bluuebunny/crossref_metadata_2025 huggingface dataset:
WE‐H‐BRA‐06: Experimental Investigation of RBE for Lung Cancer Cell Lines as a Function of Dose and LET in Proton, Helium and Carbon Beams
Authors: Patel D , Bronk L , Guan F , Peeler C , Mirkovic D , Grosshans D , Jakel O , Abdollahi A , Titt U , Mohan R
Published: July 2016

Purpose:

Investigate and quantify the effect of dose and LET on the RBE of protons, helium and carbon ions.

Methods:

High throughput, high accuracy experimental setups were custom designed to investigate the Relative Biological Effectiveness (RBE) dependence on the dose and Linear Energy Transfer (LET) values for proton, helium and carbon ion beams. The experiment was conducted at the HIT facility in collaboration with the DKFZ in Heidelberg/Germany. Clonogenic assays of two human lung cancer cell lines, H460 and H1437, were investigated in this study. γH2AX foci staining on the H460 cell line was also undertaken to facilitate the study of differential DNA double‐strand break induction and repair between low‐design available at the HIT facility. Specific points along the Bragg curve corresponding to well‐defined doses and LET values were chosen by appropriate selection of the pre‐absorber thicknesses. With a setup design for horizontal beam lines we were able to minimize ion scattering in the cell plate, resulting in narrower energy spectra and hence LET distributions in the Bragg peak and in the distal falloff regions, compared to the earlier experiments.

Results:

Approximately 16,000 samples of cancer cells were irradiated during 23 hours of beam time. The preliminary results of the survival curves for both cell lines show a distinct dependence on LET for a given dose with decreased survival fractions at increasing LET values, encountered at the Bragg peak and in the distal falloff.

Conclusion:

Our preliminary findings are indicative of the importance of novel variable‐RBE models for proton therapy and provide insight into the RBE of heavy ions for possible future heavy ion therapy facilities in the US.

Funding support: SINF 2015/16
WE‐H‐202‐00: Session in Memory of Jean Pouliot: Next‐Generation Deformable Image Registration
Authors: Hugo Geoffrey
Published: July 2016

Deformable image registration has now been commercially available for several years, with solid performance in a number of sites and for several applications including contour and dose mapping. However, more complex applications have arisen, such as assessing response to radiation therapy over time, registering images pre‐ and post‐surgery, and auto‐segmentation from atlases. These applications require innovative registration algorithms to achieve accurate alignment. The goal of this session is to highlight emerging registration technology and these new applications. The state of the art in image registration will be presented from an engineering perspective. Translational clinical applications will also be discussed to tie these new registration approaches together with imaging and radiation therapy applications in specific diseases such as cervical and lung cancers.

Learning Objectives:

To understand developing techniques and algorithms in deformable image registration that are likely to translate into clinical tools in the near future.

To understand emerging imaging and radiation therapy clinical applications that require such new registration algorithms.

Research supported in part by the National Institutes of Health under award numbers P01CA059827, R01CA166119, and R01CA166703. Disclosures: Phillips Medical systems (Hugo), Roger Koch (Christensen) support, Varian Medical Systems (Brock), licensing agreements from Raysearch (Brock) and Varian (Hugo).; K. Brock, Licensing Agreement ‐ RaySearch Laboratories. Research Funding ‐ Varian Medical Systems; G. Hugo, Research grant from National Institutes of Health, award number R01CA166119.; G. Christensen, Research support from NIH grants CA166119 and CA166703 and a gift from Roger Koch. There are no conflicts of interest.
WE‐H‐207B‐00: MRgRT
Authors: Stanescu Teo
Published: July 2016

In recent years, steady progress has been made towards the implementation of MRI in external beam radiation therapy for processes ranging from treatment simulation to in‐room guidance. Novel procedures relying mostly on MR data are currently implemented in the clinic. This session will cover topics such as (a) commissioning and quality control of the MR in‐room imagers and simulators specific to RT, (b) treatment planning requirements, constraints and challenges when dealing with various MR data, (c) quantification of organ motion with an emphasis on treatment delivery guidance, and (d) MR‐driven strategies for adaptive RT workflows. The content of the session was chosen to address both educational and practical key aspects of MR guidance.

Learning Objectives:

Good understanding of MR testing recommended for in‐room MR imaging as well as image data validation for RT chain (e.g. image transfer, filtering for consistency, spatial accuracy, manipulation for task specific);

Familiarity with MR‐based planning procedures: motivation, core workflow requirements, current status, challenges;

Overview of the current methods for the quantification of organ motion;

Discussion on approaches for adaptive treatment planning and delivery.

T. Stanescu ‐ License agreement with Modus Medical Devices to develop a phantom for the quantification of MR image system‐related distortions.; T. Stanescu, N/A
TH‐AB‐201‐04: Clinical Protocol for Reuse of Optically Stimulated Luminescence Detectors
Authors: Graeper G , Pillai S
Published: July 2016

Purpose:

OSLDs made of Al2O3:C have many useful dosimetric characteristics including their ability to be reused. The signal on an OSLD can be removed through heat or light. The objective of this study was to characterize the change in sensitivity associated with annealing OSLDs with light and in doing so define a range through which reuse is viable.

Methods:

Four groups of nanoDot OSLDs were repeatedly irradiated and bleached to create accumulated dose history. Each group's repeated irradiation remained constant at either 50, 100, 200, or 500 cGy. Before both irradiation and bleaching the OSLDs were read out, giving the dose reading and ensuring that ample bleaching had occurred, respectively. New and used OSLDs were compared in several clinical situations to verify accuracy. One final test involved correcting the readout dose based on the loss of sensitivity seen in the accumulated dose data.

