bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐364: Estimating the Minimum Number of Patients Required to Estimate the Required Planning Target Volume Margins for Prostate Glands
Authors: Bakhtiari M , Schmitt J , Sarfaraz M , Osik C
Published: June 2015

Purpose:

To establish a minimum number of patients required to obtain statistically accurate Planning Target Volume (PTV) margins for prostate Intensity Modulated Radiation Therapy (IMRT).

Methods:

A total of 320 prostate patients, consisting of a total number of 9311 daily setups, were analyzed. These patients had gone under IMRT treatments. Daily localization was done using the skin marks and the proper shifts were determined by the CBCT to match the prostate gland. The Van Herk formalism is used to obtain the margins using the systematic and random setup variations. The total patient population was divided into different grouping sizes varying from 1 group of 320 patients to 64 groups of 5 patients. Each grouping was used to determine the average PTV margin and its associated standard deviation.

Results:

Analyzing all 320 patients lead to an average Superior‐Inferior margin of 1.15 cm. The grouping with 10 patients per group (32 groups) resulted to an average PTV margin between 0.6–1.7 cm with the mean value of 1.09 cm and a standard deviation (STD) of 0.30 cm. As the number of patients in groups increases the mean value of average margin between groups tends to converge to the true average PTV of 1.15 cm and STD decreases. For groups of 20, 64, and 160 patients a Superior‐Inferior margin of 1.12, 1.14, and 1.16 cm with STD of 0.22, 0.11, and 0.01 cm were found, respectively. Similar tendency was observed for Left‐Right and Anterior‐Posterior margins.

Conclusion:

The estimation of the required margin for PTV strongly depends on the number of patients studied. According to this study at least ∼60 patients are needed to calculate a statistically acceptable PTV margin for a criterion of STD < 0.1 cm. Numbers greater than ∼60 patients do little to increase the accuracy of the PTV margin estimation.
SU‐E‐T‐365: Estimation of Neutron Ambient Dose Equivalents for Radioprotection Exposed Workers in Radiotherapy Facilities Based On Characterization Patient Risk Estimation
Authors: Irazola L , Domingo C , Romero‐Exposito M , Garcia‐Fuste M , Terron J , Sanchez‐Nieto B , Bedogni R , Sanchez‐Doblado F
Published: June 2015

Purpose:

Previous measurements with Bonner spheres1 showed that normalized neutron spectra are equal for the majority of the existing linacs2. This information, in addition to thermal neutron fluences obtained in the characterization procedure33, would allow to estimate neutron doses accidentally received by exposed workers, without the need of an extra experimental measurement.

Methods:

Monte Carlo (MC) simulations demonstrated that the thermal neutron fluence distribution inside the bunker is quite uniform, as a consequence of multiple scatter in the walls⁴. Although inverse square law is approximately valid for the fast component, a more precise calculation could be obtained with a generic fast fluence distribution map around the linac, from MC simulations⁴. Thus, measurements of thermal neutron fluences performed during the characterization procedure3, together with a generic unitary spectra2, would allow to estimate the total neutron fluences and H*(10) at any point⁵. As an example, we compared estimations with Bonner sphere measurements1, for two points in five facilities: 3 Siemens (15–23 MV), Elekta (15 MV) and Varian (15 MV).

Results:

Thermal neutron fluences obtained from characterization, are within (0.2–1.6×10⁶) cm−2•Gy−1 for the five studied facilities. This implies ambient equivalent doses ranging from (0.27–2.01) mSv/Gy 50 cm far from the isocenter and (0.03–0.26) mSv/Gy at detector location with an average deviation of ±12.1% respect to Bonner measurements.

Conclusion:

The good results obtained demonstrate that neutron fluence and H*(10) can be estimated based on: (a) characterization procedure established for patient risk estimation in each facility, (b) generic unitary neutron spectrum and (c) generic MC map distribution of the fast component.

[1] Radiat. Meas (2010) 45: 1391 – 1397; [2] Phys. Med. Biol (2012) 5 7:6167–6191; [3] Med. Phys (2015) 42:276 ‐ 281. [4] IFMBE (2012) 39: 1245–1248. [5] ICRU Report 57 (1998)
SU‐E‐T‐368: Evaluating Dosimetric Outcome of Modulated Photon Radiotherapy (XMRT) Optimization for Head and Neck Patients
Authors: McGeachy P , Villarreal‐Barajas JE , Zinchenko Y , Khan R
Published: June 2015

Purpose:

The dosimetric outcome of optimized treatment plans obtained by modulating the photon beamlet energy and fluence on a small cohort of four Head and Neck (H and N) patients was investigated. This novel optimization technique is denoted XMRT for modulated photon radiotherapy. The dosimetric plans from XMRT for H and N treatment were compared to conventional, 6 MV intensity modulated radiotherapy (IMRT) optimization plans.

Methods:

An arrangement of two non‐coplanar and five coplanar beams was used for all four H and N patients. Both XMRT and IMRT were subject to the same optimization algorithm, with XMRT optimization allowing both 6 and 18 MV beamlets while IMRT was restricted to 6 MV only. The optimization algorithm was based on a linear programming approach with partial‐volume constraints implemented via the conditional value‐at‐risk method. H and N constraints were based off of those mentioned in the Radiation Therapy Oncology Group 1016 protocol. XMRT and IMRT solutions were assessed using metrics suggested by International Commission on Radiation Units and Measurements report 83. The Gurobi solver was used in conjunction with the CVX package to solve each optimization problem. Dose calculations and analysis were done in CERR using Monte Carlo dose calculation with VMC₊₊.

