bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐260: Pediatric Total Body Irradiation Calculations and In‐Vivo Dosimetry Using Diodes and OSLD' s
Authors: Chungbin S , Fatyga M
Published: July 2014

Purpose:

To verify that a photon total body irradiation (TBI) calculation method scales properly from adult to pediatric dimensions and to determine TBI in‐vivo dosimetry correction factors for diodes and optically stimulated luminescent dosimeters (OSLD' s).

Methods:

TBI technique used is 400 SAD 18 MV opposed laterals with beam spoiler. Water bags are used to supplement narrower lateral dimensions for patient treatments. To verify that dose calculations scale properly with decreasing dimensions, CAX doses were measured and compared to calculations for different rectangular phantom geometries: (L=length(cm), H=height(cm), d=depth(cm)): L(30)xH(30) (d=3‐25), L(30)xH(12)(d=2–20), L(13)xH(13) (d=5–13), L(30)x(H=10–40) d=15, L(30–150) x H(10) (d=15). In infant geometry, measured off axis “leg” dose (L(30)xH(2.5–10.6), d=7)) was compared to CAX (“body” L(30)xH(10)(d=7) adjacent to “leg”). Entrance and exit doses were measured with surface diodes, diodes with buildup, OSLD' s, as well as ion chambers for comparison. Correction factors ((ion chamber CAX dose)/(in vivo dose)) were calculated for surface diodes, diodes with buildup, OSLD' s, and ion chamber.

Results:

All rectangular phantom measurements agree with calculated within 2.5%. For L(30)xH(30), L(30)xH(12), L(13)xH(13), L(30)x(H=10–40) and L(30–80)xH(10) agreement was within 1%. For the infant geometry, the ratio of leg dose to CAX varies from 0.956 (h=2.5) to 0.995 (h=10.6). The range of in‐vivo dosimetry entrance+exit to CAX dose correction factors varied by dosimeter (diode: 0.883–1.015, surface diode: 1.008–1.214, ion chamber: 0.924–1.084, OSLD: 0.920–1.106).

Conclusion:

TBI calculations scaled properly to pediatric dimensions. In‐vivo dosimetry with various detectors demonstrated similar trends with different magnitudes. OSLD measurements agreed well with ion chamber measurements.
SU‐E‐T‐261: Plan Quality Assurance of VMAT Using Fluence Images Reconstituted From Log‐Files
Authors: Katsuta Y , Shimizu E , Matsunaga K , Majima K
Published: July 2014

Purpose:

A successful VMAT plan delivery includes precise modulations of dose rate, gantry rotational and multi‐leaf collimator (MLC) shapes. One of the main problem in the plan quality assurance is dosimetric errors associated with leaf‐positional errors are difficult to analyze because they vary with MU delivered and leaf number. In this study, we calculated integrated fluence error image (IFEI) from log‐files and evaluated plan quality in the area of all and individual MLC leaves scanned.

Methods:

The log‐file reported the expected and actual position for inner 20 MLC leaves and the dose fraction every 0.25 seconds during prostate VMAT on Elekta Synergy. These data were imported to in‐house software that developed to calculate expected and actual fluence images from the difference of opposing leaf trajectories and dose fraction at each time. The IFEI was obtained by adding all of the absolute value of the difference between expected and actual fluence images corresponding.

Results:

In the area all MLC leaves scanned in the IFEI, the average and root mean square (rms) were 2.5 and 3.6 MU, the area of errors below 10, 5 and 3 MU were 98.5, 86.7 and 68.1 %, the 95 % of area was covered with less than error of 7.1 MU. In the area individual MLC leaves scanned in the IFEI, the average and rms value were 2.1 – 3.0 and 3.1 – 4.0 MU, the area of errors below 10, 5 and 3 MU were 97.6 – 99.5, 81.7 – 89.5 and 51.2 – 72.8 %, the 95 % of area was covered with less than error of 6.6 – 8.2 MU.

Conclusion:

The analysis of the IFEI reconstituted from log‐file was provided detailed information about the delivery in the area of all and individual MLC leaves scanned.
SU‐E‐T‐262: Planning for Proton Pencil Beam Scanning (PBS): Applications of Gradient Optimization for Field Matching
Authors: Lin H , Kirk M , Zhai H , Ding X , Liu H , Hill‐Kayser C , Lustig R , Tochner Z , Deville C , Vapiwala N , McDonough J , Both S
Published: July 2014

Purpose:

To propose the gradient optimization(GO) approach in planning for matching proton PBS fields and present two commonly used applications in our institution.

Methods:

GO is employed for PBS field matching in the scenarios that when the size of the target is beyond the field size limit of the beam delivery system or matching is required for beams from different angles to either improve the sparing of important organs or to pass through a short and simple beam path. Overlap is designed between adjacent fields and in the overlapped junction, the dose was optimized such that it gradually decreases in one field and the decrease is compensated by increase from another field. Clinical applications of this approach on craniospinal irradiation(CSI) and whole pelvis treatment were presented. Mathematical model was developed to study the relationships between dose errors, setup errors and junction lengths.

Results:

Uniform and conformal dose coverage to the entire target volumes was achieved for both applications using GO approach. For CSI, the gradient matching (6.7cm junction) between fields overcame the complexity of planning associated with feathering match lines. A slow dose gradient in the junction area significantly reduced the sensitivity of the treatment to setup errors. For whole pelvis, gradient matching (4cm junction) between posterior fields for superior target and bilateral fields for inferior target provided dose sparing to organs such as bowel, bladder and rectum. For a setup error of 3 mm in longitudinal direction from one field, mathematical model predicted dose errors of 10%, 6% and 4.3% for junction length of 3, 5 and 7cm.