Results:

In the first 40 Gy of accumulated dose the sensitivity can be broken into two regions: a region of sensitivity change and a region of no sensitivity change. From 0 cGy to an average of 1080 cGy the sensitivity does not change. After 1080 cGy the sensitivity begins to decrease linearly with an average slope of 0.00456 cGy lost per cGy accumulated after the cutoff point. The slope and cutoff point were used to correct readings in the final test, reducing the error from 6.8% to 3.9%.

Conclusion:

In the region of no sensitivity change OSLDs can be reused without concern to the validity of their result. Readings must be corrected if OSLDs are to be used in the region of sensitivity change, above 1080 cGy. After 40 Gy OSLDs must be retired because the sensitivity change reverses, making linear correction no longer feasible.
TH‐AB‐201‐08: Ion Chamber Dose Measurements ‐ Problems with the Temperature‐Pressure Correction Factor
Authors: Bourgouin A , McEwen M
Published: July 2016

Purpose:

To investigate the behavior of ionization chambers over a wide pressure range.

Methods:

Three cylindrical and two parallel‐plate designs of ion chamber were investigated. The ion chambers were placed in vessel where the pressure was varied from atmospheric (101 kPa) down to 5 kPa. Measurements were made using 60Co and high‐energy electron beams. The pressure was measured to better than 0.1% and multiple data sets were obtained for each chamber at both polarities to investigate pressure cycling and dependency on the sign of the charge collected.

Results:

For all types of chamber, the ionization current, corrected using the standard PTP, showed a similar behaviour. Deviations from the standard theory were generally small for Co‐60 but very significant for electron beams, up to 20 % below P = 10 kPa. The effect was found to be always larger when collecting negative charge, suggesting a dependence on free‐electron collection. The most likely source of such electrons is low‐energy electrons emitted from the electrodes. This signal would be independent of air pressure within the chamber cavity. The data was analyzed to extract this signal and it was found to be a non‐negligible component of the ionization current at atmospheric pressure. In the case of the parallel plate chambers, the effect was approximately 0.25 %. For the cylindrical chambers the effect was larger ‐ up to 1.2 % ‐ and dependent on the chamber type, which would be consistent with electron emission from different wall materials. For the electron beams, the correction factor was dependent on the electron energy and approximately double that observed in 60Co.

Conclusion:

Measurements have indicated significant deviations of the standard pressure correction that are consistent with electron emission from chamber electrodes. This has implications for both primary standard and reference ion chamber‐based dosimetry.
TH‐AB‐201‐09
Authors: Mirzakhanian L , Benmakhlouf H , Seuntjens J
Published: July 2016

Purpose:

To determine the k_(Q_msr,Q)^(f_msr,f_ref) factor, introduced in the small field formalism for five common type chambers used in the calibration of Leksell Gamma‐Knife Perfexion model over a range of different phantom electron densities.

Methods:

Five chamber types including Exradin‐A16, A14SL, A14, A1SL and IBA‐CC04 are modeled in EGSnrc and PENELOPE Monte Carlo codes using the blueprints provided by the manufacturers. The chambers are placed in a previously proposed water‐filled phantom and four 16‐cm diameter spherical phantoms made of liquid water, Solid Water, ABS and polystyrene. Dose to the cavity of the chambers and a small water volume are calculated using EGSnrc/PENELOPE codes. The calculations are performed over a range of phantom electron densities for two chamber orientations. Using the calculated dose‐ratio in reference and machine specific reference field, the k_(Q_msr,Q)^(f_msr,f_ref) factor can be determined.

Results:

When chambers are placed along the symmetry axis of the collimator block (z‐axis), the CC04 requires the smallest correction followed by A1SL and A16. However, when detectors are placed perpendicular to z‐axis, A14SL needs the smallest and A16 the largest correction. Moreover, an increase in the phantom electron density results in a linear increase in the k_(Q_msr,Q)^(f_msr,f_ref). Depending on the chambers, the agreement between this study and a previous study performed varies between 0.05–0.70% for liquid water, 0.07–0.85% for Solid Water and 0.00–0.60% for ABS phantoms. After applying the EGSnrc‐calculated k_(Q_msr,Q)^(f_msr,f_ref) factors for A16 to the previously measured dose‐rates in liquid water, Solid Water and ABS normalized to the dose‐rate measured with TG‐21 protocol and ABS phantom, the dose‐rate ratios are found to be 1.004±0.002, 0.996±0.002 and 0.998±0.002 (3σ) respectively.

Conclusion:

Knowing the electron density of the phantoms, the calculated k_(Q_msr,Q)^(f_msr,f_ref) values in this work will enable users to apply the appropriate correction for their own specific phantom material.

LM acknowledges partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant number: 432290)
TH‐AB‐201‐11: Small‐Field Beam Quality Corrections and Their Clinical Impact for Flattened and Unflattened Fields of the TrueBeam
Authors: Hyun M , Labby Z , DeWerd L
Published: July 2016

Purpose:

To determine small‐field beam quality correction factors for several detectors used with flattened and unflattened beams of the Varian TrueBeam accelerator, and to quantify the clinical impact of these corrections through a treatment planning study.

