bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐618: Plan Robustness Study of Volumetric‐Modulated Arc Therapy Vs. Intensity‐Modulated Radiation Therapy for Head and Neck Cancer
Authors: Liu W , Patel S , Shen J , Harrington D , Stoker J , Ding X , Hu Y , Wong W , Halyard M , Schild S , Ezzell G , Bues M
Published: June 2015

Purpose:

Lack of plan robustness may contribute to local failure in volumetric‐modulated arc therapy (VMAT) to treat head and neck (H&N) cancer. Thus we compared plan robustness of VMAT with intensity‐modulated radiation therapy (IMRT).

Methods:

VMAT and IMRT plans were created for 9 H&N cancer patients. For each plan, six new perturbed dose distributions were computed — one each for ± 3mm setup deviations along the S‐I, A‐P and L‐R directions. We used three robustness quantification tools: (1) worst‐case analysis (WCA); (2) dose‐volume histograms (DVHs) band (DVHB); and (3) root‐mean‐square‐dose deviation (RMSD) volume histogram (DDVH). DDVH represents the relative volume (y) on the vertical axis and the RMSD (x) on the horizontal axis. Similar to DVH, this means that y% of the volume of the indicated structure has the RMSD at least × Gy[RBE].The width from the first two methods at different target DVH indices (such as D95 and D5) and the area under the DDVH curves (AUC) for the target were used to indicate plan robustness. In these robustness quantification tools, the smaller the value, the more robust the plan is. Plan robustness evaluation metrics were compared using Wilcoxon test.

Results:

DVHB showed the width at D95 from IMRT to be larger than from VMAT (unit Gy) [1.59 vs 1.18 (p=0.49)], while the width at D5 from IMRT was found to be slightly larger than from VMAT [0.59 vs 0.54 (p=0.84)]. WCA showed similar results [D95: 3.28 vs 3.00 (p=0.56); D5: 1.68 vs 1.95 (p=0.23)]. DDVH showed the AUC from IMRT to be slightly smaller than from VMAT [1.13 vs 1.15 (p=0.43)].

Conclusion:

VMAT plan robustness is comparable to IMRT plan robustness. The plan robustness conclusions from WCA and DVHB are DVH parameter dependent. On the other hand DDVH captures the overall effect of uncertainties on the dose to a volume of interest.

NIH/NCI K25CA168984; Eagles Cancer Research Career Development; The Lawrence W. and Marilyn W. Matteson Fund for Cancer Research Mayo ASU Seed Grant; The Kemper Marley Foundation
SU‐E‐T‐625: Potential for Reduced Radiation Induced Toxicity for the Treatment of Inoperable Non‐Small‐Cell Lung Cancer Using RapidArc Planning
Authors: Pokhrel D , Sood S , Badkul R , Jiang H , Saleh H , Wang F
Published: June 2015

Purpose:

To investigate the feasibility of using RapidArc (RA) treatment planning to reduce irradiation volume of normal lung and other organs at risk (OARs) in the treatment of inoperable non‐small‐cell lung cancer (NSCLC) patients.

Methods:

A retrospective treatment planning and delivery study was performed to compare target coverage and the volumes of the normal lung, spinal cord, heart and esophagus on 4D‐CT scan above their dose tolerances delivered by RA vs. IMRT for ten inoperable NSCLC patients (Stage I‐IIIB). RA plans consisted of either one‐full or two‐partial co‐planar arcs used to treat 95% of the planning target volume (PTV) with 6MV beam to a prescription of 66Gy in 33 fractions. IMRT plans were generated using 5–7 co‐planar fields with 6MV beam. PTV coverage, dose‐volume histograms, homogeneity/conformity indices (CI), total number of monitor units(MUs), beam‐on time and delivery accuracy were compared between the two treatment plans.

Results:

Similar target coverage was obtained between the two techniques. RA (CI=1.02) provided more conformal plans without loss of homogeneity compared to IMRT plans (CI=1.12). Compared to IMRT, RA achieved a significant median dose reduction in V10 (3%), V20 (8%), and mean lung dose (3%) on average, respectively. On average, V5 was comparable between the two treatment plans. RA reduced mean esophagus (6%), mean heart (18%), and maximum spinal cord dose (7%), on average, respectively. Total number of MUs and beam‐on time were each reduced almost by a factor of 2 when compared to IMRT‐patient comfort, reduced intra‐fraction‐motion and leakage dose. The average IMRT and RA QA pass rate was about 98% for both types of plans for 3%/3mm criterion.

Conclusion:

Compared to IMRT plans, RA provided not only comparable target coverage, but also improved conformity, treatment time, and significant reduction in irradiation of OARs. This may potentially allow for target dose escalation without increase in normal tissue toxicity.
SU‐E‐T‐627: Precision Modelling of the Leaf‐Bank Rotation in Elekta's Agility MLC: Is It Necessary?
Authors: Vujicic M , Belec J , Heath E , Gholampourkashi S , Cygler J
Published: June 2015

Purpose:

To demonstrate the method used to determine the leaf bank rotation angle (LBROT) as a parameter for modeling the Elekta Agility multi‐leaf collimator (MLC) for Monte Carlo simulations and to evaluate the clinical impact of LBROT.

