bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐F‐T‐291: Development of Physician‐Driven, Clinically‐Relevant Pass/fail Criteria for a DVH‐Based IMRT Patient Specific QA Program
Authors: Price A , Lawrence M , Das S , Rankine L
Published: July 2016

Purpose:

To derive and implement physician driven metrics for dose volume histogram (DVH)‐based patient specific quality assurance (QA) for prostate radiation therapy based on clinically relevant deviations.

Methods:

An observership study presented 3 radiation oncology physicians from our institution with a series of artificial DVH‐based IMRT QA results, each generated by introducing iteratively larger perturbations from a planned DVH (from our pool of 20 real patient plans) to portions of the bladder, rectum, and planning target volume (PTV) DVHs. Physician responses were quantified by recording acceptable deviations from both in‐house defined clinical goals (e.g. V70Gy < 10cc for rectum, mean dose for bladder) and other accepted dosimetric indices from the literature . The newly defined DVH criteria were used to evaluate 14 clinical IMRT plans measured using ArcCHECK and processed using 3DVH (Sun Nuclear Corporation, Melbourne, FL), all of which passed the standard 3%/3mm 2D‐Γ comparison at >90% (absolute dose). In addition, 4 intentionally perturbed plans were measured, all with close to 90% 2D‐Γ passing rates (relative dose). We compared physician derived constraints against 2D‐Γ results to establish the relationship with an accepted IMRT QA metric.

Results:

Of the 14 clinical patients measured, 7 were flagged for physician review (i.e. did not pass physician‐derived DVH constraints). The rectum and bladder ΔD0.1cc constraint was the root cause of 5/7 of these QA failures. Of the 4 perturbed plans, 3/4 patients failed multiple DVH criteria in the bladder and rectum. Observership results from one physician‐derived metric (PTV ΔD0.1cc) were found to be too strict and excluded from analysis.

Conclusion:

We derived clinically relevant physician driven criteria for DVH‐based patient specific QA at our institution. In N=14 patient plans, implementation of these criteria reduced the need for physician review of patient QA results by 50%. Further refinement of PTV D0.1cc constraint is needed.

Partially funded by Sun Nuclear Corporation research agreement.
SU‐F‐T‐293: Experimental Comparisons of Ionization Chambers with Different Volumes for CyberKnife Delivery Quality Assurance
Authors: Nakayama M , Munetomo Y , Ogata T , Uehara K , Tsudou S , Nishimura H , Mayahara H , Sasaki R
Published: July 2016

Purpose:

To evaluate the practicality use of ionization chambers with different volumes for delivery quality assurance of CyberKnife plans,

Methods:

Dosimetric measurements with a spherical solid water phantom and three ionization chambers with volumes of 0.13, 0.04, and 0.01 cm3 (IBA CC13, CC04, and CC01, respectively) were performed for various CyberKnife clinical treatment plans including both isocentric and nonisocentric delivery. For each chamber, the ion recombination correction factors Ks were calculated using the Jaffe plot method and twovoltage method at a 10‐cm depth for a 60‐mm collimator field in a water phantom. The polarity correction factors Kpol were determined for 5–60‐mm collimator fields in same experimental setup. The measured doses were compared to the doses for the detectors calculated using a treatment planning system.

Results:

The differences in the Ks between the Jaffe plot method and two‐voltage method were −0.12, −0.02, and 0.89% for CC13, CC04, and CC01, respectively. The changes in Kpol for the different field sizes were 0.2, 0.3, and 0.8% for CC13, CC04, and CC01, respectively. The measured doses for CC04 and CC01 were within 3% of the calculated doses for the clinical treatment plans with isocentric delivery with collimator fields greater than 12.5 mm. Those for CC13 had differences of over 3% for the plans with isocentric delivery with collimator fields less than 15 mm. The differences for the isocentric plans were similar to those for the single beam plans. The measured doses for each chamber were within 3% of the calculated doses for the non‐isocentric plans except for that with a PTV volume less than 1.0 cm3.

Conclusion:

Although there are some limitations, the ionization chamber with a smaller volume is a better detector for verification of the CyberKnife plans owing to the high spatial resolution.
SU‐F‐T‐294: The Analysis of Gamma Criteria for Delta4 Dosimetry Using Statistical Process Control
Authors: Cho S , Ahn S , Kim J , Park K , Choi W , Lee H , Lee E , Yoon J , Keum K
Published: July 2016

Purpose:

To evaluate the sensitivity of gamma criteria for patient‐specific volumetric modulated arc therapy(VMAT) quality assurance of the Delta4 dosimetry program using the statistical process control(SPC) methodology.

Methods:

The authors selected 20 patient‐specific VMAT QA cases which were undertaken MapCHECK and ArcCHECK with gamma pass rate better than 97%. The QAs data were collected Delta4 Phantom+ and Elekta Agility six megavolts without using an angle incrementer. The gamma index(GI) were calculated in 2D planes with normalizing deviation to local dose(local gamma). The sensitivity of the GI methodology using criterion of 3%/3mm, 3%/2mm and 2%/3mm was analyzed with using process acceptability indices. We used local confidence(LC) level, the upper control limit(UCL) and lower control limit(LCL) of I‐MR chart for process capability index(Cp) and a process acceptability index (Cpk).

Results:

The lower local confidence levels of 3%/3mm, 3%/2mm and 2%/3mm were 92.0%, 83.6% and 78.8% respectively. All of the calculated Cp and Cpk values that used LC level were under 1.0 in this study. The calculated LCLs of I‐MR charts were 89.5%, 79.0% and 70.5% respectively. These values were higher than 1.0 which means good quality of QA. For the generally used lower limit of 90%, we acquired over 1.3 of Cp value for the gamma index of 3%/3mm and lower than 1.0 in the rest of GI.

