bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐425: Spherical Dose Distributions for Radiosurgery Using a Standardized MLC Plan
Authors: Popple R , Brezovich I , Wu X , Fiveash J
Published: July 2014

Purpose:

To investigate a standardized MLC treatment plan to generate small spherical dose distributions.

Methods:

The static virtual cone plan comprised six table positions with clockwise and counterclockwise arcs having collimator angles 45 and 135 degrees, respectively, at each position. The central two leaves of a 2.5 mm leaf width MLC were set to a constant gap. Control points were weighted proportional to the sine of the gantry angle. Plans were created for the 10 MV flattening‐filter‐free beam of a TrueBeam STx (Varian Medical Systems) with gaps of 1, 1.5, 2, and 3 mm and were delivered to a phantom containing radiochromic film. Dose was calculated using the Eclipse AAA (Varian Medical Systems). A dynamic plan in which the table and gantry moved simultaneously with 1.5 mm gap was also created and delivered using the TrueBeam developer mode.

Results:

The full‐width‐half‐max (FWHM) varied with leaf gap, ranging from 5.2 to 6.2 mm. Calculated FWHM was smaller than measured by 0.7 mm for the 1 mm gap and ≤ 0.4 mm for the larger gaps. The measured‐to‐calculated dose ratio was 0.93, 0.96, 1.01, and 0.99 for 1 mm, 1.5 mm, 2 mm, and 3 mm gaps, respectively. The dynamic results were the same as the static. The position deviations between the phantom target position and the center of the dose distribution were < 0.4 mm.

Conclusion:

The virtual cone can deliver spherical dose distributions suitable for radio surgery of small targets such as the trigeminal nerve. The Eclipse AAA accurately calculates the expected dose, particularly for leaf gap ≥ 1.5 mm. The measured dose distribution is slightly larger than the calculation, which is likely due to systematic leaf position error, isocenter variation due to gantry sag and table eccentricity, and inaccuracy in MLC leaf end modeling.
SU‐E‐T‐426: Dose Delivery Accuracy in Breast Field Junction for Free Breath and Deep Inspiration Breath Hold Techniques
Authors: Epstein D , Shekel E , Levin D
Published: July 2014

Purpose:

The purpose of this work was to verify the accuracy of the dose distribution along the field junction in a half beam irradiation technique for breast cancer patients receiving radiation to the breast or chest wall (CW) and the supraclavicular LN region for both free breathing and deep inspiration breath hold (DIBH) technique.

Methods:

We performed in vivo measurements for nine breast cancer patients receiving radiation to the breast/CW and to the supraclavicular LN region. Six patients were treated to the left breast/CW using DIBH technique and three patients were treated to the right breast/CW in free breath. We used five microMOSFET dosimeters: three located along the field junction, one located 1 cm above the junction and the fifth microMOSFET located 1 cm below the junction. We performed consecutive measurements over several days for each patient and compared the measurements to the TPS calculation (Eclipse, Varian™).

Results:

The calculated and measured doses along the junction were 0.97±0.08 Gy and 1.02±0.14 Gy, respectively. Above the junction calculated and measured doses were 0.91±0.08 Gy and 0.98±0.09 Gy respectively, and below the junction calculated and measured doses were 1.70±0.15 Gy and 1.61±0.09 Gy, respectively. All differences were not statistically significant. When comparing calculated and measured doses for DIBH patients only, there was still no statistically significant difference between values for all dosimeter locations. Analysis was done using the Mann‐Whitney Rank‐Sum Test.

Conclusion:

We found excellent correlation between calculated doses from the TPS and measured skin doses at the junction of several half beam fields. Even for the DIBH technique, where there is more potential for variance due to depth of breath, there is no over or underdose along the field junction. This correlation validates the TPS, as well an accurate, reproducible patient setup.
SU‐E‐T‐427: Cell Surviving Fractions Derived From Tumor‐Volume Variation During Radiotherapy for Non‐Small Cell Lung Cancer: Comparison with Predictive Assays
Authors: Chvetsov A , Yartsev S , Schwartz J , Mayr N
Published: July 2014

Purpose:

To show that a distribution of cell surviving fractions S2 in a heterogeneous group of patients can be derived from tumor‐volume variation curves during radiotherapy for non‐small cell lung cancer.

Methods:

Our analysis was based on two data sets of tumor‐volume variation curves for heterogeneous groups of 17 patients treated for nonsmall cell lung cancer with conventional dose fractionation. The data sets were obtained previously at two independent institutions by using megavoltage (MV) computed tomography (CT). Statistical distributions of cell surviving fractions S2 and cell clearance half‐lives of lethally damaged cells T1/2 have been reconstructed in each patient group by using a version of the two‐level cell population tumor response model and a simulated annealing algorithm. The reconstructed statistical distributions of the cell surviving fractions have been compared to the distributions measured using predictive assays in vitro.

Results:

Non‐small cell lung cancer presents certain difficulties for modeling surviving fractions using tumor‐volume variation curves because of relatively large fractional hypoxic volume, low gradient of tumor‐volume response, and possible uncertainties due to breathing motion. Despite these difficulties, cell surviving fractions S2 for non‐small cell lung cancer derived from tumor‐volume variation measured at different institutions have similar probability density functions (PDFs) with mean values of 0.30 and 0.43 and standard deviations of 0.13 and 0.18, respectively. The PDFs for cell surviving fractions S2 reconstructed from tumor volume variation agree with the PDF measured in vitro. Comparison of the reconstructed cell surviving fractions with patient survival data shows that the patient survival time decreases as the cell surviving fraction increases.