Results:

Both XMRT and IMRT solutions met all clinical criteria. Trade‐offs were observed between improved dose uniformity to the primary target volume (PTV1) and increased dose to some of the surrounding healthy organs for XMRT compared to IMRT. On average, IMRT improved dose to the contralateral parotid gland and spinal cord while XMRT improved dose to the brainstem and mandible.

Conclusion:

Bi‐energy XMRT optimization for H and N patients provides benefits in terms of improved dose uniformity to the primary target and reduced dose to some healthy structures, at the expense of increased dose to other healthy structures when compared with IMRT.
SU‐E‐T‐370: Evaluating Plan Quality and Dose Delivery Accuracy of Tomotherapy SBRT Treatments for Lung Cancer
Authors: Blake S , Hansen C , Deshpande S , Phan P , Franji I , Holloway L , Thwaites D
Published: June 2015

Purpose:

This study evaluated the plan quality and dose delivery accuracy of stereotactic body radiotherapy (SBRT) helical Tomotherapy (HT) treatments for lung cancer. Results were compared with those previously reported by our group for flattening filter (FF) and flattening filter free (FFF) VMAT treatments. This work forms part of an ongoing multicentre and multisystem planning and dosimetry audit on FFF beams for lung SBRT.

Methods:

CT datasets and DICOM RT structures delineating the target volume and organs at risk for 6 lung cancer patients were selected. Treatment plans were generated using the HT treatment planning system. Tumour locations were classified as near rib, near bronchial tree or in free lung with prescribed doses of 48Gy/4fr, 50Gy/5fr and 54Gy/3fr respectively. Dose constraints were specified by a modified RTOG0915 protocol used for an Australian SBRT phase II trial. Plan quality was evaluated using mean PTV dose, PTV volume receiving 100% of the prescribed dose (V100%), target conformity (CI=VD100%/VPTV) and low dose spillage (LDS=VD50%/VPTV). Planned dose distributions were compared to those measured using an ArcCheck phantom. Delivery accuracy was evaluated using a gamma‐index pass rate of 95% with 3% (of max dose) and 3mm criteria.

Results:

Treatment plans for all patients were clinically acceptable in terms of quality and accuracy of dose delivery. The following DVH metrics are reported as averages (SD) of all plans investigated: mean PTV dose was 115.3(2.4)% of prescription, V100% was 98.8(0.9)%, CI was 1.14(0.03) and LDS was 5.02(0.37). The plans had an average gamma‐index passing rate of 99.3(1.3)%.

Conclusion:

The results reported in this study for HT agree within 1 SD to those previously published by our group for VMAT FF and FFF lung SBRT treatments. This suggests that HT delivers lung SBRT treatments of comparable quality and delivery accuracy as VMAT using both FF and FFF beams.
SU‐E‐T‐374: Evaluation and Verification of Dose Calculation Accuracy with Different Dose Grid Sizes for Intracranial Stereotactic Radiosurgery
Authors: Han C , Schultheiss T
Published: June 2015

Purpose:

In this study, we aim to evaluate the effect of dose grid size on the accuracy of calculated dose for small lesions in intracranial stereotactic radiosurgery (SRS), and to verify dose calculation accuracy with radiochromic film dosimetry.

Methods:

15 intracranial lesions from previous SRS patients were retrospectively selected for this study. The planning target volume (PTV) ranged from 0.17 to 2.3 cm3. A commercial treatment planning system was used to generate SRS plans using the volumetric modulated arc therapy (VMAT) technique using two arc fields. Two convolution‐superposition‐based dose calculation algorithms (Anisotropic Analytical Algorithm and Acuros XB algorithm) were used to calculate volume dose distribution with dose grid size ranging from 1 mm to 3 mm with 0.5 mm step size. First, while the plan monitor units (MU) were kept constant, PTV dose variations were analyzed. Second, with 95% of the PTV covered by the prescription dose, variations of the plan MUs as a function of dose grid size were analyzed. Radiochomic films were used to compare the delivered dose and profile with the calculated dose distribution with different dose grid sizes.

Results:

The dose to the PTV, in terms of the mean dose, maximum, and minimum dose, showed steady decrease with increasing dose grid size using both algorithms. With 95% of the PTV covered by the prescription dose, the total MU increased with increasing dose grid size in most of the plans. Radiochromic film measurements showed better agreement with dose distributions calculated with 1‐mm dose grid size.

Conclusion:

Dose grid size has significant impact on calculated dose distribution in intracranial SRS treatment planning with small target volumes. Using the default dose grid size could lead to under‐estimation of delivered dose. A small dose grid size should be used to ensure calculation accuracy and agreement with QA measurements.
SU‐E‐T‐375: Evaluation of a MapCHECK2(tm) Planar 2‐D Diode Array for High‐Dose‐Rate Brachytherapy Treatment Delivery Verifications
Authors: Macey N , Siebert M , Shvydka D , Parsai E
Published: June 2015

Purpose:

Despite improvements of HDR brachytherapy delivery systems, verification of source position is still typically based on the length of the wire reeled out relative to the parked position. Yet, the majority of errors leading to medical events in HDR treatments continue to be classified as missed targets or wrong treatment sites. We investigate the feasibility of using dose maps acquired with a two‐dimensional diode array to independently verify the source locations, dwell times, and dose during an HDR treatment.