Conclusion:

We applied SPC methodology to evaluate the sensitivity of gamma criteria and could see the lower control limits of VMAT QA for the Delta 4 dosimetry and could see that Delta 4 phantom+ dosimetry more affected by the position error and the I‐MR chart derived values are more suitable for establishing lower limits.

Acknowledgement: This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education (No. 2015R1D1A1A01060463)
SU‐F‐T‐323: A Post‐Mastectomy Radiation Therapy Dose Distribution Study Using Nanodots and Films
Authors: Qian X , Vaidya K , Puckett L , Souri S , Chen Y , Cao Y , Jamshidi A , Lee L , Klein E , Diaz F
Published: July 2016

Purpose:

In post‐mastectomy radiation therapy (RT), skin dose must be accurately estimated to assess skin reactions such as erythema, desquamation and necrosis. Planning systems cannot always provide accurate dosimetry for target volumes distal to skin. Therefore, in‐vivo dosimetry is necessary. A female anthropomorphic phantom was used with optically stimulated luminescence dosimeters (nanoDots) to measure dose to chest wall skin. In addition, EBT2 films was employed to measure dose to left lung and heart in post‐mastectomy RT.

Methods:

Films and nanoDots were calibrated under full buildup conditions at 100cm SAD for 6MV photons. Five pieces of films were placed between slabs of Rando phantom to assess dose to left lung and heart. Two layers of 0.5cm thick bolus were used to cover the whole left chest. Six pairs of nanoDots were placed at medical and lateral aspects on the bolus surface, between the 0.5cm bolus layers, and under the bolus. Three control nanoDots were placed on chest wall to quantify imaging dose. The phantom was CT scanned with all dosimeters in place, and treatment planning was performed with tangential fields (200cGy). All dosimeters were contoured on CT and dose was extracted. NanoDots were read using nanoDot reader and films were scanned using film scanner. The measured and calculated doses were tabulated.

Results:

Dose to 12 nanoDots were evaluated. Dose variance for surface nanoDots were +3.8%, +2.7%, −5% and −9.8%. Those at lateral positions, with greater beam obliquity had larger variance than the medial positions. A similar trend was observed for other nanoDots (Table1). Point doses from films for heart and the left lung were 112.7cGy and 108.7cGy, with +10.2% and +9.04% deviation from calculated values, respectively.

Conclusion:

Dosimetry provided by the advanced planning system was verified using NanoDots and films. Both nanoDots and films provided good estimation of dose distribution in post‐mastectomy RT.
SU‐F‐T‐397: Evaluating the Impact of Bladder Filling Status for the Organs at Risk Dose Distribution in Cervical Cancer Patients with Intensity Modulated Radiotherapy
Authors: Zhang JY , Hong DL
Published: July 2016

Purpose:

To investigate the impact of bladder filling status of the organs at risk (OARs) on dose distribution during intensity modulated radiotherapy (IMRT) for cervical cancer patients.

Methods:

Twelve cervical cancer patients treated with IMRT were selected for this study. The prescription dose was 45Gy/25 fractions with the 6 MV photon beam. All patients performed two CT scans, one with an empty bladder, the other one with bladder filled. For the registration of two CT scans, the fusion was automatically carried out upon the bony anatomy. The OARs (bladder, rectum, pelvic bone and small intestine) were delineated to planning CT to evaluate the dose distributions. These dose distributions were compared between empty bladder and bladder filling.

Results:

The bladder volume with empty bladder and bladder filling was 403.2±124.13cc and 101.4±87.5cc, respectively. There were no statistical differences between empty bladder and bladder filling in the mean value of pelvic bone V10Gy, V20Gy, V40Gy; rectum V40Gy and V45Gy. The bladder V40Gy and V45Gy were lower in the bladder filling group than in the empty bladder group (63.7%±5.8% vs 87.5%±7.8%, 45.1%±9.5% vs 62.4%±11.8%, respectively). The V45Gy for small intestine in the bladder filling group was significantly less than the empty bladder group (146.7cc±95.3cc vs 245.7cc±101.8cc).

Conclusion:

Our study finds that the bladder filling status did not have a significant impact on dose distribution in the rectum and pelvic bone. However, the changes of bladder filling have a large impact on bladder and small intestine doses. A full bladder is strongly recommended during treatment for cervical cancer patients.
SU‐F‐T‐411: A Quantitative Parameter for Treatment Plan Quality
Authors: Licon A , Ford J , Defoor D , Crownover R , Li Y , Ha C , Eng T , Jones W , Papanikolaou N , Stathakis S , Mavroidis P
Published: July 2016

Purpose:

To create an IMRT treatment plan quantitative score using QUANTEC dose/volume parameters to assess plan quality.

Methods:

132 IMRT patient plans of various treatment sites were evaluated. The optimized plan's dose volume histogram (DVH) was exported to Velocity for evaluation. The proposed scoring was based on calculating the shortest distance from the QUANTEC objective to the DVH line of each organ. Each plan was normalized against the “perfect” plan where the organs at risk (OAR) received no dose and hence the distance between the QUANTEC objective and the DVH line was maximized. The normalized scores allowed comparing the quality of plans across the treatment sites, dosimetrists, and physicians. The scores were plotted and statistically analyzed to serve as basis for future plans.