Conclusion:

The data obtained in this work suggests that the cell surviving fractions S2 can be reconstructed from the tumor volume variation curves measured during radiotherapy with conventional fractionation. The proposed method can be used for treatment evaluation and adaptation.
SU‐E‐T‐428: Dosimetric Impact of Multileaf Collimator Leaf Width On Single and multiple Isocenter Stereotactic IMRT Treatment Plans for multiple Brain Tumors
Authors: Giem J , Algan O , Ahmad S , Ali I , Young J , Hossain S
Published: July 2014

Purpose:

To assess the impacts that multileaf collimator (MLC) leaf width has on the dose conformity and normal brain tissue doses of single and multiple isocenter stereotactic IMRT (SRT) plans for multiple intracranial tumors.

Methods:

Fourteen patients with 2–3 targets were studied retrospectively. Patients treated with multiple isocenter treatment plans using 9 to 12 non‐coplanar beams per lesion underwent repeat planning using single isocenter and 10 to 12 non‐coplanar beams with 2.5mm, 3mm and 5mm MLC leaf widths. Brainlab iPlan treatment planning system for delivery with the 2.5mm MLC served as reference. Identical contour sets and dose‐volume constraints were applied. The prescribed dose to each target was 25 Gy to be delivered over 5 fractions with a minimum of 99% dose to cover ≥ 95% of the target volume.

Results:

The lesions and normal brains ranged in size from 0.11 to 51.67cc (median, 2.75cc) and 1090 to 1641cc (median, 1401cc), respectively. The Paddick conformity index for single and multiple isocenter (2.5mm vs. 3mm and 5mm MLCs) was (0.79±0.08 vs. 0.79±0.07 and 0.77±0.08) and (0.79±0.09 vs. 0.77±0.09 and 0.76±0.08), respectively. The average normal brain volumes receiving 15 Gy for single and multiple isocenter (2.5mm vs. 3mm and 5mm MLCs) were (3.65% vs. 3.95% and 4.09%) and (2.89% vs. 2.91% and 2.92%), respectively.

Conclusion:

The average dose conformity observed for the different leaf width for single and multiple isocenter plans were similar, throughout. However, the average normal brain volumes receiving 2.5 to 15 Gy were consistently lower for the 2.5mm MLC leaf width, especially for single isocenter plans. The clinical consequences of these integral normal brain tissue doses are still unknown, but employing the use of the 2.5mm MLC option is desirable at sparing normal brain tissue for both single and multiple isocenter cases.
SU‐E‐T‐429: Uncertainties of Cell Surviving Fractions Derived From Tumor‐Volume Variation Curves
Authors: Chvetsov A
Published: July 2014

Purpose:

To evaluate uncertainties of cell surviving fraction reconstructed from tumor‐volume variation curves during radiation therapy using sensitivity analysis based on linear perturbation theory.

Methods:

The time dependent tumor‐volume functions V(t) have been calculated using a twolevel cell population model which is based on the separation of entire tumor cell population in two subpopulations: oxygenated viable and lethally damaged cells. The sensitivity function is defined as S(t)=[δV(t)/V(t)]/[δx/x] where δV(t)/V(t) is the time dependent relative variation of the volume V(t) and δx/x is the relative variation of the radiobiological parameter x. The sensitivity analysis was performed using direct perturbation method where the radiobiological parameter x was changed by a certain error and the tumor‐volume was recalculated to evaluate the corresponding tumor‐volume variation. Tumor volume variation curves and sensitivity functions have been computed for different values of cell surviving fractions from the practically important interval S2=0.1‐0.7 using the two‐level cell population model.

Results:

The sensitivity functions of tumor‐volume to cell surviving fractions achieved a relatively large value of 2.7 for S2=0.7 and then approached zero as S2 is approaching zero Assuming a systematic error of 3‐4% we obtain that the relative error in S2 is less that 20% in the range S2=0.4‐0.7. This Resultis important because the large values of S2 are associated with poor treatment outcome should be measured with relatively small uncertainties. For the very small values of S2<0.3, the relative error can be larger than 20%; however, the absolute error does not increase significantly.

Conclusion:

Tumor‐volume curves measured during radiotherapy can be used for evaluation of cell surviving fractions usually observed in radiation therapy with conventional fractionation.
SU‐E‐T‐430: Modeling MLC Leaf End in 2D for Sliding Window IMRT and Arc Therapy
Authors: Liang X , Zhu T
Published: July 2014

Purpose:

To develop a 2D geometric model for MLC accounting for leaf end dose leakage for dynamic IMRT and Rapidarc therapy.

Methods:

Leaf‐end dose leakage is one of the problems for MLC dose calculation and modeling. Dosimetric leaf gap used to model the MLC and to count for leakage in dose calculation, but may not be accurate for smaller leaf gaps. We propose another geometric modeling method to compensate for the MLC round‐shape leaf ends dose leakage, and improve the accuracy of dose calculation and dose verification. A triangular function is used to geometrically model the MLC leaf end leakage in the leaf motion direction, and a step function is used in the perpendicular direction. Dose measurements with different leaf gap, different window width, and different window height were conducted, and the results were used to fit the analytical model to get the model parameters.

