bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐81: A Study On Correlation Between Gamma Analysis for Midline and Lateralized Tumors Using VMAT
Authors: Kumar Syam , Anjana
Published: June 2015

Purpose:

To evaluate the fluence for the midline and lateralized tumors for VMAT technique using 2D seven29 detector array combined with the Octavius phantom.

Methods:

60 cases that are already being treated with volumetric modulated arc therapy (VMAT) have selected for this study. This includes tumors situated at the medial and lateral. Medial refers to the tumor situated at the midline of the body and lateral means toward the side or away from the midline of the body. Verification plans were created for each treatment plan in Varian Eclipse treatment planning system (version10, Varian medical systems, Palo Alto,CA) with the 2D Seven29 detector array and the Octavius phantom(PTW, Freiburg, Germany). Measurements were performed on a Varian Clinac 2100 iX, linear accelerator equipped with a millennium 120 leaf collimator. Analysis was done by comparing the fluence measured for the tumors situated on the midline and tumors situated laterally.

Results:

Fluence measured for all the delivered plans were analyzed using Verisoft software (PTW, Freiburg, Germany). The gamma pass percentage for midline tumors were found to be higher compared with the lateralized ones. The standard deviation between gamma values for midline and lateralized tumors is 2.18 and 3.5 respectively. Also the standard deviation between the point doses for midline and lateralized tumors is 0.38 and 0.29 respectively. The average gamma passing rate for midline tumors is 96.55% and for lateralized tumors are 94.94% for 3%DD and 3mm DTA criteria. From the T test, it was found that there is no significant difference between the gamma pass percentage between midline and lateralized tumors with p value of 0.28.

Conclusion:

There is no particular correlation found in the gamma pass criteria for midline and lateralized tumors.
SU‐E‐T‐87: A TG‐100 Approach for Quality Improvement of Associated Dosimetry Equipment
Authors: Manger R , Pawlicki T , Kim G
Published: June 2015

Purpose:

Dosimetry protocols devote so much time to the discussion of ionization chamber choice, use and performance that is easy to forget about the importance of the associated dosimetry equipment (ADE) in radiation dosimetry ‐ barometer, thermometer, electrometer, phantoms, triaxial cables, etc. Improper use and inaccuracy of these devices may significantly affect the accuracy of radiation dosimetry. The purpose of this study is to evaluate the risk factors in the monthly output dosimetry procedure and recommend corrective actions using a TG‐100 approach.

Methods:

A failure mode and effects analysis (FMEA) of the monthly linac output check procedure was performed to determine which steps and failure modes carried the greatest risk. In addition, a fault tree analysis (FTA) was performed to expand the initial list of failure modes making sure that none were overlooked. After determining the failure modes with the highest risk priority numbers (RPNs), 11 physicists were asked to score corrective actions based on their ease of implementation and potential impact. The results were aggregated into an impact map to determine the implementable corrective actions.

Results:

Three of the top five failure modes were related to the thermometer and barometer. The two highest RPN‐ranked failure modes were related to barometric pressure inaccuracy due to their high lack‐of‐detectability scores. Six corrective actions were proposed to address barometric pressure inaccuracy, and the survey results found the following two corrective actions to be implementable: 1) send the barometer for recalibration at a calibration laboratory and 2) check the barometer accuracy against the local airport and correct for elevation.

Conclusion:

An FMEA on monthly output measurements displayed the importance of ADE for accurate radiation dosimetry. When brainstorming for corrective actions, an impact map is helpful for visualizing the overall impact versus the ease of implementation.
SU‐E‐T‐91: Accuracy of Dose Calculation Algorithms for Patients Undergoing Stereotactic Ablative Radiotherapy
Authors: Tajaldeen A , Ramachandran P , Geso M
Published: June 2015

Purpose:

The purpose of this study was to investigate and quantify the variation in dose distributions in small field lung cancer radiotherapy using seven different dose calculation algorithms.

Methods:

The study was performed in 21 lung cancer patients who underwent Stereotactic Ablative Body Radiotherapy (SABR). Two different methods (i) Same dose coverage to the target volume (named as same dose method) (ii) Same monitor units in all algorithms (named as same monitor units) were used for studying the performance of seven different dose calculation algorithms in XiO and Eclipse treatment planning systems. The seven dose calculation algorithms include Superposition, Fast superposition, Fast Fourier Transform ( FFT) Convolution, Clarkson, Anisotropic Analytic Algorithm (AAA), Acurous XB and pencil beam (PB) algorithms. Prior to this, a phantom study was performed to assess the accuracy of these algorithms. Superposition algorithm was used as a reference algorithm in this study. The treatment plans were compared using different dosimetric parameters including conformity, heterogeneity and dose fall off index. In addition to this, the dose to critical structures like lungs, heart, oesophagus and spinal cord were also studied. Statistical analysis was performed using Prism software.

Results:

The mean±stdev with conformity index for Superposition, Fast superposition, Clarkson and FFT convolution algorithms were 1.29±0.13, 1.31±0.16, 2.2±0.7 and 2.17±0.59 respectively whereas for AAA, pencil beam and Acurous XB were 1.4±0.27, 1.66±0.27 and 1.35±0.24 respectively.