Methods:

Custom correction factors were integrated into frame‐by‐frame raw counts recorded for a Varian VariSource™ HDR afterloader Ir‐192 source located at various distances in air and in solid water from a MapCHECK2™ diode array. The resultant corrected counts were analyzed to determine the dwell position locations and doses delivered. The local maxima of polynomial equations fitted to the extracted dwell dose profiles provided the X and Y coordinates while the distance to the source was determined from evaluation of the full width at half maximum (FWHM). To verify the approach, the experiment was repeated as the source was moved through dwell positions at various distances along an inclined plane, mimicking a vaginal cylinder treatment.

Results:

Dose map analysis was utilized to provide the coordinates of the source and dose delivered over each dwell position. The accuracy in determining source dwell positions was found to be +/−1.0 mm of the preset values, and doses within +/−3% of those calculated by the BrachyVision™ treatment planning system for all measured distances.

Conclusion:

Frame‐by‐frame data furnished by a 2 ‐D diode array can be used to verify the dwell positions and doses delivered by the HDR source over the course of treatment. Our studies have verified that measurements provided by the MapCHECK2™ can be used as a routine QA tool for HDR treatment delivery verification.
SU‐E‐T‐382: Evaluation of Clinical Application and Dosimetric Comparison for Treatment Plans of Gamma Knife and CyberKnife for Arteriovenous Malformations
Authors: Kuo C
Published: June 2015

Purpose:

To analyze and compare the characteristics of dose distributions between Gamma Knife (GK) and CyberKnife (CK), in treating arteriovenous malformations (AVMs), and evaluate the influences on their clinical applications.

Methods:

Twenty four patients with AVMs treated with CK of prescribed dose (PD) of 16–25 Gy in single fraction were selected. Each patient's CT images used for CK treatment planning with contours of targets and critical organs were exported and then loaded into the GK planning system. GK treatment plan with the same PD used in CK was generated for each patient. The metrics for dose comparison between GK and CK included conformity index (CI), gradient index (GI) of 75%, 50% and 25% of the PD, heterogeneity index (HI), volume of brain tissues covered by 10 Gy and 12 Gy, maximum dose to brainstem and beam‐on time. Paired Samples t‐test was used to analyze these metrics for significance (p value).

Results:

The CI were 0.744 ± 0.075 (GK) and 0.768 ± 0.086 (CK), p = 0.281. The GI75%, GI50%, and GI25% in GK and CK were 1.735 ± 0.100 and 2.439 ± 0.338 (p < 0.001), 3.169 ± 0.265 and 4.972 ± 0.852 (p < 0.001), and 8.650 ± 0.914 and 14.261 ± 2.476 (p < 0.001). The HI were 0.728 ± 0.072 (GK) and 0.313 ± 0.069 (CK), p < 0.001. There were significant differences both for volume of brain tissues covered by 10 Gy and 12 Gy in GK and CK (p < 0.001). GK had smaller maximum dose to brainstem. CK had shorter beam‐on time.

Conclusion:

GK has similar dose conformity as CK, and has better normal tissue sparing but is less efficient than CK.
SU‐E‐T‐388: Evaluation of Electronic Brachytherapy Dose Distributions in Tissue Equivalent Materials
Authors: Johnson M , Ahmad S , Johnson D
Published: June 2015

Purpose:

To study the measured and calculated dose distributions for electronic brachytherapy (EBT) in various tissue equivalent homogenous materials.

Methods:

Calculated dose distributions in water were generated using published TG‐43 parameters in Varian BrachyVision software for a 50 kVp, 50 cm Xoft source. Dose distributions were measured within a 3D‐scanning tank using dosimeters including: PTW 0.125 cc, pin‐point, and parallel‐plate ion chambers, Sun Nuclear “Edge” diode and Gafchromic EBT3 film. Multi‐channel film dosimetry was used in film analysis. EBT3 film curves were calibrated via radial dose comparison to both independently measured and published data. The resulting film calibration was utilized to measure dose distributions created by titanium filtered source utilized in clinical brachytherapy applications. Data was collected within homogenous PMMA, vinyl, polystyrene, paraffin, and water‐equivalent plastic phantoms.

Results:

Ion‐chamber data was corrected to effective points of measurement and normalized prior to comparison between calculated and measured dose distributions. Measurements made in water and water equivalent materials compared well with results from treatment planning software. The maximum percent differences (relative to water) observed between 1 cm and 3.5 cm depth from source for each of the phantom materials are as follows: PMMA 35%, polystyrene 41%, plastic‐water 23%, vinyl 115%, and paraffin 46%.

Conclusion:

The increased probability of photoelectric interactions occurring within the patient during electronic brachytherapy will emphasize the radiological differences between varying human tissues in dose deposition. These differences can Result in clinically significant dose perturbations and it is therefore recommended to move to a model based dose calculation, as outlined in TG‐186, to improve the dosimetric accuracy of low energy EBT.
SU‐E‐T‐390: Evaluation of Heterogeneity Corrections Made by RayStation Treatment Planning
Authors: Rodriguez M , Bartolac S , Rezaee M , Bissonnette J
Published: June 2015

Purpose:

To examine the agreement between absorbed doses calculated by RayStation treatment planning algorithm to those measured with gafchromic film and ion chamber when the photon beam is perturbed by attenuation or lateral scatter of lung material.

Methods:

A gafchromic EBT2 film was placed in the center of a 30×30×20 cm3 solid water phantom with a 5 cm lung slab placed at 10 cm depth. The film was irradiated at SSD = 100 cm with a 6 MV photon beam, 10×10 and 5×5 cm2 field sizes and with the beam parallel to the film and lung slab. A CT was performed to the phantom arrangement for RayStation dose calculation. The films were scanned in an Epson 10000X flatbed scanner and analyzed using the red channel, 16 bits, 76 dpi. PDD curves at the central axis and profiles at dmax were also measured in water using a CC13 (0.13 CC) ion chamber. Measurements and calculation of PDD curves at the central axis and profiles at dmax and 20 cm depth were compared using the criteria suggested by the AAPM Task Group # 53.

