bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐448: On the Perturbation Factor P_cav of the Markus Parallel Plate Ion Chambers in Clinical Electron Beams, Monte Carlo Based Reintegration of An Historical Experiment
Authors: von Voigts‐Rhetz P , Zink K
Published: July 2014

Purpose:

All present dosimetry protocols recommend well‐guarded parallel‐plate ion chambers for electron dosimetry. For the guard‐less Markus chamber an energy dependent fluence perturbation correction pcav is given. This perturbation correction was experimentally determined by van der Plaetsen by comparison of the read‐out of a Markus and a NACP chamber, which was assumed to be “perturbation‐free”. Aim of the present study is a Monte Carlo based reiteration of this experiment.

Methods:

Detailed models of four parallel‐plate chambers (Roos, Markus, NACP and Advanced Markus) were designed using the Monte Carlo code EGSnrc and placed in a water phantom. For all chambers the dose to the active volume filled with low density water was calculated for 13 clinical electron spectra (E0=6‐21 MeV) at the depth of maximum and at the reference depth under reference conditions. In all cases the chamber's reference point was positioned at the depth of measurement. Moreover, the dose to water DW was calculated in a small water voxel positioned at the same depth.

Results:

The calculated dose ratio DNACP/DMarkus, which according to van der Plaetsen reflects the fluence perturbation correction of the Markus chamber, deviates less from unity than the values given by van der Plaetsen's but exhibits a similar energy dependence. The same holds for the dose ratios of the other well guarded chambers. But, in comparison to water, the Markus chamber reveals the smallest overall perturbation correction which is nearly energy independent at both investigated depths.

Conclusion:

The simulations principally confirm the energy dependence of the dose ratio DNACP/DMarkus as published by van der Plaetsen. But, as shown by our simulations of the ratio DW/DMarkus, the conclusion drawn in all dosimetry protocols is questionable: in contrast to all well‐guarded chambers the guard‐less Markus chamber reveals the smallest overall perturbation correction and also the smallest energy dependence.
SU‐E‐T‐449: Optimization and Evaluation: IMRT of Sinoâ,<nasal Carcinoma in Reference to ICRU 83
Authors: Maria Das Joseph K , Aziz Sait A Abdul , Senthil Kumar S , Verma M , Lal P , Kumar S
Published: July 2014

Purpose:

Estimating the potential radiobiological advantage using the sub‐volume concept (ICRU 83)in plan optimization and evaluation of IMRT for sino‐nasal carcinoma.

Methods:

Ten patients planned with IMRT were selected retrospectively. Two plans were generated for each patient with 7 coplanar field arrangements. The PTV was prescribed 60Gy/30# with an unedited expansion from CTV (CC=5mm, ML=4mm and AP=4mm) and the in‐air PTV filled with a virtual bolus 1cm thick. Proximity structures were defined as per the guidelines. Plan1 was optimized and evaluated based on ICRU 50/62, whereas plan2 was based incorporating the ICRU 83 approach. The dosimetric parameters, including CI, HI, MU were compared and radiobiological metrics estimated using the Biosuite software (NTCP‐LKB model & EUD). The paired T‐test was used to assess the statistical significance of differences of similar parameters between the plans.

Results:

The mean±sd of PTV was 230.21±88.13cc, where as the sub‐volume PTV was 3.09±1.47cc. PTV dosimetric parameters i.e., D2%, D98%, D95%, D50%, etc., CI and HI were similar for both plans, but OAR and PRV showed reduction in D2%, Dmax, Dmean for plan2 compared with plan1. D2% of optic chiasm 53.66±0.90 vs. 52.47±0.89, p = 0.004; left optic nerve 54.91±1.54 vs. 53.67±1.56, p = 0.001; left retina 54.00±1.34 vs. 51.82±2.52, p = 0.005 for plan1 and plan2 respectively while Dmax, Dmean for other OARs and PRVs demonstrated a similar trend. NTCP for optic chiasm was 7.71±2.86 vs. 5.62±2.24, p = 0.005 without fractionation effect, whereas it was 4.42±2.11 vs. 2.80±1.46, p = 0.006 with fractionation effect for plan1 and plan2 respectively. Similar trend were observed for other OARs and EUDs.7% increase in MU for plan2 was observed.

Conclusion:

Incorporating the sub volume concept of ICRU 83 in IMRT optimization and evaluation may Resultin reduction of doses to OAR and lower / safer NTCP without compromising TCP.

The authors are acknowledging Dr. Julien Uzan and Professor Alan Nahum, Clatterbridge Cancer Centre, Wirral, UK for guidance and sharing the Biosuite software for radiobiological evaluation.
SU‐E‐T‐450: Dosimetric Impact of Rotational Error On Multiple‐Target Intensity‐Modulated Radiosurgery (IMRS) with Single‐Isocenter
Authors: Jang S , Huq M
Published: July 2014

Purpose:

Evaluating the dosimetric‐impact on multiple‐targets placed away from the isocenter‐target with varying rotational‐error introduced by initial setup uncertainty and/or intrafractional‐movement

Methods:

CyberKnife‐Phantom was scanned with the Intracranial SRS‐protocol of 1.25mm slice‐thickness and the multiple‐targets(GTV) of 1mm and 10mm in diameter were contoured on the Eclipse. PTV for distal‐target only was drawn with 1mm expansion around the GTV to find out how much margin is needed to compensate for the rotational‐error. The separation between the isocenter‐target and distal‐target was varied from 3cm to 7cm. RapidArc‐based IMRS plans of 16Gy single‐fraction were generated with five non‐coplanar arcs by using Varian TrueBeam‐STx equipped with high resolution MLC leaves of 2.5mm at center and with dose‐rate of 1400MU/min at 6MV for flatteringfilter‐ free(FFF). An identical CT image with intentionally introduced 1° rotational‐error was registered with the planning CT image, and the isodose distribution and Dose‐Volume‐Histogram(DVH) were compared with the original plans. Additionally, the dosimetric‐impact of rotational error was evaluated with that of 6X photon energy which was generated with the same target‐coverage.