Methods:

A detailed model of an Elekta Infinity linac including an Agility MLC was built using the EGSnrc/BEAMnrc Monte Carlo code. The Agility 160‐leaf MLC is modelled using the MLCE component module which allows for leaf bank rotation using the parameter LBROT. A precise value of LBROT is obtained by comparing measured and simulated profiles of a specific field, which has leaves arranged in a repeated pattern such that one leaf is opened and the adjacent one is closed. Profile measurements from an Agility linac are taken with gafchromic film, and an ion chamber is used to set the absolute dose. The measurements are compared to Monte Carlo (MC) simulations and the LBROT is adjusted until a match is found. The clinical impact of LBROT is evaluated by observing how an MC dose calculation changes with LBROT. A clinical Stereotactic Body Radiation Treatment (SBRT) plan is calculated using BEAMnrc/DOSXYZnrc simulations with different input values for LBROT.

Results:

Using the method outlined above, the LBROT is determined to be 9±1 mrad. Differences as high as 4% are observed in a clinical SBRT plan between the extreme case (LBROT not modeled) and the nominal case.

Conclusion:

In small‐field radiation therapy treatment planning, it is important to properly account for LBROT as an input parameter for MC dose calculations with the Agility MLC. More work is ongoing to elucidate the observed differences by determining the contributions from transmission dose, change in field size, and source occlusion, which are all dependent on LBROT.

This work was supported by OCAIRO (Ontario Consortium of Adaptive Interventions in Radiation Oncology), funded by the Ontario Research Fund.
SU‐E‐T‐630: Predictive Modeling of Mortality, Tumor Control, and Normal Tissue Complications After Stereotactic Body Radiotherapy for Stage I Non‐Small Cell Lung Cancer
Authors: Lindsay WD , Berlind CG , Gee JC , Simone CB
Published: June 2015

Purpose:

While rates of local control have been well characterized after stereotactic body radiotherapy (SBRT) for stage I non‐small cell lung cancer (NSCLC), less data are available characterizing survival and normal tissue toxicities, and no validated models exist assessing these parameters after SBRT. We evaluate the reliability of various machine learning techniques when applied to radiation oncology datasets to create predictive models of mortality, tumor control, and normal tissue complications.

Methods:

A dataset of 204 consecutive patients with stage I non‐small cell lung cancer (NSCLC) treated with stereotactic body radiotherapy (SBRT) at the University of Pennsylvania between 2009 and 2013 was used to create predictive models of tumor control, normal tissue complications, and mortality in this IRB‐approved study. Nearly 200 data fields of detailed patient‐ and tumor‐specific information, radiotherapy dosimetric measurements, and clinical outcomes data were collected. Predictive models were created for local tumor control, 1‐ and 3‐year overall survival, and nodal failure using 60% of the data (leaving the remainder as a test set). After applying feature selection and dimensionality reduction, nonlinear support vector classification was applied to the resulting features. Models were evaluated for accuracy and area under ROC curve on the 81‐patient test set.

Results:

Models for common events in the dataset (such as mortality at one year) had the highest predictive power (AUC = .67, p < 0.05). For rare occurrences such as radiation pneumonitis and local failure (each occurring in less than 10% of patients), too few events were present to create reliable models.

Conclusion:

Although this study demonstrates the validity of predictive analytics using information extracted from patient medical records and can most reliably predict for survival after SBRT, larger sample sizes are needed to develop predictive models for normal tissue toxicities and more advanced machine learning methodologies need be consider in the future.
SU‐E‐T‐634: Pre‐Verification of FFF Prostate VMAT Plans with Gamma Method and DVHs Reconstructed Based On Measurements with 2D‐ARRAY (PTW 1500) and OCTAVIUS 4D
Authors: Kruszyna M , Adamczyk M
Published: June 2015

Purpose:

The aim of this work was to characterize the clinical correctness of FFF prostate treatment VMAT plans based on analysis of DVHs reconstructed from pre‐verification 2D‐arrays measurements.

Methods:

The new 2D ion chamber array 1500 with rotational phantom cylindrical Octavius 4D and Verisoft 6.1 software with DVH option (PTW, Freiburg) were used to determine the clinical usefulness of the treatment plans. Ten patients treated with VMAT high‐fractionated (2 fraction × 7,5 Gy) FFF prostate plans (TrueBeam, Varian) were analyzed using the 3D gamma analysis by local dose method with a 5% threshold for various tolerance parameters DTA [mm] and DD [%] were 1%/1, 2%/2, 3%/3. Additional, based on the measurements of irradiation dose distributions and patients’ CT scans with contoured structures of organs, the DVHs were reconstructed using a software. The obtained DVHs were compared to planned dose distributions and the deviations were analysed with parameters: for CTV D50, D98, D2, and D25, D50, Dmax for OARs — rectum, bladder and left/right femoral heads.

Results:

The analyzed treatment plans passed gamma criteria (3/3%; 95%), the results obtained were as follow: mean value and standard deviation of gamma score for criteria (DTA[mm]/DD[%]): 1/1% (L53.3±3.2); 2/2% (L87.0±2.2); 3/3% (L97.5±0.9). In the DVH analysis, the highest differences were observed for OARs (especially for bladder): the mean percentage differences values for rectum, bladder and left/right femoral heads were: D25 (1.67; 6.83)%, D50 (0.18; 7.18; 1.53; 0.30)%, Dmax (−0.84; −1.64; 0.37; −4.63)%, respectively. For the CTV mean relative deviations for proper parameters were in good agreement with TPS: D98 (0.95±2.21)%, D50 (1.93±0.67)%, D2 (1.76±0.76)%.

Conclusion:

The gamma method is recommended tool for pre‐verification analysis of correctness of treatment plans. Moreover, the scrutiny checking with reconstructed DVH gives additional, clinical information about quality of plan, especially about coverage of CTV and sparing of health tissues.
SU‐E‐T‐636: ProteusONE Machine QA Procedure and Stabiity Study: Half Year Clinical Operation
Authors: Freund D , Ding X , Wu H , Zhang J , Syh J , Syh J , Patel B , Song X
Published: June 2015

Purpose:

The objective of this study is to evaluate the stability of ProteusOne, the 1st commercial PBS proton system, throughout the daily QA and monthly over 6 month clinical operation.

