Cylindrical Solid Water Phantom Research Articles

Radiobiological effectiveness difference of proton arc beams versus conventional proton and photon beams

This paper aims to demonstrate the difference in biological effectiveness of proton monoenergetic arc therapy (PMAT) compared to intensity modulated proton therapy (IMPT) and conventional 6 MV photon therapy, and to quantify this difference when exposing cells of different radiosensitivity to the same experimental conditions for each modality.V79, H1299 and H460 cells were cultured in petri dishes placed in the central axis of a cylindrical and homogeneous solid water phantom of 20 cm in diameter. For the PMAT plan, cells were exposed to 13 mono-energetic proton beams separated every 15° over a 180° arc, designed to deliver a uniform dose of higher LET to the petri dishes. For the IMPT plans, 3 fields were used, where each field was modulated to cover the full target. Cells were also exposed to 6 MV photon beams in petri dishes to characterize their radiosensitivity. The relative biological effectiveness of the PMAT plans compared with those of IMPT was measured using clonogenic assays. Similarly, in order to study the quantity and quality of the DNA damage induced by the PMAT plans compared to that of IMPT and photons, γ-H2AX assays were conducted to study the relative amount of DNA damage induced by each modality, and their repair rate over time.The clonogenic assay revealed similar survival levels to the same dose delivered with IMPT or x-rays. However, a systematic average of up to a 43% increase in effectiveness in PMAT plans was observed when compared with IMPT. In addition, the repair kinetic assays proved that PMAT induces larger and more complex DNA damage (evidenced by a slower repair rate and a larger proportion of unrepaired DNA damage) than IMPT. The repair kinetics of IMPT and 6 MV photon therapy were similar.Mono-energetic arc beams offer the possibility of taking advantage of the enhanced LET of proton beams to increase TCP. This study presents initial results based on exposing cells with different radiosensitivity to other modalities under the same experimental conditions, but more extensive clonogenic and in-vivo studies will be required to confirm the validity of these results.

Clinical implementation of an exit detector-based dose reconstruction tool for helical tomotherapy delivery quality assurance.

The aim of this study was to validate the accuracy of an exit detector-based dose reconstruction tool for helical tomotherapy (HT) delivery quality assurance (DQA). Exit detector-based DQA tool was developed for patient-specific HT treatment verification. The tool performs a dose reconstruction on the planning image using the sinogram measured by the HT exit detector with no objects in the beam (i.e., static couch), and compares the reconstructed dose to the planned dose. Vendor supplied (three "TomoPhant") plans with a cylindrical solid water ("cheese") phantom were used for validation. Each "TomoPhant" plan was modified with intentional multileaf collimator leaf open time (MLC LOT) errors to assess the sensitivity and robustness of this tool. Four scenarios were tested; leaf 32 was "stuck open," leaf 42 was "stuck open," random leaf LOT was closed first by mean values of 2% and then 4%. A static couch DQA procedure was then run five times (once with the unmodified sinogram and four times with modified sinograms) for each of the three "TomoPhant" treatment plans. First, the original optimized delivery plan was compared with the original machine agnostic delivery plan, then the original optimized plans with a known modification applied (intentional MLC LOT error) were compared to the corresponding error plan exit detector measurements. An absolute dose comparison between calculated and ion chamber (A1SL, Standard Imaging, Inc., WI, USA) measured dose was performed for the unmodified "TomoPhant" plans. A 3D gamma evaluation (2%/2mm global) was performed by comparing the planned dose ("original planned dose" for unmodified plans and "adjusted planned dose" for each intentional error) to exit detector-reconstructed dose for all three "Tomophant" plans. Finally, DQA for 119 clinical (treatment length <25cm) and three cranio-spinal irradiation (CSI) plans were measured with both the ArcCHECK phantom (Sun Nuclear Corp., Melbourne, FL, USA) and the exit detector DQA tool to assess the time required for DQA and similarity between two methods. The measured ion chamber dose agreed to within 1.5% of the reconstructed dose computed by the exit detector DQA tool on a cheese phantom for all unmodified "Tomophant" plans. Excellent agreement in gamma pass rate (>95%) was observed between the planned and reconstructed dose for all "Tomophant" plans considered using the tool. The gamma pass rate from 119 clinical plan DQA measurements was 94.9%±1.5% and 91.9%±4.37% for the exit detector DQA tool and ArcCHECK phantom measurements (P=0.81), respectively. For the clinical plans (treatment length <25cm), the average time required to perform DQA was 24.7±3.5 and 39.5±4.5min using the exit detector QA tool and ArcCHECK phantom, respectively, whereas the average time required for the 3 CSI treatments was 35±3.5 and 90±5.2min, respectively. The exit detector tool has been demonstrated to be faster for performing the DQA with equivalent sensitivity for detecting MLC LOT errors relative to a conventional phantom-based QA method. In addition, comprehensive MLC performance evaluation and features of reconstructed dose provide additional insight into understanding DQA failures and the clinical relevance of DQA results.

