bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐496: Influence of the Intrafraction Patient Repositioning On Spot Scanning Proton Therapy for Prostate Cancer
Authors:   Proton Therapy for Prostate Cancer, Fujii Y , Matsuura T , Takao S , Matsuzaki Y , Yamada T , Miyamoto N , Shimizu S , Umegaki K , Shirato H
Published: June 2015

Purpose

To verify the effect of intrafraction patient repositioning based on real time image guidance on the proton dose distribution for prostate cancer.

Methods

Dose distributions were simulated using CT images of prostate cancer patients. Treatment plans were prepared using proton beam from left side of the patient with 3 and 6 mm lateral margins. During the beam delivery, prostate position was assumed to be shifted by 5 mm in anterior, posterior, superior and inferior directions. The dose distributions were calculated for two scenarios; (i) the patient position was readjusted by 5 mm just after the prostate shift occurs (ii) the patient position was not adjusted. Such a real time readjustment can be realized with real time image guidance. The differences of the values D5 and D95 in CTV (D5–95) were compared between different sizes of the lateral margins and different scenarios.

Results

D5–95 values in the case of scenario (i) are smaller than scenario (ii). The differences between scenario (i) and (ii) were not depend on the directions of the prostate shift. In the case of scenario (ii), margin expansion makes D5–95 values small. In the case of scenario (i), margin size is uncorrelated to D5–95 values.

Conclusion

These results indicate that the intrafraction patient repositioning is effective to minimize dose deterioration when the prostate moves during the irradiation. In the case without intrafraction patient repositioning, margin expansion is effective to reduce dose deterioration. In the case with intrafraction patient repositioning, 3 mm margin is enough to minimize dose deterioration.
SU‐E‐T‐497: Initial Characterization of a Novel 2D Computed Radiography (CR) Dosimeter for SBRT
Authors: Crijns W , Ramaekers S , Leblans P , Defraene G , Maes F , Haustermans K , Depuydt T
Published: June 2015

Purpose

For 2D, sub‐mm resolution dose measurements Gafchromic™ film is currently a standard in radiotherapy; mainly because of its energy independence and water equivalence. However, EBT film is disposable. Therefore, the dosimetric and uniformity characteristics needs to be estimated from a second, possibly different, (calibration) film. Moreover, EBT has a post‐irradiation coloration and a non‐linear dose dependence with saturation, which limits the applicable time interval and dose range. CR technology forms an interesting alternative for EBT. Dose dependent CRplates have a sub‐mm resolution, and additionally a linear dose dependence over decades of dose. But, CR has an inherent signal fading and energy dependence. Here, for the first time, a radiotherapy 2D CR prototype was characterized for an extended dose range (up to 35Gy), signal fading, and basic energy dependence.

Methods

The prototype was irradiated with a standard 10×10cm 6MV photon beam, and scanned with a commercial CR 15‐X(R) scanner. The time between the start of irradiation and scanning (T_Scan) was monitored. The linearity of the dose response was evaluated between 0 and 35Gy using a fixed T_Scan of 4min, if possible (i.e. ≤10Gy). Next, the signal fading was characterized for a T_Scan‐range of 4 to 20min. The energy dependence was assessed by a comparison of out‐of‐field measurements of CR, EBT, and TLD.

Results

The radiotherapy CR prototype has a linear response over the complete SBRT dose range (0– 35Gy). The prototype had a small (5%), but linear signal fading over the time interval of interest (4–20min). Out‐of‐field, the prototype has a 8% over response due to an increased amount of low energy photons. The impact of the over response on intensity modulated radiotherapy remains to be evaluated.

Conclusion

CR technology is promising for SBRT dose measurements up to 35Gy. It is a reusable linear alternative for film dosimetry.
SU‐E‐T‐501: Initial Orthovoltage Beam Profile Analysis of a Small Brass MLC
Authors: Loughery B , Snyder M
Published: June 2015

Purpose

To create brass leaves for an orthovoltage MLC and take initial beam profile measurements.

Methods

The low‐energy MLC was designed in previous work. Brass was chosen for its self‐lubrication and low cost. Stock brass rectangles (30cm × 1.0cm × 0.5cm) were ordered with pre‐cut gear rack along the topmost long edges. Leaf designs were translated into G‐code, then cut with a Tormach CNC‐1100 mill. Intense bowing was observed in the beam direction, which required straightening via an in‐house jig. Straightened leaves were placed into MLC assembly and mounted to a 320 kVp orthovoltage tube. EDR2 film was irradiated in four situations: MLC open so one edge was isocentric, and MLC open more than isocentric, completely closed MLC, and an open field shot with the MLC removed. The first two scans tested penumbra for our rectangular edges due to unfocused design. The final two scans tested transmission and interleaf leakage. All four experiments were set to 120 kVp and 10 mA for two minutes.

Results

Transmission and interleaf leakage were found to be zero. Interleaf leakage is faintly visible on film, but undetected by our film scanner despite high spatial resolution. Penumbra at isocenter was found to be 0.72mm, which matched the penumbras of true field edges. Penumbra off‐isocenter was 1.1mm. Mechanically, leaves are moving smoothly once straightened.

Conclusion

Beam profiles through our brass MLC are acceptable. Leaves attenuate and move as designed. Looking forward, we intend to animate our MLC to deliver more complicated treatment plans.
SU‐E‐T‐505: Inter‐Comparison of Clinical Plans Created On Truebeam with Trilogy and Clinac
Authors: Sharma M , Wu Y , Kim S
Published: June 2015

Purpose

The purpose of this study is to investigate the dosimetric differences in clinical treatment plans when machine parameters are switched from Varian Truebeam to older Varian models and vice‐versa. This is used to determine the clinically‐safe limits to directly transfer patients from Truebeam to Trilogy or Clinac.