Conclusion:

Our study showed significant variations among the seven different algorithms. Superposition and AcurosXB algorithms showed similar values for most of the dosimetric parameters. Clarkson, FFT convolution and pencil beam algorithms showed large differences as compared to superposition algorithms. Based on our study, we recommend Superposition and AcurosXB algorithms as the first choice of algorithms in lung cancer radiotherapy involving small fields. However, further investigation by Monte Carlo simulation is required to confirm our results.
SU‐E‐T‐94: An Advanced Rotating Gamma Ray System for Stereotactic Radiosurgery
Authors: Ma C , Chibani O , Mora G , Li J , Chen L
Published: June 2015

Purpose:

Co‐60 beams have unique dosimetric properties that are ideally suited for cranial treatments. Co‐60 sources with cone‐shaped collimators provide conformal dose distributions allowing for ablative treatments with rapid dose falloff to spare nearby critical structures. This work investigates a novel, image‐guided, rotational Gamma ray system that provides both superior dose conformity/gradient and accurate stereotaxy for stereotactic radiosurgery (SRS).

Methods:

The SupeRay system (Cyber Medical Corp., China) consists of a rotating source chamber containing 30 gamma sources focusing at the isocenter with 4 collimators measuring 3, 4, 8 and 16mm in diameter. A novel switch design enables the 30 Gamma sources to be turned off at any arbitrarily selected 60° interval in order to avoid critical structures. The 3D treatment couch provides automatic treatment positioning between individual shots and the kV imaging system provides orthogonal images with a spatial resolution of 0.24mm to facilitate target localization. Monte Carlo simulations were used to compute dose distributions and compare with measurements and other Gamma ray SRS systems.

Results:

Monte Carlo results confirmed the SupeRay design parameters including output factors and 3D dose distributions. Its beam penumbra/dose gradient is similar to or slightly better than that of the Elekta Gamma Knife. The penumbra in the (x,y,z) direction was (7.38mm,7.38mm,3.86mm) for the 16mm collimator, (4.83mm,4.83mm,3.12mm) for the 8mm collimator, and (3.03mm,3.03mm,2.38mm) for the 4mm collimator, respectively, on the SupeRay system while it was (9.5mm,10.0mm,2.9mm), (4.3mm,4.3mm,2.9mm) and (3.2mm,3.2mm,1.9mm) for the same collimator sizes, respectively, on the Perfexion system. The kV imaging system together with a non‐invasive relocatable frame provides accurate target localization (<0.5mm) for cases requiring multiple treatment fractions.

Conclusion:

Because of the unique dosimetric properties of Co‐60 sources and its accurate stereotaxy/dose delivery, SupeRay is ideally suited for single and multiple‐session ablative treatments of cranial lesions.

This work was partially supported by Cyber Medical Corp.
SU‐E‐T‐96: An Analysis of VMAT SBRT Lung Treatment Plans
Authors: Wilson D , James J , Wang B , Dunlap N , Woo S , Silverman C , Dragun A , El‐Ghamry M
Published: June 2015

Purpose:

To present an analysis of 218 lung sbrt treatment plans delivered using Varian Eclipse RapidArc software and delivered with Varian linear accelerators.

Methods:

A retrospective analysis of treatment plans generated using the Varian Eclipse PRO RapidArc VMAT optimization and AAA photon beam calculation algorithms was done for 218 plans delivered using Varian Trilogy and TrueBeam linear accelerators. Some of the patients were enrolled in the RTOG 0813 and RTOG 0915 lung protocols. All of the patients’ plans were optimized using the guidelines outlined by these protocols. The plans presented were normalized so that 95% of the PTV volume received 100% of the prescribed dose.

Results:

The dose co formality constraints from the RTOG protocols and the number of plans within those constraints were:Tumor dose between 60% of maximum dose and 90% of maximum: 218/218 = 100%.Volume of 105% tumor dose outside put less than 15% of the PTV volume: 218/218 = 100%.Maximum dose 2 cm away from PTV within protocol table guidelines: No minor deviation: 188/218 = 86.2%. No major deviation: 216/218 = 99.1%.Ratio of 50% dose volume to PTV volume less than R50 in protocol table: No minor deviation 148/218 = 67.9%. No major deviation: 213/218 = 97.7%.Percent of lung receiving 20 Gy less than 10% (no minor violation): 218/218 = 100%.99% of PTV receiving at least 90% of the tumor dose: 214/218 = 98.2%.

Conclusion:

Varian RapidArc VMAT can successfully deliver lung SBRT treatments as outlined in the RTOG 0813 and 0915 protocols. The largest deviations were seen in the R50 constraints for the larger PTV volumes.
SU‐E‐T‐97: An Analysis of Knowledge Based Planning for Stereotactic Body Radiation Therapy of the Spine
Authors: Foy J , Marsh R , Owen D , Matuszak M
Published: June 2015

Purpose:

Creating high quality SBRT treatment plans for the spine is often tedious and time consuming. In addition, the quality of treatment plans can vary greatly between treatment facilities due to inconsistencies in planning methods. This study investigates the performance of knowledge‐based planning (KBP) for spine SBRT.

Methods:

Treatment plans were created for 28 spine SBRT patients. Each case was planned to meet strict dose objectives and guidelines. After physician and physicist approval, the plans were added to a custom model in a KBP system (RapidPlan, Varian Eclipse v13.5). The model was then trained to be able to predict estimated DVHs and provide starting objective functions for future patients based on both generated and manual objectives. To validate the model, ten additional spine SBRT cases were planned manually as well as using the model objectives. Plans were compared based on planning time and quality (ability to meet the plan objectives, including dose metrics and conformity).