Methods:

Small‐field beam quality correction factors were measured and simulated in EGSnrc for several microchambers and diodes. The Exradin W1 plastic scintillation detector was used as a reference for the measured results, and its response was validated using alanine measurements traceable to a primary standard. Symmetric, jaw‐defined field sizes ranging from 6 mm to 10 cm were used. Twelve hypofractionated clinical cases were then recalculated and/or reoptimized using beam models that were modified based on the resulting output factors. Three‐dimensional dose distributions and cumulative dose‐volume‐histogram plots were compared between the models. The impact of these measurements on the flattening‐filter‐scatter‐source (FFSS) model, and its subsequent effect on treatment plans, was also investigated.

Results:

The diodes exhibited over‐response in small fields, whereas the microchambers tended to under‐respond as field size decreased. Field‐size‐dependent effects were less significant for the flattening‐filter‐free beams (i.e., corrections were closer to unity) than for flattened fields of the same size. Recalculated and reoptimized plans displayed varying levels of deviation from the original dose distributions, with larger discrepancies at effective jaw settings of 3×3 cm2 or smaller. Dose distributions calculated using uncorrected microchamber output factors and/or modified FFSS showed large discrepancies in the low‐dose regions, which manifested as DVH changes for the organs at risk (OAR).

Conclusion:

Field‐size‐dependent beam quality corrections were measured and simulated for ionization chambers and diodes typically used in small fields. Plans calculated with models based on these measured data showed dosimetric deviations mainly out of field, which affected OAR dose metrics. Accordingly, these beam quality corrections should be accounted for during commissioning to ensure greater treatment planning accuracy.
TH‐AB‐BRB‐02: ET‐Reloaded
Authors: Wu Q.
Published: July 2016

Current state‐of‐the art digital C‐arm medical linear accelerators are capable of delivering radiation treatments with high level of automation, which affords coordinated motions of gantry, couch, and multileaf collimator (MLC) with dose rate modulations. The new machine capacity has shown the potential to bring substantially improved radiation dosimetry and/or delivery efficiency to many challenging diseases. Combining an integrated beam orientation optimization algorithm with automated machine navigation, markedly improved dose conformity has been achieved using 4ρ therapy. Trajectory modulated radiation therapy (TMAT) can be used to deliver highly conformal dose to partial breast or to carve complex dose distribution for therapy involving extended volumes such as total marrow and total lymph node treatment. Dynamic electron arc radiotherapy (DEAR) not only overcomes the deficiencies of conventional electron therapy in dose conformity and homogeneity but also achieves so without patient‐specific shields. The combination of MLC and couch tracking provides improved motion management of thoracic and abdominal tumors. A substantial body of work has been done in these technological advances for clinical translation. The proposed symposium will provide a timely review of these exciting opportunities.

Learning Objectives:

Recognize the potential of using digitally controlled linacs for clinically significant improvements in delivered dose distributions for various treatment sites.

Identify existing approaches to treatment planning, optimization and delivery for treatment techniques utilizing the advanced functions of digital linacs and venues for further development and improvement.

Understand methods for testing and validating delivery system performance.

Identify tools available on current delivery systems for implementation and control for such treatments.

Obtain the update in clinical applications, trials and regulatory approval.

K. Sheng, NIH U19AI067769, NIH R43CA183390, NIH R01CA188300, Varian Medical Systems

V. Yu, Varian Medical Systems, AAPM Summer Undergraduate Fellowship, NSF graduate fellowship

S. Nill, Elekta AB. Cancer Research UK under Programme C33589/A19727, NIHR Biomedical Research Centre at The Royal Marsden and The Institute of Cancer Research.
TH‐CD‐206‐09: Learning‐Based MRI‐CT Prostate Registration Using Spare Patch‐Deformation Dictionary
Authors: Yang X , Jani A , Rossi P , Mao H , Curran W , Liu T
Published: July 2016

Purpose:

To enable MRI‐guided prostate radiotherapy, MRI‐CT deformable registration is required to map the MRI‐defined tumor and key organ contours onto the CT images. Due to the intrinsic differences in grey‐level intensity characteristics between MRI and CT images, the integration of MRI into CT‐based radiotherapy is very challenging. We are developing a learning‐based registration approach to address this technical challenge.

Methods:

We propose to estimate the deformation between MRI and CT images in a patch‐wise fashion by using the sparse representation technique. Specifically, we assume that two image patches should follow the same deformation if their patch‐wise appearance patterns are similar. We first extract a set of key points in the new CT image. Then, for each key point, we adaptively construct a coupled dictionary from the training MRI‐CT images, where each coupled element includes both appearance and deformation of the same image patch. After calculating the sparse coefficients in representing the patch appearance of each key point based on the constructed dictionary, we can predict the deformation for this point by applying the same sparse coefficients to the respective deformations in the dictionary.

Results:

This registration technique was validated with 10 prostate‐cancer patients’ data and its performance was compared with the commonly used free‐form‐deformation‐based registration. Several landmarks in both images were identified to evaluate the accuracy of our approach. Overall, the averaged target registration error of the intensity‐based registration and the proposed method was 3.8±0.4 mm and 1.9±0.3 mm, respectively.

Conclusion:

We have developed a novel prostate MR‐CT registration approach based on patch‐deformation dictionary, demonstrated its clinical feasibility, and validated its accuracy. This technique will either reduce or compensate for the effect of patient‐specific treatment variation measured during the course of radiotherapy, is therefore well‐suited for a number of MRI‐guided adaptive radiotherapy, and potentially enhance prostate radiotherapy treatment outcome.
TH‐CD‐206‐10: Clinical Application of the MIND Demons Algorithm for Symmetric Diffeomorphic Deformable MR‐To‐CT Image Registration in Spinal Interventions
Authors: Reaungamornrat S , De Silva T , Uneri A , Wolinsky J , Khanna A , Kleinszig G , Vogt S , Prince J , Siewerdsen J
Published: July 2016

Purpose.