Methods

Thirty‐two patients with cancer of different treatment sites such as lung, head‐and‐neck, prostate and breast were studied. The clinically approved IMRT, VMAT or 3D conformal treatment plans were delivered either on Truebeam or Clinac/Trilogy. Keeping the monitor units, fraction size and other machine parameters the same, the plans were recomputed on a different Varian linear accelerator (e.g., Truebeam plans were recomputed on Trilogy and vice‐versa). The plans comparison was done using the target D98 and normal organ D2, D10, D20, D30, D50 and D70 dose metrics.

Results

The perfraction dose‐difference in the PTV‐D98 for all 32 patients varied from 0.01 to −0.06 Gy with median being −0.04 Gy. For a 2Gy/fraction treatment course, this would be maximum PTV‐D98 dose‐difference of 6 cGy/fraction or 30 cGy/5fractions. For organs‐at‐risks the maximum per‐fraction dosedifference in D2, D5, D10, D20, D30, D50 and D70 between Truebeam and Trilogy/Clinac plans varied from 0.06 to −0.07 Gy with median being with — 0.02 Gy. The 3D‐CRT plans had relatively lower dose‐difference in comparison to IMRT and VMAT plans. The dose computed on Truebeam was systematically lower than the dose computed on Trilogy/ Clinac.

Conclusions

for conventional fractionation schedules, assuming the maximum uncertainty of <2%, it is clinically safe to switch the treatment machine for 5 fractions. For hypo‐fractionated treatments with higher dose per‐fraction, plans may need to be revisited before switching the linear accelerator from Truebeam to Clinac/Trilogy or vice‐versa.
SU‐E‐T‐506: Intercomparison Study On Small Field Output Factor Measurements
Authors: Talamonti C , Casati M , Compagnucci A , Arilli C , Greto D , Marrazzo L , Pallotta S , Zani M , Marinelli M , Menichelli D , Scotti L , Verona G
Published: June 2015

Purpose

In radiotherapy, uncertainties due to small field measurements (SFM) introduce systematic errors to the treatment process and the development of new dosimeters for quality assurance programs is a challenge. In this work we analyze the behavior of seven detectors measuring output factors of 6MV photon beam.

Methods

The dosimeters employed are: a single cristal diamond detector (SCCD) developed at the University of Rome Tor Vergata, a silicon diode developed within the project MAESTRO, a IBA Razor silicon diode, A1SL and A26 Exradin ion chambers, an EBT3 Gafchromic film and the Exradin W1 Scintillator.Diamond sensitive volume is a cylinder 2.2mm in diameter and 1μm thick. MAESTRO diode is 2×2mm2 active area. Razor sensitive volume is a cylinder 0.6 mm in diameter and 0.02 mm thick. A16 and A1Sl have a collecting volume of 0,015cc and 0,053cc. The W1 is an optical fiber with an active volume of 0.002cc. All measurements were performed in a water phantom, with detector positioned at the isocenter (SSD=90cm, d=10cm), MAESTRO diode being sandwiched in solid water to obtain an equivalent experimental setup.

Results

These measurements are challenging due to the absence of charged particle equilibrium conditions, detector size and positioning problems. They are in good agreement among each other, especially GAF, Razor, W1 and SCDD. Maximum deviations reported are related to the field 0.8×0.8cm2 for MAESTRO and chambers data with respect to EBT3: around 15% (A1SLvsEBT3), 16% (MAESTROvsEBT3). Razor and W1 show a deviation around 3% with respect to SCDD.

Conclusion

In this work measurements made with a variety of detectors are compared. These study show the possibility to choose different detectors for SFM and that smaller ion chambers are still not competitive with solid state detectors. Silicon, diamond and optical fiber dosimeters show a similar behavior with minor discrepancies for the smallest field.
SU‐E‐T‐508: Internal Organ Motion Effect On Radiation Dose to a Point Under Half‐Beam Block Match Line
Authors: Zhou S , Zhu X , Zhang M , Zheng D , Lei Y , Zhang Q , Li S , Driewer J , Wang S , Enke C
Published: June 2015

Purpose

Half‐beam block is a field matching technique frequently used in radiotherapy. With no setup error, a well calibrated linac, and no internal organ motion, two photon fields can be matched seamlessly dosimetry‐wise with their central axes passing the match line. However, in actual clinical situations, internal organ motion is often inevitable. This study was conducted to investigate its influence on radiation dose to patient internal points directly under the matching line.

Methods

A clinical setting is modeled as two half‐space (x<0 and x<0) radiation fields that are turned on sequentially with a time gap of integer times of the patient internal organ motion period (T0). Our point of interest moves with patient internal organs periodically and evenly in and out of the radiation fields, resulting in an average location at x=0. When the fields are delivered without any motion management, the initial phase of the point's movement is unknown. Statistical methods are used to compute the expected value () and variance (σ) of the point dose given the uncertainty.

Results

Analytical solutions are obtained for  and s of dose received by a point directly under the match line.  is proportional to the total beam‐on time (T1), and σ demonstrates previously unknown periodic behavior. /<Dσ has local maximums (minimums) when T1 is 2n*T0 ((2n+1)*T0) (n is an integer) with an upper envelope scales as 1/T1. Worst case scenario the point dose can be zero or double of the expected value.