Conclusion:

This GO approach improves the quality of the PBS treatment plan with matching fields while maintaining the safety of treatment delivery relative to potential misalignments.
SU‐E‐T‐264: Preliminary Results On New Optically Stimulated Luminescent Materials for Proton Therapy Dosimetry
Authors: Doull B , Zheng Y , Yukihara E
Published: July 2014

Purpose:

The objective of this work is to test the premise that luminescence materials with less under‐response to proton beams can be identified by testing their dose response to low‐LET radiation. The goal is to develop new Optically Stimulated Luminescence (OSL) materials with improved response for proton therapy dosimetry.

Methods:

We first measured the dose response of new OSL materials, synthesized in our laboratory, to low‐LET radiation (beta rays from a 9 0Sr/9 0Y source) and selected two materials having different OSL saturation characteristics and good dosimetric properties, namely MgB4O7:Ce,Li and MgO:Li. Commercial Al2O3:C was also used for comparison. These materials were then irradiated at several depths along a pristine proton beam. The luminescence responses of the materials, relative to the entrance response, were compared with the depth dose profile measured by a multiple‐layer ion chamber.

Results:

The OSL signals of MgB4O7:Ce,Li and MgO:Li were characterized for signal stability, dose response, and response to a clinical proton beam. The materials were also compared with the commercial Al2O3:C. The signals from both MgB4O7:Ce,Li and MgO:Li were relatively stable after a one day delay following irradiation. The low‐LET dose response of the materials showed that, over the dose range investigated (up to ∼800 Gy), MgB4O7:Ce,Li did not saturate, whereas MgO:Li and Al2O3:C saturated at doses of ∼100 Gy. MgB4O7:Ce,Li showed less underresponse to proton beams than MgO:Li and Al2O3:C.

Conclusion:

In general the material with the highest saturation doses for low‐LET radiation (MgB4O7:Ce,Li) showed the least under‐response to proton beams, which suggests that it may be possible to develop better OSL materials for proton dosimetry if the dose response can be controlled during synthesis. Nevertheless, the degree in which the response to proton beams can be controlled remains to be determined.

The research is funded by the Oklahoma Center for the Advancement of Science and Technology (OCAST), project number HR12‐055.
SU‐E‐T‐265: Presage Thin Sheet Dosimeter Characterization
Authors: Dumas M , Rakowski J
Published: July 2014

Purpose:

To quantify the sensitivity and stability of the Presage dosimeter in sheet form for different concentrations of chemicals and for a diverse range of clinical photon energies.

Methods:

Presage polymer dosimeters are formulated to investigate and optimize their sensitivity and stability. The dosimeter is composed of clear polyurethane base, leucomalachite green reporting dye, and bromoform radical initiator in 1mm thick sheets. The chemicals are well mixed together, cast in an aluminum mold, and left to cure at 60 psi for a minimum of 2 days. Dosimeter response will be characterized at multiple energies including Co‐60, 6 MV, 15 MV, 50 kVp, and 250 kVp. The dosimeters are read by an Epson 10000 XL scanner at 800 dpi, 2^16 bit depth. Red component images are analyzed with ImageJ.

Results:

Analysis of optical density verse dose for Co‐60 energies indicates that the bromoform containing Presage was able to quantify dose from 0 to 300 Gy, with saturation beyond 300 Gy. Initial results show two regions of linear response, 0–100 Gy and 150–300 Gy. The 150–300 Gy region has a sensitivity of 0.0024 net OD/Gy. Further results on other energies are still in progress.

Conclusions:

This work shows the potential for use of thin sheets of Presage dosimeter as a dosimeter capable of being analyzed with a flatbed scanner.
SU‐E‐T‐266: Proton PBS Plan Design and Robustness Evaluation for Head and Neck Cancers
Authors: Liang X , Tang S , Zhai H , Kirk M , Kalbasi A , Lin A , Ahn P , Tochner Z , McDonough J , Both S
Published: July 2014

Purpose:

To describe a newly designed proton pencil beam scanning (PBS) planning technique for radiotherapy of patients with bilateral oropharyngeal cancer, and to assess plan robustness.

Methods:

We treated 10 patients with proton PBS plans using 2 posterior oblique field (2F PBS) comprised of 80% single‐field uniform dose (SFUD) and 20% intensity‐modulated proton therapy (IMPT). All patients underwent weekly CT scans for verification. Using dosimetric indicators for both targets and organs at risk (OARs), we quantitatively compared initial plans and verification plans using student t‐tests. We created a second proton PBS plan for each patient using 2 posterior oblique plus 1 anterior field comprised of 100% SFUD (3F PBS). We assessed plan robustness for both proton plan groups, as well as a photon volumetric modulated arc therapy (VMAT) plan group by comparing initial and verification plans.

Results:

The 2F PBS plans were not robust in target coverage. D98% for clinical target volume (CTV) degraded from 100% to 96% on average, with maximum change Δ D98% of −24%. Two patients were moved to photon VMAT treatment due to insufficient CTV coverage on verification plans. Plan robustness was especially weak in the low‐anterior neck. The 3F PBS plans, however, demonstrated robust target coverage, which was comparable to the VMAT photon plan group. Doses to oral cavity were lower in the Proton PBS plans compared to photon VMAT plans due to no lower exit dose to the oral cavity.