Results:

The score for each site was evaluated and the average percentage score resulted in the abdomen having an average percent score of 1.68 ± 1.49, 34.61 ± 35.19, 11.00 ± 15.39, 28.44 ± 25.13, 1.99 ± 5.83 for abdomen, brain, chest, head/neck, and pelvis. Differences in scores between the treatment sites are largely attributed to OAR segmentation and proximity of the OAR to the PTV. Small score differences between dosimetrist were attributed to the number of plans they have completed per site. A larger sample of treatment plans is being evaluated to investigate the effect of dosimetrist's experience on plan quality.

Conclusion:

This approach allows comparison of patient treatments which will help improve patient care and treatment outcomes with a quantitative score. A larger number of patient plans is being evaluated for improved statistics.
SU‐F‐T‐414: Mathematical Formulation of Gantry Starting Angle for Right Medial Tangential Arc in Left Intact Partial Breast Irradiation Using Volumetric Modulated Arc Therapy (VMAT)
Authors: Giri U , Sarkar B , Kaur H , Ganesh T , Munshi A , Mohanti B
Published: July 2016

Purpose:

To choose appropriate gantry starting angle for partial left breast irradiation using volumetric modulated arc therapy (VMAT).

Methods:

A random patient of left breast carcinoma was selected for this study. The slice which was selected for this mathematical formulation was having maximum breast thickness and maximum medial and lateral tangential distance. After this appropriate isocenter was chosen on that CT slice. The distances between various points were measured by the measuring tool in Monaco 5.00.04. Using the various trigonometric equations, a final equation was derived which shows the relationship between Gantry start angle, isocenter Location and tissue thickness.

Results:

The final equation for gantry start for right medial tangential arc is given asStarting angle = 270°+tan^(−1)(sin(θ)/(x_1/x_2 +cosθ))The above equation was tested for 10 cases and it was found to be appropriate for all the cases.

Conclusion:

Gantry starting angle for partial arc irradiation depends upon Breast thickness, Distance between Medial and lateral tangent and isocenter location.
SU‐F‐T‐418: Evaluation of Organs at Risk (OAR) Sparing in Left Breast Irradiation Techniques
Authors: Saini A , Hwang C , Das I
Published: July 2016

Purpose:

To compare dose to organs at risk (OAR) for left sided breast radiation therapy with comparable planning target volume (PTV) coverage in three techniques; free breathing in supine position (FB), deep inspirational breath hold in supine (DIBH) and free breathing in prone position (PP).

Methods:

Ten left sided breast cancer patients suitable for this study underwent a CT scan in three techniques e.g. supine, FB and DIBH and prone position (PP). One radiation oncologist contoured the PTV and OAR (cardiac components) based on RTOG guidelines. Treatment plans were optimized using field‐in‐field technique with AAA algorithm. Each plan was optimized to provide identical coverage to PTV such that a reasonable comparison in OAR dosimetry can be evaluated. The prescribed dose to PTV were 42.56 Gy; 2.66 Gy in 16 fractions.

Results:

Average lung dose parameters; Dmean, V10, V20 and V30 were 0.5 Gy, 0.6%, 0.2%, and 0.1% respectively in PP which is significantly lower than FB (5.9 Gy, 14.2%, 10.7 %, and 8.4%) and DIBH (5.8 Gy, 14.3%, 10.2%, 7.8%). Similarly average heart Dmean, V2.5, V5, V10, and V20 were much lower in PP (1.2Gy, 9.1%, 2.1%, 0.8%, and 0.4%), and in DIBH (1.2 Gy, 10.6%, 1.7%, 0.5%, and 0.1%) compared to FB (2.6 Gy, 21%, 7.2%, 4.4% and 3.3%) respectively. Similar findings were also noted in the heart component left anterior descending artery (LAD) and Left ventricle (LV) which are correlated to radiation related toxicity.

Conclusion:

Based on the calculated dose, FB technique provides highest heart and lung dose. Prone has lowest lung dose, and DIBH has lowest LAD dose. It is found that PP is always superior for heart, LV, LAD and lung as compared to FB. This conclusion is in contrast to some published study concluding that prone position has no benefit for heart sparing.
SU‐F‐T‐424: Mitigation of Increased Surface Dose When Treating Through A Carbon Fiber Couch Top
Authors: Johnson E , Misgina F
Published: July 2016

Purpose:

To study the effect of the Varian carbon fiber couch top on surface dose for patients being treated using single PA beams in the supine position and to identify simple methods for surface dose reduction.

Methods:

Measurements of surface dose were obtained in Solid Water phantoms using both a parallel plate ionization chamber (PTW Advanced Markus) and EBT2 Radiochromic films for both 6 and 10MV photons. All measurements were referenced to a depth considered a typical for PA Spine fields. Techniques used to reduce the surface dose included introducing an air standoff using Styrofoam sheets to suspend the phantom surface above the couch top and by adding a thin high Z scattering foil on the table surface. Surface doses were evaluated for typical field sizes, standoff heights, and various scattering materials. Comparisons were made to the surface dose obtainable when treating through a Varian Mylar covered tennis racket style couch top.

Results:

Dependence on typical spine field sizes was relatively minor. Dependence on air gap was much more significant. Surface doses decreased exponentially with increases in air standoff distance. Surface doses were reduced by approximately 50% for an air gap of 10cm and 40% for a 15cm air gap. Surface doses were reduced by an additional 15% by the addition of a 1mm Tin scattering foil.