Conclusion

We have analytically analyzed the internal organ motion effect on radiation dose received by a patient internal point directly under a half‐beam block match line. Our results help us to better understand this phenomenon and facilitate the reduction of point dosimetric uncertainties in our clinical practice.
SU‐E‐T‐513: Investigating Dose of Internal Target Volume After Correcting for Tissue Heterogeneity in SBRT Lung Plans with Homogeneity Calculation
Authors: Qi P , Zhuang T , Magnelli A , Djemil T , Shang Q , Balik S , Andrews M , Stephans K , Videtic G , Xia P
Published: June 2015

Purpose

It was recommended to use the prescription of 54 Gy/3 with heterogeneity corrections for previously established dose scheme of 60 Gy/3 with homogeneity calculation. This study is to investigate dose coverage for the internal target volume (ITV) with and without heterogeneity correction.

Methods

Thirty patients who received stereotactic body radiotherapy (SBRT) to a dose of 60 Gy in 3 fractions with homogeneous planning for early stage non‐small‐cell lung cancer (NSCLC) were selected. ITV was created either from 4DCT scans or a fusion of multi‐phase respiratory scans. Planning target volume (PTV) was a 5 mm expansion of the ITV. For this study, we recalculated homogeneous clinical plans using heterogeneity corrections with monitor units set as clinically delivered. All plans were calculated with 3 mm dose grids and collapsed cone convolution algorithm. To account for uncertainties from tumor delineation and image‐guided radiotherapy, a structure ITV2mm was created by expanding ITV with 2 mm margins. Dose coverage to the PTV, ITV and ITV2mm were compared with a student paired t‐test.

Results

With heterogeneity corrections, the PTV V60Gy decreased by 10.1% ± 18.4% (p<0.01) while the maximum dose to the PTV increased by 3.7 ± 4.3% (p<0.01). With and without corrections, D99% was 65.8 ± 4.0 Gy and 66.7 ± 4.8 Gy (p=0.15) for the ITV, and 63.9 ± 3.4 Gy and 62.9 ± 4.6 Gy for the ITV2mm (p=0.22), respectively. The mean dose to the ITV and ITV2mm increased 3.6% ± 4.7% (p<0.01) and 2.3% ± 5.2% (p=0.01) with heterogeneity corrections.

Conclusion

After heterogeneity correction, the peripheral coverage of the PTV decreased to approximately 54 Gy, but D99% of the ITV and ITV2mm was unchanged and the mean dose to the ITV and ITV2mm was increased. Clinical implication of these results requires more investigation.
SU‐E‐T‐517: Investigation of Factors Contributing to Extracranial Radiation Doses From Leksell Gamma Knife
Authors: Kon D , Nakano M , Nawa K , Haga A , Nakagawa K
Published: June 2015

Purpose

The purpose of this study is to investigate dominant factors for doses to extracranial sites in treatment with Leksell Gamma Knife (LGK).

Methods

Monte Carlo simulation was implemented using EGS5 version 1.4.401. The simulation was divided into two major steps for the purpose of efficiency. As the first step, phase‐space files were obtained at a scoring plane located just below patient‐side surface of the collimator helmet of LGK. Scored particles were classified into three groups, primary, leakage and scatter, using their history information until their arrival to the scoring plane. Then classification was used at the following second step simulation to investigate which type of particle is dominant in the deposited energy at extra‐cranial sites. In the second stage, a cylindrical phantom with a semisphere shaped head was modeled such that the geometrical center of the phantom's head corresponds to the unit center point (UCP) of LGK. Scoring regions were arranged at 10 cm intervals from the UCP to 70 cm away on the central axis of the phantom. Energy deposition from each type of particles and location of interaction were recorded.

Results

The dominant factor of deposited energy depended on the collimator size. In the case of smaller collimator size, leakage was dominant. However, contribution of leakage was relatively small in the case of larger collimator size. The contribution of internal scatter varied with the distance from the UCP. In the proximal areas, internal scatter was dominant, whereas in the distal areas, particles interacting with machine components became dominant factor.

Conclusion

The Result of this study indicates that the dominant factor to dose to an extracranial site can vary with the distance from UCP and with collimator size. This means that the variation of this contribution must be considered for modeling of the extracranial dose especially in the distal area.

This work was partly supported by the JSPS Core‐to‐Core Program (No. 23003).
SU‐E‐T‐522: Investigation of Underdosage of Total Body Irradiation with Bilateral Irradiation Scheme
Authors: Lin T , Eldib A , Hossain M , Price R , Ma C
Published: June 2015

Purpose:

Patient in‐vivo measurements report lower readings than those predicted from TMR‐based treatment planning on TBI patient knees and ankles where rice was placed to fill the gap between patient's legs. This study is to understand and correct the under dosage of Total Body Irradiation(TBI) with rice tissue equivalent bolus placement at TBI treatment patient setup.

Methods:

Bilateral TBI scheme was investigated with rice bags bolus placing between patient's two legs acting as missing tissue. In‐house TMR based treatment planning system was commissioned with measurements under TBI condition at 10MV, i.e. source‐to‐reference distance 383.4cm with 40×40cm field size with 1cm thickness Lucite. Predictions of patient specific dose points are reported at different sites with 200cGy prescription at patient umbilicus point. Solid water and rice bag phantoms are used at TBI conditions for the attenuation factor verification and CT scanned to verify the CT number and electron density.