Accurate intraoperative localization of target anatomy (e.g., tumors and vertebral levels) and critical structures (e.g., nerves and vessels) is essential to safe, effective surgery. Preoperative CT/MR images can be used to identify such vital anatomy in intraoperative images through multimodality deformable registration. We proposed a deformable registration method to align preoperative MR to intraoperative CT using modality‐independent neighborhood descriptors (MIND) and a Huber metric for robust registration.

Methods.

The method, called MIND Demons, estimates time‐dependent diffeomorphisms between two images by optimizing a constrained symmetric energy functional incorporating priors on smoothness, conservation of momentum, geodesic length, invertibility, and closure under composition. Alternating optimization is performed using a regularized Gauss‐Newton (GN) method in a multiresolution scheme. Performance was measured in phantom experiments emulating image‐guided spine‐surgery and in a retrospective clinical study (N=15 patients) evaluated in terms of target registration error (TRE) and diffeomorphic properties of the resulting deformations.

Results.

The MIND Demons method outperformed free‐form deformation (FFD) methods and conventional Demons in phantom experiments (median TRE = 1.5 mm, compared to 10.2 mm for mutual information (MI) based FFD, 2.6 mm for local‐MI FFD, and 5.1 mm for normalized‐MI Demons). The resulting deformations resolved realistic deformation in clinical data with sub‐voxel TRE (<2 mm) in cases of cervical, thoracic, and lumbar spine and preserved topology with sub‐voxel invertibility (0.004 mm) and positive‐determinant nonsingular spatial‐Jacobians.

Conclusion.

A modality‐independent deformable registration method has been developed, incorporating the constrained symmetric energy functional and the Huber metric to yield stable and accurate registration, with computational efficiency improved using the GN optimization. The approach yielded registration accuracy suitable to application in image‐guided spine‐surgery across each region of the spine under realistic modes of deformation.

S. Vogt and G. Kleinszig is with Siemens Healthcare XP.
TH‐CD‐209‐02: Beam Angle Optimization Incorporating Anatomical Heterogeneities for Pencil Beam Scanning Charged‐Particle Therapy
Authors: Toramatsu C , Inaniwa T
Published: July 2016

Purpose:

In charged particle therapy with pencil beam scanning (PBS), localization of the dose in the Bragg peak makes dose distributions sensitive to lateral tissue heterogeneities. Sensitivity of a PBS plan to lateral tissue heterogeneity can be reduced by selecting suitable beam angles. The purpose of this study is to establish a fast and accurate method of beam angle selection for PBS.

Methods:

The lateral tissue heterogeneity surrounding the path of respective pencil beams at a given angle was quantified with the heterogeneity number using effective density data of patients. To shorten the computation time, one dimensional dose calculation and optimization were conducted along the central axis of pencil beams originating from the scanning magnets. Calculation was performed for all fields with possible gantry and couch angles. The angles leading the minimum mean heterogeneity number was selected as the optimal beam angle. Three head and neck (H&N) cancer patients were used to evaluate our method. Dose distributions and their robustness to range and set up errors were evaluated for all tested angles, and their relation to their heterogeneity numbers were investigated.

Results:

Variety of heterogeneity number was clearly shown due to air cavities and bony structures in the patients. The mean value of heterogeneity number in all possible fields varied between 1.2 and 10.6 mm. By selecting a field with a lower mean value of heterogeneity number, target dose coverage and robustness of the fields were improved.

Conclusion:

We developed a computational method to determine a beam angle that leads good target coverage and robustness to setup errors. The developed stand‐alone computation program is simple, fast, accurate and applicable for beam angle selection in PBS therapy.
TH‐CD‐209‐03: Feasibility of CBCT‐Based Proton Dose Calculation Using a Histogram‐Matching Algorithm in Proton Beam Therapy
Authors: Arai K , Kadoya N , Kato T , Endo H , Komori S , Abe Y , Hirose K , Nakamura T , Wada H , Kikuchi Y , Jingu K
Published: July 2016

Purpose:

The aim of this study was to confirm On‐Board Imager cone‐beam computed tomography (CBCT) using a histogram‐matching algorithm as a useful method for proton dose calculation in head and neck radiotherapy.

Methods:

We studied one head and neck phantom and ten patients with head and neck cancer treated using intensity‐modulated radiation therapy (IMRT) and proton beam therapy. We modified Hounsfield unit (HU) values of CBCT (mCBCT) using a histogram‐matching algorithm. In order to evaluate the accuracy of the proton dose calculation, we compared dose differences in dosimetric parameters (Dmean) for clinical target volume (CTV), planning target volume (PTV) and left parotid and proton ranges (PR) between the planning CT (reference) and CBCT or mCBCT, and gamma passing rates of CBCT and mCBCT. To minimize the effect of organ deformation, we also performed image registration.

Results:

For patients, the average differences in Dmean for CTV, PTV, and left parotid between planning CT and CBCT were 1.63 ± 2.34%, 3.30 ± 1.02%, and 5.42 ± 3.06%, respectively. Similarly, the average differences between planning CT and mCBCT were 0.20 ± 0.19%, 0.58 ±0.43%, and 3.53 ±2.40%, respectively. The average differences in PR between planning CT and CBCT or mCBCT of a 50° beam for ten patients were 2.1 ± 2.1 mm and 0.3 ± 0.5 mm, respectively. Similarly, the average differences in PR of a 120° beam were 2.9 ± 2.6 mm and 1.1 ± 0.9 mm, respectively. The average dose and PR differences of mCBCT were smaller than those of CBCT. Additionally, the average gamma passing rates of mCBCT were larger than those of CBCT.