Conclusion:

Using simple techniques, it is possible to reduce the surface dose when treating single PA fields through the Varian carbon fiber couch top. Surface doses can be reduced to levels observed when treating though transparent Mylar tops by adding about 15 cm of air gap. Further reductions are possible by adding thin scattering foils, such as Tin or Lead, on the couch surface. This is a low cost approach to reduce surface dose when using the Varian carbon fiber couch top.
SU‐F‐T‐433: Evaluation of a New Dose Mimicking Application for Clinical Flexibility and Reliability
Authors: Hoffman D , Nair C Kumaran , Wright C , Yamamoto T , Mayadev J , Valicenti R , Benedict S , Markham J , Rong Y
Published: July 2016

Purpose:

Clinical workflow and machine down time occasionally require patients to be temporarily treated on a system other than the initial treatment machine. A new commercial dose mimicking application provides automated cross‐platform treatment planning to expedite this clinical flexibility. The aim of this work is to evaluate the feasibility of automatic plan creation and establish a robust clinical workflow for prostate and pelvis patients.

Methods:

Five prostate and five pelvis patients treated with helical plans were selected for re‐planning with the dose mimicking application, covering both simple and complex scenarios. Two‐arc VMAT and 7‐ and 9‐field IMRT plans were generated for each case, with the objective function of achieving similar dose volume histogram from the initial helical plans. Dosimetric comparisons include target volumes and organs at risk (OARs) (rectum, bladder, small bowel, femoral heads, etc.). Dose mimicked plans were evaluated by a radiation oncologist, and patient‐specific QAs were performed to validate delivery.

Results:

Overall plan generation and transfer required around 30 minutes of dosimetrist's time once the dose‐mimicking protocol is setup for each site. The resulting VMAT and 7‐ and 9‐field IMRT plans achieved equivalent PTV coverage and homogeneity (D99/DRx = 97.3%, 97.2%, 97.2% and HI = 6.0, 5.8, and 5.9, respectively), compared to helical plans (97.6% and 4.6). The OAR dose discrepancies were up to 6% in rectum Dmean, but generally lower in bladder, femoral heads, bowel and penile bulb. In the context of 1–5 fractions, the radiation oncologist evaluated the dosimetric changes as not clinically significant. All delivery QAs achieved >90% pass with a 3%/3mm gamma criteria.

Conclusion:

The automated dose‐mimicking workflow offers a strategy to avoid missing treatment fractions due to machine down time with non‐clinically significant changes in dosimetry. Future work will further optimize dose mimicking plans and investigate other cross‐platform treatment delivery options.
SU‐F‐T‐434: Development of a Fan‐Beam Optical Scanner Using CMOS Array for Small Field Dosimetry
Authors: Brost E , Senthilkumar S , Warmington L , Watanabe Y
Published: July 2016

Purpose:

To design and construct a second generation optical computed tomography (OCT) system using a fan‐beam with a CMOS array detector for the 3D dosimetry with polymer gel and radiochromic solid dosimeters. The system was specifically designed for the small field dosimetry.

Methods:

The optical scanner used a fan‐beam laser, which was produced from a collimated red laser beam (λ=620 nm) with a 15‐degree laser‐line generating lens. The fan‐beam was sent through an index‐matching bath which holds the sample stage and a sample. The emerging laser light was detected with a 2.54 cm‐long CMOS array detector (512 elements). The sample stage rotated through the full 360 degree projection angles at 0.9‐degree increments. Each projection was normalized to the unirradiated sample at the projection angle to correct for imperfections in the dosimeter. A larger sample could be scanned by using a motorized mirror and linearly translating the CMOS detector. The height of the sample stage was varied for a full 3D scanning. The image acquisition and motor motion was controlled by a computer. The 3D image reconstruction was accomplished by a fan‐beam reconstruction algorithm. All the software was developed inhouse with MATLAB.

Results:

The scanner was used on both PRESAGE and PAGAT gel dosimeters. Irreconcilable refraction errors were seen with PAGAT because the fan beam laser line refracted away from the detector when the field was highly varying in 3D. With PRESAGE, this type of error was not seen.

Conclusion:

We could acquire tomographic images of dose distributions by the new OCT system with both polymer gel and radiochromic solid dosimeters. Preliminary results showed that the system was more suited for radiochromic solid dosimeters since the radiochromic dosimeters exhibited minimal refraction and scattering errors. We are currently working on improving the image quality by thorough characterization of the OCT system.
SU‐F‐T‐445: Effect of Triaxial Cables and Microdetectors in Small Field Dosimetry
Authors: Das I , Andersen A
Published: July 2016

Purpose:

Advances in radiation treatment especially with smaller fields used in SRS, Gamma knife, Tomotherapy, Cyberknife, and IMRT, require a high degree of precision especially with microdetectors for small field dosimetry (Das et al, Med Ph, 35, 206, 2008; Alfonso et al, Med Phys, 35, 5179, 2008). Due to small signal, the triaxial cable becomes critical in terms of signal to noise ratio (SNR) which is studied with microdetectors.

Methods:

Six high quality triaxial cables, 9.1 meters long from different manufacturers without any defects were acquired along with 5 most popular microdetectors (microdiamond, plastic scintillators, SRS‐diode, edge‐diode and pinpoint). A dedicated electrometer was used for each combination except W1 which has its own supermax electrometer. A 6MV photon beam from Varian True beam with 100 MU at a 600 MU/min was used. Measurements were made at a depth of 5 cm in water phantom. Field sizes were varied from 0.5 cm to 10 cm square fields. Readings were taken with combination of cables and microdetectors.

Results:

Signal is dependent on the quality of the connectors, cables and types of microdetector. The readings varied from nC to pC depending on the type of microdetector. The net signal, S, (Sc‐Sn), where Sc is signal with chamber and Sn is without chamber is a linear function of sensitive volume, v; (S = α+β•V), where α and β are constants. The standard deviation (SD) in 3 sets of reading with each combination of cable‐detector was extremely low <0.02%. As expected the SD is higher in small fields (<3cm). Maximum estimated error was only ±0.2% in cables‐detector combinations.