Results:

The PDD curves measured with gafchromic film and those measured in water with ion chamber agree with the ones calculated by RayStation within the uncertainty of the measurements which is within 3%. The passing rate values of the measured and calculated profiles for the 2 field sizes are within 94% for both at dmax and at 20 cm depth.

Conclusion:

Raystation dose calculation engine models inhomogeneity corrections. Differences between the calculated PDD curves and profiles and those measured with gafchromic film are within the uncertainty of the measurements and inside of the agreement tolerance suggested by TG53. Therefore, RayStation treatment planning has an acceptable algorithm to correct dose delivered by photon beams perturbed by lung tissue.
SU‐E‐T‐391: Evaluation of Image Parameters Impact On the CT Calibration Curve for Proton Therapy
Authors: Xiao Z , Reyhan M , Huang Q , Zhang M , Yue N , Chen T
Published: June 2015

Purpose:

The calibration of the Hounsfield units (HU) to relative proton stopping powers (RSP) is a crucial component in assuring the accurate delivery of proton therapy dose distributions to patients. The purpose of this work is to assess the uncertainty of CT calibration considering the impact of CT slice thickness, position of the plug within the phantom and phantom sizes.

Methods:

Stoichiometric calibration method was employed to develop the CT calibration curve. Gammex 467 tissue characterization phantom was scanned in Tomotherapy Cheese phantom and Gammex 451 phantom by using a GE CT scanner. Each plug was individually inserted into the same position of inner and outer ring of phantoms at each time, respectively. 1.25 mm and 2.5 mm slice thickness were used. Other parameters were same.

Results:

HU of selected human tissues were calculated based on fitted coefficient (Kph, Kcoh and KKN), and RSP were calculated according to the Bethe‐Bloch equation. The calibration curve was obtained by fitting cheese phantom data with 1.25 mm thickness. There is no significant difference if the slice thickness, phantom size, position of plug changed in soft tissue. For boney structure, RSP increases up to 1% if the phantom size and the position of plug changed but keep the slice thickness the same. However, if the slice thickness varied from the one in the calibration curve, 0.5%–3% deviation would be expected depending on the plug position. The Inner position shows the obvious deviation (averagely about 2.5%).

Conclusion:

RSP shows a clinical insignificant deviation in soft tissue region. Special attention may be required when using a different slice thickness from the calibration curve for boney structure. It is clinically practical to address 3% deviation due to different thickness in the definition of clinical margins.
SU‐E‐T‐392: Evaluation of Ion Chamber/film and Log File Based QA to Detect Delivery Errors
Authors: Nelson C , Ohrt J , Mason B , Kirsner S
Published: June 2015

Purpose:

Ion chamber and film (ICAF) is a method used to verify patient dose prior to treatment. More recently, log file based QA has been shown as an alternative for measurement based QA. In this study, we delivered VMAT plans with and without errors to determine if ICAF and/or log file based QA was able to detect the errors.

Methods:

Using two VMAT patients, the original treatment plan plus 7 additional plans with delivery errors introduced were generated and delivered. The erroneous plans had gantry, collimator, MLC, gantry and collimator, collimator and MLC, MLC and gantry, and gantry, collimator, and MLC errors. The gantry and collimator errors were off by 4⁰ for one of the two arcs. The MLC error introduced was one in which the opening aperture didn't move throughout the delivery of the field. For each delivery, an ICAF measurement was made as well as a dose comparison based upon log files. Passing criteria to evaluate the plans were ion chamber less and 5% and film 90% of pixels pass the 3mm/3% gamma analysis(GA). For log file analysis 90% of voxels pass the 3mm/3% 3D GA and beam parameters match what was in the plan.

Results:

Two original plans were delivered and passed both ICAF and log file base QA. Both ICAF and log file QA met the dosimetry criteria on 4 of the 12 erroneous cases analyzed (2 cases were not analyzed). For the log file analysis, all 12 erroneous plans alerted a mismatch in delivery versus what was planned. The 8 plans that didn't meet criteria all had MLC errors.

Conclusion:

Our study demonstrates that log file based pre‐treatment QA was able to detect small errors that may not be detected using an ICAF and both methods of were able to detect larger delivery errors.
SU‐E‐T‐396: Evaluation of Novel Open Face Intracranial Immobilization Devices
Authors: Wiant D , Maurer J , Liu H , Hayes L , Sintay B
Published: June 2015

Purpose:

Focal intracranial treatments, such as stereotactic radiosurgery, require that motion be minimized during treatment. This level of immobilization is typically achieved with custom thermoplastic masks that cover the patient's face. These masks they do not allow active monitoring of patient position during treatment, and in many cases they induce claustrophobia and anxiety. This work evaluates potential motion using a newly developed faceless mask system designed to reduce anxiety and allow intrafraction monitoring.

Methods:

Custom masks were created for five healthy volunteers using both the standard mask and the mask with integrated bite system (IBS). Each volunteer was monitored with surface imaging system for 15 minutes to evaluate physiological motion. They were then asked to move as much as possible to evaluate forced motion. The real time surface imaging information was recorded and analyzed off‐line.