WE‐FG‐207B‐12: Quantitative Evaluation of a Spectral CT Scanner in a Phantom Study: Results of Spectral Reconstructions

Purpose:To evaluate the quantitative image quality of spectral reconstructions of phantom data from a spectral CT scanner.Methods:The spectral CT scanner (IQon Spectral CT, Philips Healthcare) is equipped with a dual‐layer detector and generates conventional 80ߝ140 kVp images and variety of spectral reconstructions, e.g., virtual monochromatic (VM) images, virtual non‐contrast (VNC) images, iodine maps, and effective atomic number (Z) images. A cylindrical solid water phantom (Gammex 472, 33 cm diameter and 5 cm thick) with iodine (2.0ߝ20.0 mg I/ml) and calcium (50ߝ600 mg/ml) rod inserts was scanned at 120 kVp and 27 mGy CTDIvol. Spectral reconstructions were evaluated by comparing image measurements with theoretical values calculated from nominal rod compositions provided by the phantom manufacturer. The theoretical VNC was calculated using water and iodine basis material decomposition, and the theoretical Z was calculated using two common methods, the chemical formula method (Z1) and the dual‐energy ratio method (Z2).Results:Beam‐hardening‐like artifacts between high‐attenuation calcium rods (≥300 mg/ml, &gt;800 HU) influenced quantitative measurements, so the quantitative analysis was only performed on iodine rods using the images from the scan with all the calcium rods removed. The CT numbers of the iodine rods in the VM images (50∼150 keV) were close to theoretical values with average difference of 2.4±6.9 HU. Compared with theoretical values, the average difference for iodine concentration, VNC CT number and effective Z of iodine rods were −0.10±0.38 mg/ml, −0.1±8.2 HU, 0.25±0.06 (Z1) and −0.23±0.07 (Z2).Conclusion:The results indicate that the spectral CT scanner generates quantitatively accurate spectral reconstructions at clinically relevant iodine concentrations. Beam‐hardening‐like artifacts still exist when high‐attenuation objects are present and their impact on patient images needs further investigation.YY is an employee of Philips Healthcare.

FoCa: a modular treatment planning system for proton radiotherapy with research and educational purposes

FoCa is an in-house modular treatment planning system, developed entirely in MATLAB, which includes forward dose calculation of proton radiotherapy plans in both active and passive modalities as well as a generic optimization suite for inverse treatment planning.The software has a dual education and research purpose. From the educational point of view, it can be an invaluable teaching tool for educating medical physicists, showing the insights of a treatment planning system from a well-known and widely accessible software platform. From the research point of view, its current and potential uses range from the fast calculation of any physical, radiobiological or clinical quantity in a patient CT geometry, to the development of new treatment modalities not yet available in commercial treatment planning systems.The physical models in FoCa were compared with the commissioning data from our institution and show an excellent agreement in depth dose distributions and longitudinal and transversal fluence profiles for both passive scattering and active scanning modalities. 3D dose distributions in phantom and patient geometries were compared with a commercial treatment planning system, yielding a gamma-index pass rate of above 94% (using FoCa’s most accurate algorithm) for all cases considered.Finally, the inverse treatment planning suite was used to produce the first prototype of intensity-modulated, passive-scattered proton therapy, using 13 passive scattering proton fields and multi-leaf modulation to produce a concave dose distribution on a cylindrical solid water phantom without any field-specific compensator.

Poster — Thur Eve — 11: Validation of the orthopedic metallic artifact reduction tool for CT simulations at the Ottawa Hospital Cancer Centre

Metallic implants in patients can produce image artifacts in kilovoltage CT simulation images which can introduce noise and inaccuracies in CT number, affecting anatomical segmentation and dose distributions. The commercial orthopedic metal artifact reduction algorithm (O‐MAR) (Philips Healthcare System) was recently made available on CT simulation scanners at our institution. This study validated the clinical use of O‐MAR by investigating its effects on CT number and dose distributions.O‐MAR corrected and uncorrected images were acquired with a Philips Brilliance Big Bore CT simulator of a cylindrical solid water phantom that contained various plugs (including metal) of known density. CT number accuracy was investigated by determining the mean and standard deviation in regions of interest (ROI) within each plug for uncorrected and O‐MAR corrected images and comparing with no‐metal image values. Dose distributions were calculated using the Monaco treatment planning system. Seven open fields were equally spaced about the phantom around a ROI near the center of the phantom. These were compared to a “correct” dose distribution calculated by overriding electron densities a no‐metal phantom image to produce an image containing metal but no artifacts.An overall improvement in CT number and dose distribution accuracy was achieved by applying the O‐MAR correction. Mean CT numbers and standard deviations were found to be generally improved. Exceptions included lung equivalent media, which is consistent with vendor specified contraindications. Dose profiles were found to vary by ±4% between uncorrected or O‐MAR corrected images with O‐MAR producing doses closer to ground truth.