Conclusion:

The choice of cables has relatively small effect (±0.2%) with microdosimeter and should be accounted in overall error estimation in k value that is needed to convert ratio of reading to dose in small field dosimetry.
SU‐F‐T‐446: Improving Craniospinal Irradiation Technique Using Volumetric Modulated Arc Therapy (VMAT) Planning and Its Dosimetric Verification
Authors: Yang X , Tejani M , Jiang X , Elder E , Dhabaan A
Published: July 2016

Purpose:

The purpose of this study is to investigate a volumetric modulated arc therapy (VMAT) treatment planning technique for supine craniospinal irradiation (CSI). Evaluate the suitability of VMAT for CSI with dosimetric measurements and compare it to 3D conformal planning using specific plan metrics such as dose conformity, homogeneity, and dose of organs at risk (OAR).

Methods:

Ten CSI patients treated with conventional 3D technique were re‐planned with VMAT. The PTV was contoured to include the whole contents of the brain and spinal canal with a uniform margin of 5 mm. VMAT plans were generated with two partial arcs covering the brain, two partial arcs for the superior portion of the spinal cord and two partial arcs covering the remaining inferior portion of the spinal cord. Conformity index (CI), heterogeneity indexes (HI) and max and mean doses of OAR were compared to 3D plans. VMAT plans were delivered onto an anthropomorphic phantom loaded with Gafchromic films and OSLDs placed at specific positions to evaluate the plan dose at the junctions and as well as the plan dose distributions.

Results:

This VMAT technique was validated with a clinical study of 10 patients. The average CI was 1.03±0.02 for VMAT plans and 1.96±0.32 for conformal plans. And the average HI was 1.15±0.01 for VMAT plans and 1.51±0.21 for conformal plans. The mean and max doses to the all OARs for VMAT plans were significantly lower than conformal plans. The measured dose in phantom for VAMT plans was comparable to the calculated dose in Eclipse and the doses at junctions were verified.

Conclusion:

VMAT CSI was able to achieve better dose conformity and heterogeneity as well as significantly reducing the dose to Heart, esophagus and larynx. VMAT CSI appears to be a dosimterically advantageous, faster delivery, has better reproducibility CSI treatment.
SU‐F‐T‐449: Dosimetric Comparison of Acuros XB, Adaptive Convolve in Intensity Modulated Radiotherapy for Head and Neck Cancer
Authors: Uehara R , Tachibana H
Published: July 2016

Purpose:

There have been several publications focusing on dose calculation in lung for a new dose calculation algorithm of Acuros XB (AXB). AXB could contribute to dose calculation for high‐density media for bone and dental prosthesis rather than in lung. We compared the dosimetric performance of AXB, Adaptive Convolve (AC) in head and neck IMRT plans.

Methods:

In a phantom study, the difference in depth profile between AXB and AC was evaluated using Kodak EDR2 film sandwiched with tough water phantoms. 6 MV x‐ray using the TrueBeam was irradiated. In a patient study, 20 head and neck IMRT plans had been clinically approved in Pinnacle3 and were transferred to Eclipse. Dose distribution was recalculated using AXB in Eclipse while maintaining AC‐calculated monitor units and MLC sequence planned in Pinnacle. Subsequently, both the dose‐volumetric data obtained using the two different calculation algorithms were compared.

Results:

The results in the phantom evaluation for the shallow area ahead of the build‐up region shows over‐dose for AXB and under‐dose for AC, respectively. In the patient plans, AXB shows more hot spots especially around the high‐density media than AC in terms of PTV (Max difference: 4.0%) and OAR (Max. difference: 1.9%). Compared to AC, there were larger dose deviations in steep dose gradient region and higher skin‐dose.

Conclusion:

In head and neck IMRT plans, AXB and AC show different dosimetric performance for the regions inside the target volume around high‐density media, steep dose gradient regions and skin‐surface. There are limitations in skin‐dose and complex anatomic condition using even inhomogeneous anthropomorphic phantom Thus, there is the potential for an increase of hot‐spot in AXB, and an underestimation of dose in substance boundaries and skin regions in AC.
SU‐F‐T‐451: Doses to Organs‐At‐Risk in the Presence and Absence of a 1.5 T Magnetic Field for NSCLC Patients Undergoing SABR
Authors: Al‐Ward S , Kim A , McCann C , Ruschin M , Cheung P , Sahgal A , Keller B
Published: July 2016

Purpose:

To determine whether the electron return effect (ERE) has deleterious effects on lung SABR plans optimized in the presence of an orthogonal 1.5 T magnetic field.

Methods:

Data from five NSCLC‐SABR patients were used. The Dose was modeled with a 2.5 mm dose grid in the presence and absence of a magnetic field using the Monaco (Elekta) TPS with the Monte Carlo GPUMCD (v5.1) algorithm. For each patient, two plans were generated, one using our conventional Elekta Agility linac beam model and another using the Elekta MRI Linac (MRL) model. Both plans were generated on the average CT using similar dose constraints and a 5 mm PTV. The optimization was performed using our clinic's planning criteria, with normalization of the targets such that their V99% was equal to 99%. The OAR DVHs were compared for each patient.

Results:

The DVH plots revealed that there were limited differences when optimizing plans in the presence or absence of the magnetic field. The mean of the absolute differences, between the two planning types, in the equivalent uniform doses (EUDs) for the OARs were: 0.3 Gy (range of 0.0 ‐ 1.0 Gy) for the esophagus, 0.6 Gy (range of 0.1 – 1.9 Gy) for the heart, 0.5 Gy (range of 0.2 – 0.8 Gy) for the lungs, and 0.6 Gy (range of 0.2 – 1.5 Gy) for the spinal canal. Regarding the maximum doses to the serial organs, the mean of the differences were 3.0 Gy (esophagus) and 0.9 Gy (spinal canal). No trends in the differences were observed.