Conclusion:

Proton PBS plans using 2 posterior oblique fields were not robust for CTV coverage, due to variable positioning of redundant soft tissue in the posterior neck. We designed 3‐field proton PBS plans using an anterior field to avoid long heterogeneous paths in the low neck. These 3‐field proton PBS plans had significantly improved plan robustness, and the robustness is comparable to VMAT photon plans.
SU‐E‐T‐269: Quality Assurance of Spine Volumetric Modulated Arc Therapy with Flattening Filter Free Beams Using Gafchromic EBT3 Films
Authors: Choi Y , Cho B , Kwak J , Ahn S , Choi E , Kim J , Song S , Yoon S , Kim Y , Kim S , Park J
Published: July 2014

Purpose:

We implemented the Gafchromic film‐based patient specific QA of volumetric modulated arc therapy (VMAT) with flattening‐filter free (FFF) beams for spine metastases and validated the accuracy of fast arc delivery.

Methods:

EBT3 films and a homemade cylindrical QA phantom were employed for dosimetric verification of VMATs. For 14 FFF VMAT plans (10 with 10‐MV FFF beams and 4 with 6‐MV FFF beams), the doses were recalculated on the phantom and delivered by a TrueBeam STx accelerator equipped with a high‐definition 120 leaf MLC. The EBT3 films were scanned using an Epson 10000XL scanner through the FilmQA Pro software. All the irradiated film images were converted to dose map using a calibration response curve. The resulting dose map of film measurement was compared with treatment plan and evaluated using gamma analysis with dose tolerance of 2% within 2 mm. In addition, the point‐dose measurement in the phantom using an ion chamber was evaluated as a reference in a ratio of measured and planned doses.

Results:

The gamma pass rates averaged over all FFF plans for composite‐field measurements were 96.0 ± 3.6% (88.9%–99.5%). When adopting a tolerance level of 3% – 3 mm, the gamma pass rates were improved with the ranges from 98% to 100%. In addition, dose profiles and dose distributions showed that spinal cord was protected by the rapid dose fall‐off and by delivering the treatment with high precision. In point‐dose measurements, the average differences between the measured and planned doses were 0.5% ± 1.0% of the prescription dose.

Conclusion:

We demonstrated that Gafchromic EBT3 film would be an effective patient‐specific QA tool, especially for VMAT of spine SBRT with treatment of small fields and highly gradient dose distributions. The results of film QA verified that the dosimetric accuracy of spine SBRT utilizing RapidArc with FFF beams in our institution is reliable.
SU‐E‐T‐275: Radiobiological Evaluation of Intensity Modulated Radiotherapy Treatment for Locally Advanced Head and Neck Squamous Cell Carcinomas
Authors: Rekha Reddy B. , Ravikumar M. , Tanvir Pasha C.R , Anil Kumar M.R , Pyakuryal A , Varatharaj C. , Narayanasamy Ganesh
Published: July 2014

Purpose:

To evaluate the radiobiological outcome of Intensity Modulated Radiotherapy Treatment (IMRT) for locally advanced head and neck squamous cell carcinomas using HART (Histogram Analysis in Radiation Therapy; J Appl Clin Med Phys 11(1): 137–157, 2010) program and compare with the clinical outcomes.

Methods:

We have treated 20 patients of stage III and IV HNSCC Oropharynx and hypopharynx with accelerated IMRT technique and concurrent chemotherapy. Delineation of tumor and normal tissues were done using Danish Head and Neck Cancer Group (DAHANCA) contouring guidelines and radiotherapy was delivered to a dose of 70Gy in 35 fractions to the primary and involved lymph nodes, 63Gy to intermediate risk areas and 56 Gy to lower risk areas, Monday to Saturday, 6 Days/week using 6 MV Photons with an expected overall treatment time of 6 weeks. The TCP and NTCP's were calculated from the dose‐volume histogram (DVH) statistics using the Poisson Statistics (PS) and JT Lyman models respectively and the

Resultwas correlated with clinical outcomes of the patients with mean follow up of 24 months. Results:

Using HART program, the TCP (0.89± 0.01) of primary tumor and the NTCP for parotids (0.20±0.12), spinal cord (0.05±0.01), esophagus (0.30±0.2), mandible (0.35±0.21), Oral cavity (0.37±0.18), Larynx (0.30±0.15) were estimated and correlated with clinical outcome of the patients.

Conclusion:

Accelerated IMRT with Chemotherapy is a clinical feasible option in the treatment of locally advanced HNSCC with encouraging initial tumour response and acceptable acute toxicities. The correlation between the clinical outcomes and radiobiological model estimated parameters using HART programs are found to be satisfactory.
SU‐E‐T‐277: Raystation Electron Monte Carlo Commissioning and Clinical Implementation
Authors: Allen C , Sansourekidou P , Pavord D
Published: July 2014

Purpose:

To evaluate the Raystation v4.0 Electron Monte Carlo algorithm for an Elekta Infinity linear accelerator and commission for clinical use.

Methods:

A total of 199 tests were performed (75 Export and Documentation, 20 PDD, 30 Profiles, 4 Obliquity, 10 Inhomogeneity, 55 MU Accuracy, and 5 Grid and Particle History). Export and documentation tests were performed with respect to MOSAIQ (Elekta AB) and RadCalc (Lifeline Software Inc). Mechanical jaw parameters and cutout magnifications were verified. PDD and profiles for open cones and cutouts were extracted and compared with water tank measurements. Obliquity and inhomogeneity for bone and air calculations were compared to film dosimetry. MU calculations for open cones and cutouts were performed and compared to both RadCalc and simple hand calculations. Grid size and particle histories were evaluated per energy for statistical uncertainty performance. Acceptability was categorized as follows: performs as expected, negligible impact on workflow, marginal impact, critical impact or safety concern, and catastrophic impact of safety concern.