Results:

Analytical models have been obtained for stop‐and‐shoot and dynamic modes for MLC motion. Parameters a=0.4, lw' =5.0 mm for 6X and a=0.54, lw' =4.1 mm for 15x were obtained from the fitting process. The proposed MLC leaf end model improves the dose profile at the two ends of the sliding window opening. This improvement is especially significant for smaller sliding window openings, which are commonly used for highly modulated IMRT plans and arc therapy plans.

Conclusion:

This work models the MLC round leaf end shape and movement pattern for IMRT dose calculation. The theory, as well as the results in this work provides a useful tool for photon beam IMRT dose calculation and verification.
SU‐E‐T‐431: Vertically‐Oriented Farmer‐Type Chamber for Small‐Field Applications
Authors: Ahmad M , Chu A , Lincoln H , Chen Z , Deng J , Nath R
Published: July 2014

Purpose:

To introduce a non‐conventional measurement setup using Farmer‐type chambers to accommodate several situations of small‐field dose measurements without compromising accuracy. The validation of this technique was demonstrated for photon small‐field output measurements, and electron small‐field percentage depth‐dose (PDD) measurements.

Methods:

Initial chamber alignment was performed using the conventional (horizontally‐oriented) chamber setup. A PDD was acquired for a 4×4 cm2 field size using this arrangement. This PDD was used as a positional reference for the vertically‐oriented chamber (VOC) configuration. Next, a PDD was acquired for a 4×4 cm2 field size with the VOC. The PDD's were superimposed to find the effective shift of the VOC. Using the shifted VOC setup, photon small‐field output factors were measured and compared to stereotactic diode output factor measurements. Additionally, electron smallfield PDD's were acquired using the VOC setup and results were compared to electron Monte Carlo (eMC) predictions in the Eclipse treatment planning system (TPS).

Results:

(1) For photon small‐field output factors field‐sizes 2×2 cm2 and larger, the difference between the VOC setup and SFD measurements were less than 0.8%. For field sizes less than 2×2 cm2 discrepancies ranged from 4.0 to 10.6%. (2) PDD's measured by VOC setup show better than 1.6% agreement as compared to eMC for all electron energies measured down to the 80% depth on the 2×2 cm2 PDD curve. Disagreement between the VOC setup measurement and eMC calculations for depths down to the 50% depth on the PDD curve is 3.6% or less.

Conclusion:

Using the VOC setup, it is possible to use a conventional farmer chamber for small field‐size measurements down to 2×2 cm2 field size without sacrificing the accuracy of measurements.
SU‐E‐T‐432: A Rapid and Comprehensive Procedure for Daily Proton QA
Authors: Zhao T , Sun B , Grantham K , Knutson N , Santanam L , Goddu S , Klein E
Published: July 2014

Purpose:

The objective is to develop a rapid and comprehensive daily QA procedure implemented at the S. Lee Kling Proton Therapy Center at Barnes‐Jewish Hospital.

Methods:

A scribed phantom with imbedded fiducials is used for checking lasers accuracy followed by couch isocentricity and for X‐ray imaging congruence with isocenter. A Daily QA3 device (Sun Nuclear, FL) was used to check output, range and profiles. Five chambers in the central region possess various build‐ups. After converting the thickness of the inherent build‐ups into water equivalent thickness (WET) for proton, range of any beam can be checked with additional build‐up on the Daily QA3 device. In our procedure, 3 beams from 3 bands (large, small and deep) with nominal range of 20 cm are checked daily. 17cm plastic water with WET of 16.92cm are used as additional build‐up so that four chambers sit on the SOBP plateau at various depths and one sit on the distal fall off. Reading from the five chambers are fitted to an error function that has been parameterized to match the SOBP with the same nominal range. Shifting of the error function to maximize the correlation between measurements and the error function is deemed as the range shift from the nominal value.

Results:

We have found couch isocentricity maintained over 180 degrees. Imaging system exhibits accuracy in regard to imaging and mechanical isocenters. Ranges are within 1mm accuracy from measurements in water tank, and sensitive to change of sub‐millimeter. Data acquired since the start of operation show outputs, profiles and range stay within 1% or 1mm from baselines. The whole procedure takes about 40 minutes.

Conclusion:

Taking advantage of the design of Daily QA3 device turns the device originally designed for photon and electron into a comprehensive and rapid tool for proton daily QA.
SU‐E‐T‐433: Calibration Accuracy in Mailed High‐Resolution 3D Dosimetry Service for SRS/SBRT QA
Authors: Maryanski M , Keane J , Zimberg S
Published: July 2014

Purpose:

SRS/SBRT combines hypofractionation with excellent dose distributions. However, extremely steep gradients across the target along with dose escalation, if not administered accurately, may lead to serious complications, recurrences, or even fatalities. Existing commercial QA products either lack adequate spatial resolution or the 3D aspect. By contrast, the new CrystalBall™ mailed high‐resolution 3D dosimetry service removes the above limitations while reducing the overall workload on medical physics staff. The exposed dosimeters, which change optical density in proportion to local dose, are sent back to the manufacturer (MGS Research Inc., Madison, CT) for sub‐millimeter‐resolution laser‐CT scanning and QA data analysis. QA report is returned electronically within 24 hours. The purpose of this study was to evaluate the dose calibration accuracy in this system.