Conclusion:

We evaluated the accuracy of the proton dose calculation in CBCT and mCBCT with the image registration for ten patients. Our results showed that HU modification using a histogram‐matching algorithm could improve the accuracy of the proton dose calculation.
TH‐CD‐209‐04: Fuzzy Robust Optimization in Intensity‐Modulated Proton Therapy Planning to Account for Range and Patient Setup Uncertainties
Authors: An Y , Bues M , Schild S , Liu W
Published: July 2016

Purpose:

We propose to apply a robust optimization model based on fuzzy‐logic constraints in the intensity‐modulated proton therapy (IMPT) planning subject to range and patient setup uncertainties. The purpose is to ensure the plan robustness under uncertainty and obtain the best trade‐off between tumor dose coverage and organ‐at‐risk(OAR) sparing.

Methods:

Two IMPT plans were generated for 3 head‐and‐neck cancer patients: one used the planning target volume(PTV) method; the other used the fuzzy robust optimization method. In the latter method, nine dose distributions were computed ‐ the nominal one and one each for ±3mm setup uncertainties along three cardinal axes and for ±3.5% range uncertainty. For tumors, these nine dose distributions were explicitly controlled by adding hard constraints with adjustable parameters. For OARs, fuzzy constraints that allow the dose to vary within a certain range were used so that the tumor dose distribution was guaranteed by minimum compromise of that of OARs. We rendered this model tractable by converting the fuzzy constraints to linear constraints. The plan quality was evaluated using dose‐volume histogram(DVH) indices such as tumor dose coverage(D95%), homogeneity(D5%‐D95%), plan robustness(DVH band at D95%), and OAR sparing like D1% of brain and D1% of brainstem.

Results:

Our model could yield clinically acceptable plans. The fuzzy‐logic robust optimization method produced IMPT plans with comparable target dose coverage and homogeneity compared to the PTV method(unit: Gy[RBE]; average[min, max])(CTV D95%: 59 [52.7, 63.5] vs 53.5[46.4, 60.1], CTV D5% ‐ D95%: 11.1[5.3, 18.6] vs 14.4[9.2, 21.5]). It also generated more robust plans(CTV DVH band at D95%: 3.8[1.2, 5.6] vs 11.5[6.2, 16.7]). The parameters of tumor constraints could be adjusted to control the tradeoff between tumor coverage and OAR sparing.

Conclusion:

The fuzzy‐logic robust optimization generates superior IMPT with minimum compromise of OAR sparing.

This research was supported by the National Cancer Institute Career Developmental Award K25CA168984, by the Fraternal Order of Eagles Cancer Research Fund Career Development Award, by The Lawrence W. and Marilyn W. Matteson Fund for Cancer Research, by Mayo Arizona State University Seed Grant, and by The Kemper Marley Foundation. eRA Person ID(s) for the Principal Investigator: 11017970 (Research Supported by National Institutes of Health)
TH‐CD‐209‐05: Impact of Spot Size and Spacing On the Quality of Robustly‐Optimized Intensity‐Modulated Proton Therapy Plans for Lung Cancer
Authors: Liu W , Schild S , Chang J , Liao Z , Ding X , Hu Y , Shen J , Korte S , Sahoo N , Wong W , Herman M , Bues M
Published: July 2016

Purpose:

To investigate how spot size and spacing affect plan quality, especially, plan robustness and the impact of interplay effect, of robustly‐optimized intensity‐modulated proton therapy (IMPT) plans for lung cancer.

Methods:

Two robustly‐optimized IMPT plans were created for 10 lung cancer patients: (1) one for a proton beam with in‐air energy dependent large spot size at isocenter (σ: 5–15 mm) and spacing (1.53σ); (2) the other for a proton beam with small spot size (σ: 2–6 mm) and spacing (5 mm). Both plans were generated on the average CTs with internal‐gross‐tumor‐volume density overridden to irradiate internal target volume (ITV). The root‐mean‐square‐dose volume histograms (RVH) measured the sensitivity of the dose to uncertainties, and the areas under RVH curves were used to evaluate plan robustness. Dose evaluation software was developed to model time‐dependent spot delivery to incorporate interplay effect with randomized starting phases of each field per fraction. Patient anatomy voxels were mapped from phase to phase via deformable image registration to score doses. Dose‐volume‐histogram indices including ITV coverage, homogeneity, and organs‐at‐risk (OAR) sparing were compared using Student‐t test.

Results:

Compared to large spots, small spots resulted in significantly better OAR sparing with comparable ITV coverage and homogeneity in the nominal plan. Plan robustness was comparable for ITV and most OARs. With interplay effect considered, significantly better OAR sparing with comparable ITV coverage and homogeneity is observed using smaller spots.

Conclusion:

Robust optimization with smaller spots significantly improves OAR sparing with comparable plan robustness and similar impact of interplay effect compare to larger spots. Small spot size requires the use of larger number of spots, which gives optimizer more freedom to render a plan more robust. The ratio between spot size and spacing was found to be more relevant to determine plan robustness and the impact of interplay effect than spot size alone.