Results:

We found that the rice bag bolus overall density is 11% lower than the water; however, the attenuation factor of rice bags could become 15% lower than that of water at TBI condition. This overestimate of rice bag electron density could cause the lack of lateral scatter and the lack of backscatter. This could Result in an overestimate of dose at in‐vivo dosimeter measurement points with TMR‐based treatment planning systems. Observations of patient specific optically stimulated luminescent dosimeters(OSLDs) were used to confirm this overestimation. Measurements of setups with increasing the rice bag filled patient leg separation were performed to demonstrate eliminating the overdose issue.

Conclusion:

Rice bolus has a lower electron density than water does(11%) but results in 15% lower in attenuation factor at TBI condition. This effect was observed in patient delivery with OSLD measurements and can be corrected by increasing the filling rice bolus thickness with 15% longer of separation.
SU‐E‐T‐524: In‐Vivo Diode Dosimetry Proton Therapy Range Verification Validation Study for Pediatric CSI
Authors: Toltz A , Hoesl M , Schuemann J , Seuntjens J , Lu H , Paganetti H
Published: June 2015

Purpose:

With the aim of reducing acute esophageal radiation toxicity in pediatric patients receiving craniospinal irradiation (CSI), we investigated the implementation of an in‐vivo, adaptive proton therapy range verification methodology. Simulation experiments and in‐phantom measurements were conducted to validate the range verification technique for this clinical application.

Methods:

A silicon diode array system has been developed and experimentally tested in phantom for passively scattered proton beam range verification for a prostate treatment case by correlating properties of the detector signal to the water equivalent path length (WEPL). We propose to extend the methodology to verify range distal to the vertebral body for pediatric CSI cases by placing this small volume dosimeter in the esophagus of the anesthetized patient immediately prior to treatment. A set of calibration measurements was performed to establish a time signal to WEPL fit for a “scout” beam in a solid water phantom. Measurements are compared against Monte Carlo simulation in GEANT4 using the Tool for Particle Simulation (TOPAS).

Results:

Measurements with the diode array in a spread out Bragg peak of 14 cm modulation width and 15 cm range (177 MeV passively scattered beam) in solid water were successfully validated against proton fluence rate simulations in TOPAS. The resulting calibration curve allows for a sensitivity analysis of detector system response with dose rate in simulation and with individual diode position through simulation on patient CT data.

Conclusion:

Feasibility has been shown for the application of this range verification methodology to pediatric CSI. An in‐vivo measurement to determine the WEPL to the inner surface of the esophagus will allow for personalized adjustment of the treatment plan to ensure sparing of the esophagus while confirming target coverage.

A Toltz acknowledges partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant number: 432290).
SU‐E‐T‐535: Linac Based Multiple SRS Targets Plan Evaluation: Understanding Conformity Index Generated Form BrainLab IPlan RT Dose 4.1 SRS Planning System
Authors: Workie D , Hasson B , Geraghty C , Torano A , Dad L , Mary Y
Published: June 2015

Purpose:

The study intent is to provide a practical method of extracting Conformity Index for multiple Stereotactic Radiosurgery (SRS) target plans generated from BrainLab iPlan RT Dose 4.1 planning system (CIiPlan).

Methods:

CIiPlan is the ratio between normal tissue volume (VN, IDL) and target volume (VPTV, IDL) encompassed by the prescription isodose line (IDL). BrainLab iPlan RT Dose 4.1 planning and physics documents lack details on how to retrieve and interpret CIiPlan parameters of multiple SRS targets. The CIiPlan values for multiple lesions are misleading and in most cases gave bigger values than what is reflected from the plan. To understand and demonstrate the underlying issues CIiPlan values of patient plans with multiple SRS targets were extracted using two different methods. Method 1: CIiPlan values were directly obtained with all target plans present (CITN,iPlan). Method 2: CIiPlan values were extracted by assigning 100% weighting for one of the targets and 0 weighting for the rest of the targets (CIIN,iPlan). This process was t repeated for each target. In both cases CIiPlan were read from DVH graphs at the prescription IDL.

Results:

CITN,iPlan values from Method 1 were larger than CIIN,iPlan values obtained using Method 2. The difference was pronounced for smaller lesions. Even though targets were far apart and were not covered with the same prescription IDL, CIiPlan values from Method 1 were based on normal tissue volume encompassed by 5 treatment IDL from all targets. Carefully analyzing CIiPlan values from the two methods we were able to provide appropriate CIiPlan formulas that gave good estimate of both methods values.

Conclusion:

CIiPlan for multiple SRS targets need be analyzed with great caution; especially if small lesions and big lesions are part of same plan. CIIN,iPlan values based on each target normal tissue volume encompassed by the prescription IDL should be used.
SU‐E‐T‐536: LINAC‐Based Single Isocenter Frameless SRT for Brain Metastases
Authors: Liu B , Zhang L , Rigor N , Kim J
Published: June 2015

Purpose:

Single‐isocenter Stereotactic Radiotherapy of multiple brain metastases with Varian 21 IX LINAC, using Aktina Pinpoint system for patient setup.

Methods:

In 2014, five single‐isocenter RapidArc SRT plans were delivered to five patients with 2 to 8 brain metastases using Varian 21 IX. Aktina Pinpoint system was used for setup and 2mm PTV margin were used. CBCT was acquired before and after the beam delivery. The prescription is 2100 cGy in 3 fractions. Eclipse planning system was used for treatment planning. Depending on the number of metastases and their locations, 1 to 5 coplanar or non coplanar arcs were used. Typically, 2 or 3 arcs are used. IMRT QAs were performed by comparing an A1SL ion chamber point dose measurement in solid water phantom to point dose of the plan; also, based on EPID measurement, 3D spatial dose was calculated using DosimetryCheck software package from MathResolutions Inc. The EPID system has an active area of 40cm by 30cm with 1024 by 768 photodiodes, which corresponds to a resolution of 0.4mm by 0.4mm pixel dimension.

