bluuebunny/crossref_metadata_2025 huggingface dataset:
TH‐AB‐BRB‐09: Effect of Hydrogel Spacer On Achievable Rectal DVH for IMRT in Prostate Cancer
Authors: Bosch W , Olsen L , Straube W , DeWees T , Mariados N , Sylvester J , Shah D , Kurtzman S , Zimberg S , Michalski J
Published: June 2015

Purpose:

Implanted tissue spacers show great promise in reducing radiation dose to organs at risk (OAR) and/or permitting dose escalation to target volumes in IMRT plans. Dosimetric benefits have been demonstrated in a randomized study of such a device despite variations in plan optimization. The purpose of this analysis was to quantify the dosimetric effect of a tissue spacer using a validated prostate DVH prediction model trained with previous high‐quality clinical prostate plans.

Methods:

Imaging and treatment planning for men undergoing IMRT for localized stage T1‐T2 prostate cancer was performed before and after hydrogel spacer implantation between the prostate and anterior rectum. Pre‐ and post‐implant plans were independently optimized to satisfy dose‐volume criteria for rectum, bladder, and penile bulb. Image segmentation data were used to predict DVHs achievable using IMRT for rectum and bladder with institutionally‐validated model parameters [Medical Physics 39, 7446 (2012); doi: 10.1118/1.4761864]. Predicted and actual clinical DVHs (cDVH) for rectum were used to compare pre‐ and post‐implant plans.

Results:

101 pairs of plans from 5 clinical sites treating at least 14 spacer subjects each were analyzed. An average ± s.d. absolute reduction of 9.4 ± 6.5 % in V60 and 7.0 ± 4.6 % in V70 was seen for pDVH. Corresponding reductions in cDVH were 10.3 ± 6.2 % and 8.0 ± 4.3, respectively. Standard deviations of corresponding pDVH and cDVH values were similar. Correlation coefficients between predicted and actual Vx values were computed for pre‐implant, post‐implant, and pre‐post Vx differences. Correlations between pDVH and cDVH were significantly positive, but not uniform among clinical sites, suggesting differences in plan optimization.

Conclusion:

Predicted DVHs showed an absolute difference of nearly 10% in rectal volume receiving ≥ 60 Gy (50.4% relative reduction in V60) with use of peri‐rectal spacer, consistent with analysis of clincal pre‐ and post‐implant plans.

This work was supported by a research grant from Augmenix, Inc.
TH‐AB‐BRB‐10: How Many Lesions Can Be Treated with Radiosurgery? Whole Brain Dose From Radiosurgery of Multiple Targets
Authors: Becker S , Jozsef G , Presser J , Kondziolka D
Published: June 2015

Purpose:

Multiple brain metastases are a frequent indication for stereotactic radiosurgery (SRS). There is concern that as radiation doses to normal brain increase, neurocognitive toxicity increases too. In this report, we estimate how many lesions and what total volume of tumor(s) can be irradiated before a whole brain dose of 8 Gy is reached.

Methods:

Multiple tumor SRS was simulated in Matlab (Mathworks, Inc) using pre‐calculated dose distributions (kernels) created from single isocenter shots for 4mm, 8mm and 16mm targets, and composite shots for 24mm and 36mm targets in GammaPlan (Elekta, Inc Sweden). Tumors of varying size from 1–36mm were randomly placed throughout the brain and covered with dose until the mean normal brain dose reached 8Gy. Selection of tumor size, dose coverage, selectivity, normalization, and maximum dose were based off NYU Gamma Knife clinical metastases database. Scenarios were run both for mixed tumor sizes and a select few fixed tumors sizes.

Results:

The mean number of tumors treated, V12, and total tumor volume treated using mixed tumor size distribution were 40.1±8.6, 180.2±28.8cc, and 41.1±12.7cc. For the simulation runs utilizing tumor size fixed ranges of 0–4mm, 4–8mm, 8–16mm, 16–24mm, 24–36mm, the average contribution per tumor to a whole brain dose was 0.045Gy, 0.097Gy, 0.359Gy, 0.635Gy, and 1.203Gy, respectively.

Conclusion:

GK SRS would require an average of 6.7 24–36mm tumors, 177.3 0–4mm tumors, or 40.1 tumors of mixed sizes to be treated to reach 8 Gy whole normal brain dose for a single session. Therefore, this study alleviates the concern that radiosurgery for a large number of tumors or treating additional newly formed metastases after previous SRS can lead to high whole brain doses. The ability to safely retreat enables the physician to consider new SRS instead of whole brain radiotherapy when additional metastases appear.
TH‐AB‐BRA‐01: Biologically Based Prostate Cancer Treatment Planning with Combined Brachytherapy and External Beam Radiotherapy
Authors: Qiu X , Voros L , Cohen G , Mageras G , Yang J , Zaider M
Published: June 2015

Purpose:

Currently, when external beam radiotherapy (EBRT) is combined with brachytherapy, they are planned independently of one another. Standard EBRT strives to deliver homogeneous physical dose to target, ignoring the non‐homogeneous nature of brachytherapy. Here, we take advantage of the difference in dose distribution and the resulting biologic effects from both modalities, and examine an alternative approach to deliver homogeneous biological effect. Specifically, we implement the new methodology to the combination of permanent seed brachytherapy and IMRT for the treatment of prostate cancer.