Method:

Daily QA test includes IGRT position/repositioning, output in the middle of SOBP, beam flatness, symmetry, inplane and crossplane dimensions as well as energy range check. Daily range shifter QA consist of output, symmetry and field size checks to make sure its integrity. In 30 mins Daily QA test, all the measurements are performed using the MatriXXPT (IBA dosimetry). The data from these measurement was collected and compare over the first 6 month of clinical operation. In addition to the items check in daily QA, the summary also includes the monthly QA gantry star shots, absolute position check using a novel device, XRV‐100.

Results:

Average machine output at the center of the spread out bragg peak was 197.5±.8 cGy and was within 1%of the baseline at 198.4 cGy. Beam flatness was within 1% cross plane with an average of 0.67±0.12% and 2% in‐plane with an average of 1.08±0.17% compared to baseline measurements of 0.6 and 1.03, respectively. In all cases the radiation isocenter shift was less than or equal to 1mm. Output for the range shifter was within 2% for each individual measurement and averaged 34.4±.2cGy compare to a baseline reading of 34.5cGy. The average range shifter in and cross plane field size measurements were 19.8±0.5cm and 20.5±0.4cm compared with baseline values of 20.19cm and 20.79cm, respectively. Range shifter field symmetry had an average of less 1% for both in‐plane and cross plane measurements.

Conclusion:

All machine metrics over the past 6 months have proved to be stable. Although, some averages are outside the baseline measurement they are within 1% tolerance and the deviation across all measurements is minimal.
SU‐E‐T‐637: Proton Aperture Quality Assurance Using Computed Tomography
Authors: Reyhan M , Yue N , Zou J
Published: June 2015

Purpose:

To develop an automated algorithm for aperture quality assurance for double scattering proton radiotherapy.

Methods:

Anterior‐Posterior scout images were acquired on a CT scanner for five brass apertures (2 large, 3 small) with 120 kVp and 10mA. A semi‐automated algorithm was developed for the QA process. The input includes a user selecting the scout image and the associated patient plan, field and aperture size. The program automatically thresholds the scout image. Next, a connectivity algorithm is used to determine the points associated with the central shape to obtain the contour. To compare with the plan contour, the program converts the detected and plan contours into polar coordinates, interpolates the data to a 1 degree spaced grid, and determines the differences in radial distance at each grid point.

Results:

The mean and maximum difference and the percentage of points with distance differences less than 1.25 mm (due to the divergent cut of the aperture) between the detected and plan field aperture contours were obtained. A repeatability coefficient was derived based on repeated scanning and processing of three of the apertures. The mean difference for five apertures was 0.44 +/−0.08 mm. The maximum difference in distance was 1.2 +/−0.23 mm for all apertures. The Repeatability Coefficient was +/−0.038 mm, indicating the technique is highly repeatable. The mean percent of points with distance less than 1.25 mm was 97.41 +/−0.35%.

Conclusion:

Automated CT scout image based proton aperture QA is feasible, saves time, and provides a quantitative metric for proton patient specific aperture QA.
SU‐E‐T‐641: Proton Range Measurements Using a Geometrically Calibrated Liquid Scintillator Detector
Authors: Hui C , Robertson D , Alsanea F , Beddar S
Published: June 2015

Purpose:

The purpose of this work is to develop a geometric calibration method to accurately calculate physical distances within a liquid scintillator detector and to assess the accuracy, consistency, and robustness of proton beam range measurements when using a liquid scintillator detector system with the proposed geometric calibration process.

Methods:

We developed a geometric calibration procedure to accurately convert pixel locations in the camera frame into physical locations in the scintillator frame. To ensure accuracy, the geometric calibration was performed before each experiment. The liquid scintillator was irradiated with spot scanning proton beams of 94 energies in two deliveries. A CCD camera was used to capture the two‐dimensional scintillation light profile of each of the proton energies. An algorithm was developed to automatically calculate the proton range from the acquired images. The measured range was compared to the nominal range to assess the accuracy of the detector. To evaluate the robustness of the detector between each setup, the experiments were repeated on three different days. To evaluate the consistency of the measurements between deliveries, three sets of measurements were acquired for each experiment.

Results:

Using this geometric calibration procedure, the proton beam ranges measured using the liquid scintillator system were all within 0.3mm of the nominal range. The average difference between the measured and nominal ranges was −0.20mm. The delivery‐to‐delivery standard deviation of the proton range measurement was 0.04mm, and the setup‐to‐setup standard deviation of the measurement was 0.10mm.

Conclusion:

The liquid scintillator system can measure the range of all 94 beams in just two deliveries. With the proposed geometric calibration, it can measure proton range with sub‐millimeter accuracy, and the measurements were shown to be consistent between deliveries and setups. Therefore, we conclude that the liquid scintillator system provides a reliable and convenient tool for proton range measurement.

This project was supported in part by award number CA182450 from the National Cancer Institute.
SU‐E‐T‐644: QuAArC: A 3D VMAT QA System Based On Radiochromic Film and Monte Carlo Simulation of Log Files
Authors: Barbeiro A.R. , Ureba A. , Baeza J.A. , Linares R. , Jimenez‐Ortega E. , Mateos J.C. , Velazquez S. , Plaza A. Leal
Published: June 2015

Purpose:

VMAT involves two main sources of uncertainty: one related to the dose calculation accuracy, and the other linked to the continuous delivery of a discrete calculation. The purpose of this work is to present QuAArC, an alternative VMAT QA system to control and potentially reduce these uncertainties.