Methods:

Two spherical CrystalBall™ polymer gel dosimeters from the same batch, 166 mm diameter, with embedded 3D image registration markers, were mounted in a special phantom designed for reproducible positioning. For full end to end testing, the optical guidance array was mounted onto the phantom and a CT was taken. Two separate Rapid Arc SRS plans were designed. Varian Medical Systems optical guidance system was used to position the phantom and the SRS treatment plans were delivered to the two spheres on Varian's Trilogy Accelerator. Exposed dosimeters were mailed back to the manufacturer for laser CT scanning and analysis.

Results:

For each plan, 3D gamma passing rate was 100% for 2%/2mm distance‐to‐agreement criteria above 50% isodose level. The two calibration curves, generated using volumetric dose and optical density data, showed excellent mutual agreement (max difference 2.2%, median difference 0.75%).

Conclusion:

The clinical utility of new CrystalBall™ mailed QA service for SRS/SBRT and high accuracy of dose calibration have been validated. The workflow associated with the use of the CrystalBall™ in clinical setting was found to be minimal.

The presenting author is the founder of and has an ownership in MGS Research Inc which manufactures the CrystalBall system for 3D dosimetry.
SU‐E‐T‐434: Evaluation of the Machine Performance Check (MPC) Program for Truebeam 2.0
Authors: Chin E , Yu S , Hadsell M , Bush K , Mok E
Published: July 2014

Purpose:

Machine Performance Check (MPC) is a software application used to verify that the TrueBeam machine is operating within major specifications prior to treatment. Used in combination with a phantom named Isocal, it verifies beam output, beam uniformity, treatment isocenter size, coincidence of treatment and imaging isocenters, positioning accuracy of kV and MV imaging systems, accuracy of collimator and gantry rotation angle, positioning accuracy of jaws and MLC leafs, and couch positioning. The tests can be performed semi‐automatically and requires approximately 10 minutes of machine time. It is the purpose of this study to report the performance of this program.

Methods:

A pre‐release version of the MPC tool was installed on a Truebeam linac with 6D couch at our center. Baseline beam output measurements were taken for 5 photon beams (6–15 MV, 6 FFF, 10 FFF) and 5 electron beams (6–20 MeV). Deviations from the baseline output were subsequently recorded for several days and compared against independent measurements from a PTW farmer chamber and our daily QA device (Fluke Biomedical Tracker) as part of an ongoing evaluation.

Results:

The beam output deviations between the MPC and the PTW chamber measurements were within ±0.7% for photons beams and ±1.0% for electrons beams. This was similar to the tracker performance. There were some isolated incidents where the MPC measurements had unexplained spikes (>3%) that disappeared on a repeat measurement. MPC was also able to detect maximum positioning errors in the jaws (1.12 mm), MLCs (1.14 mm), and couch roll (0.11°).

Conclusion:

Overall, the ability of the MPC to monitor linac output stability was comparable to that of ionization chamber‐based measurements. MPC also provided fast daily mechanical tests not currently available in the clinic. How best to utilize this previously unavailable data is still under investigation.
SU‐E‐T‐435: Development and Commissioning of a Complete System for In‐Vivo Dosimetry and Range Verification in Proton Therapy
Authors: Samuel D , Testa M , Schneider R , Park Y , Janssens G , Orban de Xivry J , Moteabbed M , Prieels D , Lu H , Bentefour E
Published: July 2014

Purpose:

In‐vivo dose and beam range verification in proton therapy could play significant roles in proton treatment validation and improvements. Invivo beam range verification, in particular, could enable new treatment techniques one of which, for example, could be the use of anterior fields for prostate treatment instead of opposed lateral fields as in current practice. We have developed and commissioned an integrated system with hardware, software and workflow protocols, to provide a complete solution, simultaneously for both in‐vivo dosimetry and range verification for proton therapy.

Methods:

The system uses a matrix of diodes, up to 12 in total, but separable into three groups for flexibility in application. A special amplifier was developed to capture extremely small signals from very low proton beam current. The software was developed within iMagX, a general platform for image processing in radiation therapy applications. The range determination exploits the inherent relationship between the internal range modulation clock of the proton therapy system and the radiological depth at the point of measurement. The commissioning of the system, for in‐vivo dosimetry and for range verification was separately conducted using anthropomorphic phantom. EBT films and TLDs were used for dose comparisons and range scan of the beam distal fall‐off was used as ground truth for range verification.

Results:

For in‐vivo dose measurement, the results were in agreement with TLD and EBT films and were within 3% from treatment planning calculations. For range verification, a precision of 0.5mm is achieved in homogeneous phantoms, and a precision of 2mm for anthropomorphic pelvic phantom, except at points with significant range mixing.

Conclusion:

We completed the commissioning of our system for in‐vivo dosimetry and range verification in proton therapy. The results suggest that the system is ready for clinical trials on patient.
SU‐E‐T‐436: Accelerated Gated IMRT: A Feasibility Study for Lung Cancer Patients
Authors: Gilles M , Fayad H , Boussion N , Pradier O , Visvikis D
Published: July 2014

Purpose:

To evaluate the feasibility of delivering a gated Intensity Modulated Radiotherapy (IMRT) treatment using multiple respiratory phases in order to account for all anatomic changes during free breathing and accelerate the gated treatment without increasing the dose per fraction.