Results:

for all the plans, at least 95% PTV coverage was achieved for full prescription dose, with plan normalization > 75%. RTOG conformity indices are less than 1.1 and Paddick gradient indices are less than 4.5. The distance from prescription IDL to 50% IDL increases as the number of metastases increases, and it ranges from 0.6mm to 0.8mm. Treatment time varies from 10mins to 30mins, depending on the number of arcs and if the arcs are coplanar. IMRT QA shows that the ion chamber measurement agree with the eclipse calculation within 3%, and 95% of the points passed Gamma, using 3% dose difference and 3mm DTA

Conclusion:

High quality single isocenter RapidArc SRT plan can be optimized and accurately delivered using Eclipse and Varian 21IX.
SU‐E‐T‐540: MCNPX Simulation of Proton Dose Distributions in a Water Phantom
Authors: Lee C , Lee Y , Chen S , Chiang B , Tung C , Chao T
Published: June 2015

Purpose:

In this study, fluence and energy deposition of proton and proton by‐products and dose distributions were simulated. Lateral dose distributions were also been discussed to understand the difference between Monte Carlo simulations and pencil beam algorithm.

Methods:

MCNPX codes were used to build a water phantom by using “repeated structures” technique and the doses and fluences in each cell was recorded by mesh tally. This study includes, proton equilibrium and proton disequilibrium case. For the proton equilibrium case, the doses difference between proton and proton by‐products were studied. A 160 MeV proton pencil beam was perpendicularly incident into a 40 × 40 × 50 cm3 water phantom and the scoring volume was 20 × 20 × 0.2 cm3. Energy deposition and fluence were calculated from MCNPX with (1) proton only; and (2) proton and secondary particles. For the proton disequilibrium case, the dose distribution variation using different multiple Coulomb scattering were studied. A 70 MeV proton pencil beam was perpendicularly incident into a 40 × 40 × 10 cm3 water phantom and two scoring voxel sizes of 0.1 × 0.1 × 0.05 cm3 and 0.01 × 0.01 × 0.05 cm3 were used for the depth dose distribution, and 0.01 × 0.01 × 0.05 cm3 for the lateral profile distribution simulations.

Results:

In the water phantom, proton fluence and dose in depths beyond the Bragg peak were slightly perturbed by the choice of the simulated particle types. The dose from secondary particles was about three orders smaller, but its simulation consumed significant computing time. The depth dose distributions and lateral dose distributions of 70 MeV proton pencil beam obtained from MCNPX, GEANT4, and the pencil beam algorithm showed the significant deviations, probably caused by multiple Coulomb scattering.

Conclusion:

Multiple Coulomb scattering is critical when there is in proton disequilibrium.
SU‐E‐T‐541: Measurement of CT Density Model Variations and the Impact On the Accuracy of Monte Carlo (MC) Dose Calculation in Stereotactic Body Radiation Therapy for Lung Cancer
Authors: Xiang H , Li B , Russo G , Behrman R , Lu H , Fernando H , Kachnic L
Published: June 2015

Purpose:

To measure the CT density model variations between different CT scanners used for treatment planning and impact on the accuracy of MC dose calculation in lung SBRT.

Methods:

A Gammex electron density phantom (RMI 465) was scanned on two 64‐slice CT scanners (GE LightSpeed VCT64) and a 16‐slice CT (Philips Brilliance Big Bore CT). All three scanners had been used to acquire CT for CyberKnife lung SBRT treatment planning. To minimize the influences of beam hardening and scatter for improving reproducibility, three scans were acquired with the phantom rotated 120° between scans. The mean CT HU of each density insert, averaged over the three scans, was used to build the CT density models. For 14 patient plans, repeat MC dose calculations were performed by using the scanner‐specific CT density models and compared to a baseline CT density model in the base plans. All dose re‐calculations were done using the same plan beam configurations and MUs. Comparisons of dosimetric parameters included PTV volume covered by prescription dose, mean PTV dose, V5 and V20 for lungs, and the maximum dose to the closest critical organ.

Results:

Up to 50.7 HU variations in CT density models were observed over the baseline CT density model. For 14 patient plans examined, maximum differences in MC dose re‐calculations were less than 2% in 71.4% of the cases, less than 5% in 85.7% of the cases, and 5–10% for 14.3% of the cases. As all the base plans well exceeded the clinical objectives of target coverage and OAR sparing, none of the observed differences led to clinically significant concerns.

Conclusion:

Marked variations of CT density models were observed for three different CT scanners. Though the differences can cause up to 5–10% differences in MC dose calculations, it was found that they caused no clinically significant concerns.
SU‐E‐T‐640: Proton Modulated Arc Therapy Using Scanned Pencil Beams
Authors: Carabe‐Fernandez A , Kirk M , Sanchez‐Parcerisa D , Fager M , Burgdorf B , Stowe M , Solberg T
Published: June 2015

Purpose:

Rotational proton radiotherapy would be an interesting treatment modality if it proves to produce dose distributions that conform to the target as well or better than currently available treatment modalities, while reducing the dose to the surrounding organs at risk. A treatment planning study is presented, showing how this objective can be achieved using a single or small number of scanned mono‐energetic pencil beams delivered while the proton gantry rotates at the same time that they may deliver faster and more biologically effective treatments than current proton modalities.Methods Proton treatment using single‐ (SFO) and multi‐field optimized (MFO) PBS plans are compared with PMAT plans in phantoms of different shape and uniformity as well as on a brain case. The method followed in ECLIPSE to produce PMAT plans is presented and the feasibility is discussed. Results Table 1 shows that the conformity and uniformity of the PMAT plans is of similar order than those of the SFO and MFO plans. However, in the brain case, the DVHs shown in Figure 1 indicate a significant reduction of the dose to the OARs when PMAT is used. Similarly, the use of PMAT increases the LET in the treatment area, which could translate into a more biologically effective treatment.Conclusions PMAT could provide dose distributions as conformal as current PBS plans but could also offer faster beam delivery and more biologically effective proton plans.
SU‐F‐BRD‐13: A Phenomenological Relative Biological Effectiveness (RBE) Model for Proton Therapy Based On All Published in Vitro Cell Survival Data
Authors: McNamara A , Schuemann J , Paganetti H
Published: June 2015

Purpose:

Proton therapy treatments are currently planned and delivered using the assumption that the proton relative biological effectiveness (RBE) relative to photons is 1.1. This assumption ignores strong experimental evidence that suggests the RBE varies. A recent review study (Paganetti 2014), collected over 72 experimental reports on proton RBE, providing a comprehensive dataset for predicting proton RBE. Using this dataset we develop a model to predict RBE based on dose, linear energy transfer (LET) and the tissue specific parameter (α/β)x.

Methods:

The relationship of the RBE on dose, dose average LET (LETd) and (α/β)x was explored using 290 experimental data points. The RBE was calculated for each experimental point for doses ranging from 1 to 10 Gy. An RBE model, based on the linear‐quadratic model, was derived using a nonlinear regression fit to the experimental data.

Results:

The proposed RBE model predicts that the RBE increases with LET and decreases with (α/β)x. The model additionally predicts a decrease in RBE with increasing dose for low (α/β)x values (< 3 Gy). However the model predicts an increase in RBE with increasing dose for large (α/β)x (> 4 Gy). Previous phenomenological models were based on a small subset of experimental data and deviate significantly from our findings.

Conclusion:

The proposed RBE model is derived using the most comprehensive collection of proton RBE experimental data to date. The model agrees with previous theoretical predictions on the relationship between RBE, LETd and (α/β)x and also makes predictions on the relationship between RBE and dose. The proposed model shows a relationship between α and LETd but no significant relationship between β and LETd. Previously published phenomenological models based on a limited data set might have to be revised.
MO‐F‐CAMPUS‐J‐05: Toward MRI‐Only Radiotherapy: Novel Tissue Segmentation and Pseudo‐CT Generation Techniques Based On T1 MRI Sequences
Authors: Aouadi S , McGarry M , Hammoud R , Torfeh T , Perkins G , Al‐Hammadi N
Published: June 2015

Purpose:

To develop and validate a 4 class tissue segmentation approach (air cavities, background, bone and soft‐tissue) on T1 ‐weighted brain MRI and to create a pseudo‐CT for MRI‐only radiation therapy verification.

Methods:

Contrast‐enhanced T1‐weighted fast‐spin‐echo sequences (TR = 756ms, TE= 7.152ms), acquired on a 1.5T GE MRI‐Simulator, are used.MRIs are firstly pre‐processed to correct for non uniformity using the non parametric, non uniformity intensity normalization algorithm. Subsequently, a logarithmic inverse scaling log(1/image) is applied, prior to segmentation, to better differentiate bone and air from soft‐tissues. Finally, the following method is enrolled to classify intensities into air cavities, background, bone and soft‐tissue:Thresholded region growing with seed points in image corners is applied to get a mask of Air+Bone+Background. The background is, afterward, separated by the scan‐line filling algorithm. The air mask is extracted by morphological opening followed by a post‐processing based on knowledge about air regions geometry. The remaining rough bone pre‐segmentation is refined by applying 3D geodesic active contours; bone segmentation evolves by the sum of internal forces from contour geometry and external force derived from image gradient magnitude.Pseudo‐CT is obtained by assigning −1000HU to air and background voxels, performing linear mapping of soft‐tissue MR intensities in [‐400HU, 200HU] and inverse linear mapping of bone MR intensities in [200HU, 1000HU].

Results:

Three brain patients having registered MRI and CT are used for validation. CT intensities classification into 4 classes is performed by thresholding. Dice and misclassification errors are quantified. Correct classifications for soft‐tissue, bone, and air are respectively 89.67%, 77.8%, and 64.5%. Dice indices are acceptable for bone (0.74) and soft‐tissue (0.91) but low for air regions (0.48). Pseudo‐CT produces DRRs with acceptable clinical visual agreement to CT‐based DRR.

Conclusion:

The proposed approach makes it possible to use T1‐weighted MRI to generate accurate pseudo‐CT from 4‐class segmentation.
MO‐F‐CAMPUS‐I‐04: Characterization of Fan Beam Coded Aperture Coherent Scatter Spectral Imaging Methods for Differentiation of Normal and Neoplastic Breast Structures
Authors: Morris R , Albanese K , Lakshmanan M , Greenberg J , Kapadia A
Published: June 2015

Purpose:

This study intends to characterize the spectral and spatial resolution limits of various fan beam geometries for differentiation of normal and neoplastic breast structures via coded aperture coherent scatter spectral imaging techniques. In previous studies, pencil beam raster scanning methods using coherent scatter computed tomography and selected volume tomography have yielded excellent results for tumor discrimination. However, these methods don't readily conform to clinical constraints; primarily prolonged scan times and excessive dose to the patient. Here, we refine a fan beam coded aperture coherent scatter imaging system to characterize the tradeoffs between dose, scan time and image quality for breast tumor discrimination.