Conclusion:

This study has demonstrated that there were no major differences between plans optimized using a conventional linac and those optimized using an MRI linac with an orthogonal 1.5 T magnetic field. This is attributed to the consideration of the ERE in the optimization.

This project was made possible with the financial support of Elekta
SU‐F‐T‐452: Influence of Dose Calculation Algorithm and Heterogeneity Correction On Risk Categorization of Patients with Cardiac Implanted Electronic Devices Undergoing Radiotherapy
Authors: Iwai P , Lins L Nadler
Published: July 2016

Purpose:

There is a lack of studies with significant cohort data about patients using pacemaker (PM), implanted cardioverter defibrillator (ICD) or cardiac resynchronization therapy (CRT) device undergoing radiotherapy. There is no literature comparing the cumulative doses delivered to those cardiac implanted electronic devices (CIED) calculated by different algorithms neither studies comparing doses with heterogeneity correction or not. The aim of this study was to evaluate the influence of the algorithms Pencil Beam Convolution (PBC), Analytical Anisotropic Algorithm (AAA) and Acuros XB (AXB) as well as heterogeneity correction on risk categorization of patients.

Methods:

A retrospective analysis of 19 3DCRT or IMRT plans of 17 patients was conducted, calculating the dose delivered to CIED using three different calculation algorithms. Doses were evaluated with and without heterogeneity correction for comparison. Risk categorization of the patients was based on their CIED dependency and cumulative dose in the devices.

Results:

Total estimated doses at CIED calculated by AAA or AXB were higher than those calculated by PBC in 56% of the cases. In average, the doses at CIED calculated by AAA and AXB were higher than those calculated by PBC (29% and 4% higher, respectively). The maximum difference of doses calculated by each algorithm was about 1 Gy, either using heterogeneity correction or not. Values of maximum dose calculated with heterogeneity correction showed that dose at CIED was at least equal or higher in 84% of the cases with PBC, 77% with AAA and 67% with AXB than dose obtained with no heterogeneity correction.

Conclusion:

The dose calculation algorithm and heterogeneity correction did not change the risk categorization. Since higher estimated doses delivered to CIED do not compromise treatment precautions to be taken, it's recommend that the most sophisticated algorithm available should be used to predict dose at the CIED using heterogeneity correction.
SU‐F‐T‐456: Tomotherapy MLC Synchronization and Leaf Open Time Measurements Using ArcCheck
Authors: Yang B , Wong R , Lam W , Geng H , Cheung K , Yu S
Published: July 2016

Purpose:

To develop a practical method for routine QA of the MLC of a Tomotherapy unit using ArcCheck.

Methods:

Two standard test plans were used in this study. One was a helical test, in which the central leaves No. 32 and 33 opened simultaneously for 277.8ms at projections centered at 0°, 120° and 240° gantry angles. The other test plan was a static test with the gantry angle set at 0°, 45°, 90° and 135° respectively and leaves No. 32 and 33 opened sequentially for total 20s which was further divided into eleven or ten segments at each beam angle. The ArcCheck was isocentrically set up and adjusted for couch sag. Movie files which took a snapshot exposure every 50ms were recorded. The start/stop time of leaf open was decided by the ramp‐up/ramp‐down of the detectors.

Results:

The percentage differences between measured and planned leaf open time were calculated to be within 0.5% in all the tests. In static test, if leaves are synchronized perfectly, the sum of the two detectors’ signals after normalization should equal one when the leaves are in transition. Our results showed mean values of 0.982, 0.983, 0.978 and 0.995 at static gantry angle 0°, 45°, 90° and 135° respectively.

Conclusion:

A method for estimating the Tomotherapy binary MLC leaf open time using ArcCheck is proposed and proved to be precise enough to verify the planned leaf open time as small as 277.8ms. This method also makes the observation and quantification of the synchronization of leaves possible.
SU‐F‐T‐458: Tracking Trends of TG‐142 Parameters Via Analysis of Data Recorded by 2D Chamber Array
Authors: Alexandrian A , Kabat C , Defoor D , Saenz D , Rasmussen K , Kirby N , Gutierrez A , Papanikolaou N , Stathakis S
Published: July 2016

Purpose:

With increasing QA demands of medical physicists in clinical radiation oncology, the need for an effective method of tracking clinical data has become paramount. A tool was produced which scans through data automatically recorded by a 2D chamber array and extracts relevant information recommended by TG‐142. Using this extracted information a timely and comprehensive analysis of QA parameters can be easily performed enabling efficient monthly checks on multiple linear accelerators simultaneously.

Methods:

A PTW STARCHECK chamber array was used to record several months of beam outputs from two Varian 2100 series linear accelerators and a Varian NovalisTx−. In conjunction with the chamber array, a beam quality phantom was used to simultaneously to determine beam quality. A minimalist GUI was created in MatLab that allows a user to set the file path of the data for each modality to be analyzed. These file paths are recorded to a MatLab structure and then subsequently accessed by a script written in Python (version 3.5.1) which then extracts values required to perform monthly checks as outlined by recommendations from TG‐142. The script incorporates calculations to determine if the values recorded by the chamber array fall within an acceptable threshold.

Results:

Values obtained by the script are written to a spreadsheet where results can be easily viewed and annotated with a “pass” or “fail” and saved for further analysis. In addition to creating a new scheme for reviewing monthly checks, this application allows for able to succinctly store data for follow up analysis.