SU-E-J-147: Dosimetric Consequences of Intrafraction Prostate Motion: Comparison Between Phantom Measurements and Three Different Calculation Methods.

Evaluation of different calculation methods for dose modification due to intrafraction prostate motion using film measurements as ground truth. We acquired intrafraction motion data with the Calypso tumor tracking system by Varian Medical Systems Inc for 4 prostate IMRT patients treated with 35 fractions each. These motion data were transferred to a phantom platform which reproduces the observed motion and has a 20 cm diameter cylindrical solid water phantom mounted. For each patient all fractions were irradiated on one radiochromic MD-V2-55 film placed in the isocentric transversal slice of this phantom. These films serve as ground truth for three calculation Methods: 1) Recalculation of the plan with shifted target point for every segment with the segment's mean Calypso position. 2)+3) Convolution of the static dose distribution with a probability density function of the observed positions. For 2) only Calypso positions with activated beam on signal were used whereas for 3) all Calypso positions between the first and the last beam on signal for all fractions were employed. The comparisons between films and calculated dose distributions were made with the verification software VeriSoft 3.2 (PTW, Freiburg, Germany) where an 8×8 cm̂2 ROI around the isocenter was selected for gamma evaluation. The segment shifted plans reach 3%/3mm gamma values above 90% against the films for all four patients. For both convolution methods three values are above 90%, only for the patient with the largest intrafraction motion they decrease to 89%. Shifting of the target point for every segment is well suited to estimate the dosimetric consequences of intrafraction prostate motion. This may facilitate the evaluation of different margin sizes or dose prescribing recipes under different motion conditions. If such a lengthy calculation is not possible, a convolution with motion data can be used for acceptable results, too. Our work was partially supported by Siemens Healthcare and Varian Medical Systems Inc.

Evaluation of PTW Seven29 for tomotherapy patient-specific quality assurance and comparison with ScandiDos Delta 4

For routine quality assurance of helical tomotherapy plans, an alternative method, as opposed to the TomoTherapy suggested cylindrical solid water phantom with film and ionization chamber, is proposed using the PTW Seven29 2D-ARRAY inserted in a dedicated octagonal phantom, called Octavius. First, the sensitivity of the array to pitch was studied by varying the pitch during planning to 0.287, 0.433, 1.0, and 2.0. For each pitch selected, the dependence on field size was investigated by generating plans with field widths (FWs) of 1.06 cm, 2.49 cm, and 5.02 cm, for a total of 12 plans. Secondly, a total of 15 patient QA plans were delivered using helical tomotherapy with the Delta4 and Seven29/Octavius for comparison. Using the clinical gamma criteria, 3% and 3 mm, all FW and pitch plans had a passing percentage of >90%. For patient QA plans, the average gamma pass percentage was 97.0% (94.4–99.8%) for the Delta4 and 97.6% (92.5-100.0%) for the Seven29/Octavius. Both the Seven29/Octavius and Delta4 performed to a high standard of measurement accuracy and had a 90% or greater gamma percent for all plans and were considered clinically acceptable.

Superficial dose distribution in breast for tangential radiation treatment, Monte Carlo evaluation of Eclipse algorithms in case of phantom and patient geometries