Methods:

An X‐ray tube (125kVp, 400mAs) illuminated the sample with collimated fan beams of varying widths (3mm to 25mm). Scatter data was collected via two linear‐array energy‐sensitive detectors oriented parallel and perpendicular to the beam plane. An iterative reconstruction algorithm yields images of the sample's spatial distribution and respective spectral data for each location. To model in‐vivo tumor analysis, surgically resected breast tumor samples were used in conjunction with lard, which has a form factor comparable to adipose (fat).

Results:

Quantitative analysis with current setup geometry indicated optimal performance for beams up to 10mm wide, with wider beams producing poorer spatial resolution. Scan time for a fixed volume was reduced by a factor of 6 when scanned with a 10mm fan beam compared to a 1.5mm pencil beam.

Conclusion:

The study demonstrates the utility of fan beam coherent scatter spectral imaging for differentiation of normal and neoplastic breast tissues has successfully reduced dose and scan times whilst sufficiently preserving spectral and spatial resolution. Future work to alter the coded aperture and detector geometries could potentially allow the use of even wider fans, thereby making coded aperture coherent scatter imaging a clinically viable method for breast cancer detection.

United States Department of Homeland Security; Duke University Medical Center ‐ Department of Radiology; Carl E Ravin Advanced Imaging Laboratories; Duke University Medical Physics Graduate Program
MO‐F‐CAMPUS‐T‐02: Optimizing Orientations of Hundreds of Intensity‐Modulated Beams to Treat Multiple Brain Targets
Authors: Ma L , Dong P , Keeling V , Hossain S , Ahmad S , Larson D , Sahgal A
Published: June 2015

Purpose:

To investigate a new modulated beam orientation optimization (MBOO) approach maximizing treatment planning quality for the state‐of‐the‐art flattening filter free (FFF) beam that has enabled rapid treatments of multiple brain targets.

Methods:

MBOO selects and optimizes a large number of intensity‐modulated beams (400 or more) from all accessible beam angles surrounding a patient's skull. The optimization algorithm was implemented on a standalone system that interfaced with the 3D Dicom images and structure sets. A standard published data set that consisted of 1 to 12 metastatic brain tumor combinations was selected for MBOO planning. The planning results from various coplanar and non‐coplanar configurations via MBOO were then compared with the results obtained from a clinical volume modulated arc therapy (VMAT) delivery system (Truebeam RapidArc, Varian Oncology).

Results:

When planning a few number of targets (n<4), MBOO produced results equivalent to non‐coplanar multi‐arc VMAT planning in terms of target volume coverage and normal tissue sparing. For example, the 12‐Gy and 4‐Gy normal brain volumes for the 3‐target plans differed by less than 1 mL ( 3.0 mLvs 3.8 mL; and 35.2 mL vs 36.3 mL, respectively) for MBOO versus VMAT. However, when planning a larger number of targets (n≥4), MBOO significantly reduced the dose to the normal brain as compared to VMAT, though the target volume coverage was equivalent. For example, the 12‐Gy and 4‐Gy normal brain volumes for the 12‐target plans were 10.8 mL vs. 18.0 mL and 217.9 mL vs. 390.0 mL, respectively for the non‐coplanar MBOO versus the non‐coplanar VMAT treatment plans, yielding a reduction in volume of more than 60% for the case.

Conclusion:

MBOO is a unique approach for maximizing normal tissue sparing when treating a large number (n≥4) of brain tumors with FFF linear accelerators.

Dr Ma and Dr Sahgal are currently on the board of international society of stereotactic radiosurgery. Dr Sahgal has received support for educational presentations from Elekta company
MO‐F‐CAMPUS‐T‐05: Correct Or Not to Correct for Rotational Patient Set‐Up Errors in Stereotactic Radiosurgery
Authors: Briscoe M , Ploquin N , Voroney JP
Published: June 2015

Purpose:

To quantify the effect of patient rotation in stereotactic radiation therapy and establish a threshold where rotational patient set‐up errors have a significant impact on target coverage.

Methods:

To simulate rotational patient set‐up errors, a Matlab code was created to rotate the patient dose distribution around the treatment isocentre, located centrally in the lesion, while keeping the structure contours in the original locations on the CT and MRI. Rotations of 1°, 3°, and 5° for each of the pitch, roll, and yaw, as well as simultaneous rotations of 1°, 3°, and 5° around all three axes were applied to two types of brain lesions: brain metastasis and acoustic neuroma. In order to analyze multiple tumour shapes, these plans included small spherical (metastasis), elliptical (acoustic neuroma), and large irregular (metastasis) tumour structures. Dose‐volume histograms and planning target volumes were compared between the planned patient positions and those with simulated rotational set‐up errors. The RTOG conformity index for patient rotation was also investigated.

Results:

Examining the tumour volumes that received 80% of the prescription dose in the planned and rotated patient positions showed decreases in prescription dose coverage of up to 2.3%. Conformity indices for treatments with simulated rotational errors showed decreases of up to 3% compared to the original plan. For irregular lesions, degradation of 1% of the target coverage can be seen for rotations as low as 3°.

Conclusions:

This data shows that for elliptical or spherical targets, rotational patient set‐up errors less than 3° around any or all axes do not have a significant impact on the dose delivered to the target volume or the conformity index of the plan. However the same rotational errors would have an impact on plans for irregular tumours.
MO‐F‐CAMPUS‐J‐02: Automatic Recognition of Patient Treatment Site in Portal Images Using Machine Learning
Authors: Chang X , Yang D
Published: June 2015

Purpose:

To investigate the method to automatically recognize the treatment site in the X‐Ray portal images. It could be useful to detect potential treatment errors, and to provide guidance to sequential tasks, e.g. automatically verify the patient daily setup.