Results:

Overall results are: 88.8% perform as expected, 10.2% negligible, 2.0% marginal, 0% critical and 0% catastrophic. Results per test category are as follows: Export and Documentation: 100% perform as expected, PDD: 100% perform as expected, Profiles: 66.7% perform as expected, 33.3% negligible, Obliquity: 100% marginal, Inhomogeneity 50% perform as expected, 50% negligible, MU Accuracy: 100% perform as expected, Grid and particle histories: 100% negligible. To achieve distributions with satisfactory smoothness level, 5,000,000 particle histories were used. Calculation time was approximately 1 hour.

Conclusion:

Raystation electron Monte Carlo is acceptable for clinical use. All of the issues encountered have acceptable workarounds. Known issues were reported to Raysearch and will be resolved in upcoming releases.
SU‐E‐T‐284: Revisiting Reference Dosimetry for the Model S700 Axxent 50 KVp Electronic Brachytherapy Source
Authors: Hiatt JR , Rivard MJ
Published: July 2014

Purpose:

The model S700 Axxent electronic brachytherapy source by Xoft was characterized in 2006 by Rivard et al. The source design was modified in 2006 to include a plastic centering insert at the source tip to more accurately position the anode. The objectives of the current study were to establish an accurate Monte Carlo source model for simulation purposes, to dosimetrically characterize the new source and obtain its TG‐43 brachytherapy dosimetry parameters, and to determine dose differences between the source with and without the centering insert.

Methods:

Design information from dissected sources and vendor‐supplied CAD drawings were used to devise the source model for radiation transport simulations of dose distributions in a water phantom. Collision kerma was estimated as a function of radial distance, r, and polar angle, θ, for determination of reference TG‐43 dosimetry parameters. Simulations were run for 1010 histories, resulting in statistical uncertainties on the transverse plane of 0.03% at r=1 cm and 0.08% at r=10 cm.

Results:

The dose rate distribution the transverse plane did not change beyond 2% between the 2006 model and the current study. While differences exceeding 15% were observed near the source distal tip, these diminished to within 2% for r>1.5 cm. Differences exceeding a factor of two were observed near θ=150° and in contact with the source, but diminished to within 20% at r=10 cm.

Conclusions:

Changes in source design influenced the overall dose rate and distribution by more than 2% over a third of the available solid angle external from the source. For clinical applications using balloons or applicators with tissue located within 5 cm from the source, dose differences exceeding 2% were observed only for θ>110°. This study carefully examined the current source geometry and presents a modern reference TG‐43 dosimetry dataset for the model S700 source.
SU‐E‐T‐285: Revisiting the Nomogram for Intra‐Operative Planning Based Pd‐103 Brachytherapy
Authors: Narayanan S , Cho P
Published: July 2014

Purpose:

The seed implant technique at our institution involves using a published nomogram for seed ordering based on CT based volume studies of the prostate. Ultrasound volume studies are subsequently used in the operating room for planning a modified peripheral loading with urethra sparing seed implant. The purpose of this study is to determine the appropriate modality for prostate volume measurement and creating an updated nomogram for intra‐operative planning specific to our technique for pd‐103 brachytherapy for efficient seed ordering.

Methods:

Prostate volumes based on pre‐implant CT (Pre‐CT), intra‐operative ultrasound (TRUS), and post‐implant CT (post‐CT) studies as well as the total airkerma strength (AKS) of the implants were analyzed for 135 seed implant cases (69 monotherapy, 66 boost). Regression analysis was performed to derive the relationship between the total AKS and pre‐CT and TRUS volumes. The correlation between TRUS and pre‐CT volumes and TRUS and post‐CT volumes were also studied.

Results:

Ultrasound based prostate volume exhibited a stronger correlation with total AKS than the pre‐implant CT volume (R2 = 0.97 vs 0.88 for monotherapy and 0.96 vs 0.89 for boost). In general the pre‐CT overestimated the prostate volume leading to ordering of a larger number of seeds and thus leading to higher number of unused/wasted seeds. Newly derived TRUS based nomogram was better suited for our technique than the published data. The post‐implant CT volume closely followed the ultrasound volume (R2 = 0.88) as compared to pre‐implant CT volumes (R2 = 0.57).

Conclusion:

In an era of costconscious health care where waste reduction is of utmost importance, an updated technique‐specific nomogram is useful for ordering optimal number of seeds resulting in significant cost savings. In addition, our study shows that ultrasound based prostate volume is a better predictor for seed ordering for intra‐operative planning than pre‐implant CT.
SU‐E‐T‐286: Robustness of One‐ Versus Two‐Isocenter SBRT for Two Lung Lesions with Rotational Setup Errors
Authors: Palefsky S , Roper J , Dhabaan A
Published: July 2014

Purpose:

Evaluate the robustness of two different planning techniques for SBRT patients with two lung lesions: with both PTVs treated simultaneously with one isocenter, or separately with an isocenter for each PTV.

Methods:

One‐ and two‐isocenter IMRT and VMAT SBRT plans were created in Varian Eclipse for five patients who each had two lung lesions. PTV coverage for the one‐ and two‐isocenter plans was similar when possible. Rotational setup errors were simulated using couch rotations of various angles, and the resulting dose distributions were calculated.

Results:

With a 2° setup error, the average decrease in PTV volume receiving at least the prescribed dose (V100%) was 1.66% for the oneisocenter plans, and 0.05% for the two‐isocenter plans. The changes in V100% ranged from +0.33% to 4.06% for one‐isocenter plans, and from +0.23% to −0.30% for two‐isocenter plans. With a 5° setup error, the average decrease in V100% was 6.46% for a one‐isocenter plan (range: −0.19% to −13.18%), and 0.42% for a two‐isocenter plan (range: +0.21% to −1.33%). For one‐isocenter plans, the decrease in V100% tended to increase as PTV separation increased. The three patients with PTV separation less than 10 cm each showed a decrease in V100% of less than 1% for a 2° error, while the two patients with larger PTV separations showed decreases of more than 3%.