Results:

For the 1mm‐target with 6X‐FFF, PTV‐coverage(D100) of the distal‐target with 1° rotational‐error decreased from 1.00 to 0.35 as the separation between isocenter‐target and distal‐target increased from 3cm to 7cm. However, GTV‐coverage(D100) was 1.0 except that of 7cm‐separation(0.55), which resulted from the 1mm‐margin around the distal‐target. For 6X photon, GTV‐coverage remained at 1.0 regardless of the separation of targets, showing that the dosimetric‐impact of rotational error depends on the degree of rotational‐error, separation of targets, and dose distribution around targets. For 10mm‐target, PTV‐coverage of distaltarget located 3cm‐away was better than that of 1mm‐target(0.93 versus 0.7) and GTV‐coverage was 1.0 regardless of 6X‐FFF/6X photon beams.

Conclusion:

RapidArc‐based Multiple‐target IMRS may Resultin a compromised outcome due to the setup‐error for distal‐targets. With 1mm‐margin, GTV‐coverage of distal‐target was 1.0 although the separation of targets was up to 5cm.
SU‐E‐T‐451: Accuracy and Application of the Standard Imaging W1 Scintillator Dosimeter
Authors: Kowalski M , McEwen M
Published: July 2014

Purpose:

To evaluate the Standard Imaging W1 scintillator dosimeter in a range of clinical radiation beams to determine its range of possible applications.

Methods:

The W1 scintillator is a small perturbation‐free dosimeter which is of interest in absolute and relative clinical dosimetry due to its small size and water equivalence. A single version of this detector was evaluated in Co‐60 and linac photon and electron beams to investigate the following: linearity, sensitivity, precision, and dependence on electrometer type. In addition, depth‐dose and cross‐plane profiles were obtained in both photon and electron beams and compared with data obtained with wellbehaved ionization chambers.

Results:

In linac beams the precision and linearity was very impressive, with typical values of 0.3% and 0.1% respectively. Performance in a Co‐60 beam was much poorer (approximately three times worse) and it is not clear whether this is due to the lower signal current or the effect of the continuous beam (rather than pulsed beam of the linac measurements). There was no significant difference in the detector reading when using either the recommended SI Supermax electrometer or two independent high‐quality electrometers, except for low signal levels, where the Supermax exhibited an apparent threshold effect, preventing the measurement of the bremsstrahlung background in electron depth‐dose curves. Comparisons with ion chamber measurements in linac beams were somewhat variable: good agreement was seen for cross‐profiles (photon and electron beams) and electron beam depth‐dose curves, generally within the 0.3% precision of the scintillator but systematic differences were observed as a function of measurement depth in photon beam depth‐dose curves.

Conclusion:

A first look would suggest that the W1 scintillator has applications beyond small field dosimetry but performance appears to be limited to higher doserate and/or pulsed radiation beams. Further work is required to resolve discrepancies compared to ion chambers.
SU‐E‐T‐452: Impact of Respiratory Motion On Robustly‐Optimized Intensity‐Modulated Proton Therapy to Treat Lung Cancers
Authors: Liu W , Liao Z , Schild S , Park P , Li H , Li Y , Li X , Shen J , Anand A , Sahoo N , Dong L , Bues M , Zhu X , Mohan R
Published: July 2014

Purpose:

We compared conventionally optimized intensity‐modulated proton therapy (IMPT) treatment plans against the worst‐case robustly optimized treatment plans for lung cancer. The comparison of the two IMPT optimization strategies focused on the resulting plans' ability to retain dose objectives under the influence of patient set‐up, inherent proton range uncertainty, and dose perturbation caused by respiratory motion.

Methods:

For each of the 9 lung cancer cases two treatment plans were created accounting for treatment uncertainties in two different ways: the first used the conventional Method: delivery of prescribed dose to the planning target volume (PTV) that is geometrically expanded from the internal target volume (ITV). The second employed the worst‐case robust optimization scheme that addressed set‐up and range uncertainties through beamlet optimization. The plan optimality and plan robustness were calculated and compared. Furthermore, the effects on dose distributions of the changes in patient anatomy due to respiratory motion was investigated for both strategies by comparing the corresponding plan evaluation metrics at the end‐inspiration and end‐expiration phase and absolute differences between these phases. The mean plan evaluation metrics of the two groups were compared using two‐sided paired t‐tests.

Results:

Without respiratory motion considered, we affirmed that worst‐case robust optimization is superior to PTV‐based conventional optimization in terms of plan robustness and optimality. With respiratory motion considered, robust optimization still leads to more robust dose distributions to respiratory motion for targets and comparable or even better plan optimality [D95% ITV: 96.6% versus 96.1% (p=0.26), D5% – D95% ITV: 10.0% versus 12.3% (p=0.082), D1% spinal cord: 31.8% versus 36.5% (p =0.035)].