Results:

The mean root‐mean‐square (RMS) of the linear displacements over the 15 minute monitoring periods ranged from 0.3 mm – 0.7 mm with the standard mask and 0.3 mm – 0.5 mm for the mask with IBS. The maximum individual variance was 0.4 mm (2 standard deviations) both with and without IBS. The maximum RMS displacement during forced movements was 9.2 mm without IBS and 4.9 mm with IBS. The mean RMS difference in position before and after forced motion over all patients and movements (with and without IBS) was 0.5 mm +/−0.5 mm (2 SD), i.e. the volunteers were returning to within 0.5 mm of their initial position after forced movements.

Conclusion:

The intracranial mask system is able to provide sub‐millimeter immobilization for resting volunteers, and the system can limit motion to < 5 mm under the adverse conditions, providing adequate immobilization for any type of intracranial treatment while potentially improving patient comfort and reducing anxiety and claustrophobic episodes

This work was partially supported by Qfix.
SU‐E‐T‐397: Evaluation of Planned Dose Distributions by Monte Carlo (0.5%) and Ray Tracing Algorithm for the Spinal Tumors with CyberKnife
Authors: Cho H , Hepel J , Brindle J
Published: June 2015

Purpose:

To analyze and evaluate dose distribution between Ray Tracing (RT) and Monte Carlo (MC) algorithms of 0.5% uncertainty on a critical structure of spinal cord and gross target volume and planning target volume.

Methods:

Twenty four spinal tumor patients were treated with stereotactic body radiotherapy (SBRT) by CyberKnife in 2013 and 2014. The MC algorithm with 0.5% of uncertainty is used to recalculate the dose distribution for the treatment plan of the patients using the same beams, beam directions, and monitor units (MUs).

Results:

The prescription doses are uniformly larger for MC plans than RT except one case. Up to a factor of 1.19 for 0.25cc threshold volume and 1.14 for 1.2cc threshold volume of dose differences are observed for the spinal cord.

Conclusion:

The MC recalculated dose distributions are larger than the original MC calculations for the spinal tumor cases. Based on the accuracy of the MC calculations, more radiation dose might be delivered to the tumor targets and spinal cords with the increase prescription dose.
SU‐E‐T‐398: Evaluation of Radiobiological Parameters Using Serial Tumor Imaging During Radiotherapy as An Inverse Ill‐Posed Problem
Authors: Chvetsov A , Sandison G , Schwartz J , Rengan R
Published: June 2015

Purpose:

Combination of serial tumor imaging with radiobiological modeling can provide more accurate information on the nature of treatment response and what underlies resistance. The purpose of this article is to improve the algorithms related to imaging‐based radiobilogical modeling of tumor response.

Methods:

Serial imaging of tumor response to radiation therapy represents a sum of tumor cell sensitivity, tumor growth rates, and the rate of cell loss which are not separated explicitly. Accurate treatment response assessment would require separation of these radiobiological determinants of treatment response because they define tumor control probability. We show that the problem of reconstruction of radiobiological parameters from serial imaging data can be considered as inverse ill‐posed problem described by the Fredholm integral equation of the first kind because it is governed by a sum of several exponential processes. Therefore, the parameter reconstruction can be solved using regularization methods.

Results:

To study the reconstruction problem, we used a set of serial CT imaging data for the head and neck cancer and a two‐level cell population model of tumor response which separates the entire tumor cell population in two subpopulations of viable and lethally damage cells. The reconstruction was done using a least squared objective function and a simulated annealing algorithm. Using in vitro data for radiobiological parameters as reference data, we shown that the reconstructed values of cell surviving fractions and potential doubling time exhibit non‐physical fluctuations if no stabilization algorithms are applied. The variational regularization allowed us to obtain statistical distribution for cell surviving fractions and cell number doubling times comparable to in vitro data.

Conclusion:

Our results indicate that using variational regularization can increase the number of free parameters in the model and open the way to development of more advanced algorithms which take into account tumor heterogeneity, for example, related to hypoxia.
SU‐E‐T‐407: Evaluation of the Stopping Power and Imaging Visibility for Iodine Based Contrast in Proton Therapy
Authors: Singh H , Zhao L , Rana S , Chacko M , Zheng Y
Published: June 2015

Purpose:

The purpose of this study was to investigate the difference between calculated and measured relative linear stopping power (RLSP) for contrast medium in proton therapy. Furthermore, the visibility for different concentrations in prostate phantom on orthogonal X‐ray system was evaluated

Methods and Materials:

In prostate cancer patient, rectal balloon along with the contrast media was used to visualize the balloon position, thus facilitating the patient setup during each fraction of the uniform scanning proton treatment. There were no fiducial markers implanted in the prostate for this patient. A blue wax phantom with outer dimensions 10cm(H) × 14.5cm(L) × 10cm(W) was made in house. To hold iodine based contrast solution, a rectangular shaped hole with dimensions 7cm(H) × 8cm(L) × 4cm(W) was made inside the phantom. Organically bound 8.5% iodine based Cystografin Dilute contrast agent with molecular formula C11H9I3N2O4.C7H17NO5 was used in this study. Six solutions were prepared; each mixture of water and iodine based contrast agent at different concentrations as 0%, 5%, 10%, 20%, 30% and 50%. During computed tomographic(CT) simulation, solutions were placed together at the isocenter of CT and scanned at 120kVp using the same protocol as for prostate cancer patients. The treatment planning was done in CMS‐XiO system. Multi‐layer‐ion‐chamber (MLIC) was used to measure residual proton range.