PurposeThe aim of this study is to examine experimentally and by the Monte Carlo method the accuracy of the Eclipse Pencil Beam Convolution (PBC) and Analytical Anisotropic Algorithm (AAA) algorithms in the superficial region (0–2cm) of the breast for tangential photon beams in a phantom case as well as in a number of patient geometries. The aim is also to identify differences in how the patient computer tomography data are handled by the treatment planning system and in the Monte Carlo simulations in order to reduce influences of these effects on the evaluation. Materials and methodsMeasurements by thermoluminescent dosimeters and gafchromic film are performed for six MV tangential irradiation of the cylindrical solid water phantom. Tangential treatment of seven patients is investigated considering open beams. Dose distributions are obtained by the Eclipse PBC and AAA algorithms. Monte Carlo calculations are carried out by BEAMnrc/DOSXYZnrc code package. Calculations are performed with a calculation grid of 1.25×1.25×5mm3 for PBC and 2×2×5mm3 for AAA and Monte Carlo, respectively. Dose comparison is performed in both dose and spatial domains by the normalized dose difference method. ResultsExperimental profiles from the surface toward the geometrical center of the cylindrical phantom are obtained at the beam entrance and exit as well as laterally. Full dose is received beyond 2mm in the lateral superficial region and beyond 7mm at the beam entrance. Good agreement between experimental, Monte Carlo and AAA data is obtained, whereas PBC is seen to underestimate the entrance dose the first 3–4mm and the lateral dose by more than 5% up to 8mm depth. In the patient cases considered, AAA and Monte Carlo show agreement within 3% dose and 4mm spatial tolerance. PBC systematically underestimates the dose at the breast apex. The dimensions of region out of tolerance vary with the local breast shape. Different interpretations of patient boundaries in Monte Carlo and the Eclipse are found to influence the evaluation. Computer tomography marker wire may introduce local disturbance effects on the comparison as well. These factors are not related to the accuracy of the calculation algorithms and their effect is taken into account in the evaluation. ConclusionsThe accuracy of AAA in the case of the solid water phantom is comparable with that of the Monte Carlo method. The AAA–Monte Carlo differences in the patient cases considered are within 3%, 4mm tolerance. The PBC algorithm does not give equivalent results. In the phantom case, PBC underestimates the lateral dose by more than 5% up to 8mm depth. The PBC-Monte Carlo differences in the patient cases are outside the tolerance at the breast apex. The dimension of region varies with the breast shape being typically 8–10mm long and 6–8mm deep.

SU‐E‐T‐781: Reduction of Thread Effect for Superficial Tomotherapy Treatments

Purpose: Helical tomotherapy can be useful in the treatment of superficial treatment volumes. As a consequence of the helical delivery, a thread pattern is evident in measured dose distributions. This goal of this work is to reduce this effect.Methods: Radiation treatment plans were created utilizing pitches of 0.287, 0.360, 0.430 and 0.500 with a 5 cm field width with the helical tomotherapy treatment planning system (TPS). The plans were delivered to a cylindrical solid water phantom with 1.0 cm bolus. EDR2 film was placed between the solid water and the bolus in the target volume region. Back‐to‐back delivery of procedures with different pitch to constitute a single treatment fraction was considered. Results: The measured percent difference of minima and maxima of the thread pattern was determined to be 5%, 12%, 10%, and 16% for pitch values of 0.287, 0.360, 0.430 and 0.500, respectively. The calculated thread magnitude was within 2% of the measured. Combining procedures with pitches of 0.287 &amp; 0.360 and 0.287 &amp; 0.430 resulted in interference patterns in the threading. For the pitch combination of 0.287 &amp; 0.430, reduced variation (∼3%) was largely observed; however, limited regions of greater magnitude minima and maxima (± 5% of the mean dose) were also observed. An attempt was made to modify the target volume in order to create two plans with equal pitch values that combine to form a destructive interference pattern and, hence, a uniform dose. However, this was not readily achievable with the clinical version of the TPS Conclusions: The thread pattern can be mitigated with the use of a combination of delivery procedures with differing pitch, specifically 0.287 and 0.430. The TPS was able to predict the magnitude of the thread effect within 2% of the measured for the superficial target for all cases considered.

SU‐D‐BRA‐04: Technical Evaluation of TomoTherapy Roll Correction

Purpose: Evaluate the effectiveness of the roll correction feature of the TomoTherapy Hi‐ART system. Methods: A roll‐sensitive plan was designed and delivered to a cylindrical solid water phantom to test the accuracy and utility of roll corrections. Cylindrical target structures containing coaxial inner avoidance structures were placed adjacent to the plane bisecting the phantom and 7cm laterally off central axis. The phantom was positioned at isocenter with the target‐plane parallel to the couch surface. Varying degrees of phantom roll were induced and dose to the targets and inner avoidance structures was measured using Kodak EDR2 films placed in the target‐plane. Normalized point doses were compared with baseline (no roll) data to determine the sensitivity of the test and the effectiveness of the roll correction feature. Gamma analysis between baseline, roll‐corrected and uncorrected films was performed using RIT software. Results: On average, significant deviations from baseline point dose measurements (±3 standard deviations) are observed beyond 0.4° of uncorrected phantom roll. Conversely, all point dose measurements with roll correction applied are indistinguishable from baseline (&lt;3 standard deviations) up to our maximum tested roll of 5°. For measurements in the rolled phantom without roll correction, gamma pass rates drop below 90% starting at an average of 0.25°. Gamma pass rates for all measurements with roll correction applied were greater than 97%. Conclusion: Using our rigid phantom and roll‐sensitive test plan, rotational misalignment greater than 0.25° can be detected and results in observable differences between planned and delivered dose. Utilizing the roll correction feature accurately corrects for the effects of such misalignment. Further work is required to evaluate the efficacy of roll correction in clinical circumstances where deformation accompanies rotation and to assess the clinical impact of using rotational and translational corrections compared to translational corrections alone.