Conclusion:

Worst‐case robust optimization led to superior solutions for lung IMPT. Despite of the fact that robust optimization did not explicitly account for respiratory motion it produced motion‐resistant treatment plans. However, further research is needed to incorporate respiratory motion into IMPT robust optimization.
SU‐E‐T‐453: A Novel Daily QA System for Robotic Image Guided Radiosurgery with Variable Aperture Collimator
Authors: Wang L , Nelson B
Published: July 2014

Purpose:

A novel end‐to‐end system using a CCD camera and a scintillator based phantom that is capable of measuring the beam‐by‐beam delivery accuracy of Robotic Radiosurgery has been developed and reported in our previous work. This work investigates its application to end‐to‐end type daily QA for Robotic Radiosurgery (Cyberknife) with Variable Aperture Collimator (Iris).

Methods:

The phantom was first scanned with a CT scanner at 0.625 slice thickness and exported to the Cyberknife Muliplan (v4.6) treatment planning system. An isocentric treatment plan was created consisting of ten beams of 25 Monitor Units each using Iris apertures of 7.5, 10, 15, 20, and 25 mm. The plan was delivered six times in two days on the Cyberknife G4 system with fiducial tracking on the four metal fiducials embedded in phantom with re‐positioning between the measurements. The beam vectors (X, Y, Z) are measured and compared with the plan from the machine delivery file (XML file). The Iris apertures (FWHM) were measured from the beam flux map and compared with the commissioning data.

Results:

The average beam positioning accuracies of the six deliveries are 0.71 ± 0.40 mm, 0.72 ± 0.44 mm, 0.74 ± 0.42 mm, 0.70 ± 0.40 mm, 0.79 ± 0.44 mm and 0.69 ± 0.41 mm respectively. Radiation beam width (FWHM) variations are within ±0.05 mm, and they agree with the commissioning data within 0.22 mm. The delivery time for the plan is about 7 minutes and the results are given instantly.

Conclusion:

The experimental results agree with stated sub‐millimeter delivery accuracy of Cyberknife system. Beam FWHM variations comply with the 0.2 mm accuracy of the Iris collimator at SAD. The XRV‐100 system has proven to be a powerful tool in performing end‐to‐end type tests for Robotic Image Guided Radiosurgery Daily QA.
SU‐E‐T‐454: Impact of Calculation Grid Size On Dosimetry and Radiobiological Parameters for Head and Neck IMRT
Authors: Srivastava S , Cheng C , Das I
Published: July 2014

Purpose:

IMRT has become standard of care for complex treatments to optimize dose to target and spare normal tissues. However, the impact of calculation grid size is not widely known especially dose distribution, tumor control probability (TCP) and normal tissue complication probability (NTCP) which is investigated in this study.

Methods:

Ten head and neck IMRT patients treated with 6 MV photons were chosen for this study. Using Eclipse TPS, treatment plans were generated for different grid sizes in the range 1–5 mm for the same optimization criterion with specific dose‐volume constraints. The dose volume histogram (DVH) was calculated for all IMRT plans and dosimetric data were compared. ICRU‐83 dose points such as D2%, D50%, D98%, as well as the homogeneity and conformity indices (HI, CI) were calculated. In addition, TCP and NTCP were calculated from DVH data.

Results:

The PTV mean dose and TCP decreases with increasing grid size with an average decrease in mean dose by 2% and TCP by 3% respectively. Increasing grid size from 1–5 mm grid size, the average mean dose and NTCP for left parotid was increased by 6.0% and 8.0% respectively. Similar patterns were observed for other OARs such as cochlea, parotids and spinal cord. The HI increases up to 60% and CI decreases on average by 3.5% between 1 and 5 mm grid that resulted in decreased TCP and increased NTCP values. The number of points meeting the gamma criteria of ±3% dose difference and ±3mm DTA was higher with a 1 mm on average (97.2%) than with a 5 mm grid (91.3%).

Conclusion:

A smaller calculation grid provides superior dosimetry with improved TCP and reduced NTCP values. The effect is more pronounced for smaller OARs. Thus, the smallest possible grid size should be used for accurate dose calculation especially in H&N planning.
SU‐E‐T‐455: Characterization of 3D Printed Materials for Proton Beam Therapy
Authors: Zou W , McDonough J , Yin L , Fisher T , Siderits R , McKenna M , Khan A , Yue N , Teo B
Published: July 2014

Purpose:

The widespread availability of low cost 3D printing technologies provides an alternative fabrication method for customized proton range modifying accessories such as compensators and boluses. However the material properties of the printed object are dependent on the printing technology used. In order to facilitate the application of 3D printing in proton therapy, this study investigated the stopping power of several printed materials using both proton pencil beam measurements and Monte Carlo simulations.

Methods:

Five 3–4 cm cubes fabricated using three 3D printing technologies (selective laser sintering, fused‐deposition modeling and stereolithography) from five printers were investigated. The cubes were scanned on a CT scanner and the depth dose curves for a mono‐energetic pencil beam passing through the material were measured using a large parallel plate ion chamber in a water tank. Each cube was measured from two directions (perpendicular and parallel to printing plane) to evaluate the effects of the anisotropic material layout. The results were compared with GEANT4 Monte Carlo simulation using the manufacturer specified material density and chemical composition data.

Results:

Compared with water, the differences from the range pull back by the printed blocks varied and corresponded well with the material CT Hounsfield unit. The measurement results were in agreement with Monte Carlo simulation. However, depending on the technology, inhomogeneity existed in the printed cubes evidenced from CT images. The effect of such inhomogeneity on the proton beam is to be investigated.

