bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐F‐T‐97: Outlier Identification in Radiation Therapy Knowledge Modeling
Authors: Sheng Y , Ge Y , Yuan L , Li T , Yin F , Wu Q
Published: July 2016

Purpose:

To investigate the impact of outliers on knowledge modeling in radiation therapy, and develop a systematic workflow for identifying and analyzing geometric and dosimetric outliers using pelvic cases.

Methods:

Four groups (G1‐G4) of pelvic plans were included: G1 (37 prostate cases), G2 (37 prostate plus lymph node cases), and G3 (37 prostate bed cases) are all clinical IMRT cases. G4 are 10 plans outside G1 re‐planned with dynamic‐arc to simulate dosimetric outliers. The workflow involves 2 steps: 1. identify geometric outliers, assess impact and clean up; 2. identify dosimetric outliers, assess impact and clean up.1. A baseline model was trained with all G1 cases. G2/G3 cases were then individually added to the baseline model as geometric outliers. The impact on the model was assessed by comparing leverage statistic of inliers (G1) and outliers (G2/G3). Receiver‐operating‐characteristics (ROC) analysis was performed to determine optimal threshold. 2. A separate baseline model was trained with 32 G1 cases. Each G4 case (dosimetric outliers) was then progressively added to perturb this model. DVH predictions were performed using these perturbed models for remaining 5 G1 cases. Normal tissue complication probability (NTCP) calculated from predicted DVH were used to evaluate dosimetric outliers’ impact.

Results:

The leverage of inliers and outliers was significantly different. The Area‐Under‐Curve (AUC) for differentiating G2 from G1 was 0.94 (threshold: 0.22) for bladder; and 0.80 (threshold: 0.10) for rectum. For differentiating G3 from G1, the AUC (threshold) was 0.68 (0.09) for bladder, 0.76 (0.08) for rectum. Significant increase in NTCP started from models with 4 dosimetric outliers for bladder (p<0.05), and with only 1 dosimetric outlier for rectum (p<0.05).

Conclusion:

We established a systematic workflow for identifying and analyzing geometric and dosimetric outliers, and investigated statistical metrics for detecting. Results validated the necessity for outlier detection and clean‐up to enhance model quality in clinical practice.

Research Grant: Varian master research grant
SU‐F‐T‐100: Development and Implementation of a Treatment Planning Tracking System Into the Radiation Oncology Clinic
Authors: Kabat C , Cline K , Li Y , Ha C , Stathakis S
Published: July 2016

Purpose:

With increasing numbers of cancer patients being diagnosed and the complexity of radiotherapy treatments rising it's paramount that patient plan development continues to stay fluid within the clinic. In order to maintain a high standard of care and clinical efficiency the establishment of a tracking system for patient plan development allows healthcare providers to view real time plan progression and drive clinical workflow. In addition, it provides statistical datasets which can further identify inefficiencies within the clinic.

Methods:

An application was developed utilizing Microsoft's ODBC SQL database engine to track patient plan status throughout the treatment planning process while also managing key factors pertaining to the patient's treatment. Pertinent information is accessible to staff in many locations, including tracking monitors within dosimetry, the clinic network for both computers and handheld devices, and through email notifications. Plans are initiated with a CT and continually tracked through planning stages until final approval by staff. Patient's status is dynamically updated by the physicians, dosimetrists, and medical physicists based on the stage of the patient's plan.

Results:

Our application has been running over a six month period with all patients being processed through the system. Modifications have been made to allow for new features to be implemented along with additional tracking parameters. Based on in‐house feedback, the application has been supportive in streamlining patient plans through the treatment planning process and data has been accumulating to further improve procedures within the clinic.

Conclusion:

Over time the clinic will continue to track data with this application. As data accumulates the clinic will be able to highlight inefficiencies within the workflow and adapt accordingly. We will add in new features to help support the treatment planning process in the future.
SU‐F‐T‐116: Predicting IQ and the Risk of Hearing Loss Following Proton Versus Photon Radiotherapy for Pediatric Brain Tumor Patients
Authors: Fortin D , Ng A , Tsang D , Sharpe M , Laperriere N , Hodgson D
Published: July 2016

Purpose:

The increased sparing of normal tissues in intensity modulated proton therapy (IMPT) compared to photon intensity modulated radiotherapy (IMRT) in brain tumor treatments should translate into improved neurocognitive outcomes. Models were used to estimate the intelligence quotient (IQ) and the risk of hearing loss 5 years post radiotherapy and to compare outcomes of proton against photon in pediatric brain tumors.

Methods:

Patients who had received radical IMRT were randomly selected from our retrospective database: 10 cases each of craniopharyngioma, ependymoma and medulloblastoma, and 20 cases of glioma. The existing planning CT and contours were used to generate IMPT plans. The RBE‐corrected dose to brain structures and cochleas were calculated for both IMPT and IMRT. A model was applied to estimate IQ using a Markov chain Monte Carlo technique. The reported incidence of hearing loss as a function of cochlear dose was used to estimate the rate of occurrence.

Results:

The average brain dose was less in all IMPT plans compared to IMRT: ranging from a 6.7% reduction (P=0.003) in the case of medulloblastoma to 38% (P=0.007) for craniopharyngioma. This dose reduction translated into a gain in IQ of 1.9 points on average for protons vs photons for the whole cohort at 5 years post‐treatment (P=0.011). In terms of specific diseases, the gains in IQ ranged from 0.8 points for medulloblastoma, to 2.7 points for craniopharyngioma. Hearing loss probability was evaluated on a per‐ear‐basis and was found to be systematically less for proton versus photon: overall 2.9% versus 7.2% (P < 0.001).