Dosimetric characterization of the iBEAM evo carbon fiber couch for radiotherapy

This study characterizes the dosimetric properties of the iBEAM evo carbon fiber couch manufactured by Medical Intelligence and examines the accuracy of the CMS XiO and Nucletron Oncentra Masterplan (OMP) treatment planning systems for calculating beam attenuation due to the presence of the couch. To assess the homogeneity of the couch, it was CT scanned at isocentric height and a number of signal intensity profiles were generated and analyzed. To simplify experimental procedures, surface dose and central axis depth dose measurements were performed in a solid water slab phantom using Gafchromic film for 6 and 10 MV photon beams at gantry angles of 0 degree (normal incidence), 30 degrees, and 60 degrees with an inverted iBEAM couch placed on top of the phantom. Attenuation measurements were performed in a cylindrical solid water phantom with an ionization chamber positioned at the isocenter. Measurements were taken for gantry angles from 0 degree to 90 degrees in 10 degrees increments for both 6 and 10 MV photon beams. This setup was replicated in the XiO and OMP treatment planning systems. Dose was calculated using the pencil beam, collapsed cone, convolution, and superposition algorithms. The CT scan of the couch showed that it was uniformly constructed. Surface dose increased by (510 +/-0)% for a 6 MV beam and (600 +/- 20)% for a 10 MV beam passing through the couch at normal incidence. Obliquely incident beams resulted in a higher surface dose compared to normally incident beams for both open fields and fields with the couch present. Depth dose curves showed that the presence of the couch resulted in an increase in dose in the build up region. For 6 and 10 MV beams incident at 60 degrees, nearly all skin sparing was lost. Attenuation measurements derived using the ionization chamber varied from 2.7% (0 degree) to a maximum of 4.6% (50 degrees) for a 6 MV beam and from 1.9% (0 degree) to a maximum of 4.0% (50 degrees) for a 10 MV beam. The pencil beam and convolution algorithms failed to accurately calculate couch attenuation. The collapsed cone and superposition algorithms calculated attenuation within an absolute error of +/- 1.2% for 6 MV and +/- 0.8% for 10 MV for gantry angles from 0 degree to 40 degrees. Some differences in attenuation were observed dependent on how the couch was contoured. These results demonstrate that the presence of the iBEAM evo carbon fiber couch increases the surface dose and dose in the build up region. The inclusion of the couch in the planning scan is limited by the field of view employed and the couch height at the time of CT scanning.

Investigation of the effects of treatment planning variables in small animal radiotherapy dose distributions

Methods used for small animal radiation treatment have yet to achieve the same dose targeting as in clinical radiation therapy. Toward understanding how to better plan small animal radiation using a system recently developed for this purpose, the authors characterized dose distributions produced from conformal radiotherapy of small animals in a microCT scanner equipped with a variable-aperture collimator. Dose distributions delivered to a cylindrical solid water phantom were simulated using a Monte Carlo algorithm. Phase-space files for 120 kVp x-ray beams and collimator widths of 1-10 mm at isocenter were generated using BEAMnrc software, and dose distributions for evenly spaced beams numbered from 5 to 80 were generated in DOSXYZnrc for a variety of targets, including centered spherical targets in a range of sizes, spherical targets offset from centered by various distances, and various ellipsoidal targets. Dose distributions were analyzed using dose volume histograms. The dose delivered to a mouse bearing a spontaneous lung tumor was also simulated, and dose volume histograms were generated for the tumor, heart, left lung, right lung, and spinal cord. Results indicated that for centered, symmetric targets, the number of beams required to achieve a smooth dose volume histogram decreased with increased target size. Dose distributions for noncentered, symmetric targets did not exhibit any significant loss of conformality with increasing offset from the phantom center, indicating sufficient beam penetration through the phantom for targeting superficial targets from all angles. Even with variable collimator widths, targeting of asymmetric targets was found to have less conformality than that of spherical targets. Irradiation of a mouse lung tumor with multiple beam widths was found to effectively deliver dose to the tumor volume while minimizing dose to other critical structures. Overall, this method of generating and analyzing dose distributions provides a quantitative method for developing practical guidelines for small animal radiotherapy treatment planning. Future work should address methods to improve conformality in asymmetric targets.