Conclusion:

Printed blocks by three different 3D printing technologies were characterized for proton beam with measurements and Monte Carlo simulation. The effects of the printing technologies in proton range and stopping power were studied. The derived results can be applied when specific devices are used in proton radiotherapy.
SU‐E‐T‐456: Development of An EPID‐Based Output Measurement and Dosimetric Verification Tool for Electron Beam Therapy
Authors: Ding A , Han B , Xing L
Published: July 2014

Purpose:

To develop an efficient and robust method for output and absolute dosimetric measurements of electron beam therapy by using a high spatialresolution and high frame‐rate amorphous silicon flat‐panel electronic portal imaging device (EPID).

Methods:

A previously‐established EPID dosimetry system was extended to measure the output factors of electron beams of various sizes and their planar dose distributions at reference depths. Specific EPID responses to different electron energies were derived from Monte Carlo (MC) simulations to de‐convolve the EPID raw images to the incident electron fluence map. To reconstruct the 2D water‐based dose distribution map at reference depths for different energies, the fluence map was further convolved with a MC simulated pencil beam kernel. Different energies of 6, 9, 12, 16, and 20 MeV from a Varian C‐series linac were tested. Standard square fields ranging from 2×2 to 15×15 cm2 were measured and validated against film/ion chamber measurements. Small fields of different patient cases with irregular cutout shapes were also tested.

Results:

The EPIDmeasured output factors for 2×2, 3×3, 6×6, and 10×10 cm2 fields with all electron energies agree with the film/ion chamber data within 2.7%. The average discrepancy between EPID and film/ion chamber measurements was 1.4%. The differences for the three patient cases were 0.1%, 2.1%, and 1.6%. Deliveries with different monitor units were tested and the results exhibited good linearity. Measurements with different dose rate showed that the dose rate dependence was less than 1%. Using the proposed method, 2D absolute dose maps were created from the EPID raw images and the results were consistent with film measurements.

Conclusion:

The efficient data readout and portability of the newly developed EPID system provides an efficient and accurate solution for electron beam therapy. It addresses an important unmet clinical need for a fast and reliable small field output factor measurement.
SU‐E‐T‐457: Design and Characterization of An Economical 192Ir Hemi‐Brain Small Animal Irradiator
Authors: Grams M , Wilson Z , Sio T , Beltran C , Tryggestad E , Gupta S , Blackwell C , McCollough K , Sarkaria J , Furutani K
Published: July 2014

Purpose:

To describe the design and dosimetric characterization of a simple and economical small animal irradiator.

Methods:

A high dose rate 192Ir brachytherapy source from a commercially available afterloader was used with a 1.3 centimeter thick tungsten collimator to provide sharp beam penumbra suitable for hemi‐brain irradiation of mice. The unit is equipped with continuous gas anesthesia to allow robust animal immobilization. Dosimetric characterization of the device was performed with Gafchromic film. The penumbra from the small animal irradiator was compared under similar collimating conditions to the penumbra from 6 MV photons, 6 MeV electrons, and 20 MeV electrons from a linear accelerator as well as 300 kVp photons from an orthovoltage unit and Monte Carlo simulated 90 MeV protons.

Results:

The tungsten collimator provides a sharp penumbra suitable for hemi‐brain irradiation, and dose rates on the order of 200 cGy/minute were achieved. The sharpness of the penumbra attainable with this device compares favorably to those measured experimentally for 6 MV photons, and 6 and 20 MeV electron beams from a linear accelerator. Additionally, the penumbra was comparable to those measured for a 300 kVp orthovoltage beam and a Monte Carlo simulated 90 MeV proton beam.

Conclusions:

The small animal irradiator described here can be built for under $1,000 and used in conjunction with any commercial brachytherapy afterloader to provide a convenient and cost‐effective option for small animal irradiation experiments. The unit offers high dose rate delivery and sharp penumbra, which is ideal for hemi‐brain irradiation of mice. With slight modifications to the design, irradiation of sites other than the brain could be accomplished easily. Due to its simplicity and low cost, the apparatus described is an attractive alternative for small animal irradiation experiments requiring a sharp penumbra.
SU‐E‐T‐458: Determining Threshold‐Of‐Failure for Dead Pixel Rows in EPID‐Based Dosimetry
Authors: Gersh J , Wiant D
Published: July 2014

Purpose:

A pixel correction map is applied to all EPID‐based applications on the TrueBeam (Varian Medical Systems, Palo Alto, CA). When dead pixels are detected, an interpolative smoothing algorithm is applied using neighboring‐pixel information to supplement missing‐pixel information. The vendor suggests that when the number of dead pixels exceeds 70,000, the panel should be replaced. It is common for entire detector rows to be dead, as well as their neighboring rows. Approximately 70 rows can be dead before the panel reaches this threshold. This study determines the number of neighboring dead‐pixel rows that would create a large enough deviation in measured fluence to cause failures in portal dosimetry (PD).

Methods:

Four clinical two‐arc VMAT plans were generated using Eclipse's AXB algorithm and PD plans were created using the PDIP algorithm. These plans were chosen to represent those commonly encountered in the clinic: prostate, lung, abdomen, and neck treatments. During each iteration of this study, an increasing number of dead‐pixel rows are artificially applied to the correction map and a fluence QA is performed using the EPID (corrected with this map). To provide a worst‐case‐scenario, the dead‐pixel rows are chosen so that they present artifacts in the highfluence region of the field.