Conclusion:

As could be predicted by geometric calculations, when a rotational setup error occurs, the PTV coverage decreased more for a one‐isocenter plan than for a two‐isocenter plan. These findings suggest that, in general, for lung SBRT patients with two lesions, the two‐isocenter technique may be preferable. However, they may also suggest that if the PTVs are separated by less than 10 cm, and there is high confidence that setup errors are small (≤ 2°), a one‐isocenter plan may deliver similar coverage.
SU‐E‐T‐287: Robustness Study of Passive‐Scattering Proton Therapy in Lung: Is Range and Setup Uncertainty Calculation On the Initial CT Enough to Predict the Plan Robustness?
Authors: Ding X , Dormer J , Kenton O , Liu H , Simone C , Solberg T , Lin L
Published: July 2014

Purpose:

Plan robustness of the passive‐scattering proton therapy treatment of lung tumors has been studied previously using combined uncertainties of 3.5% in CT number and 3 mm geometric shifts. In this study, we investigate whether this method is sufficient to predict proton plan robustness by comparing to plans performed on weekly verification CT scans.

Methods:

Ten lung cancer patients treated with passive‐scattering proton therapy were randomly selected. All plans were prescribed 6660cGy in 37 fractions. Each initial plan was calculated using +/− 3.5% range and +/− 0.3cm setup uncertainty in x, y and z directions in Eclipse TPS(Method‐A). Throughout the treatment course, patients received weekly verification CT scans to assess the daily treatment variation(Method‐B). After contours and imaging registrations are verified by the physician, the initial plan with the same beamline and compensator was mapped into the verification CT. Dose volume histograms (DVH) were evaluated for robustness study.

Results:

Differences are observed between method A and B in terms of iCTV coverage and lung dose. Method‐A shows all the iCTV D95 are within +/− 1% difference, while 20% of cases fall outside +/−1% range in Method‐B. In the worst case scenario(WCS), the iCTV D95 is reduced by 2.5%. All lung V5 and V20 are within +/−5% in Method‐A while 15% of V5 and 10% of V20 fall outside of +/−5% in Method‐B. In the WCS, Lung V5 increased by 15% and V20 increased by 9%. Method A and B show good agreement with regard to cord maximum and Esophagus mean dose.

Conclusion:

This study suggests that using range and setup uncertainty calculation (+/−3.5% and +/−3mm) may not be sufficient to predict the WCS. In the absence of regular verification scans, expanding the conventional uncertainty parameters(e.g., to +/−3.5% and +/−4mm) may be needed to better reflect plan actual robustness.
SU‐E‐T‐289: Scintillating Fiber Based In‐Vivo Dose Monitoring System to the Rectum in Proton Therapy of Prostate Cancer: A Geant4 Monte Carlo Simulation
Authors: Tesfamicael B , Gueye P , Mahesh M , Avery S , Lyons D
Published: July 2014

Purpose:

To construct a dose monitoring system based on an endorectal balloon coupled to thin scintillating fibers to study the dose delivered to the rectum during prostate cancer proton therapy

Methods:

The Geant4 Monte Carlo toolkit version 9.6p02 was used to simulate prostate cancer proton therapy treatments of an endorectal balloon (for immobilization of a 2.9 cm diameter prostate gland) and a set of 34 scintillating fibers symmetrically placed around the balloon and perpendicular to the proton beam direction (for dosimetry measurements)

Results:

A linear response of the fibers to the dose delivered was observed within <2%, a property that makes them good candidates for real time dosimetry. Results obtained show that the closest fiber recorded about 1/3 of the dose to the target with a 1/r2 decrease in the dose distribution as one goes toward the frontal and distal top fibers. Very low dose was recorded by the bottom fibers (about 45 times comparatively), which is a clear indication that the overall volume of the rectal wall that is exposed to a higher dose is relatively minimized. Further analysis indicated a simple scaling relationship between the dose to the prostate and the dose to the top fibers (a linear fit gave a slope of −0.07±0.07 MeV per treatment Gy)

Conclusion:

Thin (1 mm × 1 mm × 100 cm) long scintillating fibers were found to be ideal for real time in‐vivo dose measurement to the rectum for prostate cancer proton therapy. The linear response of the fibers to the dose delivered makes them good candidates of dosimeters. With thorough calibration and the ability to define a good correlation between the dose to the target and the dose to the fibers, such dosimeters can be used for real time dose verification to the target
SU‐E‐T‐291: Sensitivity of a Simple 2D EPID in Vivo Dosimetry
Authors: Peca S , Brown D
Published: July 2014

Purpose:

As radiotherapy (RT) increases in complexity, so does motivation for in vivo dosimetry (IVD), which may detect errors such as: setup, beam shaping and dose delivered. We have recently developed an easy‐toimplement method for two‐dimensional IVD based on images taken with the electronic portal imaging device (EPID) in cine mode during treatment. The purpose of this work is to characterize its sensitivity to possible RT delivery errors.

Methods:

We introduced a series of modifications to a simple RT field (10×10, 100MU, 300RR, 20cm homogeneous phantom) to simulate errors. These modifications included multi‐leaf collimator (MLC) position, number of MUs, and collimator angle. We quantified the sensitivity to inhomogeneities by inserting variable amounts of solid lung and bone. Finally we delivered realistic fields to an anthropomorphic phantom to estimate sensitivity to gantry angle and setup errors.

Results:

Our EPIDIVD is sensitive to MLC positioning errors of 1mm and 3mm in the closed and open directions respectively, and to 3% MU variations. Sensitivity to collimator angle depends on field shape irregularity; in the case of a 10×10 field, we are sensitive to errors of 0.8°. The sensitivity to inhomogeneities is limited by the nature of MV imaging: approximately 1% signal change is noted when switching 5cm of water to equal amounts of bone or lung. This suggests that the EPID‐IVD is likely not sensitive to small setup or gantry angle errors, as confirmed by anthropomorphic tests.