Methods:

Prostate cancer treatments consisting of 125I (or 103Pd) implants (110 Gy or 100 Gy, respectively) followed by 45 Gy in 25 IMRT fractions are considered. A biological iso‐effective dose (IED) is calculated from clinically‐derived biologic parameters to determine the desired dose for each voxel within the target and dose constraints for normal tissue. Steep dose gradients are avoided by replacing the voxel doses required by IED with iso‐dosimetric optimization structures for IMRT planning. For sub‐volumes receiving large brachytherapy doses less than 45 Gy physical dose constraints are imposed; similarly, for cool sub‐volumes, more than 45 Gy physical dose constraints are imposed.

Results:

Retrospective evaluation of patient cases shows that improvement in therapeutic ratio is obtained with the new IED‐based treatment plan optimization. Dose distribution, IED‐volume histograms, and IED‐equivalent DVH for a 45‐fraction IMRT treatment are presented.

Conclusion:

We describe a new biological optimization‐based treatment planning implementation. IED is calculated (rather than physical dose) while respecting practical aspects of the EBRT plan. The resulting plans predict reduced toxicity to urethra, bladder and rectum. The results require further clinical verification in a larger number of cases. This new methodology can be generalized to any treatment combination.

Funding Support: Research grant from Varian Medical Systems. Conflict of Interest: Xinjie Qiu, Gig Mageras, Marco Zaider: Varian Medical Systems; Laszlo Voros, Gilad Cohen, Jie Yang: None.
TH‐CD‐210‐01: Recent Advances in SPECT/CT and PET/CT for Oncology
Authors: Kappadath S.
Published: June 2015

There have been significant recent advances in SPECT/CT and PET/CT for Oncology and Cardiology. In oncologic imaging, some commercial SPECT/CT systems now output quantitative uptake values; calibration and calculation of these quantitative uptake values will be discussed. Advances in PET detector design and PET data acquisition modes that have implemented in commercial PET/CT systems will be presented. Innovations in iterative reconstruction of SPECT/CT and PET/CT will also be described.

In cardiac imaging, novel collimator designs, such as multi‐pinhole and locally focusing collimators arranged in geometries that are optimized for cardiac imaging have been implemented to reduce imaging time and radiation dose. These new collimators have been coupled with solid‐state photon detectors. The new SPECT scanners demonstrate up to a 7‐fold increase in photon sensitivity and up to 2 times improvement in image resolution. Although PET scanners are used primarily for oncological imaging, cardiac imaging can benefit from the improved PET sensitivity of 3D systems without inter‐plane septa and implementation of the time‐of‐flight reconstruction. Additionally, all major PET vendors now implement resolution recovery techniques. These new methods improve image contrast, image resolution, and reduce image noise for cardiac imaging. New quantification techniques have been proposed for analysis of cardiac SPECT and PET images. These include direct change analysis, motion‐frozen analysis, hybrid quantification of SPECT and anatomical CT angiography data, and integration of several quantitative parameters in composite scores.

Learning Objectives:

To discuss clinical implementation of commercial quantitative SPECT/CT systems

To describe advances in PET detector design and data acquisition

To report on innovations in iterative reconstruction of SPECT/CT and PET/CT

4 Describe novel hardware designed for cardiac SPECT

Understand how recent hardware advances in PET/CT are relevant to cardiology

Describe new software techniques for quantification of cardiac SPECT and PET
TH‐E‐304‐00: Reference Dosimetry for Beam Modalities Other Than MV Photons
Authors: McEwen Malcolm
Published: June 2015

It is fair to say that the clinical determination of absorbed dose in a high‐energy photon beam is the only situation where there is currently a direct chain of traceability from the standards laboratory to the end‐user and the protocol requires only ‘simple’ conversion/ correction factors. For all other modalities the primary standard is in a different quantity (e.g., air kerma) or in the right quantity but a different radiation beam (e.g., for proton dosimetry) from what is required for the clinical dose delivery. Complex conversion factors are often then required in applying protocols to give the desired final quantity. Such conversion factors can be from kerma to dose (in the case of kilovoltage x‐rays or brachytherapy) or from one type of radiation beam to another (in the case of electron or proton beams). Even non‐standard MV photon beams (e.g. non‐standard geometry, presence of a magnetic field) can have a significant impact on the procedure that must be followed compared to what is found in TG‐51. While national and international bodies, such as the AAPM and the IAEA, primarily address the end‐user's needs with protocols and guidelines, calibration laboratories are developing new measurement standards and approaches that should simplify the calibration chain and therefore lead to simpler, more robust procedures to yield absorbed dose in the beam of interest in the radiation therapy clinic.

This session will provide a broad overview of the worldwide developments currently in progress, and also planned for the near future, of absorbed dose standards for a range of beam modalities including: Proton beams

HDR and LDR brachytherapy

Non‐standard photon fields (small fields, impact of magnetic fields, etc)

Learning Objectives:

Understand the present traceability chain for all beam modalities in regular use in the radiation therapy clinic.

Understand the current and planned development of absorbed dose standards for modalities other than linac photon beams.

Understand the physical basis of these standards and the potential impact on end‐user dosimetry.