Results:

For all eight arc‐fields deemed acceptable via a 3%/3mm gamma analysis (pass rate greater than 99%), VMAT QA yielded identical results with a 5 pixel‐width dead zone. When 10 dead lines were present, half of the fields had pass rates below the 99% pass rate. With increasing dead rows, the pass rates were reduced substantially.

Conclusion:

While the vendor still suggests to request service at the point where 70,000 dead rows are measured (as recommended by the vendor), the authors suggest that service should be requested when there are greater than 5 consecutive dead rows.
SU‐E‐T‐459: Dosimetric Consequences of Rotated Elliptical Proton Spots in Modeling In‐Air Proton Fluence for Calculating Doses in Water of Proton Pencil Beams
Authors: Matysiak W , Hsi W , Yeung D
Published: July 2014

Purpose:

We present a study of dosimetric consequences on doses in water in modeling in‐air proton fluence independently along principle axes for rotated elliptical spots.

Methods:

Phase‐space parameters for modeling in‐air fluence are the position sigma for the spatial distribution, the angle sigma for the angular distribution, and the correlation between position and angle distributions. Proton spots of the McLaren proton therapy system were measured at five locations near the isocenter for the energies of 180 MeV and 250 MeV. An elongated elliptical spot rotated with respect to the principle axes was observed for the 180 MeV, while a circular‐like spot was observed for the 250 MeV. In the first approach, the phase‐space parameters were derived in the principle axes without rotation. In the second approach, the phase space parameters were derived in the reference frame with axes rotated to coincide with the major axes of the elliptical spot. Monte‐Carlo simulations with derived phase‐space parameters using both approaches to tally doses in water were performed and analyzed.

Results:

For the rotated elliptical 180 MeV spots, the position sigmas were 3.6 mm and 3.2 mm in principle axes, but were 4.3 mm and 2.0 mm when the reference frame was rotated. Measured spots fitted poorly the uncorrelated 2D Gaussian, but the quality of fit was significantly improved after the reference frame was rotated. As a Result, phase space parameters in the rotated frame were more appropriate for modeling in‐air proton fluence of 180 MeV protons. Considerable differences were observed in Monte Carlo simulated dose distributions in water with phase‐space parameters obtained with the two approaches.

Conclusion:

For rotated elliptical proton spots, phase‐space parameters obtained in the rotated reference frame are better for modeling in‐air proton fluence, and can be introduced into treatment planning systems.
SU‐E‐T‐460: Comparison of Proton and IMRT Planning for Head and Neck Cancer
Authors: Fontenla S , Zhou Y , Mah D , Kowalski A , Leven T , Cahlon O , Lee N , Hunt M , Mechalakos J
Published: July 2014

Purpose:

A retrospective study comparing proton and intensity‐modulated radiation therapy (IMRT) for head and neck cancer

Methods:

This study consists of six H&N cancer patients that underwent proton as well as IMRT planning. Patients analyzed had unilateral target volumes, one had prior RT. 3D‐conformal proton therapy (3D‐CPT) plans with multiple field uniform scanning were generated for delivery on the inclined beam line. IMRT was planned using fixed field sliding window. Final plan evaluations were performed by a radiation oncologist and a physicist. Metrics for comparison included tumor coverage, organ sparing with respect to spinal cord, brainstem, parotids, submandibulars, oral cavity, larynx, brachial plexus, cochleas, normal brain tissue, and skin using relevant indices for these structures. Dose volume histograms were generated as well as a qualitative comparison of isodose distributions between the two modalities. Planning and treatment delivery times were compared.

Results:

Results showed that IMRT plans offered better conformality in the high dose region as demonstrated by the conformality index for each plan. Ipsilateral cochlea, submandibular gland, and skin doses were lower with IMRT than proton therapy. There was significant sparing of larynx, oral cavity, and brainstem with proton therapy compared to IMRT. This translated into direct patient benefit with no evidence of hoarseness, mucositis, or nausea. Contralateral parotid and submandibular glands were equally spared. IMRT had shorter planning/parts fabrication and treatment times which needs to be taken into account when deciding modality.

Conclusion:

Sparing of clinically significant normal tissue structures such as oral cavity and larynx for unilateral H&N cancers was seen with 3D‐CPT versus IMRT. However, this is at the expense of less conformality at the high dose region and higher skin dose. Future studies are needed with full gantry systems and pencil beam scanning as these deliveries would be expected to further improve conformality and normal tissue sparing.
SU‐E‐T‐461: Validation of Planning Algorithms in Dynamic Conformal Arc in IPlan Using ArcCHECK and 3DVH
Authors: Lei Y , Zhang Q , Li S , Morgan B , Driewer J , Zhou S
Published: July 2014

Purpose:

To study the effect of limited angular resolution of pencil beam calculation (PBC) on dynamic conformal arc plan (DCAP) in iPlan (BrainLab) using the ArcCHECK sytem and 3DVH software (Sun Nuclear Corporation).

Methods:

Four DCAPs were generated in iPlan RT Dose 4.5 treatment planning system on the ArcCHECK cylindrical phantom with central planning target volume (PTV). A cylindrical shell structure (SHELL) 2.85cm from phantom surface and 1.5 mm thickness was created to simulate the ArcCHECK diode array. Planned doses were calculated using both Monte Carlo calculation (MCC) and PBC algorithms, and exported to 3DVH software for global and target based comparisons using the 3Dgamma index. Four additional DCAPs were created and calculated on patient CT images and mapped onto the ArcCHECK phantom for measurement using a Varian TrueBeam STx. The measurements were compared against both MC and PB calculation using gamma index analysis.