Conclusion:

We have characterized a simple method of 2D dose reconstruction at isocenter depth inside the patient, which is sensitive to possible RT delivery errors. This method may be useful as a secondary safety check, to prevent large errors from being carried on to following fractions, and to record delivered dose. By using readily available hardware, it is easily implemented and may prove especially useful in centers with limited resources.
SU‐E‐T‐297: Small Field Dosimetry for Superficial Lesions
Authors: Ying J , Casto B , Wang S , Talyor T , Wichman A , Taylor M , Ku L
Published: July 2014

Purpose:

Kilo‐voltage (kV) photons and low megavoltage (MeV) electrons are the most common options for treating small superficial lesions, but they present complex dosimetry. Using a tertiary lead shield may protect the surrounding critical structures. Our goal was to quantitatively evaluate the dosimetric impact resulting from applying tertiary shields on superficial lesions.

Method:

We directly compared the beam characteristics of 80 kV (0.8 mm Al) photon setup abutting the water phantom surface and 6 MeV electron setup at 100 cm SSD. Profiles and depth doses were acquired using a 3D scanning water tank and an ion chamber (active volume 0.01 cm^3). Beam profiles were scanned at Dmax. Three lead sheets (2 mm thickness) with 2.7, 2.2, and 1.6, cm diameter circular cutouts were fabricated and placed at the water surface for both photon and electron fields.

Results:

The penumbra (80% – 20%) of the open 4×4 cm^2 electron insert was 10.7 mm, compared to an average of 7.2 mm with the tertiary cutouts. The penumbra of the open kV photon beam was 2.8 mm compared to an average of 1.8 mm with the tertiary cutouts. For field widths 2.7, 2.2, and 1.6 cm, the flatness of the electron beams was 16%, 17.3%, and 21%, respectively, and for the kV photon beams was 1.4%, 2.3%, 3.3%, respectively. The electron depth dose (PDD) shifted shallower and the photon PDD shifted deeper as the field size became smaller.

Conclusion:

The penumbra of small electron fields can be improved by adding tertiary lead shields. Both modalities are clinically feasible; however, kV photons still offer sharper penumbra and better flatness than that of 6 MeV electrons with tertiary shielding. Thus, kV photons may still be a superior option for small superficial lesions.
SU‐E‐T‐308: Systematic Characterization of the Energy Response of Different LiF TLD Crystals for Dosimetry Applications
Authors: Pena E , Caprile P , Sanchez‐Nieto B
Published: July 2014

Purpose:

The thermoluminiscense dosimeters (TLDs) are widely used in personal and clinical dosimetry due to its small size, good sensitivity and tissue equivalence, among other advantages. This study presents the characterization of Lithium Fluoride based TLDs, in terms of their absorbed dose response to successive irradiation cycles in a broad range of beam energies, measured under reference conditions.

Methods:

Four types of Harshaw TLD chips were used: TLD‐100, TLD‐600 TLD‐700 and 100‐H. They were irradiated with 10 photon beams of different energy spectrums, from 28 kVp to 18MV (in 30 consecutive cycles for 6 and 18 MV).

Results:

It was found that the response of the dosimetric system was stabilized (less than ±3%) after 10 cycles for TLD‐600 and TLD‐700. In the case of TLD‐100 and TLD‐100H this dependence was not observed. A decreased response to increasing beam energy in terms of absorbed dose to water was observed, as expected, except for TLD‐100H which showed the opposite behavior. The less energy dependent detector was the TLD‐100H exhibiting a maximum deviation of 12%. The highest variation observed was 33% for TLD‐100. The study allowed the determination of calibration factors in absorbed dose for a wide range of energies and materials for different dosimetric applications, such as in‐vivo dosimetry during imaging and radiotherapy.

Conclusion:

The study allowed the determination of calibration factors in absorbed dose for a wide range of energies and materials for different dosimetric applications, such as in‐vivo dosimetry during imaging and radiotherapy.
SU‐E‐T‐309: Tangential Modulated Arc Therapy: A Novel Technique for the Treatment of Superficial Disease
Authors: Hadsell M , Chin E , Li R , Xing L , Bush K
Published: July 2014

Purpose:

We propose a new type of treatment that employs a modulated and sliding tangential photon field to provide superior coverage of superficial targets when compared to other commonly employed methods while drastically reducing dose to the underlying sensitive structures often present in these cases.

Methods:

Modulated treatment plans were formulated for a set of three representative cases. The first was a revised treatment of a scalp sarcoma, while the second was a treatment of a right posterior chest wall sarcoma. For these cases, asymmetric jaw placement, angular limitations, and central isocenter placements were used to force the optimization algorithm into finding solutions with beamlines that were not perpendicular to the body surface. The final case targeted the chest wall of a breast cancer patient, in which standard treatments were compared to the use of modulated fields with multiple isocenters along the chest wall.

Results:

When compared with unrestricted modulated arcs, the tangential arc scalp treatment reduced the max and mean doses delivered to the brain by 33Gy (from 55 to 22Gy) and 6Gy (from 14Gy to 8Gy), respectively. In the right posterior chest wall case, the V10 in the ipsilateral lung was kept below 5% while retaining a Rx dose (45Gy) target coverage of over 97%. For the breast case, the modulated plan achieved reductions in high dose to the ipsilateral lung and heart by a factor of 2–3 when compared to classic laterally opposed tangents and reduced the V5 by 40% when compared to standard modulated arcs.