This research was supported by the National Cancer Institute Career Developmental Award K25CA168984, by the Fraternal Order of Eagles Cancer Research Fund Career Development Award, by The Lawrence W. and Marilyn W. Matteson Fund for Cancer Research, by Mayo Arizona State University Seed Grant, and by The Kemper Marley Foundation.
TH‐CD‐207B‐04: Is TTF a True Representation of the Sharpness Property of a Non‐Linear CT System?
Authors: Robins M , Solomon J , Samei E
Published: July 2016

Purpose:

To investigate if the task‐transfer‐function (TTF) accurately models the transfer properties of a CT system for lung nodule imaging.

Methods:

An anthropomorphic lung phantom was imaged using a standard chest protocol on a clinical CT scanner with and without 24 physically inserted synthetic lesions (nominal diameter: 8 – 10 mm). Images were reconstructed using FBP and iterative algorithm (SAFIRE, Siemens Healthcare). 3D TTF was measured using an established technique. Corresponding idealized virtual lesions were blurred with the TTF and superimposed onto lesion‐less phantom images. Images of the physically and virtually inserted lesions were compared in terms of rendition of spatial features (blurriness of the edges), lesion morphology, and lesion volume. Feature rendition was measured in terms root‐mean‐square (RMS) of the frequency power of the native and TTF‐transferred lesion edge transition. Morphology was assessed with the Regional Hausdorff Distance (RHD) using custom written code (MATLAB v2015b). Volumes were measured using a clinical segmentation tool (iNtution, TeraRecon).

Results:

The RMS was less than 0.02 and 0.05 for FBP and IR respectively. Using the nonlinear mixed effect (nlme) package (R, www.r‐project.org), the difference in RHD between virtual and physical lesions was 5% on average. There was less than 1 ± 5% (R2 > 0.97) and 3 ± 4% (R2 > 0.97) difference between the volumes of the physical lesions and the corresponding virtual lesions for FBP and IR respectively. Additionally, was closer concordance for images reconstructed with FBP than iterative reconstruction.

Conclusion:

The TTF was found to accurately model the transfer properties of the CT imaging system on lung lesions for both FBP and iterative reconstruction algorithms. TTF was found to offer slightly better lesion renditions when modeling images reconstructed with FBP versus iterative reconstruction. This methodology will be used in future investigation of more complex imaging tasks such as low contrast detectability of known lesions.
TH‐CD‐207B‐06: Swank Factor of Segmented Scintillators in Multi‐Slice CT Detectors: Pulse Height Spectra and Light Escape
Authors: Howansky A , Peng B , Lubinsky A , Zhao W
Published: July 2016

Purpose:

Pulse height spectra (PHS) have been used to determine the Swank factor of a scintillator by measuring fluctuations in its light output per x‐ray interaction. The Swank factor and x‐ray quantum efficiency of a scintillator define the upper limit to its imaging performance, i.e. DQE(0). The Swank factor below the K‐edge is dominated by optical properties, i.e. variations in light escape efficiency from different depths of interaction, denoted e(z). These variations can be optimized to improve tradeoffs in x‐ray absorption, light yield, and spatial resolution. This work develops a quantitative model for interpreting measured PHS, and estimating e(z) on an absolute scale. The method is used to investigate segmented ceramic GOS scintillators used in multi‐slice CT detectors.

Methods:

PHS of a ceramic GOS plate (1 mm thickness) and segmented GOS array (1.4 mm thick) were measured at 46 keV. Signal and noise propagation through x‐ray conversion gain, light escape, detection by a photomultiplier tube and dynode amplification were modeled using a cascade of stochastic gain stages. PHS were calculated with these expressions and compared to measurements. Light escape parameters were varied until modeled PHS agreed with measurements. The resulting estimates of e(z) were used to calculate PHS without measurement noise to determine the inherent Swank factor.

Results:

The variation in e(z) was 67.2–89.7% in the plate and 40.2–70.8% in the segmented sample, corresponding to conversion gains of 28.6–38.1 keV−1 and 17.1–30.1 keV−1, respectively. The inherent Swank factors of the plate and segmented sample were 0.99 and 0.95, respectively.

Conclusion:

The high light escape efficiency in the ceramic GOS samples yields high Swank factors and DQE(0) in CT applications. The PHS model allows the intrinsic optical properties of scintillators to be deduced from PHS measurements, thus it provides new insights for evaluating the imaging performance of segmented ceramic GOS scintillators.
TH‐CD‐207B‐10: Effect of CT Reconstruction Filter On Measured Hounsfield Values in Lung Nodules
Authors: Little K , Reiser I , Sanchez A , Chung J , MacMahon H , Lu Z
Published: July 2016

Purpose:

Measured Hounsfield numbers in CT are used by radiologists to determine the presence of calcium or fat in lung nodules, either of which suggests a benign diagnosis. However, substantial variations in Hounsfield number may arise due to the use of different reconstruction parameters such as the filter/kernel, leading to measurement inaccuracies. This quality improvement project was developed to demonstrate measurement pitfalls and to identify acceptable conditions for incorporating Hounsfield values as a factor in lung nodule diagnosis.

Methods:

12 mm‐diameter spheres of polyurethane and urethane foam were placed into an anthropomorphic chest phantom, and 10 mm‐diameter tubes with varying iodine concentrations were placed into a 16 cm PMMA cylindrical phantom. Additionally, 11 mm‐diameter PMMA and HDPE spheres were placed in a 10 cm PMMA cylindrical phantom. Phantoms were scanned at 120 kVp using a Siemens Biograph mCT and on a Philips iCT and reconstructed using various reconstruction filters.