Results:

For the ArcCHECK phantom, the dose distribution agreement quantified with 3D‐gamma index is better (average‐gamma (<γ>)=99.9%vs.79.1% and 96.8%vs45.7%, p=0.0294, 0.0286 for gamma (2mm,2%) and (1mm,1%) criteria respectively using Mann‐Whitney U test) in the PTV than in the SHELL. The measurements show better agreement with MCC than the PB (<γ>=100%vs.86.7%, 99.6%vs.72.3%, 85.5%vs.50.8%, p=0.021, 0.026, 0.029 for gamma (3mm,3%), (2mm,2%) and (1mm,1%) criteria using Mann‐Whitney U test respectively). The effect due to limited (10 degree) angular resolution of the PBC was observed, and it can be one of the possible reasons for poor agreement between measurement and PB calculation.

Conclusion:

The PBC of iPlan shows poor peripheral dose calculation accuracy for dynamic conformal arc plans due to limited angular resolution, but it performs well in the area close to target volume without considering heterogeneity. Since the user cannot change the 10 degree angular resolution of PBC, MCC is more appropriate for dynamic conformal arc plans.
SU‐E‐T‐462: Fixed‐Jaw Optimization for Critical Structure Sparing in IMRT Treatment Planning: Beam Modeling Cautions for Non‐Routine Use
Authors: Price R , Veltchev I , Cherian G , Ma C
Published: July 2014

Purpose:

Multiple publications exist concerning fixed‐jaw utilization to avoid linac carriage shifts and reduce intensity modulated radiotherapy (IMRT) treatment times. The purpose of this work is to demonstrate delivery QA discrepancies and illustrate the need for improved treatment planning system (TPS) commissioning for non‐routine use.

Methods:

A 6cm diameter spherical target was delineated on a virtual phantom containing the Iba Matrixx linear array within the Varian Eclipse TPS. Optimization was performed for target coverage for the following 3 scenarios: a single open, zero degree field where the X and Y jaws completely cover the target; the same field using an asymmetric, fixed‐jaw technique where the upper Y jaw does not cover the superior 2cm of the target; and both of the aforementioned directed at the target at 315 and 45 degree gantry angles, respectively. This final orientation was also irradiated on a linac for delivery analysis. A sarcoma patient case was also analyzed where the fixed jaw technique was utilized for kidney sparing.

Results:

The open beam results were as predicted but the fixed‐jaw results demonstrate a pronounced fluence increase along the asymmetric, upper jaw. Analysis of the delivery of the combined beam plan Resultin 83% of pixels evaluated passing gamma criteria of 3%, 3mm DTA. Analysis for the sarcoma patient, in the plane of the shielded kidney, indicated 93% passing although the maximum dose discrepancies in this region were approximately 23%.

Conclusion:

Optimization within the target is routinely performed using MLC leaf‐end characteristics. The fixed‐jaw technique forces optimization of target coverage to utilize the penumbra profiles of the associated beamdefining jaw. If the profiles were collected using a common 0.125cc ionization chamber, the resolution may be insufficient resulting in a planvs.‐delivery mismatch. It is recommended that high‐resolution beam characteristics be considered when non‐routine planning methods are utilized.
SU‐E‐T‐463: Quantification of Rotational Variation in Mouse Setup for IGRT
Authors: McCarroll R , Rubinstein A , Kingsley C , Yang J , Yang P , Court L
Published: July 2014

Purpose:

New small‐animal irradiators include extremely precise IGRT capabilities. However, mouse immobilization and localization remains a challenge. In particular, unlike week‐to‐week translational displacements, rotational changes in positioning are not easily corrected for in subject setup. Using two methods of setup, we aim to quantify week‐to‐week rotational variation in mice for the purpose of IGRT planning in small animal studies.

Methods:

Ten mice were imaged weekly using breath‐hold CBCT (X‐RAD 225 Cx), with the mouse positioned in a half‐pipe support, providing 40 scans. A second group of two mice were positioned in a 3D printed immobilization device, which was created using a CT from a similarly shaped mouse, providing 10 scans. For each mouse, the first image was taken to be the reference image. Subsequent CT images were then rigidly registered, based on bony anatomy. Rotations in the axial (roll), sagittal (pitch), and coronal (yaw) planes were recorded and used to quantify variation in angular setup.

Results:

For the mice imaged in the half pipe, average magnitude of roll was found to be 5.4±4.6° (range: −12.9:18.86°), of pitch 1.6±1.3° (range: −1.4:4.7°), and of yaw 1.9±1.5° (range −5.4:1.1°). For the mice imaged in the printed setup; average magnitude of roll was found to be 0.64±0.6° (range: −2.1:1.0°), of pitch 0.6±0.4° (range: 0.0:1.3°), and of yaw 0.2±0.1° (range: 0.0:0.4°). The printed setup provided reduction in roll, pitch, and yaw by 88, 62, and 90 percent, respectively.