SU-FF-T-248: Quality Assurance for Total Marrow Irradiation (TMI) Using Helical Tomotherapy

Purpose: Helical Tomotherapy (HT) offers advantages for total marrow irradiation (TMI) patients.The purpose of this study was to perform a more rigorous validation of TMI treatment plans using two types of radiation QA phantoms. Methods and Materials: Cylindrical solid water phantom: Several different delivery quality assurance (DQAs) were generated at different anatomical sites of the PTV of an actual TMI patient. The DQAs were performed using an ion chamber for point absolute dose measurement and EDR film for relative dose profiles. The ion chamberdose and film dose profiles were compared with the expected dose and dose profiles calculated from the treatment planning station (TPS). Rando Phantom: A HT TMI treatment plan was generated using a Rando phantom as the patient. The prescription was 80% of the PTV receives12 Gy. A DQA was also generated based on this plan. The same CTimage set acquired for planning the patient was also used as the CT scan for the DQA phantom. Film profile dose measurements were performed in transverse planes in between slabs making up the phantom. Results: For Rando TMI treatment plan, the mean dose of the PTV was 12.34±0.65Gy and the other organdoses were also well below currently accepted normal tissue tolerances. The absolute dose measurement and isodose distribution comparisons between planned and delivered doses on both coronal and transverse direction were within the clinical tolerance limit. Conclusion: Dose verification to the large TMI PTV delivered by the HT system was performed using an ion chamber and film in a cylindrical solid water phantom at multiple treatment sites along the patient's axis. Both the absolute point doses and relative dose profiles were within the clinical tolerance of +/−3%. A Rando phantom can be used for more rigorous QA by eliminating differences between an actual patient and a phantom.

SU-FF-T-671: Investigation of Effects of Treatment Planning Variables On Small Animal Therapy Dose Distributions

Purpose: Methods used for small animal radiation treatment have yet to achieve the same dose targeting as in clinical radiation therapy. Toward understanding how to better plan small animal radiation, we characterized dose distributions of conformal radiotherapy of small animals in a microCT scanner with a variable aperture collimator.Method and Materials:Dose distributions were simulated on a cylindrical solid water phantom (radius 2.5mm, resolution 195μm) using a Monte Carlo algorithm. Phase‐space files for collimator widths of 1–10mm were generated using BEAMnrc software, and dose distributions were generated in DOSXYZ for centered spherical targets with radii of 1, 2, 5, and 10mm; 2mm spherical targets offset by 10%, 20%, 30%, and 40% of the phantom diameter; and asymmetric elliptical targets of 2×4×2mm, 2×6×2mm, and 2×8×2mm. For each target, treatment plans included evenly‐space, matched‐isocenter beams ranging in number from 5–100, with multiple beam widths allowed for asymmetric targets. Dose distributions were analyzed in RT_Image. Results: For centered, symmetric targets, the number of beams required to achieve a smooth dose‐volume histogram decreased with target size; for a 1mm target, no significant increase in smoothness was achieved with beam numbers beyond 60; for a 2mm target, beyond 60; for a 5mm target, beyond 50; and for a 10mm target, beyond 40. Dose distributions for non‐centered, symmetric targets did not exhibit any significant loss of conformality with increasing offset from the phantom center, indicating sufficient beam penetration through the phantom for targeting superficial targets from all angles. Even with variable collimator widths, targeting of asymmetric targets was found to have less conformality than that of spherical targets. Conclusion: This method of generating and analyzing dose distributions provides a quantitative method for developing practical guidelines for small animal radiotherapytreatment planning. Future work should address methods to improve conformality in asymmetric targets.

SU‐FF‐T‐354: Evaluation of Dosimetric Characteristics of Electron Beam Arc Therapy Using Gafchromic EBT Film

Purpose: Evaluation of dosimetric characteristics of therapeutic electron beam arc therapy using GAFCHROMIC‐EBT film. Method and Materials: Output, PDDs and profiles were obtained using EBT film for 6 and 9MeV static and arc beam from Clinac2100CD linear accelerator using slab and cylindrical solid water phantom with gel bolus of thickness 1.5/2.0cm. For static beams, the measurements were carried out at SSD 100cm while for arc various SSDs of 80, 82, 85, 87 and 90cm were used. Measurements of EBT film were validated with ion chamber (PPC05 and FC65G). The effect of isocenter depth and the dose rate dependence were evaluated. Films were scanned with the Vidar dosimetri‐pro(red) scanner and analyzed with Omni‐PRO software. Results: Relationship of the dose vs optical density was linear within ±1% for 6 and 9MeV arc beam. PDD of arc beams show higher penetration and the dose‐maximum of 6 and 9MeV beam occurs at a slightly greater depth, 2 and 4mm respectively as compared to static beam. Arc beam resulted in higher x‐ray contamination towards the center of rotation and lesser surface dose, however, surface dose was found to be decreasing with increase in SSD as compared to static beam. An increase in relative dose was observed with SSD, however for larger SSDs (&gt;85cm) the increase was rapid. Ion chamber measurements showed a higher relative output (6%) for the lower dose rates (100–300MU/min), whereas less dependent(&lt;1%) for higher dose rates (400–1000MU/min). The response of EBT film was found to be dose rate dependent (3%) for lower dose rate in the range of 100–200MU/min, however, profiles obtained for various dose rates were independent of the dose rate. Conclusion: All the parameters evaluated in this study were found to be within acceptable limits, hence Gafchromic film can be a valid choice of dosimeter for electron beam arc therapy.