Conclusion:

A novel method was developed to predict neurocognitive outcomes in pediatric brain tumor patients on a case‐by‐case basis. A modest gain in IQ was systematically observed for proton in all patients. Given the uncertainties within the model used and our reinterpretation, these gains may be underestimated.
SU‐F‐T‐129: Impact of Radial Fluctuations in RBE for Therapeutic Proton Beams
Authors: Butkus M , Palmer T
Published: July 2016

Purpose:

To evaluate the off axis relative biological effectiveness (RBE) for actively scanned proton beams and determine if a constant radial RBE can be assumed.

Methods:

The PHITS Monte Carlo code paired with a microscopic analytical function was used to determine probability distribution functions of the lineal energy in 0.3µm diameter spheres throughout a water phantom. Twenty million primary protons were simulated for a 0.6cm diameter pencil beam. Beam energies corresponding to Bragg Peak depths of 50, 100, 150, 200, 250, and 300mm were used and evaluated transversely every millimeter and radially for annuli of 1.0, 2.0, 3.0, 3.2, 3.4, 3.6, 4.0, 5.0, 10.0, 15.0, 20.0 and 25.0mm outer radius. The acquired probability distributions were reduced to dose‐mean lineal energies and applied to the modified microdosimetric kinetic model, for human submandibular gland (HSG) cells, to calculate relative biological effectiveness (RBE) compared to 60Co beams at the 10% survival threshold.

Results:

RBE was generally seen to increase as distance from the central axis (CAX) increased. However, this increase was only seen in low dose regions and its overall effects on the transverse biological dose remains low. In the entrance region of the phantom (10mm depth), minimum and maximum calculated RBEs varied between 15.22 and 18.88% for different energies. At the Bragg peak, this difference ranged from 3.15 to 26.77%. Despite these rather large variations the dose‐weighted RBE and the CAX RBE varied by less than 0.14% at 10mm depth and less than 0.16% at the Bragg peak. Similarly small variations were found at all depths proximal of the Bragg peak.

Conclusion:

Although proton RBE does vary radially, its overall effect on biological dose is minimal and the use of a radially constant RBE in treatment planning for scanned proton beams would not produce large errors.
SU‐F‐T‐142: An Analytical Model to Correct the Aperture Scattered Dose in Clinical Proton Beams
Authors: Sun B , Liu S , Zhang T , Zhao T , Yang D , Grantham K , Goddu S , Bradley J , Mutic S
Published: July 2016

Purpose:

Apertures or collimators are used to laterally shape proton beams in double scattering (DS) delivery and to sharpen the penumbra in pencil beam (PB) delivery. However, aperture‐scattered dose is not included in the current dose calculations of treatment planning system (TPS). The purpose of this study is to provide a method to correct the aperture‐scattered dose based on an analytical model.

Methods:

A DS beam with a non‐divergent aperture was delivered using a single‐room proton machine. Dose profiles were measured with an ion‐chamber scanning in water and a 2‐D ion chamber matrix with solid‐water buildup at various depths. The measured doses were considered as the sum of the non‐contaminated dose and the aperture‐scattered dose. The non‐contaminated dose was calculated by TPS and subtracted from the measured dose. Aperture scattered‐dose was modeled as a 1D Gaussian distribution. For 2‐D fields, to calculate the scatter‐dose from all the edges of aperture, a sum of weighted distance was used in the model based on the distance from calculation point to aperture edge. The gamma index was calculated between the measured and calculated dose with and without scatter correction.

Results:

For a beam with range of 23 cm and aperture size of 20 cm, the contribution of the scatter horn was ∼8% of the total dose at 4 cm depth and diminished to 0 at 15 cm depth. The amplitude of scatter‐dose decreased linearly with the depth increase. The 1D gamma index (2%/2 mm) between the calculated and measured profiles increased from 63% to 98% for 4 cm depth and from 83% to 98% at 13 cm depth. The 2D gamma index (2%/2 mm) at 4 cm depth has improved from 78% to 94%.

Conclusion:

Using the simple analytical method the discrepancy between the measured and calculated dose has significantly improved.
SU‐F‐T‐157: Physics Considerations Regarding Dosimetric Accuracy of Analytical Dose Calculations for Small Field Proton Therapy: A Monte Carlo Study
Authors: Geng C , Daartz J , Cheung K , Bussiere M , Shih H , Paganetti H , Schuemann J
Published: July 2016

Purpose:

To evaluate the accuracy of dose calculations by analytical dose calculation methods (ADC) for small field proton therapy in a gantry based passive scattering facility. Methods: 50 patients with intra‐cranial disease were evaluated in the study. Treatment plans followed standard prescription and optimization procedures of proton stereotactic radiosurgery. Dose distributions calculated with the Monte Carlo (MC) toolkit TOPAS were used to represent delivered treatments. The MC dose was first adjusted using the output factor (OF) applied clinically. This factor is determined from the field size and the prescribed range. We then introduced a normalization factor to measure the difference in mean dose between the delivered dose (MC dose with OF) and the dose calculated by ADC for each beam. The normalization was determined by the mean dose of the center voxels of the target area. We compared delivered dose distributions and those calculated by ADC in terms of dose volume histogram parameters and beam range distributions.

Results:

The mean target dose for a whole treatment is generally within 5% comparing delivered dose (MC dose with OF) and ADC dose. However, the differences can be as great as 11% for shallow and small target treated with a thick range compensator. Applying the normalization factor to the MC dose with OF can reduce the mean dose difference to less than 3%. Considering range uncertainties, the generally applied margins (3.5% of the prescribed range + 1mm) to cover uncertainties in range might not be sufficient to guarantee tumor coverage. The range difference for R90 (90% distal dose falloff) is affected by multiple factors, such as the heterogeneity index.