Conclusion:

By utilizing this tool, parameters recommended by TG‐142 for multiple linear accelerators can be rapidly obtained and analyzed which can be used for evaluation of monthly checks.
SU‐F‐T‐459: ArcCHECK Machine QA : Highly Efficient Quality Assurance Tool for VMAT, SRS & SBRT Linear Accelerator Delivery
Authors: Mhatre V , Patwe P , Dandekar P
Published: July 2016

Purpose:

Quality assurance (QA) of complex linear accelerators is critical and highly time consuming. ArcCHECK Machine QA tool is used to test geometric and delivery aspects of linear accelerator. In this study we evaluated the performance of this tool.

Methods:

Machine QA feature allows user to perform quality assurance tests using ArcCHECK phantom. Following tests were performed 1) Gantry Speed 2) Gantry Rotation 3) Gantry Angle 4)MLC/Collimator QA 5)Beam Profile Flatness & Symmetry. Data was collected on trueBEAM stX machine for 6 MV for a period of one year. The Gantry QA test allows to view errors in gantry angle, rotation & assess how accurately the gantry moves around the isocentre. The MLC/Collimator QA tool is used to analyze & locate the differences between leaf bank & jaw position of linac. The flatness & Symmetry test quantifies beam flatness & symmetry in IEC‐y & x direction. The Gantry & Flatness/Symmetry test can be performed for static & dynamic delivery.

Results:

The Gantry speed was 3.9 deg/sec with speed maximum deviation around 0.3 deg/sec. The Gantry Isocentre for arc delivery was 0.9mm & static delivery was 0.4mm. The maximum percent positive & negative difference was found to be 1.9 % & – 0.25 % & maximum distance positive & negative diff was 0.4mm & – 0.3 mm for MLC/Collimator QA. The Flatness for Arc delivery was 1.8 % & Symmetry for Y was 0.8 % & X was 1.8 %. The Flatness for gantry 0°,270°,90° & 180° was 1.75,1.9,1.8 & 1.6% respectively & Symmetry for X & Y was 0.8,0.6% for 0°, 0.6,0.7% for 270°, 0.6,1% for 90° & 0.6,0.7% for 180°.

Conclusion:

ArcCHECK Machine QA is an useful tool for QA of Modern linear accelerators as it tests both geometric & delivery aspects. This is very important for VMAT, SRS & SBRT treatments.
SU‐F‐T‐460: Dosimetric Matching Between Trilogy Tx and TrueBeam STx
Authors: Choi Y , Kwak J , Jeong C , Ahn S , Lee S , Cho B
Published: July 2016

Purpose:

To compare the commissioned beam data for one flattening filter photon mode (6 MV) and two flattening filter‐free (FFF) photon modes (6 and 10 MV‐FFF) between Trilogy Tx and TrueBeam STx and evaluate the possibility of dosimetric matching

Methods:

Dosimetric characteristics of the new Trilogy Tx including percent depth doses (PDDs), profiles, and output factors were measured for commissioning. Linear diode array detector and ion chambers were used to measure dosimetric data. The depth of dose maximum (dmax) and PDD at 10 cm (PDD10) were evaluated: 3×3 cm2, 10×10 cm2, and 40×40 cm2. The beam profiles were compared and then penumbras were evaluated. As a further test of the dosimetric matching, the same VMAT plans were delivered, measured with film, and compared with TPS calculation.

Results:

All the measured PDDs matched well across the two units. PDD10 showed less than 0.5% variation and dmax were within 1.5 mm at the field sizes evaluated. Within the central 80% of transverse axis, profile data were almost identical. TrueBeam data resulted in a slightly greater penumbra width (up to 1.9 mm). The greatest differences of output factors were found at 40 × 40 cm2: 2.40%, 2.03%, and 2.22% for 6 MV, 6 MV‐FFF, and 10 MV‐FFF, respectively. For smaller field sizes, less than 1% differences were observed. The film measurements demonstrated over 97.3% pixels passing‐gamma analysis (2%/2mm). The results showed excellent agreement between measurements of two machines.

Conclusion:

The differences between Trilogy Tx and TrueBeam STx found could possibly affect small field and also very large field sizes in dosimetric matching considerations. These differences encountered are mostly related with the changes in the head design of the TrueBeam. Although it cannot guarantee full interchangeability of two machines, dosimetric matching by field size of 25 × 25 cm2 might be clinically acceptable.
SU‐F‐T‐462: Lessons Learned From a Machine Incident Reporting System
Authors: Sutlief S , Hoisak J
Published: July 2016

Purpose:

Linear accelerators must operate with minimal downtime. Machine incident logs are a crucial tool to meet this requirement. They providing a history of service and demonstrate whether a fix is working. This study investigates the information content of a large department linear accelerator incident log.

Methods:

Our department uses an electronic reporting system to provide immediate information to both key department staff and the field service department. This study examines reports for five linac logs during 2015. The report attributes for analysis include frequency, level of documentation, who solved the problem, and type of fix used.

Results:

Of the reports, 36% were documented as resolved. In another 25% the resolution allowed treatment to proceed although the reported problem recurred within days. In 5% only intermediate troubleshooting was documented. The remainder lacked documentation. In 60% of the reports, radiation therapists resolved the problem, often by clearing the appropriate faults or reinitializing a software or hardware service. 22% were resolved by physics and 10% by field service engineers. The remaining 8% were resolved by IT, Facilities, or resolved spontaneously. Typical fixes, in order of scope, included clearing the fault and moving on, closing and re‐opening the patient session or software, cycling power to a sub‐unit, recalibrating a device (e.g., optical surface imaging), and calling in Field Service (usually resolving the problem through maintenance or component replacement).