Methods:

For 7 patients with lung cancer, IMRT treatment plans were generated on a full inspiration (FI) Computed Tomography (CT) and a Mid Intensity Position (MIP) CT. Moreover, in order to achieve an accelerated gated IMRT, multiple respiratory phase plans were calculated: 2‐phase plans including the FI and the full expiration phases, and 3‐phase plans by adding the mid‐inspiration phase. In order to assess the tolerance limits, plans' doses were registered and summed to the FI‐based plan. Mean dose received by Organs at Risk (OARs) and target volumes were used to compare obtained plans.

Results:

The mean dose differences between the FI plans and the multi‐phase plans never exceeded 0.4 Gy (Fig. 1). Concerning the clinical target volume these differences were even smaller: less than 0.1 Gy for both the 2‐phase and 3‐phase plans. Regarding the MIP treatment plan, higher doses in different healthy structures were observed, with a relative mean increase of 0.4 to 1.5 Gy. Finally, compared to the prescribed dose, the FI as well as the multi‐phase plans were associated with a mean difference of 0.4 Gy, whereas in the case of MIP a higher mean difference of 0.6 Gy was observed.

Conclusion:

The doses obtained while planning a multi‐phase gated IMRT treatment were within the tolerance limits. Compared to MIP, a better healthy tissue sparing was observed in the case of treatment planning based on one or multiple phases. Future work will consist in testing the multi‐phase treatment delivery while accounting for the multileaf collimator speed constraints.
SU‐E‐T‐437: Dosimetric Assessment of Brass Mesh Bolus for Postmastectomy Chest Wall Irradiation
Authors: Manger R , Paxton A , Cervino L
Published: July 2014

Purpose:

It has been suggested that the use of a brass mesh bolus for chest wall irradiation sufficiently increases surface dose while having little effect on the dose at depth. This work quantified the increase in surface dose when using a brass mesh bolus in postmastectomy chest wall radiotherapy compared to tissue‐equivalent bolus and assessed its effect on dose at depth.

Methods:

Percent depth doses with brass bolus, 5mm tissue‐equivalent bolus, and no bolus were determined for a 6 MV photon beam in a solid water phantom using a parallel plate ionization chamber. Gafchromic film was used to determine the surface dose for the same three experimental setups. For comparison to a realistic treatment setup, gafchromic film and OSLDs were used to determine the surface dose over the irradiated area of a 6 MV chest wall plan with tangential beams delivered to a heterogeneous thorax phantom. The plan was generated using a CT of the phantom and delivered using brass mesh bolus, 5mm tissue‐equivalent bolus, and no bolus.

Results:

For the en face beam, the central surface dose increased to 90% of maximum with the tissue‐equivalent bolus, but to only 62% of maximum with the brass mesh. Using tangential beams on the thorax phantom, the surface dose increased from 40–72% to 75–110% of prescribed dose, with the brass mesh, and to 85–109% with the tissue‐equivalent bolus. At depths beyond dmax in the plastic water phantom, the dose with and without brass mesh bolus differed by less than 0.5%.

Conclusion:

A brass mesh may be considered as a substitute for tissue‐equivalent bolus to increase the superficial dose of 6 MV chest wall tangent plans. The brass mesh does not significantly change the dose at depth, so a non‐bolus plan could be used for bolus and non‐bolus treatments.
SU‐E‐T‐438: Commissioning of An In‐Vivo Quality Assurance Method Using the Electronic Portal Imaging Device
Authors: Morin O , Held M , Pouliot J
Published: July 2014

Purpose:

Patient specific pre‐treatment quality assurance (QA) using arrays of detectors or film have been the standard approach to assure the correct treatment is delivered to the patient. This QA approach is expensive, labor intensive and does not guarantee or document that all remaining fractions were treated properly. The purpose of this abstract is to commission and evaluate the performance of a commercially available in‐vivo QA software using the electronic portal imaging device (EPID) to record the daily treatments.

Methods:

The platform EPIgray V2.0.2 (Dosisoft), which machine model compares ratios of TMR with EPID signal to predict dose was commissioned for an Artiste (Siemens Oncology Care Systems) and a Truebeam (Varian medical systems) linear accelerator following the given instructions. The systems were then tested on three different phantoms (homogeneous stack of solid water, anthropomorphic head and pelvis) and on a library of patient cases. Simple and complex fields were delivered at different exposures and for different gantry angles. The effects of the table attenuation and the EPID sagging were evaluated. Gamma analysis of the measured dose was compared to the predicted dose for complex clinical IMRT cases.

Results:

Commissioning of the EPIgray system for two photon energies took 8 hours. The difference between the dose planned and the dose measured with EPIgray was better than 3% for all phantom scenarios tested. Preliminary results on patients demonstrate an accuracy of 5% is achievable in high dose regions for both 3DCRT and IMRT. Large discrepancies (>5%) were observed due to metallic structures or air cavities and in low dose areas. Flat panel sagging was visible and accounted for in the EPIgray model.

Conclusion:

The accuracy achieved by EPIgray is sufficient to document the safe delivery of complex IMRT treatments. Future work will evaluate EPIgray for VMAT and high dose rate deliveries.

This work is supported by Dosisoft, Cachan, France.
SU‐E‐T‐439: An Improved Formula of Scatter‐To‐Primary Ratio for Photon Dose Calculation
Authors: Zhu T
Published: July 2014

Purpose:

Scatter‐to‐primary ratio (SPR) is an important dosimetric quantity that describes the contribution from the scatter photons in an external photon beam. The purpose of this study is to develop an improved analytical formula to describe SPR as a function of circular field size (r) and depth (d) using Monte Carlo (MC) simulation.