Results:

The average dose to the spinal cord and the cord PRV differed between the validation and control plans by <0.25% demonstrating iso‐toxicity. Six out of 10 validation plans met all dose objectives without the need for modifications, and overall, target dose coverage was increased by about 4.8%. If the validation plans did not meet the dose requirements initially, only 1–2 iterations of modifying the planning parameters were required before an acceptable plan was achieved. While manually created plans usually required 30 minutes to 3 hours to create, KBP can be used to create similar quality plans in 15–20 minutes.

Conclusion:

KBP for spinal tumors has shown to greatly decrease the amount of time required to achieve high quality treatment plans with minimal human intervention and could feasibly be used to standardize plan quality between institutions.

Supported by Varian Medical Systems
SU‐E‐T‐109: An Investigation of Including Variable Relative Biological Effectiveness in Intensity Modulated Proton Therapy Planning Optimization for Head and Neck Cancer Patients
Authors: Cao W , Zaghian M , Randeniya K , Lim G , Titt U , Mohan R
Published: June 2015

Purpose:

The current practice of considering the relative biological effectiveness (RBE) of protons in intensity modulated proton therapy (IMPT) planning is to use a generic RBE value of 1.1. However, RBE is indeed a variable depending on the dose per fraction, the linear energy transfer, tissue parameters, etc. In this study, we investigate the impact of using variable RBE based optimization (vRBE‐OPT) on IMPT dose distributions compared by conventional fixed RBE based optimization (fRBE‐OPT).

Methods:

Proton plans of three head and neck cancer patients were included for our study. In order to calculate variable RBE, tissue specific parameters were obtained from the literature and dose averaged LET values were calculated by Monte Carlo simulations. Biological effects were calculated using the linear quadratic model and they were utilized in the variable RBE based optimization. We used a Polak‐Ribiere conjugate gradient algorithm to solve the model. In fixed RBE based optimization, we used conventional physical dose optimization to optimize doses weighted by 1.1. IMPT plans for each patient were optimized by both methods (vRBE‐OPT and fRBE‐OPT). Both variable and fixed RBE weighted dose distributions were calculated for both methods and compared by dosimetric measures.

Results:

The variable RBE weighted dose distributions were more homogenous within the targets, compared with the fixed RBE weighted dose distributions for the plans created by vRBE‐OPT. We observed that there were noticeable deviations between variable and fixed RBE weighted dose distributions if the plan were optimized by fRBE‐OPT. For organs at risk sparing, dose distributions from both methods were comparable.

Conclusion:

Biological dose based optimization rather than conventional physical dose based optimization in IMPT planning may bring benefit in improved tumor control when evaluating biologically equivalent dose, without sacrificing OAR sparing, for head and neck cancer patients.

The research is supported in part by National Institutes of Health Grant No. 2U19CA021239‐35.
SU‐E‐T‐115: Analysis of Patient Specific QA for VMAT by Disease Site and Planning‐Delivery System Using the ScandiDos Delta4 Phantom
Authors: Giaddui T , Hardin M , Keller J , Kremmel E , Fu M , Peng C , Doyle L , Yu Y , Xiao Y , Harrison A
Published: June 2015

Purpose:

To evaluate patient specific quality assurance (PSQA) for the delivery of volumetric modulated arc therapy (VMAT) by disease site. To compare planning‐delivery system (PDS) PSQA pass rates in a dual vendor institution.

Methods:

PSQA is performed for VMAT plans using a ScandiDos Delta4 phantom. Verification plans are calculated using Varian Eclipse and Elekta Monaco treatment planning systems (TPS) for patients treated using Varian Truebeam and Elekta linear accelerators respectively. Individual arcs are delivered to the Delta4 phantoms and assessed using the gamma index pass criterion(3% Dose Deviation(DD%), 3mm Distance to Agreement(DTA),10% dose threshold and 90% gamma index).

Results:

A total of 287 VMAT plans and 680 arcs were analyzed. The passing rates for VMAT QA plans were 95% and 98% for head/neck and pelvis/prostate plans respectively, and 100% for chest/abdomen, spine, lung Stereotactic Body Radiotherapy (SBRT) and Stereotactic Radiosurgery(SRS) plans. Average gamma indices were: (99 ± 2) % for pelvis/prostate, chest/abdomen and lung SBRT plans, (97 ± 4) % for head and neck plans and (98 ± 3) % for spine plans. The average DD% and DTA pass rates ranged from 82% to 90% and 98% to 99% respectively for plans in different disease sites. Paired t‐test analysis (two tails) indicated no significant differences in the gamma indices between plans delivered using different PDS; the P values were: 0.08, 0.45, and 0.94 for lung SBRT, head/neck and pelvis/prostate plans respectively. The statistical power for comparing PDS in different disease sites with an alpha of 0.05 is 1.

Conclusion:

The Gamma indices based on 3% DD%, 3 mm DTA and 10% dose threshold for the VMAT QA plans in all disease sites were well above 90%, suggesting the possibility of using a more stringent PSQA criterion. No significant differences were observed in the QA of VMAT plans delivered using different PDS.
SU‐E‐T‐117: Analysis of the ArcCHECK Dosimetry Gamma Failure Using the 3DVH System
Authors: Cho S , Choi W , Lee H , Yoon J , Lee E , Park K , Keum K
Published: June 2015

Purpose:

To evaluate gamma analysis failure for the VMAT patient specific QA using ArcCHECK cylindrical phantom. The 3DVH system(Sun Nuclear, FL) was used to analyze the dose difference statistic between measured dose and treatment planning system calculated dose.