Methods:

An automated MC simulation of log files, recorded during VMAT treatment plans delivery, was implemented in order to simulate the actual treatment parameters. The linac head models and the phase‐space data of each Control Point (CP) were simulated using the EGSnrc/BEAMnrc MC code, and the corresponding dose calculation was carried out by means of BEAMDOSE, a DOSXYZnrc code modification. A cylindrical phantom was specifically designed to host films rolled up at different radial distances from the isocenter, for a 3D and continuous dosimetric verification. It also allows axial and/or coronal films and point measurements with several types of ion chambers at different locations. Specific software was developed in MATLAB in order to process and evaluate the dosimetric measurements, which incorporates the analysis of dose distributions, profiles, dose difference maps, and 2D/3D gamma index. It is also possible to obtain the experimental DVH reconstructed on the patient CT, by an optimization method to find the individual contribution corresponding to each CP on the film, taking into account the total measured dose, and the corresponding CP dose calculated by MC.

Results:

The QuAArC system showed high reproducibility of measurements, and consistency with the results obtained with the commercial system implemented in the verification of the evaluated treatment plans.

Conclusion:

A VMAT QA system based on MC simulation and high resolution dosimetry with film has been developed for treatment verification. It shows to be useful for the study of the real VMAT capabilities, and also for linac commissioning and evaluation of other verification devices.
SU‐E‐T‐645: Qualification of a 2D Ionization Chamber Array for Beam Steering and Profile Measurement
Authors: Gao S , Balter P , Rose M , Simon W
Published: June 2015

Purpose:

Establish a procedure for beam steering and profile measurement using a 2D ionization chamber array and show equivalence to a water scanning system.

Methods:

Multiple photon beams (30×30cm2 field) and electron beams (25×25cm2 cone) were steered in the radial and transverse directions using Sun Nuclear's IC PROFILER (ICP). Solid water was added during steering to ensure measurements were beyond the buildup region. With steering complete, servos were zeroed and enabled. Photon profiles were collected in a 30×30cm2 field at dmax and 2.9 cm depth for flattened and FFF beams respectively. Electron profiles were collected with a 25×25cm2 cone and effective depth (solid water + 0.9 cm intrinsic buildup) as follows: 0.9 cm (6e), 1.9 cm (9e), 2.9 cm (12e, 16e, 20e). Profiles of the same energy, field size and depth were measured in water with Sun Nuclear's 3D SCANNER (3DS). Profiles were re‐measured using the ICP after the in‐water scans. Profiles measured using the ICP and 3DS were compared by (a) examining the differences in Varian's “Point Difference Symmetry” metric, (b) visual inspection of the overlaid profile shapes and (c) calculation of point‐by‐point differences.

Results:

Comparing ICP measurements before and after water scanning showed very good agreement indicating good stability of the linac and measurement system. Comparing ICP Measurements to water phantom measurements using Varian's symmetry metric showed agreement within 0.5% for all beams. The average magnitude of the agreement was within 0.2%. Comparing ICP Measurements to water phantom measurements using point‐by‐point difference showed agreement within 0.5% inside of 80% area of the field width.

Conclusion:

Profile agreement to within 0.5% was observed between ICP and 3DS after steering multiple energies with the ICP. This indicates that the ICP may be used for steering electron beams, and both flattened and FFF photon beams.

Song Gao: Sun Nuclear's invitation of speak and financial support for attending the 8th QA & Dosimetry Symposium in Orlando, Florida 2015.
SU‐E‐T‐647: Quality Assurance of VMAT by Gamma Analysis Dependence On Low‐Dose Threshold
Authors: Song J , Kim M , Park S , Lee S , Lee M , Suh T
Published: June 2015

Purpose:

The AAPM TG‐119 instructed institutions to use low‐dose threshold (LDT) of 10% or a ROI determined by the jaw when they collected gamma analysis QA data of planar dose distribution. Also, based on a survey by Nelms and Simon, more than 70% of institutions use a LDT between 0% and 10% for gamma analysis. However, there are no clinical data to quantitatively demonstrate the impact of the LDT on the gamma index. Therefore, we performed a gamma analysis with LDTs of 0% to 15% according to both global and local normalization and different acceptance criteria: 3%/3 mm, 2%/2 mm, and 1%/1 mm.

Methods:

A total of 30 treatment plans—10 head and neck, 10 brain, and 10 prostate cancer cases—were randomly selected from the Varian Eclipse TPS, retrospectively. For the gamma analysis, a predicted portal image was acquired through a portal dose calculation algorithm in the Eclipse TPS, and a measured portal image was obtained using a Varian Clinac iX and an EPID. Then, the gamma analysis was performed using the Portal Dosimetry software.

Results:

For the global normalization, the gamma passing rate (%GP) decreased as the LDT increased, and all cases of low‐dose thresholds exhibited a %GP above 95% for both the 3%/3 mm and 2%/2 mm criteria. However, for local normalization, the %GP increased as LDT increased. The gamma passing rate with LDT of 10% increased by 6.86%, 9.22% and 6.14% compared with the 0% in the case of the head and neck, brain and prostate for 3%/3 mm criteria, respectively.