Understand the likely impact of these standards developments on dosimetry protocols in the coming 5–10 years.
TH‐E‐304‐01: Overview of Primary Standards for Reference Dosimetry
Authors: McEwen M.
Published: June 2015

It is fair to say that the clinical determination of absorbed dose in a high‐energy photon beam is the only situation where there is currently a direct chain of traceability from the standards laboratory to the end‐user and the protocol requires only ‘simple’ conversion/ correction factors. For all other modalities the primary standard is in a different quantity (e.g., air kerma) or in the right quantity but a different radiation beam (e.g., for proton dosimetry) from what is required for the clinical dose delivery. Complex conversion factors are often then required in applying protocols to give the desired final quantity. Such conversion factors can be from kerma to dose (in the case of kilovoltage x‐rays or brachytherapy) or from one type of radiation beam to another (in the case of electron or proton beams). Even non‐standard MV photon beams (e.g. non‐standard geometry, presence of a magnetic field) can have a significant impact on the procedure that must be followed compared to what is found in TG‐51. While national and international bodies, such as the AAPM and the IAEA, primarily address the end‐user's needs with protocols and guidelines, calibration laboratories are developing new measurement standards and approaches that should simplify the calibration chain and therefore lead to simpler, more robust procedures to yield absorbed dose in the beam of interest in the radiation therapy clinic.

This session will provide a broad overview of the worldwide developments currently in progress, and also planned for the near future, of absorbed dose standards for a range of beam modalities including: Proton beams

HDR and LDR brachytherapy

Non‐standard photon fields (small fields, impact of magnetic fields, etc)

Learning Objectives:

Understand the present traceability chain for all beam modalities in regular use in the radiation therapy clinic.

Understand the current and planned development of absorbed dose standards for modalities other than linac photon beams.

Understand the physical basis of these standards and the potential impact on end‐user dosimetry.

Understand the likely impact of these standards developments on dosimetry protocols in the coming 5–10 years.
TH‐E‐304‐02: Standards for Proton and Heavy Ion Beams
Authors: Tosh R.
Published: June 2015

It is fair to say that the clinical determination of absorbed dose in a high‐energy photon beam is the only situation where there is currently a direct chain of traceability from the standards laboratory to the end‐user and the protocol requires only ‘simple’ conversion/ correction factors. For all other modalities the primary standard is in a different quantity (e.g., air kerma) or in the right quantity but a different radiation beam (e.g., for proton dosimetry) from what is required for the clinical dose delivery. Complex conversion factors are often then required in applying protocols to give the desired final quantity. Such conversion factors can be from kerma to dose (in the case of kilovoltage x‐rays or brachytherapy) or from one type of radiation beam to another (in the case of electron or proton beams). Even non‐standard MV photon beams (e.g. non‐standard geometry, presence of a magnetic field) can have a significant impact on the procedure that must be followed compared to what is found in TG‐51. While national and international bodies, such as the AAPM and the IAEA, primarily address the end‐user's needs with protocols and guidelines, calibration laboratories are developing new measurement standards and approaches that should simplify the calibration chain and therefore lead to simpler, more robust procedures to yield absorbed dose in the beam of interest in the radiation therapy clinic.

This session will provide a broad overview of the worldwide developments currently in progress, and also planned for the near future, of absorbed dose standards for a range of beam modalities including: Proton beams

HDR and LDR brachytherapy

Non‐standard photon fields (small fields, impact of magnetic fields, etc)

Learning Objectives:

Understand the present traceability chain for all beam modalities in regular use in the radiation therapy clinic.

Understand the current and planned development of absorbed dose standards for modalities other than linac photon beams.

Understand the physical basis of these standards and the potential impact on end‐user dosimetry.

Understand the likely impact of these standards developments on dosimetry protocols in the coming 5–10 years.
TH‐E‐304‐04: Standards for Non‐Standard Photon Beams
Authors: Seuntjens J.
Published: June 2015

It is fair to say that the clinical determination of absorbed dose in a high‐energy photon beam is the only situation where there is currently a direct chain of traceability from the standards laboratory to the end‐user and the protocol requires only ‘simple’ conversion/ correction factors. For all other modalities the primary standard is in a different quantity (e.g., air kerma) or in the right quantity but a different radiation beam (e.g., for proton dosimetry) from what is required for the clinical dose delivery. Complex conversion factors are often then required in applying protocols to give the desired final quantity. Such conversion factors can be from kerma to dose (in the case of kilovoltage x‐rays or brachytherapy) or from one type of radiation beam to another (in the case of electron or proton beams). Even non‐standard MV photon beams (e.g. non‐standard geometry, presence of a magnetic field) can have a significant impact on the procedure that must be followed compared to what is found in TG‐51. While national and international bodies, such as the AAPM and the IAEA, primarily address the end‐user's needs with protocols and guidelines, calibration laboratories are developing new measurement standards and approaches that should simplify the calibration chain and therefore lead to simpler, more robust procedures to yield absorbed dose in the beam of interest in the radiation therapy clinic.

This session will provide a broad overview of the worldwide developments currently in progress, and also planned for the near future, of absorbed dose standards for a range of beam modalities including: Proton beams

HDR and LDR brachytherapy

Non‐standard photon fields (small fields, impact of magnetic fields, etc)

Learning Objectives:

Understand the present traceability chain for all beam modalities in regular use in the radiation therapy clinic.

Understand the current and planned development of absorbed dose standards for modalities other than linac photon beams.

Understand the physical basis of these standards and the potential impact on end‐user dosimetry.

Understand the likely impact of these standards developments on dosimetry protocols in the coming 5–10 years.
TH‐EF‐BRD‐02: An Analytic Linear Accelerator Source Model and Automatic Source Commissioning for GPU‐Based Fast Monte Carlo Dose Calculation
Authors: Tian Z , Folkerts M , Li Y , Shi F , Jiang S , Jia X
Published: June 2015

Purpose:

GPU‐based Monte Carlo (MC) dose calculations have become available. Linac source modeling for it is still a problem to be addressed. The model should be carefully designed for sufficient accuracy, ease of automatic commissioning, and more importantly suitability for parallelization. This abstract presents an analytical source model for GPU‐based MC dose engines with these factors considered.