Conclusion:

The reports with undocumented resolution represent a missed opportunity for learning. Frequency of who resolves a problem scales with the proximity of the person's role (therapist, physicist, or service engineer), which is inversely related to the permanence of the resolution. Review of lessons learned from machine incident logs can form the basis for guidance to radiation therapists and medical physicists to minimize equipment downtime and ensure safe operation.
SU‐F‐T‐463: Light‐Field Based Dynalog Verification
Authors: Atwal P , Ramaseshan R
Published: July 2016

Purpose:

To independently verify leaf positions in so‐called dynalog files for a Varian iX linac with a Millennium 120 MLC. This verification provides a measure of confidence that the files can be used directly as part of a more extensive intensity modulated radiation therapy / volumetric modulated arc therapy QA program.

Methods:

Initial testing used white paper placed at the collimator plane and a standard hand‐held digital camera to image the light and shadow of a static MLC field through the paper. Known markings on the paper allow for image calibration. Noise reduction was attempted with removal of ‘inherent noise’ from an open‐field light image through the paper, but the method was found to be inconsequential. This is likely because the environment could not be controlled to the precision required for the sort of reproducible characterization of the quantum noise needed in order to meaningfully characterize and account for it. A multi‐scale iterative edge detection algorithm was used for localizing the leaf ends. These were compared with the planned locations from the treatment console.

Results:

With a very basic setup, the image of the central bank A leaves 15–45, which are arguably the most important for beam modulation, differed from the planned location by [0.38±0.28] mm. Similarly, for bank B leaves 15–45 had a difference of [0.42±0.28] mm

Conclusion:

It should be possible to determine leaf position accurately with not much more than a modern hand‐held camera and some software. This means we can have a periodic and independent verification of the dynalog file information. This is indicated by the precision already achieved using a basic setup and analysis methodology. Currently, work is being done to reduce imaging and setup errors, which will bring the leaf position error down further, and allow meaningful analysis over the full range of leaves.
SU‐F‐T‐464: Development of a Secondary Check Procedure to Evaluated Flatness and Symmetry Discrepancies Detected During Daily Morning QA
Authors: Wagar M , Friesen S , Lyatskaya Y , O'Farrell D , Bhagwat M
Published: July 2016

Purpose:

A daily QA device is used to monitor output, flatness and symmetry constancy for all linac photon and electron energies. If large deviations from baseline in flatness or symmetry are reported it becomes necessary to crosscheck the measurements with a second device. Setting up another device such as Matrixx (IBA Dosimetry) can be time consuming, due to its warm‐up time, and trained personnel may not be readily available to analyze the results. Furthermore, this discrepancy is frequently isolated to a single energy. Unaffected energies could still be used, avoiding further patient delays, if a method to gather data for offline analysis could be developed. We find that optically stimulated luminescent dosimeters (OSLDs) provide a quick, simple, and inexpensive solution to this important clinical problem.

Methods:

The exact geometry of the detectors on the daily tracker (Keithley Therapy Beam Evaluator) was reproduced by placing nanoDot OSLDs (Landauer) on a solid water phantom. A combination of bolus and solid water was placed on top to provide buildup and prevent air gaps. Standard daily measurements of output, flatness and symmetry were taken for 2 photon energies (6x,10x) and 5 electron energies (6e,9e,12e,15e,18e) using the tracker. These measurements were then repeated with the OSLD phantom.

Results:

The time it took to set up the OSLD phantom was comparable to that of the tracker. The inline and crossline OSLD phantom measurements of flatness and symmetry agreed with the tracker results to within 2%.

Conclusion:

OSLDs provide a good solution for a quick second check when questionable flatness and symmetry results are detected with the tracker during daily QA.
SU‐F‐T‐472: Validation of Absolute Dose Measurements for MR‐IGRT With and Without Magnetic Field
Authors: Green O , Li H , Goddu S , Mutic S , Kawrakow I
Published: July 2016

Purpose:

To validate absolute dose measurements for a MR‐IGRT system without presence of the magnetic field.

Methods:

The standard method (AAPM's TG‐51) of absolute dose measurement with ionization chambers was tested with and without the presence of the magnetic field for a clinical 0.32‐T Co‐60 MR‐IGRT system. Two ionization chambers were used ‐ the Standard Imaging (Madison, WI) A18 (0.123 cc) and the PTW (Freiburg, Germany). A previously reported Monte Carlo simulation suggested a difference on the order of 0.5% for dose measured with and without the presence of the magnetic field, but testing this was not possible until an engineering solution to allow the radiation system to be used without the nominal magnetic field was found. A previously identified effect of orientation in the magnetic field was also tested by placing the chamber either parallel or perpendicular to the field and irradiating from two opposing angles (90 and 270). Finally, the Imaging and Radiation Oncology Core provided OSLD detectors for five irradiations each with and without the field ‐ with two heads at both 0 and 90 degrees, and one head at 90 degrees only as it doesn't reach 0 (IEC convention).

Results:

For the TG‐51 comparison, expected dose was obtained by decaying values measured at the time of source installation. The average measured difference was 0.4%±0.12% for A18 and 0.06%±0.15% for Farmer chamber. There was minimal (0.3%) orientation dependence without the magnetic field for the A18 chamber, while previous measurements with the magnetic field had a deviation of 3.2% with chamber perpendicular to magnetic field. Results reported by IROC for the OSLDs with and without the field had a maximum difference of 2%.

Conclusion:

Accurate absolute dosimetry was verified by measurement under the same conditions with and without the magnetic field for both ionization chambers and independently‐verifiable OSLDs.