Results:

The 50% concentration contrast solution was used during treatment for better visualization on orthogonal X‐ray image. The measured RLSP for 5%, 10%, 20%, 30% & 50% solutions were 1.005, 1.010, 1.018, 1.023 & 1.033; and similarly calculated RLSP from XiO were 1.090, 1.135, 1.222, 1.299 & 1.448 respectively.

Conclusion:

The treatment planning system could overestimates the relative stopping power of contrast solution with high concentrations. It is recommended to override the contrast with measured RLSP for high atomic number based contrast solution in treatment planning for proton therapy.
SU‐E‐T‐411: Evaluation of Treatment Plan Quality Between Two Treatment Planning Systems for VMAT
Authors: Yang T , Xu W , Ge R , Qu B , Xu S
Published: June 2015

Purpose:

To investigate the treatment plan quality between two treatment planning systems (TPS) for volumetric modulated Arc therapy (VMAT).

Methods:

VMAT plans based on Varian Eclipse (Ver10.0) and Philips Pinnacle (Ver9.10) TPS were respectively generated by 10 cervical cancer patients by using 2, 3 arcs, controlling the gantry movement with the space of 2 degrees and setting the same optimization constraint as soon as possible. The delivery system with VMAT was Varian Clinac iX machine. The prescription dose was 50Gy/23f for regions at high risk (PTV1), and 45Gy/23f for regions at low risk (PTV). The difference of treatment plans was evaluated by dose distribution and delivery efficiency.

Results:

The VMAT 2‐arcs plans based on Pinnacle in the conformal indexes of PTV(CI: 0.77±0.05 vs 0.75±0.02, p>0.05), and OARs (such as V30, V40 of rectum and bladder) showed slightly better than those of Eclipse TPS, and the homogeneity indexes (HI) of in Pinnacle were slightly worse than the Eclipse (PTV: 0.17±0.01 vs 0.14±0.01, p <0.05; PTV1: 0.06±0.01 vs 0.04±0.002, p <0.05), there were statistical difference. The MUs with 2‐arcs (750±91 vs 1118±84, p <0.05) and 3‐arcs (837±86 vs 1215±208, p <0.05) of the Eclipse plans had a clear advantages over the Pinnacle ones, with the statistical difference. The 3‐arc plans in Eclipse showed an advantage over the 2‐arcs ones and the plan quality of 3‐arcs in Pinnacle had no advantage comparing with 2 arcs, but the MUs of 3 Arcs in both Eclipse (837±86 vs 750±92, p <0.05) and Pinnacle (1215±208 vs 1118±84, p >0.05) were more than those of 2‐arcs.

Conclusion:

For patients with cervical cancer, the VMAT treatment plans of two TPSs could achieve clinical acceptable dose distribution. However, we will still need more cases to further study and determine the optimization and performance characteristics of the two commercial TPSs.
SU‐E‐T‐413: Examining Acquisition Rate for Using MatriXX Ion Chamber Array to Measure HDR Brachytherapy Treatments
Authors: Wagar M , Bhagwat M , O'Farrell D , Friesen S , Buzurovic I , Damato A , Devlin P , Cormack R
Published: June 2015

Purpose:

There are unique obstacles to implementing the MatriXX ionchamber array as a QA tool in Brachytherapy given that the device is designed for use in the MV energy range. One of the challenges we investigate is the affect of acquisition rates on dose measurement accuracy for HDR treatment plans.

Methods:

A treatment plan was optimized in Oncentra Brachy TPS to deliver a planar dose to a 5×5cm region at 10mm depth. The applicator was affixed to the surface of the MatriXX array. The plan was delivered multiple times using a Nucleatron HDR afterloader with a 2.9Ci Ir192 source. For each measurement the sampling rate of the MatriXX movie mode was varied (30ms and 500ms). This experiment was repeated with identical parameters, following a source exchange, with an 11.2Ci Ir192 source. Finally, a single snap measurement was acquired. Analysis was preformed to evaluate the fidelity of the dose delivery for each iteration of the experiment. Evaluation was based on the comparison between the measured and TPS predicted dose.

Results:

Higher sample rates induce a greater discrepancy between the predicted and measured dose. Delivering the plan using a lower activity source also produced greater discrepancy in the measurement due to the increased delivery time. Analyzing the single snap measurement showed little difference from the 500ms integral dose measurement.

Conclusion:

The advantage of using movie mode for HDR treatment delivery QA is the ability for real time source tracking in addition to dose measurement. Our analysis indicates that 500ms is an optimal frame rate.
SU‐E‐T‐416: Experimental Evaluation of a Commercial GPU‐Based Monte Carlo Dose Calculation Algorithm
Authors: Paudel M R , Kim A , Beachey D J , Ahmad S , Sarfehnia A , Sahgal A , Keller B
Published: June 2015

Purpose:

A new commercial GPU‐based Monte Carlo dose calculation algorithm (GPUMCD) developed by the vendor Elekta™ to be used in the Monaco Treatment Planning System (TPS) is capable of modeling dose for both a standard linear accelerator and for an Elekta MRI‐Linear accelerator (modeling magnetic field effects). We are evaluating this algorithm in two parts: commissioning the algorithm for an Elekta Agility linear accelerator (the focus of this work) and evaluating the algorithm's ability to model magnetic field effects for an MRI‐linear accelerator.