Conclusion:

This study indicates insufficient accuracy calculating proton doses using ADC. Our results suggest that uncertainties of target doses are reduced using MC techniques, improving the dosimetric accuracy for proton stereotactic radiosurgery.

The work was supported by NIH/NCI under CA U19 021239. CG was partially supported by the Chinese Scholarship Council (CSC) and the National Natural Science Foundation of China (Grant No. 11475087).
SU‐F‐T‐158: Experimental Characterization of Field Size Dependence of Dose and Lateral Beam Profiles of Scanning Proton and Carbon Ion Beams for Empirical Model in Air
Authors: Li Y , Hsi W , Zhao J
Published: July 2016

Purpose:

The Gaussian model for the lateral profiles in air is crucial for an accurate treatment planning system. The field size dependence of dose and the lateral beam profiles of scanning proton and carbon ion beams are due mainly to particles undergoing multiple Coulomb scattering in the beam line components and secondary particles produced by nuclear interactions in the target, both of which depend upon the energy and species of the beam. In this work, lateral profile shape parameters were fitted to measurements of field size dependence dose at the center of field size in air.

Methods:

Previous studies have employed empirical fits to measured profile data to significantly reduce the QA time required for measurements. From this approach to derive the weight and sigma of lateral profiles in air, empirical model formulations were simulated for three selected energies for both proton and carbon beams.

Results:

The 20%–80% lateral penumbras predicted by the double model for proton and single model for carbon with the error functions agreed with the measurements within 1 mm. The standard deviation between measured and fitted field size dependence of dose for empirical model in air has a maximum accuracy of 0.74% for proton with double Gaussian, and of 0.57% for carbon with single Gaussian.

Conclusion:

We have demonstrated that the double Gaussian model of lateral beam profiles is significantly better than the single Gaussian model for proton while a single Gaussian model is sufficient for carbon. The empirical equation may be used to double check the separately obtained model that is currently used by the planning system. The empirical model in air for dose of spot scanning proton and carbon ion beams cannot be directly used for irregular shaped patient fields, but can be to provide reference values for clinical use and quality assurance.
SU‐F‐T‐159: Monte Carlo Simulation Studies of Three‐Dimensional Dose Distribution for Polymer Gel Dosimeter and Radiochromic Gel Dosimeter in a Proton Beam
Authors: Park M , Kim G , Jung H , Park S , Park S , Ji Y , Yun J
Published: July 2016

Purpose:

The purpose of this simulation study is to evaluate the proton detectability of gel dosimeters, and estimate the three‐dimensional dose distribution of protons in the radiochromic gel and polymer gel dosimeter compared with the dose distribution in water.

Methods:

The commercial composition ratios of normoxic polymer gel and LCV micelle radiochromic gel were included in this simulation study. The densities of polymer and radiochromic gel were 1.024 and 1.005 g/cm3, respectively. The 50, 80 and 140 MeV proton beam energies were selected. The dose distributions of protons in the polymer and radiochromic gel were simulated using Monte Carlo radiation transport code (MCNPX 2.7.0, Los Alamos Laboratory). The water equivalent depth profiles and the dose distributions of two gel dosimeters were compared for the water.

Results:

In case of irradiating 50, 80 and 140 MeV proton beam to water phantom, the reference Bragg‐peak depths are represented at 2.22, 5.18 and 13.98 cm, respectively. The difference in the water equivalent depth is represented to about 0.17 and 0.37 cm in the radiochromic gel and polymer gel dosimeter, respectively. The proton absorbed doses in the radiochromic gel dosimeter are calculated to 2.41, 3.92 and 6.90 Gy with increment of incident proton energies. In the polymer gel dosimeter, the absorbed doses are calculated to 2.37, 3.85 and 6.78 Gy with increment of incident proton energies. The relative absorbed dose in radiochromic gel (about 0.47 %) is similar to that of water than the relative absorbed dose of polymer gel (about 2.26 %). In evaluating the proton dose distribution, we found that the dose distribution of both gel dosimeters matched that of water in most cases.

Conclusion:

As the dosimetry device, the radiochromic gel dosimeter has the potential particle detectability and is feasible to use for quality assurance of proton beam therapy beam.
SU‐F‐T‐162: Comparison of Beam Data for Two Varian ProBeam Pencil Beam Scanning Proton Systems
Authors: Langner U , Langen K , Eley J , Zhu M , Yu J , Chung H , Polf J , Dong L
Published: July 2016

Purpose:

To compare the beam data for proton spot scanning beams for the first two Varian ProBeam sites in the US at the Maryland Proton Treatment Center (MPTC) and Scripps Proton Therapy Center (Scripps). To investigate how well beams can be matched between rooms.

Method:

Beam data were acquired with independent dosimetry systems and compared for two sites. Integrated depth dose curves (IDDs) were acquired with large volume PTW Bragg peak chambers in a 3D water tank for pencil beams at both sites. Spot profiles were acquired at different distances from the isocenter at a gantry angle of 0 degrees as well as function of gantry angle.

Results:

IDDs for the two Varian ProBeam proton sites are similar, except in the plateau region were the Scripps data is on average 4.5% higher relative to each other, but only 1.5% relative to the maximum at the Bragg Peak. This increase in the plateau region decrease as energy increase and for energies larger than 180 MeV there is no marked difference. Range in water coincide for all energies within 0.5mm. The sigma of the spot profiles in air are within 10% of each other at isocenter. This difference increase as the detector distance from the isocenter is increased.