SU-FF-T-649: A Comparison of Constant Dose Rate SmartArc and Helical Tomotherapy

Purpose: The purpose of this study was to compare constant dose rate and gantry rotation speed intensity modulated arc delivery planned with the new Pinnacle SmartArc planning system and delivered on a Varian 21EX with helical tomotherapy (HT). Materials and Methods: A series of thirty patients that were treated with HT were randomly selected and re‐planned for SmartArc treatment delivery on a Varian 21EX equipped with a 120‐leaf MLC. All SmartArc plans were optimized for delivery using a constant dose rate and gantry speed, which eliminated the need for special VMAT licensees. The same contours, objectives, and prescriptions were used for the HT and SmartArc plans. SmartArc dosimetric verification was performed using a cylindrical solid water phantom with radiographic film and with an ionization chamber array. The MLC log files for the SmartArc treatment deliveries were also downloaded and analyzed. Results: 25 of the test cases were planned with a single arc, while 5 cases were planned with double arcs. The mean delivery time was 2.5 minutes, and the mean number of monitor units was 547. For the prostate test cases, the SmartArc plans were remarkably similar to the tomotherapy plans. The dose uniformity of the PTV for the SmartArc cases was actually superior to the tomotherapy dose uniformity for most of the prostate cases. For both the lung cases and H&N cases, the PTV uniformity and normal tissue DVHs were slight worse in the SmarArc plans as compared to the HT plans. Conclusion: SmartArc treatment plans can be created for delivery using a constant dose rate and gantry rotation speed that are very similar to helical tomotherapy plans for prostate, lung, and simple H&N patients. These plans can be delivered in 2–3 minutes per arc without the need to purchase special VMAT licenses on most accelerators.

SU‐FF‐T‐342: Evaluation of Superficial Dose for Helical Tomotherapy with GAFCHROMIC® EBT Film

Purpose: To measure the surface dose using EBT films for helical tomotherapy. Materials and methods: A cylindrical solid water phantom was scanned on a Siemens Somatom CT scanner with 3mm slice thickness. Four types of tomotherapy plans were studied: 1. The PTV touching the surface, planned and treated with different thicknesses of bolus (0,3,5,8 and 10mm), 2. Same as first, but treated without the bolus, 3. The PTV extended by 5mm outside the surface, planned and treated with and without a 10mm bolus, and 4. The PTV withdrawn from the surface by 3mm to 5mm, planned and treated without bolus. For each case, a plan was created to deliver 2Gy to 95% of the PTV per fraction. Gafchromic EBT films were used to measure the surface and the target doses. We also measured the surface doses of five clinical patients to verify the phantom study results. Results: For type 1 cases, when planned and treated without the bolus, the surface dose was about 65% of the prescribed dose; when different thicknesses of bolus were used, the surface doses were slightly higher than the prescribed dose. For type 2 cases, the surface dose decreased by about 40% to 45% of the prescribed dose, whereas the dose to the rest of PTV only increased by less than 3%. For type 3 cases, the surface doses were similar to the other cases with and without the bolus. For type 4 cases, the surface dose decreased by about 40% relative to the prescribed dose. Conclusion: When the PTV touches the surface, using bolus to ensure sufficient dose coverage of the tumor volume is recommended. Conversely, when the target volume is withdrawn from the surface, adequate tumor coverage can be achieved without bolus while reducing the surface dose by about 40% to 45%.