Methods:

The portal images were exported from MOSAIQ as DICOM files, and were 1) processed with a threshold based intensity transformation algorithm to enhance contrast, and 2) where then down‐sampled (from 1024×768 to 128×96) by using bi‐cubic interpolation algorithm. An appearance‐based vector space model (VSM) was used to rearrange the images into vectors. A principal component analysis (PCA) method was used to reduce the vector dimensions. A multi‐class support vector machine (SVM), with radial basis function kernel, was used to build the treatment site recognition models. These models were then used to recognize the treatment sites in the portal image. Portal images of 120 patients were included in the study. The images were selected to cover six treatment sites: brain, head and neck, breast, lung, abdomen and pelvis. Each site had images of the twenty patients. Cross‐validation experiments were performed to evaluate the performance.

Results:

MATLAB image processing Toolbox and scikit‐learn (a machine learning library in python) were used to implement the proposed method. The average accuracies using the AP and RT images separately were 95% and 94% respectively. The average accuracy using AP and RT images together was 98%. Computation time was ∼0.16 seconds per patient with AP or RT image, ∼0.33 seconds per patient with both of AP and RT images.

Conclusion:

The proposed method of treatment site recognition is efficient and accurate. It is not sensitive to the differences of image intensity, size and positions of patients in the portal images. It could be useful for the patient safety assurance.

The work was partially supported by a research grant from Varian Medical System.
MO‐F‐CAMPUS‐T‐03: Verification of Range, SOBP Width, and Output for Passive‐Scattering Proton Beams Using a Liquid Scintillator Detector
Authors: Henry T , Robertson D , Therriault‐Proulx F , Beddar S
Published: June 2015

Purpose:

Liquid scintillators have been shown to provide fast and high‐resolution measurements of radiation beams. However, their linear energy transfer‐dependent response (quenching) limits their use in proton beams. The purpose of this study was to develop a simple and fast method to verify the range, spread‐out Bragg peak (SOBP) width, and output of a passive‐scattering proton beam with a liquid scintillator detector, without the need for quenching correction.

Methods:

The light signal from a 20×20×20 cm3 liquid scintillator tank was collected with a CCD camera. Reproducible landmarks on the SOBP depth‐light curve were identified which possessed a linear relationship with the beam range and SOBP width. The depth‐light profiles for three beam energies (140, 160 and 180 MeV) with six SOBP widths at each energy were measured with the detector. Beam range and SOBP width calibration factors were obtained by comparing the depth‐light curve landmarks with the nominal range and SOBP width for each beam setting. The daily output stability of the liquid scintillator detector was also studied by making eight repeated output measurements in a cobalt‐60 beam over the course of two weeks.

Results:

The mean difference between the measured and nominal beam ranges was 0.6 mm (σ=0.2 mm), with a maximum difference of 0.9 mm. The mean difference between the measured and nominal SOBP widths was 0.1 mm (σ=1.8 mm), with a maximum difference of 4.0 mm. Finally an output variation of 0.14% was observed for 8 measurements performed over 2 weeks.

Conclusion:

A method has been developed to determine the range and SOBP width of a passive‐scattering proton beam in a liquid scintillator without the need for quenching correction. In addition to providing rapid and accurate beam range and SOBP measurements, the detector is capable of measuring the output consistency with a high degree of precision.

This project was supported in part by award number CA182450 from the National Cancer Institute.
MO‐F‐CAMPUS‐T‐05: Design of An Innovative Beam Monitor for Particle Therapy for the Simultaneous Measurement of Beam Fluence and Energy
Authors: Sacchi R , Cartiglia N , Cenna F , Guarachi L Fanola , Ferrero M , Giordanengo S , Marchetto F , Monaco V , Vignati A , Anvar M Varasteh , Cirio R
Published: June 2015

Purpose:

Monitoring the prescribed dose in particle therapy is typically carried out by using parallel plate ionization chambers working in transmission mode. The use of gas detectors has several drawbacks: they need to be calibrated daily against standard dosimeters and their dependence on beam quality factors need to be fully characterized and controlled with high accuracy. A detector capable of single particle counting is proposed which would overcome all these limitations. Combined with a gas ionization chamber, it will allow determining the average particle stopping power, thus providing an effective method for the online verification of the selected particle energy and range.

Methods:

Low‐Gain Avalanche Detectors (LGADs) are innovative n‐in‐p silicon sensors with moderate internal charge multiplication occurring in the strong field generated by an additional p+ doping layer implanted at a depth of a few µm in the bulk of the sensor. The increased signal‐to‐noise ratio allows designing very thin, few tens of microns, segmented LGADs, called Ultra Fast Silicon Detectors (UFSD), optimized for very fast signal, which would be suitable for charged particle counting at high rates. A prototype UFSD is being designed for this purpose.

Results:

Different LGAD diodes have been characterized both in laboratory and beam tests, and the results compared both with those obtained with similar diodes without the gain layer and with a program simulating the signal in the sensors. The signal is found to be enhanced in LGADs, while the leakage current and the noise is not affected by the gain. Possible alternative designs and implementations are also presented and discussed.

Conclusion:

Thanks to their excellent counting capabilities, UFSD detectors are a promising technology for future beam monitor devices in hadron‐therapy applications. Studies are ongoing to better understand their properties and optimize the design in view of this application.