Conclusion:

Applying the LDT in the global normalization does not have critical impact to judge patient‐specific QA results. However, LDT for the local normalization should be carefully selected because applying the LDT could affect the average of the %GP to increase rapidly.
SU‐E‐T‐652: Quantification of Dosimetric Uncertainty of I‐125 COMS Eye Plaque
Authors: Ferreira C , Ahmad S , Firestone B , Johnson D , Matthiesen C , De La Fuente Herman T
Published: June 2015

Purpose:

To compare dosimetrically three plan calculation systems (Plato, Varian Brachytherapy, and in‐house‐made Excel) available for I‐125 COMS eye plaque treatment with measurement.

Methods:

All systems assume homogeneous media and calculations are based on a three‐dimensional Cartesian coordinates, Plato and Brachytherapy Planning are based on AAPM TG‐43 and the in‐house Excel program only on inverse square corrections. Doses at specific depths were measured with EBT3 Gafchromic film from a fully loaded and a partially loaded 16 mm plaque (13 and 8 seeds respectively, I‐125, model 6711 GE, Oncura). Measurements took place in a water tank, utilizing solid water blocks and a 3D‐printed plaque holder. Taking advantage that gafchromic film has low energy dependence, a dose step wedge was delivered with 6 MV photon beam from a Varian 2100 EX linac for calibration. The gray‐scale to dose in cGy was obtained with an Epson Expression 10000 XL scanner in the green channel. Treatment plans were generated for doses of 2200 cGy to a depth of 7 mm, and measurements were taken on a sagittal plane.

Results:

The calculated dose at the prescription point was 2242, 2344, and 2211 cGy with Excel, Brachyvision and Plato respectively for a fully loaded plaque, for the partially loaded plaque the doses were 2266, 2477, and 2193 cGy respectively. At 5 mm depth the doses for Brachyvision and Plato were comparable (3399 and 3267 cGy respectively), however, the measured dose in film was 3180 cGy which was lower by as much as 6.4% in the fully loaded plaque and 7.6% in the partially loaded plaque.

Conclusion:

Careful methodology and calibration are essential when measuring doses at specific depth due to the sensitivity and rapid dose fall off of I‐125.
SU‐E‐T‐653: Quantifying Inter‐Fraction Range Uncertainty for Input Into Robust Proton Planning
Authors: McGowan S , Holloway M , Burnet N , Thomas S
Published: June 2015

Purpose:

To provide a method for quantifying centre‐ and tumour site‐specific population range uncertainty due to inter‐fraction motion for use within robust proton planning and plan analysis. PTV‐based and probabilistic/robust plans depend upon accurate knowledge of geometric and range uncertainties. If the uncertainty model is inaccurate, both types of plan, PTV or probabilistic, could produce under‐dosing of the target and/or overdosing of OAR.

Methods:

Daily mega‐voltage volumetric imaging data from previously treated radiotherapy patients has been used to investigate inter‐fraction changes to water equivalent path‐length (WEPL). Daily image‐guidance scans were carried out for each patient and corrected for changes in CTV position (using rigid transformations). An effective depth algorithm was used to determine residual range changes, after corrections were applied, by comparing WEPL within the CTV at each fraction for several beam angles. This method has been demonstrated for nine prostate and seven head and neck.

Results:

Inter‐fraction range changes for the head and neck patient sample were Σ=3.39mm, σ=4.72mm and overall mean =−1.82mm; for prostate Σ=5.64mm, σ=5.91mm and overall mean =0.98 mm. The choice of beam angle for head and neck did not affect the inter‐fraction range error significantly; however this was not the same for prostate. Greater range changes were seen using a lateral beam compared to an anterior beam for prostate due to relative motion of the prostate and femoral heads.

Conclusion:

A method has been developed to quantify population range changes due to inter‐fraction motion. The results show a non‐zero mean for these samples, as well as significant Σ and σ, highlighting the importance of quantifying uncertainties and use of image guidance. Such information can be used in robust optimisation algorithms and for analysing treatment plan robustness. It may also help in establishing beam start conditions at planning and for establishing adaptive planning protocols.
SU‐E‐T‐654: Quantifying Plan Quality Can Effectively Distinguish Between Competing Equivocal IMRT Prostate Plans
Authors: Price A , Lo J , Das S
Published: June 2015

Purpose:

The purpose of this study was to create a prostate IMRT plan quality index (PQI) that may be used to quantitatively compare competing plans using a methodology that mimics physician preference. This methodology allows planners to choose between plans with equivocal characteristics, prior to physician scrutiny.

Methods:

An observer study was conducted to gather data from 3 radiation oncology physicians who ranked a set of 20 patients (each with 5 plans). The rankings were used to optimize a PQI that combined weighted portions of the rectum, bladder, and planning target volume DVHs, such that the relative PQI mimicked physician rankings as best as possible. Once optimized, a test study assessed the PQI by comparison to physician rankings in a new set of 25 patients (each with 4 plans). The physician group in the test study included 6 physicians, 5 of whom were not included in the modeling study. PQI scores were evaluated against the physicians’ rank list using Spearman rank correlation.

Results:

The optimized plan quality index combined the following DVH features: high dose regions above 40Gy/60Gy (rectum/bladder), organ weightings, and PTV shoulder coverage. Mean correlation of the PQI vs. physicians’ rankings in the modeling study was 0.507 (range: 0.345–0.706). By comparison, the mean correlation between physicians was 0.301 (range: 0.242–0.334). The mean correlation of the PQI vs. physician rankings in test study was 0.726 (range: 0.416–0.936), indicating robustness of the PQI by virtue of producing similar results to the modeling study. Intra‐physician correlation was 0.564 (range: 0.398–0.689).