Methods:

Four case of gamma analysis failure were selected retrospectively. Our institution gamma analysis indexes were absolute dose, 3%/3mm and 90%pass rate in the ArcCHECK dosimetry. The collapsed cone convolution superposition (CCCS) dose calculation algorithm for VMAT was used. Dose delivery was performed with Elekta Agility. The A1SL(standard imaging, WI) and cavity plug were used for point dose measurement. Delivery QA plans and images were used for 3DVH Reference data instead of patient plan and image. The measured data of ‘.txt’ file was used for comparison at diodes to acquire a global dose level. The,.acml’ file was used for AC‐PDP and to calculated point dose.

Results:

The global dose of 3DVH was calculated as 1.10 Gy, 1.13, 1.01 and 0.2 Gy respectively. The global dose of 0.2 Gy case was induced by distance discrepancy. The TPS calculated point dose of was 2.33 Gy to 2.77 Gy and 3DVH calculated dose was 2.33 Gy to 2.68 Gy. The maximum dose differences were −2.83% and −3.1% for TPS vs. measured dose and TPS vs. 3DVH calculated respectively in the same case. The difference between measured and 3DVH was 0.1% in that case. The 3DVH gamma pass rate was 98% to 99.7%.

Conclusion:

We found the TPS calculation error by 3DVH calculation using ArcCHECK measured dose. It seemed that our CCCS algorithm RTP system over estimated at the central region and underestimated scattering at the peripheral diode detector point. The relative gamma analysis and point dose measurement would be recommended for VMAT DQA in the gamma failure case of ArcCHECK dosimetry.
SU‐E‐T‐119: Analysis the Efficacy of Different Radiotherapy Methods and Failure Mode in No‐Metastasis Esophageal Squamous Cell Carcinoma
Authors: Yankun C , Zhihui T , Runxiao L , Shen W
Published: June 2015

Purpose:

To evaluate the curative effect of radio (chemo) therapy and mode of treatment failure in no‐metastasis and lesion length ≤ 5.0cm esophageal squamous cell carcinoma (ESCC).

Methods:

There were 158 eligible patients were retrospectively analyzed, to analysis the curative effect of radio (chemo) therapy, prognosis factors, toxicity and prognostic index model.

Results:

To all patients the 1, 3, 5 overall survival rate were 83.54%, 52.53%, 32.58%, the local recurrence rate were 15.08%, 33.60% and 38.14%; distant metastasis rate were 10.64%, 25.21% and 36.06%; tumor specific survival rate were 76.64%, 54.07% and 44.51%. Multivariate analysis showed that patients with ECOG grade (χ2=13.945, P=0.000), short‐term effect (χ2=19.360, P=0.000) and different radiotherapy methods (χ2=9.866, P=0.002) as the independent prognostic factors. Prognostic index model showed that the survival rate was significantly higher in the lower value of PI group than in the larger value of PI group (χ2=49.19, P=0.0000). In our whole group, there were simple locoregional recurrence (LR) 40 cases (25.3%), simple Distant metastasis (DM) 31 cases (19.6%), LR and DM in 14 cases (8.9%) after treatment. The chi‐square test showed that there were no significant difference in the incidence of Elective Nodal Irradiation (ENI )and Involved Field Irradiation (IFI) patients with LR and DM ( χ2=2.363, 2.950, P=0.124, 0.085).

Conclusion:

Radio (chemo) therapy has a good curative effect in no‐metastasis and lesion length ≤ 5.0cm ESCC patients.
SU‐E‐T‐139: Automated Daily EPID Exit Dose Analysis Uncovers Treatment Variations
Authors: Olch A
Published: June 2015

Purpose:

To evaluate a fully automated EPID exit dose system for its ability to detect daily treatment deviations including patient setup, delivery, and anatomy changes.

Methods:

PerFRACTION (Sun Nuclear Corporation) software is a system that uses integrated EPID images taken during patient treatment and automatically pulled from the Aria database and analyzed based on user‐defined comparisons. This was used to monitor 20 plans consisting of a total of 859 fields for 18 patients, for a total of 251 fractions. Nine VMAT, 5 IMRT, and 6 3D plans were monitored. The Gamma analysis was performed for each field within a plan, comparing the first fraction against each of the other fractions in each treatment course. A 2% dose difference, 1 mm distance‐to‐agreement, and 10% dose threshold was used. These tight tolerances were chosen to achieve a high sensitivity to treatment variations. The field passed if 93% of the pixels had a Gamma of 1 or less.

Results:

Twenty‐nine percent of the fields failed. The average plan passing rate was 92.5%.The average 3D plan passing rate was less than for VMAT or IMRT, 84%, vs. an average of 96.2%. When fields failed, an investigation revealed changes in patient anatomy or setup variations, often also leading to variations of transmission through immobilization devices.

Conclusion:

PerFRACTION is a fully automated system for determining daily changes in dose transmission through the patient that requires no effort other than for the imager panel to be deployed during treatment. A surprising number of fields failed the analysis and can be attributed to important treatment variations that would otherwise not be appreciated. Further study of inter‐fraction treatment variations is possible and warranted.