Methods:

A beam model was developed in the Monaco TPS (v.5.09.06) using the commissioned beam data for a 6MV Agility linac. A heterogeneous phantom representing tumor‐in‐lung, lung, bone‐in‐tissue, and prosthetic was designed/built. Dose calculations in Monaco were done using the current clinical algorithm (XVMC) and the new GPUMCD algorithm (1 mm3 voxel size, 0.5% statistical uncertainty) and in the Pinnacle TPS using the collapsed cone convolution (CCC) algorithm. These were compared with the measured doses using an ionization chamber (A1SL) and Gafchromic EBT3 films for 2×2 cm2, 5×5 cm2, and 10×10 cm2 field sizes.

Results:

The calculated central axis percentage depth doses (PDDs) in homogeneous solid water were within 2% compared to measurements for XVMC and GPUMCD. For tumor‐in‐lung and lung phantoms, doses calculated by all of the algorithms were within the experimental uncertainty of the measurements (±2% in the homogeneous phantom and ±3% for the tumor‐in‐lung or lung phantoms), except for 2×2 cm2 field size where only the CCC algorithm differs from film by 5% in the lung region. The analysis for bone‐in‐tissue and the prosthetic phantoms are ongoing.

Conclusion:

The new GPUMCD algorithm calculated dose comparable to both the XVMC algorithm and to measurements in both a homogeneous solid water medium and the heterogeneous phantom representing lung or tumor‐in‐lung for 2×2 cm2‐10×10 cm2 field sizes.

Funding support was obtained from Elekta.
SU‐E‐T‐420: Failure Effects Mode Analysis for Trigeminal Neuralgia Frameless Radiosurgery
Authors: Howe J
Published: June 2015

Purpose:

Functional radiosurgery has been used successfully in the treatment of trigeminal neuralgia but presents significant challenges to ensuring the high prescription dose is delivered accurately. A review of existing practice should help direct the focus of quality improvement for this treatment regime.

Method:

Failure modes and effects analysis was used to identify the processes in preparing radiosurgery treatment for TN. The map was developed by a multidisciplinary team including: neurosurgeon, radiation oncology, physicist and therapist. Potential failure modes were identified for each step in the process map as well as potential causes and end effect. A risk priority number was assigned to each cause.

Results:

The process map identified 66 individual steps (see attached supporting document). Corrective actions were developed for areas of high risk priority number. Wrong site treatment is at higher risk for trigeminal neuralgia treatment due to the lack of site specific pathologic imaging on MR and CT – additional site specific checks were implemented to minimize the risk of wrong site treatment. Failed collision checks resulted from an insufficient collision model in the treatment planning system and a plan template was developed to address this problem.

Conclusion:

Failure modes and effects analysis is an effective tool for developing quality improvement in high risk radiotherapy procedures such as functional radiosurgery.
SU‐E‐T‐422: Fast Analytical Beamlet Optimization for Volumetric Intensity‐Modulated Arc Therapy
Authors: Chan Kenny S K , Lee Louis K Y , Xing L , Chan Anthony T C
Published: June 2015

Purpose:

To implement a fast optimization algorithm on CPU/GPU heterogeneous computing platform and to obtain an optimal fluence for a given target dose distribution from the pre‐calculated beamlets in an analytical approach.

Methods:

The 2D target dose distribution was modeled as an n‐dimensional vector and estimated by a linear combination of independent basis vectors. The basis set was composed of the pre‐calculated beamlet dose distributions at every 6 degrees of gantry angle and the cost function was set as the magnitude square of the vector difference between the target and the estimated dose distribution. The optimal weighting of the basis, which corresponds to the optimal fluence, was obtained analytically by the least square method. Those basis vectors with a positive weighting were selected for entering into the next level of optimization. Totally, 7 levels of optimization were implemented in the study.Ten head‐and‐neck and ten prostate carcinoma cases were selected for the study and mapped to a round water phantom with a diameter of 20cm. The Matlab computation was performed in a heterogeneous programming environment with Intel i7 CPU and NVIDIA Geforce 840M GPU.

Results:

In all selected cases, the estimated dose distribution was in a good agreement with the given target dose distribution and their correlation coefficients were found to be in the range of 0.9992 to 0.9997. Their root‐mean‐square error was monotonically decreasing and converging after 7 cycles of optimization. The computation took only about 10 seconds and the optimal fluence maps at each gantry angle throughout an arc were quickly obtained.

Conclusion:

An analytical approach is derived for finding the optimal fluence for a given target dose distribution and a fast optimization algorithm implemented on the CPU/GPU heterogeneous computing environment greatly reduces the optimization time.
SU‐E‐T‐423: Fast Photon Convolution Calculation with a 3D‐Ideal Kernel On the GPU
Authors: Moriya S , Tachibana H , Sato M
Published: June 2015

Purpose:

The calculation time is a trade‐off for improving the accuracy of convolution dose calculation with fine calculation spacing of the KERMA kernel. We investigated to accelerate the convolution calculation using an ideal kernel on the Graphic Processing Units (GPU).

Methods:

The calculation was performed on the AMD graphics hardware of Dual FirePro D700 and our algorithm was implemented using the Aparapi that convert Java bytecode to OpenCL. The process of dose calculation was separated with the TERMA and KERMA steps. The dose deposited at the coordinate (x, y, z) was determined in the process. In the dose calculation running on the central processing unit (CPU) of Intel Xeon E5, the calculation loops were performed for all calculation points. On the GPU computation, all of the calculation processes for the points were sent to the GPU and the multi‐thread computation was done. In this study, the dose calculation was performed in a water equivalent homogeneous phantom with 1503 voxels (2 mm calculation grid) and the calculation speed on the GPU to that on the CPU and the accuracy of PDD were compared.

Results:

The calculation time for the GPU and the CPU were 3.3 sec and 4.4 hour, respectively. The calculation speed for the GPU was 4800 times faster than that for the CPU. The PDD curve for the GPU was perfectly matched to that for the CPU.