Conclusion:

The correlation coefficients of the PQI vs. physicians were similar to the correlation coefficients of the physicians with each other, implying that the PQI developed in this work shows promise in reflecting physician clinical preference when selecting between competing, dosimetrically equivocal plans.
SU‐E‐T‐664: Radiobiological Modeling of Prophylactic Cranial Irradiation in Mice
Authors: Smith D , Debeb B , Woodward W
Published: June 2015

Purpose:

Prophylactic cranial irradiation (PCI) is a clinical technique used to reduce the incidence of brain metastasis and improve overall survival in select patients with ALL and SCLC, and we have shown the potential of PCI in select breast cancer patients through a mouse model (manuscript in preparation). We developed a computational model using our experimental results to demonstrate the advantage of treating brain micro‐metastases early.

Methods:

MATLAB was used to develop the computational model of brain metastasis and PCI in mice. The number of metastases per mouse and the volume of metastases from four‐ and eight‐week endpoints were fit to normal and log‐normal distributions, respectively. Model input parameters were optimized so that model output would match the experimental number of metastases per mouse. A limiting dilution assay was performed to validate the model. The effect of radiation at different time points was computationally evaluated through the endpoints of incidence, number of metastases, and tumor burden.

Results:

The correlation between experimental number of metastases per mouse and the Gaussian fit was 87% and 66% at the two endpoints. The experimental volumes and the log‐normal fit had correlations of 99% and 97%. In the optimized model, the correlation between number of metastases per mouse and the Gaussian fit was 96% and 98%. The log‐normal volume fit and the model agree 100%. The model was validated by a limiting dilution assay, where the correlation was 100%. The model demonstrates that cells are very sensitive to radiation at early time points, and delaying treatment introduces a threshold dose at which point the incidence and number of metastases decline.

Conclusion:

We have developed a computational model of brain metastasis and PCI in mice that is highly correlated to our experimental data. The model shows that early treatment of subclinical disease is highly advantageous.
SU‐E‐T‐677: Reproducibility of Production of Ionization Chambers
Authors: Kukolowicz P , Bulski W , Ulkowski P
Published: June 2015

Purpose:

To compare the reproducibility of the production of several cylindrical and plane‐parallel chambers popular in Poland in terms of a calibration coefficient.

Methods:

The investigation was performed for PTW30013 (20 chambers), 30001 (10 chambers), FC65‐G (17 chambers) cylindrical chambers and for PPC05 (14 chambers), Roos 34001 (8 chambers) plane parallel chambers. The calibration factors were measured at the same accredited secondary standard laboratory in terms of a dose to water. All the measurements were carried out at the same laboratory, by the same staff, in accordance with the same IAEA recommendations. All the chambers were calibrated in the Co60 beam. Reproducibility was described in terms of the mean value, its standard deviation and the ratio of the maximum and minimum value of calibration factors for each set of chambers separately. The combined uncertainty budged (1SD) calculated according to the IAEA‐TECDOC‐1585 of the calibration factor was of 0.25%.

Results:

The calibration coefficients for PTW30013, 30001, and FC65‐G chambers were 5.36±0.03, 5.28±0.06, 4.79±0.015 nC/Gy respectively and for PPC05, and Roos chambers were 59±2, 8.3±0.1 nC/Gy respectively. The maximum/minimum ratio of calibration factors for PTW30013, 30001, FC65‐G, and for PPC05, Roos chambers were 1.03, 1.03, 1.01, 1.14 and 1.03 respectively.

Conclusion:

The production of all ion chambers was very reproducible except the Markus type PPC05 for which the ratio of maximum/minimum calibration coefficients of 1.14 was obtained.
SU‐E‐T‐684: Robustness Versus Plan Quality for IMPT
Authors: Gorissen BL , Depauw N , Unkelbach J
Published: June 2015

Purpose:

Range and set‐up uncertainties in intensity‐modulated proton therapy cannot be addressed with margins. Instead, several robust optimization (RO) models have been developed that explicitly take uncertainty into account. RO optimizes the worst case at the cost of the treatment outcome in more likely scenarios. The goal is to provide insight in the trade‐off between robustness and plan quality and the behavior of different trade‐off methods.

Methods:

We present four methods to trade‐off robustness with plan quality that can be applied to any of the worst case methods found in literature. Each trade‐off is a mixture between worst case, expected value, and non‐robust planning, which effectively corresponds to assigning different importance weights to error scenarios. Each method is tested on several worst case methods for a sarcoma case, a paraspinal case and a benchmark case from literature.

Results:

Each trade‐off method yields a unique dose distribution in the border region between target volume and adjacent normal tissues, corresponding to a specific trade‐off between robustness and nominal plan quality. The fully robust solutions perform badly when the realized errors are smaller than the maximum projected errors. Compared to fully robust solutions, significant improvements are possible for non‐extreme scenarios while only slightly deteriorating plan quality at an extreme scenario. It is further observed that trade‐off methods cannot be mimicked by putting different weights on objectives for the tumor and the normal tissue.

Conclusion:

The method to trade‐off robustness with plan quality should be chosen carefully, as each method has a different impact on plan quality. Two of the methods can directly be implemented in any framework for multicriteria optimization, hopefully leading to their quick dissemination.
SU‐E‐T‐685: Root Cause Analysis of in Vivo Dosimetry Controls for Dynamic Arc Treatments
Authors: Celi S , Wessels C , Francois P
Published: June 2015

Purpose:

To produce an insight on the influence of selected parameters on the pass/fail rates of in‐vivo dosimetry (IVD) controls for dynamic arc treatments.