Methods:

MC simulation was performed for Mohan photon spectra (Co‐60, 4, 6, 10, 15, 23 MV) using EGSNRC code. Point‐spread scatter dose kernels in water are generated. The scatter‐to‐primary ratio (SPR) is also calculated using MC simulation as a function of field size for circular field size with radius r and depth d. The doses from forward scatter and backscatter photons are calculated using a convolution of the point‐spread scatter dose kernel and by accounting for scatter photons contributing to dose before (z' d) reaching the depth of interest, d, where z' is the location of scatter photons, respectively. The depth dependence of the ratio of the forward scatter and backscatter doses is determined as a function of depth and field size.

Results:

We are able to improve the existing 3‐parameter (a, w, d0) empirical formula for SPR by introducing depth dependence for one of the parameter d0, which becomes 0 for deeper depths. The depth dependence of d0 can be directly calculated as a ratio of backscatter‐to‐forward scatter doses for otherwise the same field and depth. With the improved empirical formula, we can fit SPR for all megavoltage photon beams to within 2%. Existing 3‐parameter formula cannot fit SPR data for Co‐60 to better than 3.1%.

Conclusion:

An improved empirical formula is developed to fit SPR for all megavoltage photon energies to within 2%.
SU‐E‐T‐440: Elliptical Source Model for IMRT and VMAT with Complex Modulation
Authors: Tang G , Lim S , LoSasso T
Published: July 2014

Purpose:

To improve dose calculation accuracy for highly modulated fields in IMRT and VMAT using an elliptical source model.

Methods:

For 2 TrueBeam linacs equipped with Millennium and HD MLCs, the source model of 6 and 15 MV beams was optimized with adjustments of the focal spot dimensions in the Varian Eclipse TPS. Starting from the measured focal spot size of 1.75 × 1.75 mm, the y‐dimension of the source was varied while the x‐dimension was kept constant. In addition, all other modeling parameters such as the extrafocal source and dosimetric leaf gap remained unchanged. The different models were compared with measured penumbra for jaws and MLC using ion chamber and diode. For 2 IMRT and 2 VMAT cases, the calculated doses were compared with radiochromic film and EPID measurements.

Results:

A source size of 1.75 × 0.75 mm was found to be optimal for beam modeling. The calculated penumbra (80–20) of a MLCdefined open field was within 0.8 mm compare to measurement for both the inline and crossline profile for various field sizes and depths. The dose gradients in the direction perpendicular to the MLC travel direction was better modeled compared to the source model with a circular focal spot of 1.75 mm for the fields with rigorous MLC modulation. Local differences up to 8% were found in those regions.

Conclusion:

The conventional source model using a symmetric focal spot tends to overestimate the penumbra perpendicular to the MLC travel direction, resulting in an inaccurate dose calculation especially for fields with complex MLC modulation. This can be improved with an elliptical source model and it can be considered as a standard as IMRT and VMAT treatments are becoming a norm for the clinic.
SU‐E‐T‐441: Comparison of Dose Distributions for Spot‐Scanned Pencil‐Beam and Scattered‐Beam Proton Treatments of Ocular Tumors
Authors: Deisher A , Whitaker T , Kooy H , Trofimov A , Kruse J
Published: July 2014

Purpose:

To study the cross‐field and depth dose profiles of spot‐scanned pencil beam configurations for the treatment of ocular tumors and to compare their performance to a simulated scattered beam.

Methods:

Dose distributions in a cubic water phantom were compared for beams that passed through a final 24mm diameter aperture to deposit maximum dose at 2.4cm depth. The pencil‐beam spots formed a hexagonally‐packed ring with a center‐to‐center spacing of 4mm. The protons exited the nozzle with energy 95.5MeV, traversed a 4.5cm water‐equivalent range shifter, and travelled either 42.5cm or 100cm to the phantom surface. The aperture‐to‐phantom distance (APD) was 5.7cm to allow room for eye‐tracking hardware. A configuration with APD=0 was also tested. The scattered beam was generated with energy 159MeV, passed through 127mm of Lexan, exited the final aperture, and travelled 5.7cm to the phantom surface. This latter configuration is comparable to the MGH single scattered beamline. All beams were modelled with TOPAS1.0‐beta6 compiled with GEANT4.9.6p2.

Results:

The modeled scattered beam produced a distal fall‐off along the central axis of zd90%‐zd10%=3.6mm. For the pencil beam, the zd90%‐zd10% was 1.6mm in all configurations. The scattered beam's cross‐field penumbra at depth of maximum dose was r90%‐ r10%=1.9mm. For the spot‐scanned configuration with the range‐shifter‐tophantom distance (RsPD) of 100cm, similar cross‐field profiles were achieved with r90%‐r10%=2.0mm. At shorter RsPD of 42.5cm, the crossfield penumbras were 5.6mm and 7.7mm for APD=0cm and APD=5.7cm, respectively.

Conclusion:

For proton treatments employing a range shifter, the cross‐field and central axis dose profiles depend on the quality of the original beam, the size of the range shifter, the distance from the range shifter exit to the patient, and the distance from the final aperture to the patient. A spot‐scanned pencil beam configuration can achieve cross‐field penumbras equal to a scattered beam and superior distal gradients.
SU‐E‐T‐442: Sensitivity of Quality Assurance Tools to Delivery Errors On a Magnetic Resonance‐Imaging Guided Radiation Therapy (MR‐IGRT) System
Authors: Rodriguez V , Li H , Yang D , Kashani R , Wooten H , Dempsey J , Mutic S , Green O
Published: July 2014

Purpose:

To test the sensitivity of the quality assurance (QA) tools actively used on a clinical MR‐IGRT system for potential delivery errors.