Methods:

Analytical source models are preferred for GPU comparing to phase‐space‐file based models due to the avoidance of loading and transferring large data. In our model, we introduced a phase‐space‐ring (PSR) concept. A PSR contains particles of the same type and close in energy and radial distance on a plane above jaws. We parameterized probability densities of particle location, energy, and direction for three sub‐source types: primary photon PSR, scattered photon PSR and electron PSR. Model parameters were derived by analyzing reference phase‐space files. To incorporate this model into our GPU dose engine, we designed an efficient sampling scheme to reduce GPU thread divergence. Automatic commissioning was achieved by solving an optimization problem to adjust PSR weight.

Results:

Our model built for Varian Truebeam 6MV photon beam was able to represent the reference phase‐space. Comparing open‐field doses in water computed using our model and the phase‐space file, average distance‐to‐agreement(DTA)≤1mm for high‐gradient regions, root‐mean‐square(RMS) difference≤1.1% for low‐gradient regions and output factor difference≤0.5% were observed. Good agreements were seen in IMRT cases. Our model improved efficiency by 1.70–4.41 times compared to phase‐space‐based source model. It took ∼20sec to perform commissioning. Compared to uncommissioned model, DTA was reduced from 0.04–0.28cm to 0.04–0.12cm for depth dose at build‐up region, and RMS from 0.32%–0.67% to 0.21%–0.48%, 0.31%–2.0% to 0.06%–0.78% and 0.20%–1.25% to 0.10%–0.51% for depth dose after build‐up, lateral profiles of inner beam and outer beam, respectively.

Conclusion:

The analytic source model achieved high accuracy, efficiency, and suitability for GPU‐based dose calculations.
TH‐EF‐BRD‐07: Knowledge Based Automatic Lung IMRT Planning with Non‐Coplanar Beams
Authors: Yuan L , Ge Y , Sheng Y , Hedrick K , Yin F , Wu Q J
Published: June 2015

Purpose:

We present a lung IMRT planning method which automatically determines both plan optimization objectives and beam configurations with non‐coplanar beams by analyzing patient‐specific anatomical information and learning from past planning experiences.

Methods:

A beam navigation index map is constructed to characterize the geometry of the tumor relative to the OARs at each candidate beam direction. It is defined as the ratio of the dose deposited inside the OARs to that in the PTV along the beam path. An additional term which is a function of the beam spread is added to the navigation index to account for the effect of multiple beams on PTV dose conformity. For each case, a fixed number of beams which minimizes the total index values are selected. The relative weights of different OARs and the dose conformity term in the index are learned from the beam configurations in a database of mostly co‐planar beam plans. The OAR dose sparing objectives are generated by OAR DVH prediction models which are trained by prior IMRT plans. This completely automated method was validated by re‐planning 10 clinical lung plans which have used non‐coplanar beams. Important dosimetric parameters of the automatically generated plans are compared with those of the clinical plans by Wilcoxon signed rank tests.

Results:

All dosimetric parameters in the automatically generated plans are statistically better or comparable with those in clinical plans. The most significant improvements are: lung V10Gy with average reduction of 6% of OAR volume, Esophagus V20Gy with average reduction of 3% and spinal cord D2% with reduction of 19% of prescription dose on average.

Conclusion:

Knowledge models learned from mostly co‐planar beam plans can be utilized to generate IMRT plans with non‐coplanar beams. This method demonstrates the potential to improve both the planning efficiency and plan quality by automatically incorporating non‐coplanar beams.

Partially supported by NIH/NCI under grant #R21CA161389 and a master research grant from Varian Medical Systems.
TH‐EF‐BRD‐09: Dosimetric Analysis of Patient‐Specific Planning Target Volumes Using Population Statistical Modeling of Interfractional Prostate Motion
Authors: Vile D , Christensen G , Ford J , Mukhopadhyay N , Williamson J
Published: June 2015

Purpose:

To use a population‐based statistical motion model to create a patient‐specific planning target volume (PTV) for patients undergoing radiotherapy for definitive treatment of low‐risk prostate cancer and to compare the dosimetric impact of this PTV against current clinically used PTVs.

Methods:

From mapping 10–13 fractional CT images to the planning image, systematic and random deformations were calculated for 19 prostate patients. These deformations were mapped to a reference image using inter‐patient deformable image registration (DIR). In the reference image coordinate system, principal component analysis (PCA) was used to create 3D statistical motion models of the systematic and random displacements. Using these models to sample synthetic displacements and mapping them back to a patient's planning coordinate system, organ occupancy maps were created for the prostate. The organ occupancy map was thresholded at the 95% coverage level to create a PCA‐based PTV. For bony aligned and prostate centroid aligned patient setups, a virtual clinical trial was conducted to determine the dosimetric differences between a PCA‐based PTV and a conventional PTV (using a van Herk margin for bony alignment and 5 mm fixed [3 mm posterior] for centroid alignment). Dose volume histogram (DVH) metrics were used in the analysis.

Results:

All PTVs provided prescription dose coverage to the prostate. For bony aligned setup, the PCA‐based PTV significantly (p<0.05) reduced D₃₀, D₂₀, and D5 to bladder and D5₀ to rectum, while increasing rectal D₂₀ and D5. For the centroid aligned setup, the PCA PTV significantly reduced all bladder DVH metrics and trended to lower rectal DVH metrics.