Conclusion:

Beam data of the Varian ProBeam proton spot scanning beams for two sites are very well matched. Differences will not be clinically relevant.
SU‐F‐T‐165: Daily QA Analysis for Spot Scanning Beamline
Authors: Poenisch F , Gillin M , Sahoo N , Martin C , Zhu X
Published: July 2016

Purpose:

The dosimetric results of our daily quality assurance over the last 8 years for discrete pencil beam scanning proton therapy will be presented.

Methods:

To perform the dosimetric checks, a multi‐ion chamber detector is used, which consists of an array of 5 single parallel plate ion chambers that are aligned as a cross separated by 10cm each. The Tracker is snapped into a jig, which is placed on the tabletop. Different amounts of Solid Water buildup are added to shift the dose distribution. The dosimetric checks consist of 3 parts: position check, range check and volume dose check.

Results:

The average deviation of all position‐check data were 0.2±1.3%. For the range check, the average deviation was 0.1%±1.2%, which also corresponds to a range stability of better than 1 mm over all measurements. The volumetric dose output readings were all within ±1% with the exception of 2 occasions when the cable to the dose monitor was being repaired.

Conclusion:

Morning QA using the Tracker device gives very stable dosimetric readings but is also sensitive to mechanical and output changes in the proton therapy delivery system.
SU‐F‐T‐166: On the Nature of the Background Visible Light Observed in Fiber Optic Dosimetry of Proton Beams
Authors: Darafsheh A , Taleei R , Kassaee A , Finlay J
Published: July 2016

Purpose:

The nature of the background visible light observed during fiber optic dosimetry of proton beams, whether it is due to Cherenkov radiation or not, has been debated in the literature recently. In this work, experimentally and by means of Monte Carlo simulations, we shed light on this problem and investigated the nature of the background visible light observed in fiber optics irradiated with proton beams.

Methods:

A bare silica fiber optics was embedded in tissue‐mimicking phantoms and irradiated with clinical proton beams with energies of 100–225 MeV at Roberts Proton Therapy Center. Luminescence spectroscopy was performed by a CCD‐coupled spectrograph to analyze in detail the emission spectrum of the fiber tip across the visible range of 400–700 nm. Monte Carlo simulation was performed by using FLUKA Monte Carlo code to simulate Cherenkov light and ionizing radiation dose deposition in the fiber.

Results:

The experimental spectra of the irradiated silica fiber shows two distinct peaks at 450 and 650 nm, whose spectral shape is different from that of Cherenkov radiation. We believe that the nature of these peaks are connected to the point defects of silica including oxygen‐deficiency center (ODC) and non‐bridging oxygen hole center (NBOHC). Monte Carlo simulations confirmed the experimental observations that Cherenkov radiation cannot be solely responsible for such a signal.

Conclusion:

We showed that Cherenkov radiation is not the dominant visible signal observed in bare fiber optics irradiated with proton beams. We observed two distinct peaks at 450 and 650 nm whose nature is connected with the point defects of silica fiber including oxygen‐deficiency center and non‐bridging oxygen hole center.
SU‐F‐T‐167: Measurement of Proton Beam Spot Size Using a 2D Ion Chamber Array for Daily QA
Authors: Thorne N , Kassaee A
Published: July 2016

Purpose:

To develop an algorithm which can calculate the Full Width Half Maximum (FWHM) of a Proton Pencil Beam from a 2D dimensional ion chamber array (IBA Matrixx) with limited spatial resolution (7.6 mm inter chamber distance). The algorithm would allow beam FWHM measurements to be taken during daily QA without an appreciable time increase.

Methods:

Combinations of 147 MeV single spot beams were delivered onto an IBA Matrixx and concurrently on EBT3 films for a standard. Data were collected around the Bragg Peak region and evaluated by a custom MATLAB script based on our algorithm using a least squared analysis. A set of artificial data, modified with random noise, was also processed to test for robustness.

Results:

The Matlab script processed Matixx data shows acceptable agreement (within 5%) with film measurements with no single measurement differing by more than 1.8 mm. In cases where the spots show some degree of asymmetry, the algorithm is able to resolve the differences. The algorithm was able to process artificial data with noise up to 15% of the maximum value. Time assays of each measurement took less than 3 minutes to perform, indicating that such measurements may be efficiently added to daily QA treatment.

Conclusion:

The developed algorithm can be implemented in daily QA program for Proton Pencil Beam scanning beams (PBS) with Matrixx to extract spot size and position information. The developed algorithm may be extended to small field sizes in photon clinic.
SU‐F‐T‐168: Development and Implementation of An Anthropomorphic Head & Neck Phantom for the Assessment of Proton Therapy Treatment Procedures
Authors: Branco D , Taylor P , Frank S , Li H , Zhang X , Mehrens H , Guindani M , Followill D
Published: July 2016

Purpose:

To design a Head and Neck (H&N) anthropomorphic QA phantom that the Imaging and Radiation Oncology Core Houston (IROC‐H) can use to verify the quality of intensity modulated proton therapy (IMPT) H&N treatments for institutions participating in NCI clinical trials.

Methods:

The phantom was created to serve as a remote auditing tool for IROC‐H to evaluate an institution's IMPT planning and delivery abilities. The design was based on the composition, size, and geometry of a generalized oropharyngeal tumor and contains critical structures (parotids and spinal cord). Radiochromic film in the axial and sagittal planes and thermoluminescent dosimeters (TLD)‐100 capsules were embedded in the phantom and used to perform the dose delivery evaluation. A CT simulation was used to create a passive scatter and a spot scanning treatment plan with typical clinical constraints for H&N cancer. The IMPT plan was approved by a radiation oncologist and the phantom was irradiated multiple times. The measured dose distribution using a 7%/4mm gamma analysis (85% of pixels passing) and point doses were compared with the treatment planning system calculations.