Conclusion:

Tangential modulated arc therapy has outperformed the conventional modalities of treatment for superficial lesions used in our clinic. We hope that with the advent of digitally controlled linear accelerators, we can uncover further benefits of this new technique and extend its applicability to a wider section of the patient population.
SU‐E‐T‐310: Targeting Safety Improvements Through Analysis of Near‐Miss Error Detection Points in An Incident Learning Database
Authors: Novak A , Nyflot M , Sponseller P , Howard J , Logan W , Holland L , Jordan L , Carlson J , Ermoian R , Kane G , Ford E , Zeng J
Published: July 2014

Purpose:

Radiation treatment planning involves a complex workflow that can make safety improvement efforts challenging. This study utilizes an incident reporting system to identify detection points of near‐miss errors, in order to guide our departmental safety improvement efforts. Previous studies have examined where errors arise, but not where they are detected or their patterns.

Methods:

1377 incidents were analyzed from a departmental nearmiss error reporting system from 3/2012–10/2013. All incidents were prospectively reviewed weekly by a multi‐disciplinary team, and assigned a near‐miss severity score ranging from 0–4 reflecting potential harm (no harm to critical). A 98‐step consensus workflow was used to determine origination and detection points of near‐miss errors, categorized into 7 major steps (patient assessment/orders, simulation, contouring/treatment planning, pre‐treatment plan checks, therapist/on‐treatment review, post‐treatment checks, and equipment issues). Categories were compared using ANOVA.

Results:

In the 7‐step workflow, 23% of near‐miss errors were detected within the same step in the workflow, while an additional 37% were detected by the next step in the workflow, and 23% were detected two steps downstream. Errors detected further from origination were more severe (p<.001; Figure 1). The most common source of near‐miss errors was treatment planning/contouring, with 476 near misses (35%). Of those 476, only 72(15%) were found before leaving treatment planning, 213(45%) were found at physics plan checks, and 191(40%) were caught at the therapist pre‐treatment chart review or on portal imaging. Errors that passed through physics plan checks and were detected by therapists were more severe than other errors originating in contouring/treatment planning (1.81 vs 1.33, p<0.001).

Conclusion:

Errors caught by radiation treatment therapists tend to be more severe than errors caught earlier in the workflow, highlighting the importance of safety checks in dosimetry and physics. We are utilizing our findings to improve manual and automated checklists for dosimetry and physics.
SU‐E‐T‐311: Technical Aspects of Credentialing for the NSABP B‐51 / RTOG 1304 Trial
Authors: Lowenstein J , Nguyen H , Hollan A , Galvin J , Followill D
Published: July 2014

Purpose:

For each of this trials 3 treatment scenarios an institution must be credentialed. The purpose of credentialing is to verify all personnel involved with treatment planning have read the protocol and can follow the protocol specifications prior to placing a patient on protocol with the end result of limiting the number of protocol deviations. Credentialing also provides feedback to an institution prior to treatment of a patient on trial to correct any mistakes that may occur.

Methods:

An institution needs to update their Facility Questionnaire and download the 3 CT benchmark cases representing the 3 treatment groups (1A, 2A and 2B) of the study from the RPCs website. The institution may decide which modality (3DCRT/IMRT) to use for each benchmark, however at least one benchmark must be completed for each modality for which the institution wants to be credentialed. The RPC reviews these benchmarks using MIM for verification that dose requirements are per protocol.

Results:

Presently, 101 institutions have initiated the credentialing process. 94 institutions have submitted their benchmarks, 7% failed and never resubmitted a benchmark for re‐review. The breakdown of credentialed techniques is; 4% IMRT only, 52% 3DCRT only and 44% 3DCRT/IMRT. Of those sites that have been credentialed: 6% of the institutions failed the benchmark for 1A, 11% failed for 2B and 40% failed for 2A at least once, before being credentialed. 9% of the institutions failed 2A twice and 1 institution failed three times.

Conclusion:

Currently 87 institutions have been credentialed for this study. The protocol has accrued 9 patients which have all been pre‐treatment reviews where 2 of them had to be re‐submitted for a re‐review due to volume contouring.

This work was supported by PHS grant CA 10953 awarded by NCI, DHHS.
SU‐E‐T‐316: The Design of a Risk Index Method for 3D Patient Specific QA
Authors: Cho W , Xing L , Suh T , Wu H
Published: July 2014

Purpose:

To suggest a new guidance for the evaluation of 3D patient specific QA, a structure‐specific risk‐index (RI) method was designed and implemented.

Methods:

A new algorithm was designed to assign the score of Pass, Fail or Pass with Risk to all 3D voxels in each structure by improving a conventional Gamma Index (GI) algorithm, which implied the degree of the risk of under‐dose to the treatment target or over‐dose to the organ at risks (OAR). Structure‐specific distance to agreement (DTOA), dose difference and minimum checkable dose were applied to the GI algorithm, and additional parameters such as dose gradient factor and dose limit of structures were used to the RI method. Maximum passing rate (PR) and minimum PR were designed and calculated for each structure with the RI method. 3D doses were acquired from a spine SBRT plan by simulating the shift of beam iso‐center, and tested to show the feasibility of the suggested method.

Results:

When the iso‐center was shifted by 1 mm, 2 mm, and 3 mm, the PR of conventional GI method between shifted and non‐shifted 3D doses were 99.9%, 97.4%, and 89.7% for PTV, 99.8%, 84.8%, and 63.2% for spinal cord, and 100%, 99.5%, 91.7% for right lung. The minimum PRs from the RI method were 98.9%, 96.9%, and 89.5% for PTV, and 96.1%, 79.3%, 57.5% for spinal cord, and 92.5%, 92.0%, 84.4% for right lung, respectively. The maximum PRs from the RI method were equal or less than the PRs from the conventional GI evaluation.