Conclusion:

Statistical motion modeling provides a systematic method for creating PTVs. Plans created using these PTVs provide adequate prescription dose coverage of the prostate while lowering the bladder dose. PCA‐based PTVs tended to increase rectal dose for bony‐aligned patients and lower it for centroid aligned patients.
TH‐EF‐BRD‐11: Clinical Skin Toxicity Comparison and Phantom Dose Measurements for Head and Neck Patients Treated with IMRT Vs. VMAT
Authors: Bredfeldt JS , Sapir E , Masi KJ , Schipper MJ , Eisbruch A , Matuszak MM
Published: June 2015

Purpose:

Observations in our clinic have suggested a trend towards increased skin‐toxicity for head and neck (HN) patients treated with Volumetric Modulated Arc Therapy (VMAT) compared with Intensity Modulated Radiation Therapy (IMRT). Here, we report on these observations and quantify surface dose differences between VMAT and IMRT treatment plans for HN cancer patients.

Methods:

We retrospectively compared skin‐toxicity scores gathered by the treating physician according to the Common Terminology Criteria for Adverse Events (CTCAE v4.0) for head and neck squamous cell carcinoma (HNSCC) patients treated with IMRT (102) and VMAT (88) . A Cochran‐Armitage test evaluated the relationship between treatment modality, chemotherapy and toxicity. Six patients with grade 3 skin‐toxicities were selected from this cohort and the target/organ at risk volumes were transferred onto an anthropomorphic phantom using a deformable image registration based atlas (SmartSegmentation, Varian Medical). Two‐arc VMAT and 9‐field IMRT plans were optimized and delivered to the anthropomorphic phantom to produce similar, clinically‐acceptable, dose distributions. Surface dose was measured using optically‐stimulated luminescent dosimeters placed at 2 positions on the phantom's neck which were identical between VMAT and IMRT deliveries. N‐factor ANOVA was performed to identify statistically significant differences in surface dose.

Results:

Our retrospective study showed a marginally significant higher skin‐toxicity (Grade≥ 2) for VMAT compared with IMRT (35%vs.20%, p=0.06) for patients treated with radiation alone. Phantom measurements showed a significant effect of treatment modality on surface dose (F=42.5,p<0.001) with VMAT delivering 8% higher surface doses on average. No interaction was found between use of a thermoplastic mask and treatment with VMAT (F=0.02,p=0.884).

Conclusion:

This work indicates that marginal increases in skin dose and subsequent toxicity may be expected from HN patients treated with VMAT compared with IMRT. Our results motivate the need for techniques to spare the skin during VMAT treatment planning and for the early assessment of skin‐toxicity.
TH‐EF‐BRB‐02: Combination of Multiple EPID Imager Layers Improves Image Quality and Tracking Performance of Low Contrast Objects
Authors: Yip S , Rottmann J , Chen H , Morf D , Fueglistaller R , Star‐Lack J , Zentai G , Berbeco R
Published: June 2015

Purpose:

We hypothesize that an electronic portal imaging device (EPID) consisting of multiple layers, each containing a scintillator coupled to amorphous silicon flat panel imager (AMFPI), increases photon detection efficiency, improving image quality and tracking accuracy of low contrast targets during radiotherapy.

Methods:

The prototype 4‐layer imager evaluated in this study contained four individually programmable layers each with a copper‐Gd₂O₂S scintillator coupled to an AMFPI with a complete set of readout electronics. The imager was placed on a Varian TrueBeam linac. A Las‐Vegas phantom programmed with sinusoidal motion (Peak‐to‐peak amplitude=20cm, Period=3.5sec) was imaged at a frame rate of 10Hz with one to four layers activated (activation of only the top layer is equivalent to using a conventional EPID). Image quality was assessed by counting the number of visible circles, and computing the contrast‐to‐noise ratio (CNR) of the least visible circle (depth=0.5mm, diameter=7mm). A previously validated tracking algorithm was employed for auto‐tracking. Tracking error was defined as the difference between the programmed and tracked positions of the circle. The Pearson correlation coefficient of the CNR and tracking error was also computed.

Results:

Motion‐induced EPID image blurring significantly reduced circle visibility. During one cycle of phantom motion, the number of visible circles ranged from 11–23, 13–24, 15–25, and 16–26 when imaged with one, two, three and four layer imagers, respectively. Compared with using only a single layer, combining 2, 3, and 4 layers increased the median CNR by factors of 1.19, 1.42, and 1.71, respectively and reduced the average tracking error from 3.32mm to 1.67mm to 1.47mm, and 0.74mm, respectively. Significant correlations (p∼10−9) were found between the tracking error and CNR.

Conclusion:

Combination of four amorphous silicon layers, instead of single layer, significantly improves the EPID image quality and tracking accuracy for a poorly visible object that replicates lung tumor radiotherapy conditions.

The project described was supported, in part, by a grant from Varian Medical Systems, Inc., and Award No. R01CA188446‐01 from the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health.
TH‐EF‐BRB‐06: A Method to Synthesize Hybrid KV/MV Projections for Metal Artifact Corrections in CBCT
Authors: Altunbas C , Kavanagh B , Miften M
Published: June 2015

Purpose:

Simultaneous utilization of kV and MV projections is a promising approach to correct metal artifacts in CBCT. However, due to inherent differences in attenuation, beam hardening, and scattering properties of kV and MV beams, it is challenging to employ kV and MV projections in the same CBCT set. To address these limitations, a novel method was developed to generate hybrid kV/MV projections, and to correct metal artifacts in CBCT images.