Sun Nuclear Corporation provided a license to the software described.
SU‐E‐T‐141: Automated Dose Point Placement for Cervical Cancer Brachytherapy Using Tandem and Ovoid Applicators
Authors: Kang H , Padilla L , Hasan Y , Al‐Hallaq H
Published: June 2015

Purpose:

To develop a standalone application, which automatically and consistently calculates the coordinates of points A and H based solely on the implanted applicator geometry for cervical cancer HDR brachytherapy.

Methods:

Manchester point A and ABS point H are both located 2cm lateral from the central tandem plane. While both points are located 2cm above the cervical os, surrogates for the os differ. Point A is defined relative to the anatomical cervical os. Point H is defined relative to the intersection of the tandem with the superior aspects of the ovoids. The application takes an input text file generated by the treatment planning system (TPS, BrachyVision, Varian) that specifies the source geometries. It then outputs the 3D coordinates of points A and H in both the left and right directions. The algorithm was implemented and tested on 34 CT scans of 7 patients treated with HDR brachytherapy delivered using tandem and ovoids. A single experienced user retrospectively and manually placed points A and H on the CT scans, whose coordinates were used as the gold standard for the comparison to the automatically calculated points.

Results:

The automatically calculated coordinates of points A and H agree within 0.7mm with the gold standard. The averages and standard deviations of the 3D coordinate difference between points placed by the two methods are 0.3±0.1 and 0.4±0.1mm for points A and H, respectively. The maximum difference in 3D magnitude is 0.7mm.

Conclusion:

The algorithm consistently calculates dose point coordinates independently of the planner for cervical cancer brachytherapy treated with tandem and ovoids. Automated point placement based on the geometry of the implanted applicators agrees in sub‐millimeter with careful manual placements by an experienced user. This algorithm expedites the planning process and eliminates dependencies on either user input or TPS visualization tools.
SU‐E‐T‐143: Automatic PBS Treatment Planning for Prostate Cancer
Authors: Zhang J , Durci M , Wu H , Rosen L , Ding X
Published: June 2015

Purpose:

This study presents a novel idea of using automated planning technique for parallel‐opposed prostate plans with pencil beam scanning proton therapy.

Methods:

A random 5 clinical prostate patients treated with parallel‐opposed modulated‐scanning proton beam in our proton center were selected in this study. All of the 5 cases were re‐planned with in‐house developed python scripts integrated into RayStation 4.0 clinical version. This automatic tool was designed to perform patient DICOM data import, external structure contours, CT density assignment, prescription to PTV assignment, beam placement, optimization, and dose calculation.

Results:

The mean time to generate a complete treatment plan was 1 minute per plan. For the automatically generated plans, 5 out of 5 plans (100%) were deemed clinically acceptable by one experienced radiation oncologist. In addition, the automated plans were overall dosimetrically equivalent to the clinical plans when scored for target coverage, rectum and bladder dose. Mann‐Whitney U tests did not show significant dosimetric differences for normal tissue structures between the clinical and automated plans.

Conclusion:

We have developed a robust and automated method for fully inversed planned paralleled‐opposed prostate proton treatment planning. This efficient method can be readily integrated into clinical practice. The tool produces clinically acceptable plans using only manually contoured anatomic structures and simulation CT data set. We anticipate that this tool will not only improve patient access to high‐quality proton treatment by simplifying the planning process and will also reduce the effort and cost of incorporating more advanced planning into clinical practice in the near future.
SU‐E‐T‐144: Bayesian Inference of Local Relapse Data Using a Poisson‐Based Tumour Control Probability Model
Authors: La Russa D
Published: June 2015

Purpose:

The purpose of this project is to develop a robust method of parameter estimation for a Poisson‐based TCP model using Bayesian inference.

Methods:

Bayesian inference was performed using the PyMC3 probabilistic programming framework written in Python. A Poisson‐based TCP regression model that accounts for clonogen proliferation was fit to observed rates of local relapse as a function of equivalent dose in 2 Gy fractions for a population of 623 stage‐I non‐small‐cell lung cancer patients. The Slice Markov Chain Monte Carlo sampling algorithm was used to sample the posterior distributions, and was initiated using the maximum of the posterior distributions found by optimization. The calculation of TCP with each sample step required integration over the free parameter α, which was performed using an adaptive 24‐point Gauss‐Legendre quadrature. Convergence was verified via inspection of the trace plot and posterior distribution for each of the fit parameters, as well as with comparisons of the most probable parameter values with their respective maximum likelihood estimates.

Results:

Posterior distributions for α, the standard deviation of α (σ), the average tumour cell‐doubling time (Td), and the repopulation delay time (Tk), were generated assuming α/β = 10 Gy, and a fixed clonogen density of 10⁷ cm−3. Posterior predictive plots generated from samples from these posterior distributions are in excellent agreement with the observed rates of local relapse used in the Bayesian inference. The most probable values of the model parameters also agree well with maximum likelihood estimates.

Conclusion:

A robust method of performing Bayesian inference of TCP data using a complex TCP model has been established.
SU‐E‐T‐149: Brachytherapy Patient Specific Quality Assurance for a HDR Vaginal Cylinder Case
Authors: Barbiere J , Napoli J , Ndlovu A
Published: June 2015

Purpose:

Commonly Ir‐192 HDR treatment planning system commissioning is only based on a single absolute measurement of source activity supplemented by tabulated parameters for multiple factors without independent verification that the planned distribution corresponds to the actual delivered dose. The purpose on this work is to present a methodology using Gafchromic film with a statistically valid calibration curve that can be used to validate clinical HDR vaginal cylinder cases by comparing the calculated plan dose distribution in a plane with the corresponding measured planar dose.