Results:

The designed phantom could not achieve the target dose prescription and organ at risk dose constraints with the passive scatter treatment plan. The target prescription dose could be met but not the parotid dose constraint. The average TLD point dose ratio in the target was 0.975, well within the 5% acceptance criterion. The dose distribution analysis using various acceptance criteria, 5%/4mm, 5%/3mm, 7%/4mm and 7%/5mm, had average pixel passing rates of 85.9%, 81.8%, 89.6% and 91.6%, and respectively.

Conclusion:

An anthropomorphic IMPT H&N phantom was designed that can assess the dose delivery of proton sites wishing to participate in clinical trials using a 5% TLD dose and 7%/4mm gamma analysis acceptance criteria.
SU‐F‐T‐173: One‐Scan Protocol: Verifying the Delivery of Spot‐Scanning Proton Beam
Authors: Chan M , Chen C , Li J , Tang X , Li X , Tang G , Mah D
Published: July 2016

Purpose:

Radiochromic film for spot‐scanning QA provides high spatial resolution and efficiency gains from one‐shot irradiation for multiple depths. However, calibration can be a tedious procedure which may limit widespread use. Moreover, since there may be an energy dependence, which manifests as a depth dependence, this may require additional measurements for each patient. We present a one‐scan protocol to simplify the procedure.

Methods:

We performed the calibration using an EBT3 film at depths of 18, 20, 24cm of Plastic Water exposed by a 6‐level step‐wedge plan on a Proteus Plus proton system (IBA, Belgium). The calibration doses ranged 65–250 cGy(RBE) for proton energies of 170–200MeV. A clinical prostate+nodes plan was used for validation. The planar doses at selected depths were measured with EBT3 films and analyzed using one‐scan protocol (one‐scan digitization of QA film and at least one film exposed to known dose). The Gamma passing rates, dose‐difference maps, and profiles of 2D planar doses measured with EBT3 film, IBA MatriXX PT, versus TPS calculations were analyzed and compared.

Results:

The EBT3 film measurement results matched well with the TPS calculation data with an average passing rate of ∼95% for 2%/2mm and slightly lower passing rates were obtained from an ion chamber array detector. We were able to demonstrate that the use of a proton step‐wedge provided clinically acceptable results and minimized variations between film‐scanner orientation, inter‐scan, and scanning conditions. Furthermore, it could be derived from no more than two films exposed to known doses (one could be zero) for rescaling the master calibration curve at each depth.

Conclusion:

The use of a proton step‐wedge for calibration of EBT3 film increases efficiency. The sensitivity of the calibration to depth variations has been explored. One‐scan protocol results appear to be comparable to that of the ion chamber array detector.

One author has a research grant from Ashland Inc., the manufacturer of the GafChromic film.
SU‐F‐T‐177: Impacts of Gantry Angle Dependent Scanning Beam Properties for Proton Treatment
Authors: Lin Y , Clasie B , Lu H , Flanz J , Jee K
Published: July 2016

Purpose:

In pencil beam scanning (PBS), the delivered spot MU, position and size are slightly different at different gantry angles. We investigated the level of delivery uncertainty at different gantry angles through a log file analysis.

Methods:

34 PBS fields covering full 360 degrees gantry angle spread were collected retrospectively from 28 patients treated at our institution. All fields were delivered at zero gantry angle and the prescribed gantry angle, and measured at isocenter with the MatriXX 2D array detector at the prescribed gantry angle. The machine log files were analyzed to extract the delivered MU per spot and the beam position from the strip ionization chambers in the treatment nozzle. The beam size was separately measured as a function of gantry angle and beam energy. Using this information, the dose was calculated in a water phantom at both gantry angles and compared to the measurement using the 3D γ‐index at 2mm/2%.

Results:

The spot‐by‐spot difference between the beam position in the log files from the delivery at the two gantry angles has a mean of 0.3 and 0.4 mm and a standard deviation of 0.6 and 0.7 mm for × and y directions, respectively. Similarly, the spot‐by‐spot difference between the MU in the log files from the delivery at the two gantry angles has a mean 0.01% and a standard deviation of 0.7%. These small deviations lead to an excellent agreement in dose calculations with an average γ pass rate for all fields being approximately 99.7%. When each calculation is compared to the measurement, a high correlation in γ was also found.

Conclusion:

Using machine logs files, we verified that PBS beam delivery at different gantry angles are sufficiently small and the planned spot position and MU. This study brings us one step closer to simplifying our patient‐specific QA.
SU‐F‐T‐181: Proton Therapy Tissue‐Equivalence of 3D Printed Materials
Authors: Taylor P , Craft D , Followill D , Howell R
Published: July 2016

Purpose:

This work investigated the proton tissue‐equivalence of various 3D printed materials.

Methods:

Three 3D printers were used to create 5 cm cubic phantoms made of different plastics with varying percentages of infill. White resin, polylactic acid (PLA), and NinjaFlex plastics were used. The infills ranged from 15% to 100%. Each phantom was scanned with a CT scanner to obtain the HU value. The relative linear stopping power (RLSP) was then determined using a multi‐layer ion chamber in a 200 MeV proton beam. The RLSP was measured both parallel and perpendicular to the print direction for each material.