Conclusion:

For the typical setup routine, roll in mouse position is the dominant source of rotational variation. However, when a printed device was used, drastic improvements in mouse immobilization were seen. This work provides a promising foundation for mouse immobilization, required for full scale small animal IGRT. Currently, we are making improvements to allo±w the use of a similar system for MR, PET, and bioluminescence.
SU‐E‐T‐464: On the Equivalence of the Quality Correction Factor for Pencil Beam Scanning Proton Therapy
Authors: Sorriaux J , Paganetti H , Testa M , Giantsoudi D , Schuemann J , Bertrand D , Orban de Xivry J. , Lee J , Palmans H , Vynckier S , Sterpin E
Published: July 2014

Purpose:

In current practice, most proton therapy centers apply IAEA TRS‐398 reference dosimetry protocol. Quality correction factors (kQ) take into account in the dose determination process the differences in beam qualities used for calibration unit and for treatment unit. These quality correction factors are valid for specific reference conditions. TRS‐398 reference conditions should be achievable in both scattered proton beams (i.e. DS) and scanned proton beams (i.e. PBS). However, it is not a priori clear if TRS‐398 kQ data, which are based on Monte Carlo (MC) calculations in scattered beams, can be used for scanned beams. Using TOPAS‐Geant4 MC simulations, the study aims to determine whether broad beam quality correction factors calculated in TRS‐398 can be directly applied to PBS delivery modality.

Methods:

As reference conditions, we consider a 10×10×10 cm3 homogeneous dose distribution delivered by PBS system in a water phantom (32/10 cm range/modulation) and an air cavity placed at the center of the spread‐out‐Bragg‐peak. In order to isolate beam differences, a hypothetical broad beam is simulated. This hypothetical beam reproduces exactly the same range modulation, and uses the same energy layers than the PBS field. Ion chamber responses are computed for the PBS and hypothetical beams and then compared.

Results:

For an air cavity of 2×2×0.2 cm3, the ratio of ion chamber responses for the PBS and hypothetical beam qualities is 0.9991 ± 0.0016.

Conclusion:

Quality correction factors are insensitive to the delivery pattern of the beam (broad beam or PBS), as long as similar dose distributions are achieved. This investigation, for an air cavity, suggests that broad beam quality correction factors published in TRS‐398 can be applied for scanned beams.

J. Sorriaux is financially supported by a public‐private partnership involving the company Ion Beam Applications (IBA).
SU‐E‐T‐465: Dose Calculation Method for Dynamic Tumor Tracking Using a Gimbal‐Mounted Linac
Authors: Sugimoto S , Utsunomiya S , Ebe K , Inoue T , Kurokawa C , Usui K , Sasai K
Published: July 2014

Purpose:

Dynamic tumor tracking using the gimbal‐mounted linac (Vero4DRT, Mitsubishi Heavy Industries, Ltd., Japan) has been available when respiratory motion is significant. The irradiation accuracy of the dynamic tumor tracking has been reported to be excellent. In addition to the irradiation accuracy, a fast and accurate dose calculation algorithm is needed to validate the dose distribution in the presence of respiratory motion because the multiple phases of it have to be considered. A modification of dose calculation algorithm is necessary for the gimbal‐mounted linac due to the degrees of freedom of gimbal swing. The dose calculation algorithm for the gimbal motion was implemented using the linear transformation between coordinate systems.

Methods:

The linear transformation matrices between the coordinate systems with and without gimbal swings were constructed using the combination of translation and rotation matrices. The coordinate system where the radiation source is at the origin and the beam axis along the z axis was adopted. The transformation can be divided into the translation from the radiation source to the gimbal rotation center, the two rotations around the center relating to the gimbal swings, and the translation from the gimbal center to the radiation source. After operating the transformation matrix to the phantom or patient image, the dose calculation can be performed as the no gimbal swing. The algorithm was implemented in the treatment planning system, PlanUNC (University of North Carolina, NC). The convolution/superposition algorithm was used. The dose calculations with and without gimbal swings were performed for the 3 × 3 cm2 field with the grid size of 5 mm.

Results:

The calculation time was about 3 minutes per beam. No significant additional time due to the gimbal swing was observed.

Conclusions:

The dose calculation algorithm for the finite gimbal swing was implemented. The calculation time was moderate.
SU‐E‐T‐466: Dosimetric Assessment of the Salivary Glands in Head and Neck Tumour Patients Treated with Radiation Therapy
Authors: Peixoto Xavier C , Costa Ferreira B , Khouri L , Lopes M , Stathakis S , Papanikolaou N , Mavroidis P
Published: July 2014

Purpose:

To estimate the correlation between different dosimetric indices and the clinical outcome, which was recorded at different time points after the treatment, regarding the structure of salivary glands in head & neck cancer radiotherapy.

Methods:

218 salivary glands from patients with head and neck tumours treated at IPOCFG from 2007 to 2013 were included in this study. The follow up of those patients consisted on weekly medical visits during the treatment that take about seven weeks. After that the patients had consults every three months. Treatment complications were graded using the RTOG/EORTC guidelines. The response of the salivary glands (both parotids, oral cavity and both submandibular glands) was analyzed for six time periods: 7th week, 3rd, 7th, 12th, 18th and 24th months after the start of the radiotherapy. The total dose distributions, converted to a fractionation scheme of 2Gy fractional doses, were used to calculate the DVHs and dose‐response plots of salivary glands.

Results:

The small differences obtained between the mean DVHs showed that patients should be grouped into: with complications (G1+G2) and without complications (G0). The mean dose/2Gy was 22.2±13.2 Gy (G0) and 33.2±8.0 Gy (G1+G2) for the 7th week, 31.9±9.6 Gy (G0) and 34.6±6.8 Gy (G1+G2) for the 12th month and, 32.9±9.3 Gy (G0) and 37.2±8.5 Gy (G1+G2) for the 24th month. The dose‐response plots for the 7th week and 24th month were similar to the dose‐response curves published in the literature.