Methods:

A vaginal cylinder plan was created with Oncentra treatment planning system. The 3D dose matrix was exported to a Varian Eclipse work station for convenient extraction of a 2D coronal dose plane corresponding to the film position. The plan was delivered with a sheet of Gafchromic EBT3 film positioned 1mm from the catheter using an Ir‐192 Nucletron HDR source. The film was then digitized with an Epson 10000 XL color scanner. Film analysis is performed with MatLab imaging toolbox. A density to dose calibration curve was created using TG43 formalism for a single dwell position exposure at over 100 points for statistical accuracy. The plan and measured film dose planes were registered using a known dwell position relative to four film marks. The plan delivered 500 cGy to points 2 cm from the sources.

Results:

The distance to agreement of the 500 cGy isodose between the plan and film measurement laterally was 0.5 mm but can be as much as 1.5 mm superior and inferior. The difference between the computed plan dose and film measurement was calculated per pixel. The greatest errors up to 50 cGy are near the apex.

Conclusion:

The methodology presented will be useful to implement more comprehensive quality assurance to verify patient‐specific dose distributions
SU‐E‐T‐156: Can Sr‐90 Check Sources Replace Co‐60 Measurements for Monitoring of Reference Chamber Stability?
Authors: McEwen M , Miksys N , Niven D
Published: June 2015

Purpose:

To determine the ultimate precision of a system for monitoring reference‐class ion chamber stability using a commercial Sr‐90 check source.

Methods:

A detailed investigation of a commercial Sr‐90 check source (PTW48002) was carried out using a series of Farmer‐type ionization chambers. Investigations included: positioning repeatability (angular variation as chamber is rotated in source, variation in ionization current with vertical alignment); chamber settling; short and long term repeatability

Results:

i) Measurement precision – the ionization current was typically 10 pA, and therefore a high‐precision electrometer is required to prevent electrometer noise/resolution/leakage biaising the results. ii) Chamber settling ‐ the chamber response stabilizes after approximately 10 minutes, which is longer than reported for linac beams and is likely due to the low doserate of the source.iii) The measured response depended at the 1 % level on the orientation of the chamber with respect to the source. However, consistent positioning resulted in repeatability at the 0.05 % level. Care was also required to ensure that the chamber was consistently positioned vertically with respect to the source. The sensitivity to vertical position was found to be > 1 % per mm.iv)With a uniform procedure the long‐term (> 6 month) repeatability was found to be better than 0.1 % for multiple chamber types and potentially a precision of 0.05 % is achievable.

Conclusion:

A Sr‐90 check source is easy to use and is a viable alternative to Co‐60 for monitoring reference chamber stability.
SU‐E‐T‐162: Characterization of a New Oblong Cone for Use with the Intraoperative Mobetron Unit
Authors: Cantley J , Colussi V
Published: June 2015

Purpose:

To evaluate the dosimetric characteristics of a new flat 7 cm × 12 cm oblong cone for intraoperative radiation therapy (IORT) procedures.

Methods:

Percent depth dose (PDD) curves and profile measurements at dose depths of D100, D90, D80, D50, and D30 were measured using a parallel‐plate ion chamber and a 3‐D water tank Blue phantom II (iba) for the three energies (6, 9, and 12 MeV) available with the new generation Mobetron 2000. In addition, PDD curves and profiles were made with Gafchromic EBT3 films in solid water phantom. H&D curves were created for each energy for absolute calibration. The films were analyzed using the RIT image analysis software and then compared with the ion chamber results. Output values were obtained by normalizing the dose/MU at D100 at the clinical axis of the oblong cone by the dose/MU at D100 for the flat 10cm cylindrical standard cone using ion chamber measurements.

Results:

Ion chamber results show that the PDDs from the flat oblong cone, at the clinical axis, match with the PDDs for its “cylindrical brother” 7cm flat cone. These PDDs are comparable with those from the 10cm standard cone. The D100 coincides for all three energies. The cone output factors for the oblong cone are 1.07 for all energies. Ion chamber profiles measurements and film analysis show that the use of flat oblong cone with the 6 and 9 MeV energies

Result

in horns of approximately 12% and 8%, respectively. Conversely, 12 MeV profiles show loss of flatness near the field edge.

Conclusion:

The potential advantage of the oblong cones is the elimination of the well‐known difficulties of electron field matching, both dosimetrically and in clinical setup.
SU‐E‐T‐167: Characterization of In‐House Plastic Scintillator Detectors Array for Radiation Therapy
Authors: Zhu T , Liu H , Dimofte A , Darafsheh A , Lin H , Kassaee A , Finlay J , Both S
Published: June 2015

Purpose:

To characterize basic performance of plastic scintillator detectors (PSD) array designed for dosimetry of radiation therapy.