Results:

For the Siemens system, both sharp kernels and smooth kernels altered the Hounsfield numbers. Hounsfield numbers varied within a range of 8.9 HU for urethane foam and varied within 58.7 HU for polyurethane. The iodine measurements varied up to 37.9 HU for the lowest concentration. For the Philips system, Hounsfield numbers were relatively consistent but were higher for the “Detail” and “Lung Enhanced” filters, varying by 36.9 HU for PMMA and 15.9 HU for HDPE.

Conclusion:

Reconstruction filters can change the measured Hounsfield numbers of nodular objects, especially with detail‐enhancing (sharpening) filters commonly used in lung imaging. Measured values should only be used for diagnostic decision support with filters that have demonstrated accuracy and consistency. While filter accuracy statements are available from manufacturers, radiologists are likely not aware of the extent of potential variations that can occur in a clinical setting.
TH‐D‐BRC‐02: Panel Member
Authors: Hahn S.
Published: July 2016

Interest in proton therapy has increased dramatically in the past couple of years, especially in the United States. There certainly is an important place for proton therapy in the arsenal of cancer treatments. Its dosimetric advantage and potential for low toxicity makes it the perfect partner for photons and other cancer treatment modalities.

Often there is a belief that the new technology ought to be better but many believe that they should be widely adopted in the clinic only after evidence has shown that they are at least as safe and efficacious as existing technologies, which are often less expensive. This is the case for proton radiotherapy.

This session being both educational and debate will provide a good review of basics and technological advancement as well as a comprehensive clinical update for both proton and photon therapy.

Future technology developments and how they will impact the potential dosimetric advantage of proton therapy will be discussed. Particularly the debate will focus on whether these developments will close or widen the gap between photon and proton therapy.

Learning Objectives:

To review challenges, limitations, and recent and future developments in proton therapy.

To review challenges, limitations, recent and future developments in photon radiotherapy.

To review current clinical trials and challenging clinical cases.

C. Yu, No company or organization that my presentation mentions provided me with financial support. I am the Founder and CEO of Xcision Medical Systems, LLC. However, my presentation will not mention any product or technology developed by Xcision.
TH‐EF‐204‐04: Experience of IMRT and Other Conformal Techniques in Russia
Authors: Krylova T.
Published: July 2016

Joanna E. Cygler, Jan Seuntjens, J. Daniel Bourland, M. Saiful Huq, Josep Puxeu Vaque, Daniel Zucca Aparicio, Tatiana Krylova, Yuri Kirpichev, Eric Ford, Caridad Borras

Stereotactic Radiation Therapy (SRT) utilizes small static and dynamic (IMRT) fields, to successfully treat malignant and benign diseases using techniques such as Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiation Therapy (SBRT). SRT is characterized by sharp dose gradients for individual fields and their resultant dose distributions. For appropriate targets, small field radiotherapy offers improved treatment quality by allowing better sparing of organs at risk while delivering the prescribed target dose. Specialized small field treatment delivery systems, such as robotic‐controlled linear accelerators, gamma radiosurgery units, and dynamic arc linear accelerators may utilize rigid fixation, image guidance, and tumor tracking, to insure precise dose delivery to static or moving targets. However, in addition to great advantages, small field delivery techniques present special technical challenges for dose calibration due to unique geometries and small field sizes not covered by existing reference dosimetry protocols such as AAPM TG‐51 or IAEA TRS 398. In recent years extensive research has been performed to understand small field dosimetry and measurement instrumentation. AAPM, IAEA and ICRU task groups are expected to provide soon recommendations on the dosimetry of small radiation fields. In this symposium we will: 1] discuss the physics, instrumentation, methodologies and challenges for small field radiation dose measurements; 2] review IAEA and ICRU recommendations on prescribing, recording and reporting of small field radiation therapy; 3] discuss selected clinical applications and technical aspects for specialized image‐guided, small field, linear accelerator based treatment techniques such as IMRT and SBRT.

Learning Objectives:

To learn the physics of small fields in contrast to dosimetry of conventional fields

To learn about detectors suitable for small fields

To learn about the role of Monte Carlo simulations in determination of small field output factors

To provide an overview of the IAEA small field dosimetry recommendations

To provide an overview of the content of the ICRU report on Prescribing, Reporting and Recording of Small Field Radiation Therapy.

To learn about special technical considerations in delivering IMRT and SBRT treatments

To appreciate specific challenges of IMRT implementation

J. Seuntjens, Natural Sciences and Engineering Research Council; Canadian Institutes of Health Research
TH‐EF‐BRB‐01: BEST IN PHYSICS (THERAPY): Dosimetric Comparison of 4π and Clinical IMRT for Cortex‐Sparing High‐Grade Glioma Treatment
Authors: Woods K , Karunamuni R , Tran A , Yu V , Nguyen D , Hattangadi‐Gluth J , Sheng K
Published: July 2016

Purpose:

Thinning of the cerebral cortex has been observed in patients treated with fractionated partial brain radiation therapy and may contribute to cognitive decline following treatment. The extent of this thinning is dose‐dependent, and was shown comparable to that of neurodegenerative diseases such as Alzheimer's disease at one year post‐therapy. This study investigates whether 4π radiotherapy can enable better sparing of the cortex and other critical structures when compared to conventional clinical IMRT plans.