Conclusion:

The convolution calculation with the ideal kernel on the GPU was clinically acceptable for time and may be more accurate in an inhomogeneous region. Intensity modulated arc therapy needs dose calculations for different gantry angles at many control points. Thus, it would be more practical that the kernel uses a coarse spacing technique if the calculation is faster while keeping the similar accuracy to a current treatment planning system.
SU‐E‐T‐424: Feasibility of 3D Printed Radiological Equivalent Customizable Tissue Like Materials
Authors: Johnson D , Ferreira C , Ahmad S
Published: June 2015

Purpose:

To investigate the feasibility of 3D printing CT# specific radiological equivalent tissue like materials.

Methods:

A desktop 3D printer was utilized to create a series of 3 cm × 3 cm × 2 cm PLA plastic blocks of varying fill densities. The fill pattern was selected to be hexagonal (Figure 1). A series of blocks was filled with paraffin and compared to a series filled with air. The blocks were evaluated with a “GE Lightspeed” 16 slice CT scanner and average CT# of the centers of the materials was determined. The attenuation properties of the subsequent blocks were also evaluated through their isocentric irradiation via “TrueBeam” accelerator under six beam energies. Blocks were placed upon plastic‐water slabs of 4 cm in thickness assuring electronic equilibrium and data was collected via Sun Nuclear “Edge” diode detector. Relative changes in dose were compared with those predicted by Varian “Eclipse” TPS.

Results:

The CT# of 3D printed blocks was found to be a controllable variable. The fill material was able to narrow the range of variability in each sample. The attenuation of the block tracked with the density of the total fill structure. Assigned CT values in the TPS were seen to fall within an expected range predicted by the CT scans of the 3D printed blocks.

Conclusion:

We have demonstrated that it is possible to 3D print materials of varying tissue equivalencies, and that these materials have radiological properties that are customizable and predictable.
SU‐E‐T‐427: Feasibility Study for Evaluation of IMRT Dose Distribution Using Geant4‐Based Automated Algorithms
Authors: Choi H , Shin W , Kim J , Testa M , Min C
Published: June 2015

Purpose:

For intensity‐modulated radiation therapy (IMRT) treatment planning validation using Monte Carlo (MC) simulations, a precise and automated procedure is necessary to evaluate the patient dose distribution. The aim of this study is to develop an automated algorithm for IMRT simulations using DICOM files and to evaluate the patient dose based on 4D simulation using the Geant4 MC toolkit.

Methods:

The head of a clinical linac (Varian Clinac 2300 IX) was modeled in Geant4 along with particular components such as the flattening filter and the multi‐leaf collimator (MLC). Patient information and the position of the MLC were imported from the DICOM‐RT interface. For each position of the MLC, a step‐ and‐shoot technique was adopted. PDDs and lateral profiles were simulated in a water phantom (50×50×40 cm3) and compared to measurement data. We used a lung phantom and MC‐dose calculations were compared to the clinical treatment planning used at the Seoul National University Hospital.

Results:

In order to reproduce the measurement data, we tuned three free parameters: mean and standard deviation of the primary electron beam energy and the beam spot size. These parameters for 6 MV were found to be 5.6 MeV, 0.2378 MeV and 1 mm FWHM respectively. The average dose difference between measurements and simulations was less than 2% for PDDs and radial profiles. The lung phantom study showed fairly good agreement between MC and planning dose despite some unavoidable statistical fluctuation.

Conclusion:

The current feasibility study using the lung phantom shows the potential for IMRT dose validation using 4D MC simulations using Geant4 tool kits.

This research was supported by Korea Institute of Nuclear safety and Development of Measurement Standards for Medical Radiation funded by Korea research Institute of Standards and Science. (KRISS‐2015‐15011032)
SU‐E‐T‐428: Feasibility Study of 4D Image Reconstruction by Organ Motion Vector Extension Based On Portal Images
Authors: Yoon J , Jung J , Yeo I , Kim J , Yi B
Published: June 2015

Purpose:

To develop and to test a method to generate a new 4D CT images of the treatment day from the old 4D CT and the portal images of the day when the motion extent exceeded from that represented by plan CTs.

Methods:

A motion vector of a moving tumor in a patient may be extended to reconstruct the tumor position when the motion extent exceeded from that represented by plan CTs. To test this, 1. a phantom that consists of a polystyrene cylinder (tumor) embedded in cork (lung) was placed on a moving platform with 4 sec/cycle and amplitudes of 1 cm and 2 cm, and was 4D‐scanned. 2. A 6MV photon beam was irradiated on the moving phantoms and cineEPID images were obtained. 3. A motion vector of the tumor was acquired from 4D CT images of the phantom with 1 cm amplitude. 4. From cine EPID images of the phantom with the 2 cm amplitude, various motion extents (0.3 cm, 0.5 cm, etc) were acquired and programmed into the motion vector, producing CT images at each position. 5. The reconstructed CT images were then compared with pre‐acquired “reference” 4D CT images at each position (i.e. phase).

Results:

The CT image was reconstructed and compared with the reference image, showing a slight mismatch in the transition direction limited by voxel size (slice thickness) in CT image. Due to the rigid nature of the phantom studied, the modeling the displacement of the center of object was sufficient. When deformable tumors are to be modeled, more complex scheme is necessary, which utilize cine EPID and 4D CT images.

Conclusion:

The new idea of CT image reconstruction was demonstrated. Deformable tumor movements need to be considered in the future.