Conclusion:

In some cases, there were not major differences between the mean DVHs of the groups with and without complications in the salivary glands, when comparing them at different time periods. More radiobiological analysis should thus be made to estimate the clinical impact of those differences.
SU‐E‐T‐467: Definition of the Proper Tolerances for Beam‐Position Accuracy and Beam Width for Quality Assurance in Active Proton Pencil Beam Scanning
Authors: Safai S
Published: July 2014

Purpose:

To estimate the correlation between different dosimetric indices and the clinical outcome, which was recorded at different time points after the treatment, regarding the structure of salivary glands in head & neck cancer radiotherapy.

Methods:

218 salivary glands from patients with head and neck tumours treated at IPOCFG from 2007 to 2013 were included in this study. The follow up of those patients consisted on weekly medical visits during the treatment that take about seven weeks. After that the patients had consults every three months. Treatment complications were graded using the RTOG/EORTC guidelines. The response of the salivary glands (both parotids, oral cavity and both submandibular glands) was analyzed for six time periods: 7th week, 3rd, 7th, 12th, 18th and 24th months after the start of the radiotherapy. The total dose distributions, converted to a fractionation scheme of 2Gy fractional doses, were used to calculate the DVHs and dose‐response plots of salivary glands.

Results:

The small differences obtained between the mean DVHs showed that patients should be grouped into: with complications (G1+G2) and without complications (G0). The mean dose/2Gy was 22.2±13.2 Gy (G0) and 33.2±8.0 Gy (G1+G2) for the 7th week, 31.9±9.6 Gy (G0) and 34.6±6.8 Gy (G1+G2) for the 12th month and, 32.9±9.3 Gy (G0) and 37.2±8.5 Gy (G1+G2) for the 24th month. The dose‐response plots for the 7th week and 24th month were similar to the dose‐response curves published in the literature.

Conclusion:

In some cases, there were not major differences between the mean DVHs of the groups with and without complications in the salivary glands, when comparing them at different time periods. More radiobiological analysis should thus be made to estimate the clinical impact of those differences.
SU‐E‐T‐468: A New 3D Gamma Analysis Algorithm for Better VMAT/IMRT QA Results
Authors: Cao D , Ye J , Shepard D
Published: July 2014

Purpose:

The gamma dose distribution comparison method is used in VMAT QA to score the agreement between planned and measured dose distributions. Unfortunately, this dose comparison technique can Resultin compromised passing rates when the treatment plan includes a high dose gradient perpendicular to the measurement plane. To address this, we have developed and tested a 3D gamma analysis tool.

Methods:

An in‐house 3D‐gamma analysis algorithm was developed. Five VMAT patient specific QA plans were delivered and measured using a MatriXX 2D ion chamber array. A 3D dose volume was generated in the Pinnacle TPS, and exported to the analysis software. Both 2D and 3D gamma analysis were performed. The results determined using a 3%/3mm passing criteria were then evaluated and compared.

Results:

Cases that did not include a high dose gradient perpendicular and adjacent to the detector plane showed only minor differences between the 2D and 3D gamma analysis results. Cases including a high dose gradient (e.g. as a means to avoid an adjacent sensitive structure), the use of 3D gamma analysis resulted in improved passing rates with lower gamma values. For the five cases tested, the average gamma index value was reduced from 0.361 to 0.345 when 3D analysis was used. The average passing rate increased from 94.7% to 96%.

Conclusion:

A 3D gamma analysis tool has been developed and tested. For patient specific IMRT/VMAT QA performed using the composite dose measurement method, the use of 3D gamma analysis can effectively reduce the uncertainty in QA results due to factors such as setup errors. This technique can more effectively account for a high dose gradient perpendicular to the measurement plan. Consequently, improved and more meaningful QA results can be achieved using this technique.
SU‐E‐T‐469: A Practical Approach for the Determination of Small Field Output Factors Using Published Monte Carlo Derived Correction Factors
Authors: Calderon E , Siergiej D
Published: July 2014

Purpose:

Output factor determination for small fields (less than 20 mm) presents significant challenges due to ion chamber volume averaging and diode over‐response. Measured output factor values between detectors are known to have large deviations as field sizes are decreased. No set standard to resolve this difference in measurement exists. We observed differences between measured output factors of up to 14% using two different detectors. Published Monte Carlo derived correction factors were used to address this challenge and decrease the output factor deviation between detectors.

Methods:

Output factors for Elekta's linac‐based stereotactic cone system were measured using the EDGE detector (Sun Nuclear) and the A16 ion chamber (Standard Imaging). Measurements conditions were 100 cm SSD (source to surface distance) and 1.5 cm depth. Output factors were first normalized to a 10.4 cm × 10.4 cm field size using a daisy‐chaining technique to minimize the dependence of field size on detector response. An equation expressing the relation between published Monte Carlo correction factors as a function of field size for each detector was derived. The measured output factors were then multiplied by the calculated correction factors. EBT3 gafchromic film dosimetry was used to independently validate the corrected output factors.

Results:

Without correction, the deviation in output factors between the EDGE and A16 detectors ranged from 1.3 to 14.8%, depending on cone size. After applying the calculated correction factors, this deviation fell to 0 to 3.4%. Output factors determined with film agree within 3.5% of the corrected output factors.

Conclusion:

We present a practical approach to applying published Monte Carlo derived correction factors to measured small field output factors for the EDGE and A16 detectors. Using this method, we were able to decrease the percent deviation between both detectors from 14.8% to 3.4% agreement.