Results:

The HU values of the materials ranged from lung‐equivalent (−820 HU σ160) when using a low infill, to soft‐tissue‐equivalent 159 (σ12). The RLSP of the materials depended on the orientation of the beam relative to the print direction. When the proton beam was parallel to the print direction, the RLSP was generally higher than the RLSP in the perpendicular orientation, by up to 45%. This difference was smaller (less than 6%) for the materials with 100% infill. For low infill cubes irradiated parallel to the print direction, the SOBP curve showed extreme degradation of the beam in the distal region. The materials with 15–25% infill had wide‐ranging agreement with a clinical HU‐RLSP conversion curve, with some measurements falling within 1% of the curve and others deviating up to 45%. The materials with 100% infill all fell within 7% of the curve.

Conclusion:

While some materials tested fall within 1% of a clinical HU‐RLSP curve, caution should be taken when using 3D printed materials with proton therapy, as the orientation of the beam relative to the print direction can result in a large change in RLSP. Further investigation is needed to measure how the infill pattern affects the material RLSP.

This work was supported by PHS grant CA180803.
SU‐F‐T‐188: A Robust Treatment Planning Technique for Proton Pencil Beam Scanning Cranial Spinal Irradiation
Authors: Zhu M , Yam M , Mehta M , Badiyan S , Young K , Malyapa R , Regine W , Langen K
Published: July 2016

Purpose:

To propose a proton pencil beam scanning (PBS) cranial spinal irradiation (CSI) treatment planning technique robust against patient roll, isocenter offset and proton range uncertainty.

Method:

Proton PBS plans were created (Eclipse V11) for three previously treated CSI patients to 36 Gy (1.8 Gy/fractions). The target volume was separated into three regions: brain, upper spine and lower spine. One posterior‐anterior (PA) beam was used for each spine region, and two posterior‐oblique beams (15° apart from PA direction, denoted as 2PO_15) for the brain region. For comparison, another plan using one PA beam for the brain target (denoted as 1PA) was created. Using the same optimization objectives, 98% CTV was optimized to receive the prescription dose. To evaluate plan robustness against patient roll, the gantry angle was increased by 3° and dose was recalculated without changing the proton spot weights. On the re‐calculated plan, doses were then calculated using 12 scenarios that are combinations of isocenter shift (±3mm in X, Y, and Z directions) and proton range variation (±3.5%). The worst‐case‐scenario (WCS) brain CTV dosimetric metrics were compared to the nominal plan.

Results:

For both beam arrangements, the brain field(s) and upper‐spine field overlap in the T2–T5 region depending on patient anatomy. The maximum monitor unit per spot were 48.7%, 47.2%, and 40.0% higher for 1PA plans than 2PO_15 plans for the three patients. The 2PO_15 plans have better dose conformity. At the same level of CTV coverage, the 2PO_15 plans have lower maximum dose and higher minimum dose to the CTV. The 2PO_15 plans also showed lower WCS maximum dose to CTV, while the WCS minimum dose to CTV were comparable between the two techniques.

Conclusion:

Our method of using two posterior‐oblique beams for brain target provides improved dose conformity and homogeneity, and plan robustness including patient roll.
SU‐F‐T‐193: Evaluation of a GPU‐Based Fast Monte Carlo Code for Proton Therapy Biological Optimization
Authors: Taleei R , Peeler C , Qin N , Jiang S , Jia X
Published: July 2016

Purpose:

Biological treatment plan optimization is of great interest for proton therapy. It requires extensive Monte Carlo (MC) simulations to compute physical dose and biological quantities. Recently, a gPMC package was developed for rapid MC dose calculations on a GPU platform. This work investigated its suitability for proton therapy biological optimization in terms of accuracy and efficiency.

Methods:

We performed simulations of a proton pencil beam with energies of 75, 150 and 225 MeV in a homogeneous water phantom using gPMC and FLUKA. Physical dose and energy spectra for each ion type on the central beam axis were scored. Relative Biological Effectiveness (RBE) was calculated using repair‐misrepair‐fixation model. Microdosimetry calculations were performed using Monte Carlo Damage Simulation (MCDS).

Results:

Ranges computed by the two codes agreed within 1 mm. Physical dose difference was less than 2.5 % at the Bragg peak. RBE‐weighted dose agreed within 5 % at the Bragg peak. Differences in microdosimetric quantities such as dose average lineal energy transfer and specific energy were < 10%. The simulation time per source particle with FLUKA was 0.0018 sec, while gPMC was ∼ 600 times faster.

Conclusion:

Physical dose computed by FLUKA and gPMC were in a good agreement. The RBE differences along the central axis were small, and RBE‐weighted dose difference was found to be acceptable. The combined accuracy and efficiency makes gPMC suitable for proton therapy biological optimization.
SU‐F‐T‐196: Hypo‐Fractionation with Intensity Modulated Proton Therapy for Unilateral Metallic Prosthesis Prostate Cancer Patients
Authors: Rana S , Zheng Y , Zhang Y , Pokharel  , Cheng C , Park S
Published: July 2016

Purpose:

The purpose of this study is to investigate the dosimetric feasibility of hypo‐fractionated intensity modulated proton therapy (IMPT) for unilateral metallic prosthesis prostate cancer patients based on proton collaborative group (PCG)‐GU002‐10 (NCT01230866) protocol criteria.

Methods:

A total of five unilateral metallic prosthesis prostate cancer cases were included in this retrospective study. For each case, IMPT plans were generated for treatment to be delivered with 7.6 Gy[RBE] per fraction in 5 fractions per week for a total dose of 38 Gy(RBE). Each plan was generated using two anterior‐oblique beams and one lateral beam. Treatment plans were optimized with an objective meeting PCG‐GU002‐10 (NCT01230866) protocol criteria: (i) planning target volume (PTV): D99.5% > 36.1 Gy[RBE], (ii) rectum: V24 < 35%, V33.6 < 10%, (iii) bladder: V39 < 8 cc, and (iv) femoral head: V23 < 1cc.