Methods:

Patient‐specific QA procedures have been implemented for a commercially available Cobalt‐60 MR‐IGRT system. The QA tools utilized were a MR‐compatible cylindrical diode‐array detector (ArcCHECK) with a custom insert which positions an ionization chamber (Exradin A18) in the middle of the device, as well as an in‐house treatment delivery verification program. These tools were tested to investigate their sensitivity to delivery errors. For the ArcCHECK and ion chamber, a baseline was established with a static field irradiation to a known dose. Variations of the baseline were investigated which included rotated gantry, altered field size, directional shifts, and different delivery time. In addition, similar variations were tested with the automated delivery verification program that compared the treatment parameters in the machine delivery logs to the ones in the plan. To test the software, a 3‐field conformal plan was generated as the baseline.

Results:

ArcCHECK noted at least a 13% decrease in passing rate from baseline in the following scenarios: gantry rotation of 1 degree from plan, 5mm change in field size, 2mm lateral shift, and delivery time decrease. Ion chamber measurements remained consistent for these variations except for the 5 second decrease in delivery time scenario which resulted in an 8% difference from baseline. The delivery verification software was able to detect and report the simulated errors such as when the gantry was rotated by 0.6 degrees, the beam weighting was changed by a percent, a single multileaf collimator was moved by 1cm, and the dose was changed from 2 to 1.8Gy.

Conclusion:

The results show that the current tools used for patient specific QA are capable of detecting small errors in RT delivery with presence of magnetic field.
SU‐E‐T‐443: Developmental Technique for Proton Pencil Beam Measurements: Depth Dose
Authors: Arjomandy B , Lee T , Schultz T , Hsi W , Park S
Published: July 2014

Purpose:

Measurements of depth dose distribution (DDD) of pencil beam in proton therapy can be challenging and time consuming. We have developed a technique that uses two Bragg peak chambers to expedite these measurements with a high accuracy.

Methods and Material:

We used a PTW water tank and two PTW 10.5 cm3 Bragg peak chambers; one as a field chamber and the other as a reference chamber to measure DDDs for 100–250 MeV proton pencil beams. The reference chamber was positioned outside of the water tank upstream with respect to field chamber. We used Geant4 Monte Carlo Simulation (MCS) to model the ProTom proton beam to generate DDDs. The MCS generated DDDs were used to account for halo effects of proton pencil beam that are not measureable with Bragg peak chambers. We also used PTW PEAKFINDER to measure DDDs for comparison purpose.

Results:

We compared measured and MCS DDDs with Continuous Slowing Down Approximation (CSDA) ranges to verify the range of proton beams that were supplied by the manufacturer. The agreements between all DDD with respect to CSDA were within ±0.5 mm. The WET for Bragg peak chamber for energies between 100–250 MeV was 12.7 ± 0.5 mm. The correction for halo effect was negligible below 150 MeV and was in order of ∼5‐10% for 150–250 MeV.

Conclusion:

Use of Bragg Peak chamber as a reference chamber can facilitate DDD measurements in proton pencil beam with a high accuracy. Some corrections will be required to account for halo effect in case of high energy proton beams due to physical size of chamber.
SU‐E‐T‐444: Quantitatively Comparison of Low Dose Spillage Outside of PTV Edge in Arc Therapy Modalities
Authors: Gong J , Sarwan R , Pavord D
Published: July 2014

Purpose:

To quantitatively compare low dose spillage outside of PTV edge in arc therapy modalities

Methods:

The machines used in the study are Tomotherapy Hi‐Arc and Varian 21EX with millennium120 MLC. TPS are TomoPlaning and RayStation for VMAT, respectively. The phantom is a 30cm diameter cylindrical solid water (TOMOTHERAPY, TOMOPHANTOM ASSY). The PTV is 4cm length with ellipsoidal sectional shape with major axis=5cm, minor axis=3cm in the axial plane and reversed in the coronal plane. The PTV volume is created with interpolation. It is located at the center of the phantom. The prescribed dose is 1000×5 cGy to 95% the PTV. The isocenter is set co‐centered with the PTV. EBT‐3 film was used to measure iso‐dose lines at the center plane. Film dosimetry is performed with the RIT, v6.2.

Results:

the study shows: (1) dose falloff gradient is usually uneven, depending on the PTV shape in the gantry rotation plane. For an elliptical shape, the low dose spillage is wider in the minor axis direction than that in the major axis direction. The more a shape is closer to circular, the more even gradient is all directions; (2)for a circular shape (CAX plane in this study), the maximum dose in % of Rx dose at 2cm from PTV is 55% for Tomo, vs. 70% for VMAT (3) the most rapid dose falloff rang is between 95%–80% IDL for both modalities.

Conclusion:

Tomo has more rapid dose falloff outside of PTV. In some areas, the gradient is double for Tomo helical than that for LINAC VMAT at same points. Future work will examine the differences between optimization of doses and inherent delivery limitations.
SU‐E‐T‐445: Lateral Optical Density Variation in Flatbed Scanners in Combination with Gafchromic Film
Authors: van Battum LJ , Heukelom S
Published: July 2014

Purpose

This study investigates the origin of lateral optical density (OD) variation for Gafchromic film (EBT and EBT2) scanned in transmission mode with Epson flatbed scanners (1680 Expression Pro and 10000XL). Effects investigated are: cross talk, optical path length and polarization.