Methods:

First, a calibration dataset‐based method was developed to correct beam hardening and scatter in kV and MV projections, and to convert MV projections to kV‐equivalent attenuation maps. In the next stage, hybrid kV/MV projections were synthesized by weighted summation of corrected kV and MV projections; a user‐controlled, generalized logistic curve was utilized as the weighting function, and its shape determined the relative weight of kV and MV projections with respect to relative attenuation differences on a pixel‐by‐pixel basis.

Results:

Phantoms with integrated metal hardware were imaged using 125 kVp and 6 MV beams from a linac with on‐board KV imaging. CBCT images were reconstructed from KV‐only projections and from hybrid kV/MV projections. In kV‐only CBCT sets, metal hardware lead to severe artifacts and HU variations. HU values were underestimated by — 400±117 in a Catphan phantom with femoral implants. On the other hand, CBCT reconstructed from hybrid kV/MV projections exhibited significantly less metal artifacts, and the underestimation in HU values was reduced to — 100±73. Similar results were observed in all imaging experiments.

Conclusion:

Our method was successful in reducing metal artifacts, and improving CT number accuracy in CBCT images. Consistency of corrections appeared to be robust under diverse imaging conditions. The user‐controlled weighting function allowed fine tuning the balance between the level of desired metal artifact suppression and the preservation of soft tissue contrast in CBCT images.
TH‐EF‐BRB‐06: Intensity Modulated Imaging?: Clinical Workflow for Fluence Field Modulated CT On a TomoTherapy System
Authors: Szczykutowicz T , Hermus J , Geurts M , Smilowitz J
Published: June 2015

Purpose:

This work presents a new approach that uses the multi leaf collimator present on TomoTherapy devices to modulate the imaging beam. This allows for targeted imaging in which only regions of clinical importance for IGRT (patient positioning confirmation) receive a high level of image quality. This allows the total imaging dose to decrease, especially for healthy tissue.

Methods:

A clinical TomoTherapy machine was programmed to deliver imaging dose such that small pre‐defined volumes of interest (VOI) received relatively higher levels of image quality with respect to surrounding regions. Four different size ROIs were placed at varying distances from isocenter. The noise and mean CT number were compared between VOI and un‐modulated scans. Dose distributions were generated using a treatment planning dose calculator. A clinically feasible workflow was developed to implement this technique that used treatment planning contours to define VOI positions.

Results:

The VOI‐FFMCT technique produced an image noise within 5% of an unmodulated scan for all size VOIs. The VOI technique required a total imaging dose of 0.61, 0.69, 0.60, and 0.50 times the “full dose” acquisition dose for VOI sizes/locations of 10/13/10/6 cm in diameter and located 0/2/5/6 cm from isocenter respectively. Mean CT numbers for the VOI scans were within 1% of the unmodulated case.

Conclusion:

This approach, which combines state‐of‐the‐art radiation therapy fluence control with a novel imaging approach has been implemented on a clinical system with no hardware or software changes. A clinically feasible method for implementing this technique was developed which requires no additional user input relative to the current procedures in our clinic. The method allows physicians to choose between: (1) better image quality at no dose penalty or (2) equal image quality while reducing dose levels relative to today's standard of care.

Research grants from GE HealthCare and TomoTherapy.
TH‐EF‐BRB‐09: Real‐Time Ultrasound Monitoring with Speckle Tracking in Abdominal Stereotactic Body Radiation Therapy
Authors: Su L , O'shea T , Ng S K , Zhang Y , Iordachita I , Wong J , Harris E , Bamber J , Sen H T , Kazanzides P , Bell M Lediju , Ding K
Published: June 2015

Purpose:

Ultrasound is ideal for real‐time monitoring with high soft tissue contrast, non‐ionization, portability, and cost effectiveness. No studies have investigated clinical feasibility of real‐time ultrasound monitoring for abdominal stereotactic body radiation therapy (SBRT) under active breath control (ABC). We are able to monitor target motion using 4D ultrasound and speckle tracking.

Methods:

An arm‐bridge system (ABS) was designed to allow clinician to freely move and lock ultrasound probe. A ceiling mounted infrared camera was calibrated to track probe position using an reflector on the probe. An abdominal ultrasound phantom was secured on a programmable respiratory motion platform to simulate the 20 second expiration and inspiration phases of the ABC with 10 mm inferior translation for 6 cycles. The probe was coupled to the phantom using gel. 4D ultrasound B‐mode images were simultaneously acquired at each breath‐hold for monitoring. A 3D normalized cross‐correlation template matching algorithm was developed to track speckle and feature Point‐of‐interest (POI) motion at shallow, medium and deep positions in B‐mode images. The reproducibility of breath‐hold was evaluated by comparing speckle tracking and motion platform position as a function of ROI size.

Results:

During the repeated respiration movement, the probe position variation was minimal (< 0.5 mm at all time in all directions). For target motion monitoring, ultrasound speckle tracking showed high reproducibility (0.005±0.005 mm AP and 0.01±0.01 mm SI for all ROIs, and 0.03±0.02 mm RL for the shallow and medium ROI). For the deep speckle ROI, the RL error was 0.04±0.04 mm likely due to reduced resolution with depth. Deep ROI showed increased RL tracking accuracy with increased ROI size.

Conclusion:

Our ABS was able to maintain fixed probe position during the respiration movement with high fidelity. Ultrasound speckle tracking was able to precisely determine the motion over repeated breath‐hold for real time monitoring.