Methods:

A group of 25 patient treatment plans optimized with the treatment planning system Eclipse™ (Varian) for dual‐arc RapidArc treatment was analyzed. All plans were tracked with the IVD solution EPIgray (DOSIsoft S.A) over all sessions of the treatment course. Based on the acquired a‐Si EPID images, the dose differences to the TPS at given reference points and the corresponding gamma indices were computed. In addition, the dose difference variations over the course were determined post‐treatment. Five parameters — considered as possible fail or pass causes of IVD for dynamic arc plans — were chosen. Based on the treatment plan, two types of modulation indices qualifying the movement of the MLC were calculated. An “in‐field” coefficient and a “weighted‐in‐field” coefficient were established to quantify the position of the dose reference points in the planned volume. Finally, the dose rate variations for each arc were also determined. The described parameters were confronted to the results of the IVD controls.

Results:

The analyzed plans have a mean relative dose difference of 0.2% (±7.2%). No correlation could be established between the modulation indices and the dose differences, the gamma indices or the dose difference variation. Slightly better passing rates for individual sessions and total treatment plans were observed for “in‐field” coefficients superior to 50%. Similar passing rates were observed for arcs with below and above average dose rate deviations.

Conclusion:

Though all the analyzed parameters are cited as possible pass/fail criteria, only the position of the dose reference point can be linked to dose differences in IVD controls. Automatically generated dose measurement points could avoid false interpretations and an overflow of false negative alerts.

The author's PhD research is funded by DOSIsoft S.A.
SU‐E‐T‐686: SBRT 4‐Dimensional Treatment Planning for Vero Dynamic Tumor Tracking Lung Patients
Authors: Su Z , Dagan R , Li Z
Published: June 2015

Purpose:

To investigate 4‐dimensional (4D) treatment planning for Vero dynamic tumor tracking (DTT) SBRT lung patients and to compare its dosimetry to one single‐phase (50%) CT based treatment plan as well as to 4D average‐intensity CT based plan.

Methods:

7 lung patients with total of 10 tumors were retrospectively selected. Tumor and organ‐at‐risks (OARs), which include lung, heart, esophagus, spinal cord and proximal airways, were contoured on each phase of the 4DCTs and 4D average‐intensity CTs. GTV averages and standard deviations (SD) among phases were obtained for target deformation evaluation. For each tumor, treatment plans of 50Gy in 5 fractions with same beam arrangements were generated for the 3 planning approaches. MimVista was used to deform phase dose matrices to the 50% phase. Doses to the OARs were compared between 4D deformed plans and single‐phase plans (clinical DTT plans). Single‐phase CT plans were also compared to 4D average‐intensity CT plans.

Results:

The relative SD of phase GTV volume is mostly in an inverse relationship with GTV volume. Compared to 4D average‐intensity plan, single‐phase DTT plan shows up to 253.9cc reduction of lung volume at 12.5Gy, and reduction of 28.1Gy, 4.8Gy, 5.3Gy of maximum dose to proximal airways, to heart and to spinal cord, respectively; Compared to single‐phase DTT plan, 4D deformed plan shows up to 70.3cc reduction of lung volume at 12.5Gy, and reduction of 1.6Gy, 3.4Gy, 1.3Gy of maximum dose to proximal airways, to heart and to spinal cord, respectively.

Conclusion:

This study showed that small tumors are potentially prone to phase deformation. This study also demonstrated that DTT can significantly reduce doses to OARs for lung SBRT patients. Furthermore, 4D treatment planning with phase dose deformation to 50% phase is more representative of actual dose to OARs than single‐phase CT based planning for lung patients treated with DTT.

This study is partially supported by Brainlab Inc.
SU‐E‐T‐692: Should the Machine‐Related Range Shifter (MRS) Be Replaced by a Patient‐Related Range Shifter (PRS) to Preserve the Scanning Beam Dosimetric Advantages?
Authors: Lin H , Shen J , McDonough J , Tochner Z , Both S
Published: June 2015

Purpose:

To present and evaluate the impact of PRS relative to MRS on PBS spot size.

Methods:

Three PRS: U‐shape bolus (USB), anterior‐lateral bridge bolus (ALBB), couch top bolus (CTB)) and MRS mounted on the nozzle are currently employed in our institution. Homogeneous materials with different densities were employed for each device. In‐air spot sizes were simulated and compared for 6 conditions: open field, MRS with 15cm, 20cm, and 35 cm air gaps and PRS with 2cm and 8cm air gaps to reproduce clinical treatment conditions. The pros and cons of MRS are evaluated against PRS.

Results:

Spot sizes increased with greater air gaps, which are more significant for low energy protons. PRS can be consistently placed close to the patient surface; therefore spot sizes don't change significantly relative to the open beam. MRS may have the same effect for small air gaps. While PRS is overall superior in avoiding patient collision, its impact on the quality of the verification imaging increases as Z increases, in particular for CTB. However, lower Z‐material CTB may increase collision and still affect CBCT quality. The main advantages of the PRS are spot size preservation (<2mm change for air gaps of 2 to 8cm), increased uniformity and conformality of the dose distributions and patient collision avoidance. This cannot be achieved consistently for Brain and HN treatment using MRS as the air gap may increase greater than 15cm (maximal increase of 4.5 mm on spot size) due to clearance issues.

Conclusion:

The spot size and PBS dosimetrical advantages can be maintained with minimized air gap for both PRS and MRS. However; the PRS non‐removable device (CTB) affects CBCT imaging quality. Therefore, an ideal approach would be a combination between the USB for the brain and HN treatments and an automated MRS for body and extremities.
SU‐E‐T‐697: Simple and Efficient Modeling of the Varian TrueBeam Linac Using the GATE Monte Carlo Platform
Authors: Roth B , Ploquin N , Wiebe J
Published: June 2015

Purpose:

To investigate the use of the relatively new Geant4 Application for Tomographic Emission (GATE) Monte Carlo (MC) platform for radiotherapy applications, by developing a model of the Varian TrueBeam™ linac.