Conclusion:

Designed 3D RI method showed more strict acceptance level than the conventional GI method, especially for OARs. The RI method is expected to give the degrees of risks in the delivered doses, as well as the degrees of agreements between calculated 3D doses and measured (or simulated) 3D doses.
SU‐E‐T‐319: The Effect of Slice Thickness On IMRT Planning
Authors: Srivastava S , Cheng C , Das I
Published: July 2014

Purpose:

The accuracy of volume estimated of a treatment planning system is investigated in this study. In addition, the effect of slice thickness on IMRT planning is also studied.

Methods:

The accuracy in volume determination was investigated using a water phantom containing various objects with known volumes ranging from 1–100cm3. The phantom was scanned with different slice thickness (1–10 mm). The CT data sets were sent to Eclipse TPS for contour delineation and volume calculation. The effect of slice thickness on IMRT planning was studied using a commercial phantom containing four different shaped objects. The phantom was scanned with different slice thickness (1–5 mm). IMRT plans were generated for the different CT datasets to calculate TCP, homogeneity (HI) and conformity indices (CI).

Results:

The variability of volumes with CT slice thickness was significant especially for small volume structures. The minimum and maximum error in the volume estimation is in the range of −2.3% to 92%. On the other hand, with increasing slice thickness, the PTV mean dose and TCP values decreases. Maximum variation of ∼5% was observed in mean dose and ∼2% in TCP with slice thickness change from 1–5 mm. The relative decrease in target volume receiving 95% of prescribed dose is ∼5% slice thickness change from 1–5 mm. HI increases up to 163% and CI decreases by 4% between 1–5 mm slice thickness change, producing highly inhomogeneous and least conformal plan.

Conclusion:

Accuracy of volume estimation is dependent on CT slice thickness and the contouring algorithm in a TPS. During TPS commissioning and for all clinical protocols, evaluation of volume should be included to provide the limit of accuracy in DVH calculation. A smaller slice thickness provides superior dosimetry with improved TCP values. Thus, the smallest possible slice thickness should be used for IMRT planning
SU‐E‐T‐320: The Effect of Survivin Perturbation On the Radiation Response of Breast Cancer Cell Lines
Authors: Smith D , Debeb B , Woodward W
Published: July 2014

Purpose:

Survivin is the smallest member of the inhibitor of apoptosis protein family and is well‐known for its universal over‐expression in human cancers. Due to its role in apoptosis and cellular proliferation, survivin is implicated in the radiation response in several cancer types, and antisurvivin treatments have had success as a radiation sensitizer in many preclinical cancer models. As no studies to date have reported survivin as a factor affecting radiation resistance in breast cancer models, we sought to evaluate the synergistic relationship between survivin function and irradiation in breast cancer cell lines.

Methods:

Information regarding survivin protein expression in breast cancer was retrieved from three public databases: Oncomine, Kaplan‐Meier Plotter, and GOBO. For the in vitro studies, survivin function was compromised by transducing a non‐functional mutant form (survivin‐DN) into two breast cancer cell lines, the estrogen receptor‐positive MCF7 and the triple‐negative, inflammatory SUM149. Cell growth was compared in the survivin‐DN and control populations with colony‐formation assays. To assess how survivin affects radiation response, clonogenic assays were performed by irradiating the cell lines up to 6 Gy.

Results:

From the public databases, survivin is more highly expressed in triple‐negative breast cancer compared to all other subtypes, and is prognostic of poor survival in all breast cancer patients. In MCF7, the survivin‐DN population had decreased colony‐formation potential; the opposite was true in SUM149. In the clonogenic assays, abrogation of survivin function radio‐protected MCF7 cells in monolayer and 3D growth conditions, while SUM149 survivin‐DN cells were radiosensitized in monolayer conditions.

Conclusion:

We observed synergy between survivin function and radiation, although the results between the two cell lines were disparate. Further investigation is required to identify the mechanism of this discrepancy, including evaluation of the potential role of estrogen receptor status, before proceeding with small‐animal experiments.
SU‐E‐T‐321: The Effects of a Dynamic Collimation System On Proton Pencil Beams to Improve Lateral Tissue Sparing in Spot Scanned Proton Therapy
Authors: Hill P , Wang D , Flynn R , Hyer D
Published: July 2014

Purpose:

To evaluate the lateral beam penumbra in pencil beam scanning proton therapy delivered using a dynamic collimator device capable of trimming a portion of the primary beam in close proximity to the patient.

Methods:

Monte Carlo simulations of pencil beams were performed using MCNPX. Each simulation transported a 125 MeV proton pencil beam through a range shifter, past acollimator, and into a water phantom. Two parameters were varied among the simulations, the source beam size (sigma in air from 3 to 9 mm), and the position of the edge of the collimator (placed from 0 to 30 mm from the central axis of the beam). Proton flux was tallied at the phantom surface to determine the effective beam sizefor all combinations of source beam size and collimator edge position.

Results:

Quantifying beam size at the phantom surface provides a useful measure tocompare performance among varying source beam sizes and collimation conditions. For arelatively large source beam size (9 mm) entering the range shifter, sigma at thesurface was found to be 10 mm without collimation versus 4 mm with collimation. Additionally, sigma at the surface achievable with collimation was found to be smallerthan for any uncollimated beam, even for very small source beam sizes. Finally, thelateral penumbra achievable with collimation was determined to be largely independentof the source beam size.

Conclusion:

Collimation can significantly reduce proton pencil beam lateral penumbra.Given the known dosimetric disadvantages resulting from large beam spot sizes,employing a dynamic collimation system can significantly improve lateral tissuesparing in spot‐scanned dose distributions.