Methods

Cross talk has been examined with triangular shaped light‐transmission sheets with OD ranging from 0 to opaque. Optical path length has been studied with absorptive and reflective OD‐filters (OD range 0.2 to 2.0). Dependency on light‐polarization on the scanner read out has been investigated using linear polarizer sheets. All experiments have been performed at centre scanner position (norm point) and at several lateral scan positions, without and with (un)irradiated EBT‐film. Dose values used ranged between 0.2 to 9 Gy, yielding an OD‐range between 0.25 to 1.1.

Results

The lateral OD variation is dose dependent and increases up to 14% at most lateral position for dose up to 9 Gy. Cross talk effect contributes to 0.5% in clinical used OD ranges but equals 2% for extreme high dose gradients. Film induced optical path length will effect the lateral OD variation up to 3% at most lateral points. Light polarization is inherent present in these scanners due to multiple reflection on mirrors. In addition film induced polarization is the most important effect generating the observed lateral OD variation. Both Gafchromic film base and sensitive layer have polarizing capabilities; for the sensitive layer its influence is dose dependent.

Conclusions

Lateral OD variation origins from optical physics (i.e. polarization and reflection) related to scanner and film construction. Cross talk can be ignored in film dosimetry for clinical used dose values and gradients. Therefore it is recommended to determine the lateral OD variation per film type and scanner.
SU‐E‐T‐446: Evaluation of the Dosimetric Properties of a Diode Detector to Proton Radiosurgery
Authors: Teran A , McAuley G , Slater J D , Slater J M , Wroe A
Published: July 2014

Purpose:

To test the PTW PR60020 proton dosimetry diode in radiation fields relevant to proton radiosurgery applications and evaluate its suitability as a high resolution, real time dosimetry device.

Methods:

Data was collected using our standard nominal radiosurgery energies of 126 MeV and 155 MeV through a single stage scattering system, corresponding to a range of 9.7 and 15 cm in water respectively. Various beam modulations were tested as part of this study. Depth dose and beam profile measurements were completed with the PTW PR60020 dosimetry diode with comparative measurements using a PTW Markus ionization chamber and EBT2 Gafchromic film. Monte Carlo simulations were also completed for comparison.

Results:

The single 1 mm2 by 20 μm thick sensitive volume allowed for high spatial resolution measurements while maintaining sufficient sensitive volume to ensure that measurements could be completed without excessive beam delivery. Depth dose profiles exhibited negligible LET dependence which typically impacts film and other solid state dosimetry devices, while beam ranges measured with the PTW diode were within 1 mm of ion chamber data. In an edge on arrangement beam profiles were also measured within 0.5 mm full‐width at half‐maximum at various depths as compared to film and simulation data.

Conclusion:

The PTW PR60020 proved to be a very useful radiation metrology apparatus for proton radiosurgery applications. Its waterproof and rugged construction allowed for easy deployment in phantoms or water tanks that are commonly used in proton radiosurgery QA. Dosimetrically, the diode exhibited negligible LET dependence as a function of depth, while in edge on arrangement to the incident proton beam it facilitated the measurement of beam profiles with a spatial resolution comparable to both Monte Carlo and film measurements.

This project was sponsored in part by funding from the Department of Defense (DOD# W81XWH‐BAA‐10‐1).
SU‐E‐T‐447: Electronic Brachytherapy (EBT) Treatment of Cervical Cancer – First Clinical Experience
Authors: Johnson D , Johnson M , Chan L , Hausen H , Thompson J , Ahmad S
Published: July 2014

Purpose:

To study the first trial patient in which an electronic brachytherapy (EBT) x‐ray source is utilized for treatment of cervical cancer.

Methods:

During patient treatment, a miniaturized x‐ray source was used in combination with a customized titanium tandem and ovoid applicator set. The semi‐specialized source was modeled with formalisms outlined by AAMP Task Group 43. Multiple models were used to compensate for variable attenuation conditions as a function of source positions. Varian Brachyvision treatment planning software was utilized on CT data sets for dose calculations prior to treatment delivery. The dose was prescribed to “point A” as defined by American Brachytherapy society. Additional treatments plans were created from those clinically utilized in patient care and were recalculated for an existing Ir‐192 source model. Dose volume histograms (DVH) and point dose calculations were compared between the modalities for the clinical condition present in patients treated with EBT.

Results:

Clinical treatment times, though longer than those typically experienced by Ir‐192 users, were manageable. Instantaneous dose rates at personal positions within the treatment vault were lower than those measured during intra operative radiation therapy and breast EBT treatments. Due to lower average photon energy in EBT, dose gradients within the treatment plans were as expected steeper than those observed in Ir‐192 based brachytherapy. DVH comparisons between Ir‐192 and EBT treatments showed an expected decrease in the integral dose to normal tissues of interest for EBT. In comparing plans created for EBT delivery with those calculated for Ir‐192, average dose values for EBT were more than 4%, 11%, and 9% lower at predefined bladder, rectum and “point B” positions, respectively.

Conclusion:

For the first time, we have demonstrated that the utilizing electronic brachytherapy system for tandem and ovoid based treatment of cancer of the cervix is feasible, and potentially beneficial, in clinical environment.