SU‐FF‐T‐431: Uncertainties in Superficial Dose From Helical Tomotherapy Delivery Due to Target Localization Errors

Purpose: To quantitate shallow‐depth dose calculation accuracy for IMRT breast treatment plans created with Tomotherapy system and to estimate the dose errors caused by setup errors. Methods: A cylindrical solid water phantom with radiochromic film was used for this purpose. The split phantom can hold a film in coronal or sagittal planes. A 5cm thick PTV was contoured by extending it to the surface of the phantom. A plan was optimized to treat the superficial PTV while limiting the dose to the surrounding critical structures. To evaluate the setup uncertainties multiple phantom plans were generated by shifting the phantom 5, 10 and 20 mm to right and up (7, 14, and 28mm distance) and simulated doses were compared against the measured doses. The 3D dose matrices from TPS for no‐shift and shifted phantom plans were exported in DICOM and were analyzed in MatLab. Doses from eleven transverse slices were analyzed to estimate the dose differences in the shifted phantom plans. Shifted phantom plan dose distributions were transposed back onto the patient PTV and computed dose difference histograms (DDHs) in the PTV volume. Results: The planning system overestimated the shallow‐depth dose by as much as 14%, but was smaller than 3% at depths greater than 5mm. PTV DDHs for no and 7 mm shifts showed &lt;5% dose differences, while for 14 mm, the dose differences were ⩽10% and were very large for the 28 mm shift. Conclusion: Tomotherapy planning system computes the shallow‐depth dose within 14% accuracy at 1.0 mm and better than 3% for depths greater than 5mm. Setup errors ⩽7mm showed negligible dose errors, consistent with the use of image‐guided therapy. Large shifts were required to induce large dose errors in the PTV. Conflict of Interest: This work supported in part by Tomotherapy, Inc.

WE-D-AUD-04: Dose Engine Verification of a Dual-Level MMLC-Based Treatment Planning System Benchmarked by MCNPX and Pinnacle

Purpose: To evaluate and verify a dose engine by comparing 3D dose matrices calculated by a dual-level micro multileaf collimator based treatment planning system (TPS) benchmarked by MCNPX and Pinnacle3 TPS Method and Materials: An irregular aperture shape was created by a dual-level micro multileaf collimator (mMLC) and dose was delivered through the aperture to a cylindrical solid water phantom with a 6MV Varian 600C linear accelerator. A Monte Carlo model was designed by using MCNPX according to the configurations of the linac source, jaws, aperture shape and phantom. The computed dose matrices from CrossPlan, a specially design TPS for mMLC, and MCNPX were compared against each other and the radiographic films. The isodose distributions, profiles, and gamma analysis were parameters used for comparison. 3D dose matrices from CrossPlan and MCNPX were converted and exported to Pinnacle3 TPS for further comparison. Results: The isodose overlay, gamma analysis, and dose profiles generated by both CrossPlan and MCNPX closely matched each other. The mean dose difference was less than 1.4%, and the gamma index values were less than one. The computed dose images from CrossPlan and MCNPX matched well with the radiographic films. The isodose and DVH comparisons by Pinnacle3 further demonstrate the agreement of the 3D dose matrices from CrossPlan and MCNPX. Conclusion: The good agreement between the computed dose from CrossPlan and MCNPX compared with the radiographic films indicated that both CrossPlan and MCNPX dose engines produce reliable and accurate dose distributions. The CrossPlan based dual-level micro-MLC is capable of precise aperture shaping, potentially allowing for an improved intensity modulated radiation therapy (IMRT) plan and delivery. Conflict of Interest: Supported by Initia Medical Technology.

SU‐FF‐T‐292: Inter‐Institution Comparison of Patient Quality Assurance Analysis for Tomotherapy

Purpose: HiArt Tomotherapy unit has been used in the clinics for a couple of years. However, the patient‐specific intensity modulated radiotherapy (IMRT) quality assurance (QA) methods vary from center to center. The purpose of this investigation is to analyze and compare the patient specific QA results for the HiArt TomoTherapy between two different institutions. Method and Materials: A patient‐specific IMRT QA procedure was developed and implemented in two different institutions. Once a QA plan is calculated, the dose to a phantom is measured using ion chamber and film. The ion chamber is placed 5 mm below the film which in turn is placed in the equator of a cylindrical solid water phantom. After the plan is delivered to the phantom, the point dose is recorded and the film is processed. Dosimetric analysis is performed after the film and planar dose are co‐registered in the TomoTherapy planning station. In total, sixty‐three patient QA from one center and fifty‐four patients QA from the other were analyzed. Results: For 52.99% of QA measurements done, the difference between measured and calculated doses was less than 1%. In both centers, the patient specific QA tolerance is set to 3%. In total, 92.31% of patient specific QA can pass the QA. Failure to pass the QA can be attributed to: (1) setup uncertainty (2) resolution differences in scanned QA phantom (3) the machine output fluctuation. Conclusions: A comprehensive patient QA program has been developed and the results of 117 patients from two different centers are analyzed in this paper. Given the novel approach of TomoTherapy towards IMRT, it is important to see that for the majority of the patients the deviation between planned and delivered doses is less than 3%.