Results:

All five cases satisfied PTV D99.5% (average=36.82 Gy[RBE]; range, 36.36–37.13 Gy[RBE]). PTV D95% ranged from 36.66 Gy[RBE] to 38.65 Gy[RBE] and PTV V100 ranged from 95.47% to 97.95%. For the rectum, V24 was less than 35% (average=14.07 Gy[RBE]; range, 6.22–18.42%, whereas V33.6 Gy[RBE] was less than 10% (average=6.83; range, 3.06 – 9.15%). Rectal mean dose ranged from 4.22 Gy[RBE] to 9.97 Gy[RBE]. For the bladder, V39 was found to be less than 8 cc (average=3.69 cc; range, 0.19–7.68 cc). Bladder mean dose ranged from 4.22 Gy[RBE] to 18.83 Gy[RBE]. For the femoral head, V23 was 0 in all five cases.

Conclusion:

All five unilateral metallic prosthesis prostate cancer IMPT plans generated with one lateral and two anterior‐oblique beams satisfied the dosimetric criteria of PCG‐GU002‐10 (NCT01230866) protocol.
SU‐F‐T‐204: A Preliminary Approach of Reducing Contralateral Breast and Heart Dose in Left Sided Whole Breast Cancer Patients Utilizing Proton Beams
Authors: Islam M , Algan O , Jin H , Ahmad S , Hossain S
Published: July 2016

Purpose:

To investigate the plan quality and feasibility of a hybrid plan utilizing proton and photon fields for superior coverage in the internal mammary (IM) and supraclavicular (S/C) regions while minimizing heart and contralateral breast dose for the left‐sided whole breast cancer patient treatment.

Methods:

This preliminary study carried out on single left‐sided intact breast patient involved IM and S/C nodes. The IM and S/C node fields of the 5‐Field 3DCRT photon‐electron base plan were replaced by two proton fields. These two along with two Field‐in‐Field tangential photon fields were optimized for comparable dose coverage. The treatment plans were done using Eclipse TPS for the total dose of 46Gy in 23 fractions with 95% of the prescription dose covering 95% of the RTOG PTV. The 3DCRT photon‐electron and 4‐Field photon‐proton hybrid plans were compared for the PTV dose coverage as well as dose to OARs.

Results:

The overall RTOG PTV coverage for proton‐hybrid and 3DCRT plan was comparable (95% of prescription dose covers 95% PTV volume). In proton‐hybrid plan, 99% of IM volume received 100% dose whereas in 3DCRT only 77% received 100% dose. For S/C regions, 97% and 77% volume received 100% prescription dose in proton‐hybrid and 3DCRT plans, respectively. The heart mean dose, V3Gy(%), and V5Gy(%) was 2.2Gy, 14.4%, 9.8% for proton‐hybrid vs. 4.20 Gy, 21.5%, and 39% for 3DCRT plan, respectively. The maximum dose to the contralateral breast was 39.75Gy for proton‐hybrid while 56.87Gy for 3DCRT plan. The mean total lung dose, V20Gy(%), and V30Gy(%) was 5.68Gy, 11.3%, 10.5% for proton‐hybrid vs. 5.90Gy, 9.8%, 7.2% for 3DCRT, respectively.

Conclusion:

The protonhybrid plan can offer better dose coverage to the involved lymphatic tissues while lower doses to the heart and contralateral breast. More treatment plans are currently in progress before being implemented clinically.
SU‐F‐T‐208: An Efficient Planning Approach to Posterior Fossa Tumor Bed Boosts Using Proton Pencil Beam Scanning in Fixed‐Beam Room
Authors: Ju N , Chen C , Gans S , Hug E , Cahlon O , Wolden S , Chon B , Yeh B , Tsai H , Sine K , Mah D
Published: July 2016

Purpose:

A fixed‐beam room could be underutilized in a multi‐room proton center. We investigated the use of proton pencil beam scanning (PBS) on a fixed‐beam as an alternative for posterior fossa tumor bed (PF‐TB) boost treatments which were usually treating on a gantry with uniform scanning.

Methods:

Five patients were treated with craniospinal irradiation (CSI, 23.4 or 36.0 Gy(RBE)) followed by a PF‐TB boost to 54 Gy(RBE) with proton beams. Three PF‐TB boost plans were generated for each patient: (1) a uniform scanning (US) gantry plan with 4–7 posterior fields shaped with apertures and compensators (2) a PBS plan using bi‐lateral and vertex fields with a 3‐mm planning organ‐at‐risk volume (PRV) expansion around the brainstem and (3) PBS fields using same beam arrangement but replacing the PRV with robust optimization considering a 3‐mm setup uncertainty.

Results:

A concave 54‐Gy(RBE) isodose line surrounding the brainstem could be achieved using all three techniques. The mean V95% of the PTV was 99.7% (range: 97.6% to 100%) while the V100% of the PTV ranged from 56.3% to 93.1% depending on the involvement of the brainstem with the PTV. The mean doses received by 0.05 cm3 of the brainstem were effectively identical: 54.0 Gy(RBE), 53.4 Gy(RBE) and 53.3 Gy(RBE) for US, PBS optimized with PRV, and PBS optimized with robustness plans respectively. The cochlea mean dose increased by 23% of the prescribed boost dose in average from the bi‐lateral fields used in the PBS plan. Planning time for the PBS plan with PRV was 5–10 times less than the US plan and the robustly optimized PBS plan.