Methods:

Multiple 6 MV phase space files were provided by Varian, serving as a simulation particle source. Modeling of the jaws and multileaf collimator (MLC) in GATE's macro structured environment was automated through the use of an in‐house developed Python program. Profiles and percentage depth doses (PDD) were compared between MC calculations, diode, ion chamber and radiochromic film measurements for six square fields ranging in size from 1 × 1 cm2 to 40 × 40 cm2 at five different depths. The MLC model was evaluated through comparison to radiochromic film for an “E” shaped field defined by the MLCs at two different depths.

Results:

Profiles and PDDs of MC calculated values for each field size and depth were evaluated against measurements using a tight 3%/1 mm gamma criteria. It was found that 28 out of 30 square field profiles have a gamma pass rate of ≥90%, 20 of which achieved at least a 95% pass rate. All PDDs were found to have at least a 97% pass rate. When compared to film measurements, 97% and 99% of points met the gamma criteria for the MLC shaped field along the inline and crossline axes respectively, while 100% of points achieved the criteria along both axes at a depth of 50 mm.

Conclusion:

GATE provides a valid, efficient and simple to use MC platform to model medical linear accelerators for the purpose of radiotherapy dose calculation.
SU‐E‐T‐704: Skin Dose in Proton Pencil Beam Scanning for Pediatric Patients
Authors: Axente M , Moskvin V , Shin J , Tsiamas P , Merchant T , Farr J
Published: June 2015

Purpose:

Optimize intensity modulated proton therapy (IMPT) treatment planning guidelines with regard to skin dose reduction in pediatric patients.

Methods:

Phantom and patient CTs were used to evaluate skin dose using a pre‐clinical proton beam modeled using Monte Carlo (TOPAS) in a commercial radiotherapy planning system (RTP). Single‐spot and single‐beam doses were analyzed in phantom environment. Fractional surface dose was evaluated vs. spot spacing and maximum range for beams optimized for increasingly deep targets. Both RTP automatic variable spacing (0.5xσprojected) and constant spacing (0.2–1.2cm) were evaluated. 3 spine IMPT plans were obtained with/without objectives for skin dose, with automatic and constant spot spacing. Dosimetric parameters and spot weight/energy statistics were evaluated.

Results:

Single‐spot primary fluence depletion raises the spot axial relative skin dose above Bragg peak dose for E0>139MeV. Dose heterogeneity and maximum skin dose increases with maximum beam energy and spot spacing non‐linearly. Exponential fitting of maximum skin‐to‐target dose ratio against spot spacing suggested an optimum constant spacing <3mm for minimizing skin dose. Automatic spacing did not produce optimal results for deep tumors. For patient plans, heterogeneous and high skin dose were observed where >50% MU weight was carried by spots with energy >135MeV, and had spots >155MeV. Introducing skin optimization objectives/constraints, decreased high energy spots weights, while that for >135MeV spots increased. Hence, skin D50 increased with skin‐optimized plans, while coverage remained constant. Average dose, D1 and D0.1cc decreased compared to plans without skin objectives. Constant spacing did not produce superior skin dose distributions without skin optimization objectives.

Conclusion:

IMPT inverse planning can produce high skin doses with heterogeneous distributions, effects that can be mitigated via optimization. Careful coordination between RTP techniques and accurate skin dosimetric parameters will allow minimization of clinical skin effects and establishment of threshold doses for acute/late effects in pediatric population treated with IMPT.
SU‐E‐T‐719: Surface Dose Evaluation of the Equivalent Quality Flattening Filter‐Free Photon Beams
Authors: Meshram M , Pramanik S , Ranjith C P , Meshram M
Published: June 2015

Purpose:

To provide data on the relative surface dose of photon beams of Versa HD linear accelerator designed to delivered the Flattening Filter Free (FFF) beam having beam quality equivalent to the corresponding Flattened beam.This study involves the comparative evaluation of the entrance dose of FFF and Flattened photon beams in the buildup region.

Methods:

Surface dose was measured using Markus parallel plate chamber with the thin flat entrance window thickness of 0.025mm of water equivalent material in solid water phantom. Data were collected for square fields of 2×2cm2, 3×3cm2,4×4cm2,5×5cm2,7×7cm2,10×10cm2 and 20×20cm2 created in Monaco Treatment Planning System.The surface dose was define for this study as the relative dose at depth of 0.5mm with respect to the dose at dmax.The same Markus chamber was used to investigate the dose in the buildup region for all the square fields.The buildup depths for 6MV flattened and FFF beams were 0 to 18mm and for 10MV Flattened and FFF beams were 0 to 24mm. All values were determined in relation to the depth at dose maximum dmax.

Results:

The relation of surface dose for both Flattened and FFF beams with field sizes were linear. The surface dose for smaller field size upto 10×10cm2 for both FFF beams were higher than Flattened beams but the variation was marginal. The difference was 0.3 percentage at 6MV and 1.9 percentage at 10MV for a 10×10cm2 field but for the larger field the relative surface dose was lower for FFF beam with difference of 3.6 percentage at 6MV and 4.3 percentage at 10MV for a 20×20cm2 field.

Conclusion:

The equivalent quality FFF Photons produce higher surface dose for smaller field sizes upto 10×10cm2 but lower surface dose for large field sizes than the conventional flattened beam. The percentage variation was marginal for smaller field sizes and may not be clinically significant.