This work is supported by NIT grant R01CA161613.
TH‐EF‐BRA‐03: Size‐Specific, Scanner‐Independent Fetal Dose Estimates in Abdominal and Pelvic CT Examinations of Pregnant Patients
Authors: McMillan K , Bostani M , Angel E , McNitt‐Gray M
Published: June 2015

Purpose:

The purpose of this work is to develop patient size‐specific, scanner‐independent CTDIvol‐to‐fetal‐dose conversion coefficients for abdominal and pelvic CT examinations of pregnant patients of various gestational ages.

Methods:

For 18 pregnant patients of gestational age ranging from 12 to 36 weeks who underwent clinically‐indicated CT examinations, models of maternal and fetal (or embryo) anatomy were created from the image data, and dose to the fetus was estimated using Monte Carlo simulations of fixed tube current helical scans of the abdomen and pelvis for 64‐slice MDCT scanners from four major manufacturers. Fetal doses were normalized by scanner‐specific 32 cm CTDIvol values and averaged across all scanners to obtain a scanner‐independent CTDIvol‐to‐fetal‐dose conversion coefficient for each patient. Patient size was described using water equivalent diameter (WED) measured at the image containing the three‐dimensional geometric centroid of the fetus. The coefficient of variation (CoV) across scanners for the simulated fetal dose was calculated before and after CTDIvol normalization to demonstrate the utility of scanner‐independent CTDIvol‐to‐fetal‐dose conversion coefficients for each patient. The relationship between the WED patient size metric and CTDIvol‐to‐fetal‐dose conversion coefficients was also examined to determine if a correlation exists.

Results:

The CoV across scanners for fetal dose in all patients before and after CTDIvol normalization ranged from 25.10%–30.16% and 5.60%–9.48%, respectively. An exponential relationship between CTDIvol‐to‐fetal‐dose conversion coefficients and patient size was observed with a coefficient of determination of 0.84.

Conclusion:

Strong correlation exists between CTDIvol normalized fetal dose and WED. These results indicate that fetal dose from abdominal and pelvic CT examinations of pregnant patients of various gestational ages may be accurately estimated with: (a) fetal dose normalized by CTDIvol to account for scanner variation and (b) a WED patient size metric to account for patient size variation.

Disclosures — Michael McNitt‐Gray: Institutional Research Agreement, Siemens AG; Research Support, Siemens AG; Consultant, Flaherty Sensabaugh Bonasso PLLC; Consultant, Fulbright and Jaworski; Disclosures — Erin Angel: Employee of Toshiba America Medical Systems, Inc.
TH‐EF‐BRA‐04: Direct Organ Dose Measurements in a Human Cadaver Scanned with Pediatric CT Protocols
Authors: Lipnharski I , Lamoureux R , Carranza C , Cormack B , Bidari S , Rill L , Arreola M
Published: June 2015

Purpose:

To measure pediatric organ doses and assess clinical image quality in a cadaver scanned with pediatric protocols on a clinical CT scanner.

Methods:

Organ doses were measured in the thoracic and abdominal regions of a small sized cadaver by inserting optically stimulated luminescent dosimeters (OSLDs) into various organs. The subject's effective diameter was measured as 22.9 cm in the chest, and 22.3 cm in the abdomen, corresponding to the size of an 11‐year old pediatric patient, based on AAPM Report No. 204. The subject was scanned with a pediatric chest and pediatric abdomen protocol on a Toshiba Aquilion ONE CT scanner. Scans were acquired with filtered back projection (FBP) and iterative reconstruction (IR) to quantify organ dose reduction capabilities of IR in pediatric patients. The subject was scanned with reduced dose protocols by increasing the noise tolerance level (SD) from 12.5 to 17.5 and 25.0. Image quality was assessed by a blinded observer study utilizing pediatric radiologists. Readers graded the images with a three point scale on the basis of diagnostic ability.

Results:

In comparing IR versus FBP reconstructed images, we measured average organ dose reductions of 25% and 32% for the chest and abdomen scans, respectively. Increasing the SD from 12.5 to 17.5 offered organ dose savings of 51% and 58% for the chest and abdomen scans. Further dose reduction was achieved by increasing the SD to 25, allowing substantial dose savings of 75% and 80% for the chest and abdomen scans. The majority of the images were found to be of acceptable image quality.

Conclusion:

Organ dose quantification and reduction were investigated in this clinical CT study. Iterative reconstruction algorithms provide significant dose reduction to pediatric patients undergoing CT scans. Further dose reductions can be achieved by increasing noise tolerance levels while still maintaining acceptable image quality.
TH‐EF‐BRA‐08: Extending the Concept of Weighted CT Dose Index to Elliptical Phantoms of Various Aspect Ratios
Authors: Markovich A , Morgan A , Dong F , Primak A , Li X
Published: June 2015

Purpose:

Standard CT dosimetry phantoms are currently perfectly circular cylinders. Lately elliptical cylinders have been proposed as an alternative because they better represent the shapes of the human body and allow the evaluation of tube current modulation systems. The purpose of this study was to extend the concept of CTDIw to elliptical phantoms of various aspect ratios.