Methods:

Clinical cortex‐sparing IMRT plans for 15 high‐grade glioma patients were included in this study. 4π radiotherapy plans were created for each patient with 20 intensity‐modulated non‐coplanar fields selected with a greedy column‐generation optimization. All plans were normalized to deliver 100% of the prescribed dose to 95% of the planning target volume (PTV). The mean and maximum dose to the cerebral cortex and other organs at risk (OARs) were compared for the two plan types, as well as the conformity index (CI), homogeneity index (HI), and 50% dose spillage volume (R50).

Results:

The 4π plans significantly reduced the mean cortex dose by an average of 16% (range 6% to 27%) compared to the clinical plans. The mean dose to every other OAR compared was also reduced by 15% to 43%, with statistically significant reductions to the brainstem, chiasm, eyes, optic nerves, subcortical whit, and hippocampus. The average maximum doses were also reduced for 10/12 OARs. The R50 was significantly reduced with the 4π plans (>14%) and the homogeneity index was significantly improved.

Conclusion:

4π enables significant sparing of the cerebral cortex when treating high‐grade gliomas with fractionated partial brain radiation therapy, potentially reducing the risk of harmful dose‐dependent cortical thinning.

NIH R43CA183390, NIH R01CA188300, Varian Medical Systems
TH‐EF‐BRB‐02: Feasibility of Optimization for Dynamic Trajectory Radiotherapy
Authors: Fix MK , Frei D , Volken W , Terribilini D , Aebersold DM , Manser P
Published: July 2016

Purpose:

Over the last years, volumetric modulated arc therapy (VMAT) has been widely introduced into clinical routine using a coplanar delivery technique. However, VMAT might be improved by including dynamic couch and collimator rotations, leading to dynamic trajectory radiotherapy (DTRT). In this work the feasibility and the potential benefit of DTRT was investigated.

Methods:

A general framework for the optimization was developed using the Eclipse Scripting Research Application Programming Interface (ESRAPI). Based on contoured target and organs at risk (OARs), the structures are extracted using the ESRAPI. Sampling potential beam directions, regularly distributed on a sphere using a Fibanocci‐lattice, the fractional volume‐overlap of each OAR and the target is determined and used to establish dynamic gantry‐couch movements. Then, for each gantry‐couch track the most suitable collimator angle is determined for each control point by optimizing the area between the MLC leaves and the target contour. The resulting dynamic trajectories are used as input to perform the optimization using a research version of the VMAT optimization algorithm and the ESRAPI. The feasibility of this procedure was tested for a clinically motivated head and neck case. Resulting dose distributions for the VMAT plan and for the dynamic trajectory treatment plan were compared based on DVH‐parameters.

Results:

While the DVH for the target is virtually preserved, improvements in maximum dose for the DTRT plan were achieved for all OARs except for the inner‐ear, where maximum dose remains the same. The major improvements in maximum dose were 6.5% of the prescribed dose (66 Gy) for the parotid and 5.5% for the myelon and the eye.

Conclusion:

The result of this work suggests that DTRT has a great potential to reduce dose to OARs with similar target coverage when compared to conventional VMAT treatment plans. This work was supported by Varian Medical Systems.

This work was supported by Varian Medical Systems.
TH‐EF‐BRB‐03: Significant Cord and Esophagus Dose Reduction by 4π Non‐Coplanar Spine Stereotactic Body Radiation Therapy and Stereotactic Radiosurgery
Authors: Yu V , Tran A , Nguyen D , Woods K , Cao M , Kaprealian T , Chin R , Low D , Sheng K
Published: July 2016

Purpose:

To demonstrate significant organ‐at‐risk (OAR) sparing achievable with 4π non‐coplanar radiotherapy on spine SBRT and SRS patients.

Methods:

Twenty‐five stereotactic spine cases previously treated with VMAT (n = 23) or IMRT (n = 2) were included in this study. A computer‐aided‐design model of a Linac with a 3D‐scanned human surface was utilized to determine the feasible beam space throughout the 4π steradian and beam specific source‐to‐target‐distances (STD) required for collision avoidance. 4π radiotherapy plans integrating beam orientation and fluence map optimization were then created using a column‐generation algorithm. Twenty optimal beams were selected for each case. To evaluate the tradeoff between dosimetric benefit and treatment complexity, 4π plans including only isocentrically deliverable beams were also created. Beam angles of all standard and isocentric 4π plans were imported into Eclipse to recalculate the dose using the same calculation engine as the clinical plans for unbiased comparison. OAR and PTV dose statistics for the clinical, standard‐4π, and isocentric‐4π plans were compared.

Results:

Comparing standard‐4π to clinical plans, particularly significant average percent reduction in the [mean, maximum] dose of the cord and esophagus of [41%, 21.7%], and [38.7%, 36.4%] was observed, along with global decrease in all other OAR dose statistics. The average cord volume receiving more than 50% prescription dose was substantially decreased by 76%. In addition, improved PTV coverage was demonstrated with a maximum dose reduction of 0.93% and 1.66% increase in homogeneity index (D95/D5). All isocentric‐4π plans achieved dosimetric performance equivalent to that of the standard‐4π plans with higher delivery complexity.

Conclusion:

4π radiotherapy significantly improves stereotactic spine treatment dosimetry. With the substantial OAR dose sparing, PTV dose escalation is considerably safer. Isocentric‐4π is sufficient to achieve the dosimetric gain. The successful implementation of 4π using an FDA approved planning system paves the way for a prospective clinical trial.

Varian Medical Systems, NIH R43CA183390 and R01CA188300, NSF graduate research fellowship DGE‐1144087