Methods:

An in‐house PSD array has been developed by placing single point PSD into customized 2D holder. Each point PSD is a plastic scintillating fiber‐based detector designed for highly accurate measurement of small radiotherapy fields used in patient plan verification and machine commissioning and QA procedures. A parallel fiber without PSD is used for Cerenkov separation by subtracting from PSD readings. Cerenkov separation was confirmed by optical spectroscopy. Alternative Cerenkov separation approaches are also investigated. The optical signal was converted to electronic signal with a photodiode and then subsequently amplified. We measured its dosimetry performance, including percentage depth dose and output factor, and compared with reference ion chamber measurements. The PSD array is then placed along the radiation beam for multiple point dose measurement, representing subsets of PDD measurements, or perpendicular to the beam for profile measurements.

Results:

The dosimetry results of PSD point measurements agree well with reference ion chamber measurements. For percentage depth dose, the maximal differences between PSD and ion chamber results are 3.5% and 2.7% for 6MV and 15MV beams, respectively. For the output factors, PSD measurements are within 3% from ion chamber results. PDD and profile measurement with PSD array are also performed.

Conclusions:

The current design of multichannel PSD array is feasible for the dosimetry measurement in radiation therapy. Dose distribution along or perpendicular to the beam path could be measured. It might as well be used as range verification in proton therapy.A PS hollow fiber detector will be investigated to eliminate the Cerenkov radiation effect so that all 32 channels can be used.
SU‐E‐T‐168: Characterization of Neutrons From the TrueBeam Treatment Head
Authors: Sawkey D , Svatos M
Published: June 2015

Purpose:

Calculate neutron production and transport in the TrueBeam treatment head, as input for vault design and phantom dose calculations.

Methods:

A detailed model of the treatment head, including shielding components off the beam axis, was created from manufacturer's engineering drawings. Simulations were done with Geant4 for the 18X, 15X, 10X and 10FFF beams, tuned to match measured dose distributions inside the treatment field. Particles were recorded on a 70 cm radius sphere surrounding the treatment head enabling input into simulations of vaults.

Results:

For the 18X beam, 11×10^9 neutrons/MU were observed. The energy spectrum was a broad peak with average energy 0.37 MeV. With jaws closed, 48% of the neutrons were generated in the primary collimator, 18% in the jaws, 12% in the target, and 10% in the flattening filter. With wide open jaws, few neutrons were produced in the jaws and consequently total neutron production dropped to 8.5×10^9 neutrons/MU. Angular distributions were greatest along the beam axis (12×10^9 neutrons/MU/sr, within 2 deg of the beam axis) and antiparallel to the beam axis (7×10^9 neutrons/MU/sr). Peaks were observed in the neutron energy spectrum, corresponding to elastic scattering resonances in the shielding materials. Neutron production was lower for the other beams studied: 4.1×10^9 neutrons/MU for 15X, 0.38×10^9 neutrons/MU for 10X, and 0.22×10^9 neutrons/MU for 10FFF. Despite dissimilar treatment head geometries and materials, the neutron production and energy spectrum were similar to those reported for Clinac accelerators.

Conclusion:

Detailed neutron production and leakage calculations for the TrueBeam treatment head were done. Unlike other studies, results are independent of the surrounding vault, enabling vault design calculations.
SU‐E‐T‐169: Characterization of Pacemaker/ICD Dose in SAVI HDR Brachytherapy
Authors: Kalavagunta C , Lasio G , Yi B , Zhou J , Lin M
Published: June 2015

Purpose:

It is important to estimate dose to pacemaker (PM)/Implantable Cardioverter Defibrillator (ICD) before undertaking Accelerated Partial Breast Treatment using High Dose Rate (HDR) brachytherapy. Kim et al. have reported HDR PM/ICD dose using a single‐source balloon applicator. To the authors knowledge, there have so far not been any published PM/ICD dosimetry literature for the Strut Adjusted Volume Implant (SAVI, Cianna Medical, Aliso Viejo, CA). This study aims to fill this gap by generating a dose look up table (LUT) to predict maximum dose to the PM/ICD in SAVI HDR brachytherapy.

Methods:

CT scans for 3D dosimetric planning were acquired for four SAVI applicators (6−1‐mini, 6−1, 8−1 and 10−1) expanded to their maximum diameter in air. The CT datasets were imported into the Elekta Oncentra TPS for planning and each applicator was digitized in a multiplanar reconstruction window. A dose of 340 cGy was prescribed to the surface of a 1 cm expansion of the SAVI applicator cavity. Cartesian coordinates of the digitized applicator were determined in the treatment leading to the generation of a dose distribution and corresponding distance‐dose prediction look up table (LUT) for distances from 2 to 15 cm (6‐mini) and 2 to 20 cm (10–1).The deviation between the LUT doses and the dose to the cardiac device in a clinical case was evaluated.

Results:

Distance‐dose look up table were compared to clinical SAVI plan and the discrepancy between the max dose predicted by the LUT and the clinical plan was found to be in the range (−0.44%, 0.74%) of the prescription dose.

Conclusion:

The distance‐dose look up tables for SAVI applicators can be used to estimate the maximum dose to the ICD/PM, with a potential usefulness for quick assessment of dose to the cardiac device prior to applicator placement.
SU‐E‐T‐170: Characterization of the Location, Extent, and Proximity to Critical Structures of Target Volumes Provides Detail for Improved Outcome Predictions Among Pancreatic Cancer Patients
Authors: Cheng Z , Moore J , Rosati L , Mian O , Narang A , Herman J , McNutt T
Published: June 2015

Purpose:

In radiotherapy, size, location and proximity of the target to critical structures influence treatment decisions. It has been shown that proximity of the target predicts dosimetric sparing of critical structures. In addition to dosimetry, precise location of disease has further implications such as tumor invasion, or proximity to major arteries that inhibit surgery. Knowledge of which patients can be converted to surgical candidates by radiation may have high impact on future treat/no‐treat decisions. We propose a method to improve our characterization of the location of pancreatic cancer and treatment volume extent with respect to nearby arteries with the goal of developing features to improve clinical predictions and decisions.