Methods:

Based on published adult and pediatric data, eight body aspect ratios were chosen from the full range between 1 (perfectly circular) and 1.72 (extremely elliptical). For each value, two elliptical cylinders were created digitally to represent adult and pediatric bodies. They had the same cross‐sectional areas as the standard 32‐cm and 16‐cm CTDI phantoms. For each digital phantom, CTDI₁₀₀ at central and peripheral locations were simulated for tube voltages between 70–140 kVp using a Monte Carlo program previously validated for a clinical CT system (SOMATOM Definition Flash, Siemens Healthcare). The simulation also output the average dose over the cross‐sectional area, Dxsec, the quantity CTDIw is intended to represent. Values of Dxsec and CTDI₁₀₀ allowed linear systems of equations to be established, from which central and peripheral weighting coefficients were solved.

Results:

All elliptical phantoms had the same Dxsec as the standard circular phantoms. Regardless of phantom shape, only two weighting coefficients (w1 and w2) were needed: w1 for central CTDI₁ ₀₀ and w2 for the average of the four peripheral CTDI₁₀₀'s. For perfectly circular phantoms, w1 and w2 were 0.37 and 0.63, respectively, agreeing well with the conventional weighting coefficients of 1/3 and 2/3. Over the full range of aspect ratios, w1 increased linearly from 0.37 to 0.46, whereas w2 decreased linearly from 0.63 to 0.54.

Conclusion:

CTDI₁₀₀ weighting coefficients are linear functions of phantom aspect ratio, allowing the concept of CTDIw to be easily extended to elliptical phantoms of various shapes.

This research is supported in part by a Faculty Startup Fund from Cleveland State University.
TH‐EF‐BRA‐10: Monte Carlo Simulation of CT Dose Index and Equilibrium Dose‐Pitch Product: Effect of Ion Chamber Model On Simulated Dose Response
Authors: Liptak C , Morgan A , Dong F , Primak A , Li X
Published: June 2015

Purpose:

In Monte Carlo simulations of patient dose from a CT scan, accurate knowledge about the x‐ray energy spectra is essential. Simulated CT dose index (CTDI₁₀₀) free‐in‐air is often used together with measured values to calibrate the intensity of the spectra. To simulate CTDI₁₀₀, a computational model of the ion chamber is required. Various chamber modeling methods have been reported. The purpose of this study was to investigate systematically how chamber modeling affects simulated dose response. Both the pencil chamber and the thimble chamber recently recommended by AAPM TG111 were studied.

Methods:

A Monte Carlo program previously validated for a clinical CT system (SOMATOM Definition Flash, Siemens Healthcare) was used. To examine the effect of chamber modeling, a highly detailed model of an actual CTDI₁₀₀ pencil chamber (model 10×5−3CT, Radcal Corporation) was first defined. Five additional models with reduced detail were then created. They differed from the first model in terms of their dimensions, component parts, material composition, and the use of quadric or voxel geometry. Two models of a TG111 thimble chamber were also created, representing a highly detailed and a rather crude rendition of an actual thimble chamber (model 10×0.6−3CT, Radcal Corporation). Single axial scans for CTDI₁₀₀ chamber models and helical scans for TG111 chamber models were simulated free‐in‐air at 70 and 120 kVp. Simulated dose responses were then compared amongst different chamber models.

Results:

For the six models of the CTDI₁₀₀ pencil chamber, the coefficient of variation of the simulated CTDI₁₀₀ was 0.9% at 70 kVp and 1.0% at 120 kVp. For the two models of the TG111 thimble chamber, the difference in the simulated equilibrium‐dose‐pitch product was 1.9% at 70 kVp and 2.1% at 120 kVp.

Conclusion:

In Monte Carlo simulations of CT ion chambers, detailed modeling of chamber geometry and material composition is not necessary.

This research is supported in part by a Faculty Startup Fund from Cleveland State University.
TH‐EF‐BRA‐11: Investigation of Effects of Adipose Tissue Shielding On Organ Doses and Equation Modeling in Post‐Mortem Subjects for CT
Authors: Lamoureux R , Lipnharski I , Carranza C , Mench A , Sinclair L , Cormack B , Bidari S , Rill L , Arreola M
Published: June 2015

Purpose:

To determine the quantitative effect of adipose tissue shielding on organ dose measurements in computed tomography (CT) and model the effects with organ dose equations.

Methods:

The post‐mortem dose measurement methodology established in house was utilized to perform organ dose measurements on subjects of varying body habitus for a clinically standardized chest/abdomen/pelvis (CAP) protocol and chest protocol on a 320‐slice CT scanner. The outer effective diameter was calculated obtaining the anterior/posterior and lateral dimensions of the entire anatomy imaged at the middle of the scan range, while the inner effective diameter used the AP and Lat dimensions of the anatomy excluding adipose tissue. These parameters determined effective diameter using Effective Diameter= SQRT(AP *Lat). The subjects were matched by their inner effective diameter and the effective shielding radius was calculated by subtracting the inner effective diameter from the outer effective diameter and dividing by two. The relationship to organ specific CTDI‐to‐organ dose conversion factors was analyzed using linear regression analysis.

Results:

Of 7 subjects scanned with both the CAP and chest protocols, three were found to have the same inner effective diameters. The calculated CTDI‐to‐organ‐dose conversion factors relationship with the effective internal shielding radius was analyzed using a linear regression analysis. The R2 value for organs within the inner effective diameter space to include lungs, liver, stomach, small intestine, colon, uterus, and ovary ranged from 0.74 to 0.99, with an average of 0.91 for the CAP equations. While the R2 values for the chest scans for lungs, liver, and stomach ranged from 0.98 to 0.9991 with an average of 0.99, showing a good linear fit.

Conclusion:

The effective internal shielding radius shows a strong correlation when matching similar inner effective diameters to the CTDI‐to‐organ‐dose conversion factors.