models showed that 90.4 % of voxels agreed within a gamma of (1%, 1 mm). This tolerance was violated near air‐tissue interfaces due to differences in the measured PDDs between both models. The devised technique allowed most measurements necessary for beam modeling, except for output factors, to be made for a single energy in under 8 hours. Conclusions: The IC array with solid phantom setup allows substantially faster beam acquisition than a water tank. Further investigation is required for measuring exact PDDs in the IC array setup. Future work will study lower photon energies and electron beam modeling with IC array acquisitions. Funding for the presenting author was provided for this study by Sun Nuclear Corporation during the summer of 2010.

SU-E-T-485: Clinical Evaluation of Inverse Planning Models and IMRT Delivery Systems in the Framework of AAPM TG-119 Protocol

Purpose/Objective: To evaluate the overall performance of treatment planning and MLC delivery systems for clinical implementation of IMRT modality. Materials/Methods: A 6-MV beam of photons from Elekta Synergy linac, modeled into Pinnacle treatment planning system (TPS), (Version 9.0) was used to develop plans from a suite of benchmark test cases provided in the TG-119 protocol. Each test case had a planning tumor volume (PTV) and an organ-at-risk (OAR) embedded into the rectangular phantom. These tests were planned, delivered, measured and analyzed using state-of-the-art dosimetry systems. Per-field planar isodose distributions were measured with the MapCHECK-2 diode-array device (Sun Nuclear Corporation, Melbourne, FL). A radiographic EDR-2 film, placed at one coronal plane in the phantom, was exposed to all fields with planned gantry and collimator angles. RIT113 scanning software (V5.3) was used to perform film dosimetry. Measured and planned dose distributions were analyzed using gamma criteria of 3%/3-mm. For absolute dosimetry, each test plan was delivered on a 30-cm×30-cm×20-cm homogenous solid-water phantom to assess agreement between planned and measured dose in high and low dose regions using a calibrated 0.125cc cylindrical ion chamber. Upon achieving quantitative confidence limits on baseline values established in the TG-119 protocol, the H&N phantom of the Radiation Physics Center (RPC) was irradiated with its planned IMRT fields and returned to RPC for dosimetric analysis. Results: The MapCHECK-2 and EDR-2 film measurements reproduced the planned isodose distributions with 99–100% gamma-test passing rates for all IMRT test plans. Ion-chamber dosimetry showed <2% difference between the planned and measured doses in the high and low dose regions. Our institution passed the RPC IMRT credentialing test using the RPCˈs H&N dosimetry phantom. Conclusions: AAPM TG- 119 protocol provides a useful resource for the clinical evaluation of the IMRT modality consisting of Pinnacle TPS and Elekta synergy MLC delivery system.

Characterization of a multi‐axis ion chamber array

The aim of this work was to characterize a multi-axis ion chamber array (IC PROFILER; Sun Nuclear Corporation, Melbourne, FL, USA) that has the potential to simplify the acquisition of LINAC beam data. The IC PROFILER (or panel) measurement response was characterized with respect to radiation beam properties, including dose, dose per pulse, pulse rate frequency (PRF), and energy. Panel properties were also studied, including detector-calibration stability, power-on time, backscatter dependence, and the panel's agreement with water tank measurements [profiles, fractional depth dose (FDD), and output factors]. The panel's relative deviation was typically within (+/-) 1% of an independent (or nominal) response for all properties that were tested. Notable results were (a) a detectable relative field shape change of approximately 1% with linear accelerator PRF changes; (b) a large range in backscatter thickness had a minimal effect on the measured dose distribution (typically less than 1%); (c) the error spread in profile comparison between the panel and scanning water tank (Blue Phantom, CC13; IBA Schwarzenbruck, DE) was approximately (+/-) 0.75%. The ability of the panel to accurately reproduce water tank profiles, FDDs, and output factors is an indication of its abilities as a dosimetry system. The benefits of using the panel versus a scanning water tank are less setup time and less error susceptibility. The same measurements (including device setup and breakdown) for both systems took 180 min with the water tank versus 30 min with the panel. The time-savings increase as the measurement load is increased.

Wide field array calibration dependence on the stability of measured dose distributions

The aim of this work was to simulate the effect of dose distribution changes on detector array calibrations and to explore compensatory methods that are used during calibration measurements. The array calibration technique that was investigated is known as wide field (WF) calibration. Using this method, a linear array [y-axis (65 detectors) of the IC PROFILER (Sun Nuclear Corporation, Melbourne, FL)] is calibrated with three measurements (alpha, theta, and lamda); each measurement uses the same radiation field, which is larger than the array. For measurement configuration theta, the array is rotated by 180 degrees from its position in a; for lamda, the array is shifted by one detector from its position in theta. The relative detector sensitivities are then determined through ratios of detector readings at the same field locations (using theta and lamda). This method results in error propagation that is proportional to the number of detectors in the array. During the procedure, the calibration protocol operates under three postulates, which state that (a) the beam shape does not change between measurements; (b) the relative sensitivities of the detectors do not change; and (c) the scatter to the array does not change as the array is moved. The WF calibration's sensitivity to a postulate (a) violation was quantified by applying a sine shaped perturbation (of up to 0.1%) to a, theta, or lamda, and then determining the change relative to a baseline calibration. Postulate (a) violations were minimized by using a continuous beam and mechanized array movement during theta and lamda. A continuously on beam demonstrated more stable beam symmetry as compared to cycling the beam on and off between measurements. Additional side-scatter was also used to satisfy postulate (c). Simulated symmetry perturbations of 0.1% to theta or lamda resulted in calibration errors of up to 2%; alpha was relatively immune to perturbation (<0.1% error). Wide field calibration error on a linear accelerator with similar symmetry variations was +/- 1.6%. Using a continuous beam during theta and lamda with additional side-scatter reduced the calibration error from +/- 1.6% to +/- 0.48%. This work increased the reproducibility of WF calibrations by limiting the effect of measurement perturbations primarily due to linear accelerator symmetry variations. The same technique would work for any array using WF calibration.