Methods:

Oncospace is a local learning health system that systematically captures clinical outcomes and all aspects of radiotherapy treatment plans, including overlap volume histograms (OVH) – a measure of spatial relationships between two structures. Minimum and maximum distances of PTV and OARs based on OVH, PTV volume, anatomic location by ICD‐9 code, and surgical outcome were queried. Normalized distance to center from the left and right kidney was calculated to indicate tumor location and laterality. Distance to critical arteries (celiac, superior mesenteric, common hepatic) is validated by surgical status (borderline resectable, locally advanced converted to resectable).

Results:

There were 205 pancreas stereotactic body radiotherapy patients treated from 2009–2015 queried. Location/laterality of tumor based on kidney OVH show strong trends between location by OVH and by ICD‐9. Compared to the locally advanced group, the borderline resectable group showed larger geometrical distance from critical arteries (p=0.03).

Conclusion:

Our platform enabled analysis of shape/size‐location relationships. These data suggest that PTV volume and attention to distance between PTVs and surrounding OARs and major arteries may be promising for improving characterization of treatment anatomy that can refine our ability for outcome predictions and decision making.

Elekta, Toshiba
SU‐E‐T‐173: Clinical Comparison of Treatment Plans and Fallback Plans for Machine Downtime
Authors: Cruz W , Papanikolaou P , Mavroidis P , Stathakis S
Published: June 2015

Purpose:

The purpose of this study was to determine the clinical effectiveness and dosimetric quality of fallback planning in relation to machine downtime.

Methods:

Plans for a Varian Novalis TX were mimicked, and fallback plans using an Elekta VersaHD machine were generated using a dual arc template. Plans for thirty (n=30) patients of various treatment sites optimized and calculated using RayStation treatment planning system. For each plan, a fall back plan was created and compared to the original plan. A dosimetric evaluation was conducted using the homogeneity index, conformity index, as well as DVH analysis to determine the quality of the fallback plan on a different treatment machine. Fallback plans were optimized for 60 iterations using the imported dose constraints from the original plan DVH to give fallback plans enough opportunity to achieve the dose objectives.

Results:

The average conformity index and homogeneity index for the NovalisTX plans were 0.76 and 10.3, respectively, while fallback plan values were 0.73 and 11.4. (Homogeneity =1 and conformity=0 for ideal plan) The values to various organs at risk were lower in the fallback plans as compared to the imported plans across most organs at risk. Isodose difference comparisons between plans were also compared and the average dose difference across all plans was 0.12%.

Conclusion:

The clinical impact of fallback planning is an important aspect to effective treatment of patients. With the complexity of LINACS increasing every year, an option to continue treating during machine downtime remains an essential tool in streamlined treatment execution. Fallback planning allows the clinic to continue to run efficiently should a treatment machine become offline due to maintenance or repair without degrading the quality of the plan all while reducing strain on members of the radiation oncology team.
SU‐E‐T‐179: Clinical Impact of IMRT Failure Modes at Or Near TG‐142 Tolerance Criteria Levels
Authors: Faught J Tonigan , Balter P , Johnson J , Kry S , Court L , Stingo F , Followill D
Published: June 2015

Purpose:

Quantitatively assess the clinical impact of 11 critical IMRT dose delivery failure modes.

Methods:

Eleven step‐and‐shoot IMRT failure modes (FMs) were introduced into twelve Pinnacle v9.8 treatment plans. One standard and one highly modulated plan on the IROC IMRT phantom and ten previous H&N patient treatment plans were used. FMs included physics components covered by basic QA near tolerance criteria levels (TG‐142) such as beam energy, MLC positioning, and MLC modeling. Resultant DVHs were compared to those of failure‐free plans and the severity of plan degradation was assessed considering PTV coverage and OAR and normal tissue tolerances and used for FMEA severity scoring. Six of these FMs were physically simulated and phantom irradiations performed. TLD and radiochromic film results are used for comparison to treatment planning studies.

Results:

Based on treatment planning studies, the largest clinical impact from the phantom cases was induced by 2 mm systematic MLC shift in one bank with the combination of a D95% target under dose near 16% and OAR overdose near 8%. Cord overdoses of 5%–11% occurred with gantry angle, collimator angle, couch angle, MLC leaf end modeling, and MLC transmission and leakage modeling FMs. PTV coverage and/or OAR sparing was compromised in all FMs introduced in phantom plans with the exception of CT number to electron density tables, MU linearity, and MLC tongue‐and‐groove modeling. Physical measurements did not entirely agree with treatment planning results. For example, symmetry errors resulted in the largest physically measured discrepancies of up to 3% in the PTVs while a maximum of 0.5% deviation was seen in the treatment planning studies. Patient treatment plan study results are under analysis.

Conclusion:

Even in the simplistic anatomy of the IROC phantom, some basic physics FMs, just outside of TG‐142 tolerance criteria, appear to have the potential for large clinical implications.