Conclusion:

We have demonstrated that a fixed‐beam with PBS can deliver a dose distribution comparable to a gantry plan using uniform scanning. Planning time can be reduced substantially using a PRV around the brainstem instead of robust optimization.
SU‐F‐T‐213: Commissioning Results of the Prototype Active Scanning Irradiation System of Korea Heavy Ion Medical Accelerator
Authors: Kim C , Seduk J , Yang T
Published: July 2016

Purpose:

A prototype actives scanning beam delivery system was designed, manufactured and installed as a part of the Korea Heavy Ion Medical Accelerator Project. The prototype system includes the most components for steering, modulating, detecting incident beam to patient. The system was installed in MC‐50 cyclotron beam line and tested to extract the normal operation conditions.

Methods:

The commissioning process was completed by using 45 MeV of proton beam. To measure the beam position accuracy along the scanning magnet power supply current, 25 different spots were scanning and measured. The scanning results on GaF film were compared with the irradiation plan. Also, the beam size variation and the intensity reduction using range shifter were measured and analyzed. The results will be used for creating a conversion factors for asymmetric behavior of scanning magnets and a dose compensation factor for longitudinal direction.

Results:

The results show asymmetry operations on both scanning × and y magnet. In case of scanning magnet × operation, the current to position conversion factors were measured 1.69 mm/A for positive direction and 1.74 mm/A for negative direction. The scanning magnet y operation shows 1.38mm/A and 1.48 mm/A for both directions. The size of incoming beam which was 18 mm as sigma becomes larger up to 55 mm as sigma while using 10 mm of the range shifter plate. As the beam size becomes large, the maximum intensity of the was decreased. In case of using 10 mm of range shifter, the maximum intensity was only 52% compared with no range shifter insertion.

Conclusion:

For the appropriate operation of the prototype active scanning system, the commissioning process were performed to measure the beam characteristics variation. The obtained results would be applied on the irradiation planning software for more precise dose delivery using the active scanning system.
SU‐F‐T‐216: Evaluating Dosimetry Accuracy of a Treatment Planning System On Small Proton Fields
Authors: Zhang M , Xiao Z , Zou J , Chen T , Yue N
Published: July 2016

Purpose:

This study is aiming to identify the smallest field size for which a treatment planning system (TPS) can accurately calculate the relative dose distribution. The finding would be used as a guideline to choose the smallest proton field for clinical treatment.

Methods:

Mevion S250™ double scattering proton delivery system and Eclipse™ TPS (Varian) with pencil beam convolution (PBC) dose algorithm were used in this study. Square sized fields of 1 cm, 2 cm, 3 cm, 4 cm, 5 cm, and 10 cm were planned on a cubical water phantom with iso‐center placed at 10 cm depth. All beams used the same proton beam option: range 15 cm and modulation 10 cm. Dose in water was calculated without any compensator. Gafchromic™ EBT3 film and diode detectors were used to measure the central axis dose distribution and lateral dose profiles at 5 cm, 10 cm, and 14 cm depth.

Results:

The preliminary film measurement shows good agreement between Eclipse calculated lateral dose profiles for all tested field sizes. The differences on full width half maximum were ≤ 1 mm while the differences on the penumbras were between 1 mm and 2 mm between Eclipse and film. For the depth dose, Eclipse results matched well with film measurements for field sizes down to 2 cm2. With smaller field size of 1 cm2, Eclipse was able to predict the decreasing of SOBP due to the lack of lateral charged particle equilibrium in depth. However, it did not match the film measurement. Diode measurement results will be available at the time of presentation.

Conclusion:

The PBC dose algorithm in Eclipse can accurately calculate relative dose distribution in double scattered proton system for field size down to 2 cm2.
SU‐F‐T‐217: A Comprehensive Monte‐Carlo Study of Out‐Of‐Field Secondary Neutron Spectra in a Scanned‐Beam Proton Therapy Treatment Room
Authors: Englbrecht F , Trinkl S , Mares V , Ruehm W , Wielunski M , Wilkens J , Hillbrand M , Parodi K
Published: July 2016

Purpose:

To simulate secondary neutron radiation‐fields produced at different positions during phantom irradiation inside a scanning proton therapy gantry treatment room. Further, to identify origin, energy distribution and angular emission as function of proton beam energy.

Methods:

GEANT4 and FLUKA Monte‐Carlo codes were used to model the relevant parts of the treatment room in a gantry‐equipped pencil beam scanning proton therapy facility including walls, floor, metallic gantry‐components, patient table and the homogeneous PMMA target. The proton beams were modeled based on experimental beam ranges in water and spot shapes in air. Neutron energy spectra were simulated at 0°, 45°, 90° and 135° relative to the beam axis at 2m distance from isocenter, as well as 11×11 cm2 fields for 75MeV, 140MeV, 200MeV and for 118MeV with 5cm PMMA range‐shifter. The total neutron energy distribution was recorded for these four positions and proton energies. Additionally, the room‐components generating secondary neutrons in the room and their contributions to the total spectrum were identified and quantified.

Results:

FLUKA and GEANT4 simulated neutron spectra showed good general agreement in the whole energy range of 10−9 to 102 MeV. Comparison of measured spectra with the simulated contributions of the various room components helped to limit the complexity of the room model, by identifying the dominant contributions to the secondary neutron spectrum. The iron of the bending magnet and counterweight were identified as sources of secondary evaporation‐neutrons, which were lacking in simplified room models.

Conclusion:

Thorough Monte‐Carlo simulations have been performed to complement Bonner‐sphere spectrometry measurements of secondary neutrons in a clinical proton therapy treatment room. Such calculations helped disentangling the origin of secondary neutrons and their dominant contributions to measured spectra, besides providing a useful validation of widely used Monte‐Carlo packages in comparison to experimental data.

Cluster of Excellence of the German Research Foundation (DFG) “Munich‐Centre for Advanced Photonics (MAP)”