bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐F‐T‐509: Investigation into the Impact of the Linear Accelerator Treatment Table On Skin Dose to Prone Breast Patients
Authors: Pedersen K , Irwin J , Sansourekidou P , Kriminski S , Pavord D
Published: July 2016

Purpose:

To investigate the impact of the treatment table on skin dose for prone breast patients for which the breast contacts the table and to develop a method to decrease skin dose.

Methods:

We used 12cm stack of 15cmx15cm solid water slabs to imitate breast. Calibrated EBT3 radiochromic film was affixed to the bottom of the phantom. Treatments for 32 patients were analyzed to determine typical prone breast beam parameters. Based on the analysis, a field size and a range of gantry angles were chosen for the test beams. Three experimental setups were used. The first represented the patient setup currently used in our clinics with the phantom directly on the table. The second was the skin sparing setup, with a 1.5cm Styrofoam slab between the phantom and the table. The third used a 7.5cm Styrofoam slab to examine the extent of skin sparing potential. The calibration curve was applied to each film to determine dose. Percent difference in dose between the current and skin sparing setups was calculated for each gantry angle and gantry angle pair.

Results:

Data showed that beams entering through the table showed a skin dose decrease ranging from 13%–30% with the addition of 7.5cm Styrofoam, while beams exiting through the table showed no significant difference. The addition of 1.5cm Styrofoam resulted in differences ranging from 0.5%–13% with the skin sparing setup.

Conclusion:

The results demonstrate that skin in contact with the table receives increased dose from beams entering through the table. By creating separation between the breast and the table with Styrofoam the skin dose can be lowered, but 1.5 cm did not fully mitigate the effect. Further investigation will be performed to identify a clinically practical thickness that maximizes this mitigation.
SU‐F‐T‐510: Single Isocenter Radiosurgery for the Simultaneous Treatment of Multiple Brain Metastases: Volumetric Modulated Arc Therapy Or Dynamic Conformal Arc?
Authors: Maurer J , Sintay B , Manning M , Wiant D , Liu H
Published: July 2016

Purpose:

This study evaluates a novel algorithm that can be used with any treatment planning system for simple and rapid generation of stereotactic radiosurgery (SRS) plans for treating multiple brain metastases using a single isocenter dynamic conformal arc (DCA) approach. This technique is compared with a single isocenter volumetric modulated arc therapy (VMAT) technique in terms of delivery time, conformity, low dose spread and delivery accuracy.

Methods:

Five patients, with a total of 37 (5 – 11) targets were planned using a previously published method for generating optimal VMAT plans and using the proposed DCA algorithm. All planning target volumes (PTVs) were planned to 20 Gy, meeting a minimum 99% coverage and maximum 135 % hot spot for both techniques. Quality assurance was performed using radiochromic film, with films placed in the high dose regions of each PTV. Normal tissue volumes receiving 12 Gy and 6 Gy (V12 and V6) were computed for each plan. Conformity index (CI) and gamma evaluations (95% of points passing 4%/0.5mm) were computed for each PTV.

Results:

Delivery times, including beam on and table rotation times, were comparable: 17 – 22 minutes for all deliveries. V12s for DCA plans were (18.5±15.2 cc) vs. VMAT (19.7±14.4 cc). V6s were significantly lower for DCA (69.0±52.0 cc) compared with VMAT (154.0±91.0 cc) (p <<0.05). CIs for VMAT targets were (1.38±0.50) vs. DCA (1.61±0.41). 36 of 37 DCA planned targets passed gamma tests, while 29 of 37 VMAT planned targets passed.

Conclusion:

Single isocenter DCA plans were easily achieved. The evaluation suggests that DCA may represent a favorable technique compared with VMAT for multiple target SRS by reducing dose to normal tissue and more accurately depicting deliverable dose.
SU‐F‐T‐514: Evaluation of the Accuracy of Free‐Breathing and Deep Inspiration Breath‐Hold Gated Beam Delivery Using An Elekta Linac
Authors: Jermoumi M , Xie R , Cao D , Housley D , Shepard D
Published: July 2016

Purpose:

In this study, we evaluated the performance of an Elekta linac in the delivery of gated radiotherapy. We examined whether the use of either a short gating window or a long beam hold impacts the accuracy of the delivery

Methods:

The performance of an Elekta linac in the delivery of gated radiotherapy was assessed using a 20cmX 20cm open field with the radiation delivered using a range of beam‐on and beam‐off time periods. Two SBRT plans were used to examine the accuracy of gated beam delivery for clinical treatment plans. For the SBRT cases, tests were performed for both free‐breathing based gating and for gated delivery with a simulated breath‐hold. A MatriXX 2D ion chamber array was used for data collection, and the gating accuracy was evaluated using gamma score.

Results:

For the 20cmX20cm open field, the gated beam delivery agreed closely with the non‐gated delivery results. Discrepancies in the agreement, however, began to appear with a 5‐to‐1 ratio of the beam‐off to beam‐on. For these tight gating windows, each beam‐on segment delivered a small number of monitor units. This finding was confirmed with dose distribution analysis from the delivery of the two VMAT plans where the gamma score(±1%,2%/1mm) showed passing rates in the range of 95% to 100% for gating windows of 25%, 38%, 50%, 63%, 75%, and 83%. Using a simulated sinusoidal breathing signal with a 4 second period, the gamma score of freebreathing gating and breath‐hold gating deliveries were measured in the range of 95.7% to 100%.

Conclusion:

The results demonstrate that Elekta linacs can be used to accurately deliver respiratory gated treatments for both free‐breathing and breath‐hold patients. The accuracy of beams delivered in a gated delivery mode at low small MU proved higher than similar deliveries performed in a non‐gated (manually interrupted) fashion.
SU‐F‐T‐516: Effects of Inter‐Fraction Organ Displacement/deformation On the Delivered Doses to the Heart, Esophagus, and Lungs in Patients Receiving Thoracic Radiotherapy
Authors: Hammers J , Matney J , Kaidar‐Person O , Zagar T , Marks L , Das S , Mavroidis P
Published: July 2016

Purpose:

To quantitatively assess the effects of inter‐fraction changes in organ shape and location on the delivered dose distribution to the organs at risk (OAR) in lung cancer patients.

Methods:

This study analyzes treatment data of 10 patients, who were treated to 60Gy in 30 fractions. In each fraction a cone beam CT (CBCT) was acquired. Each CBCT was registered with the planning CT using deformable registration tools within MIM Software. The daily setup shifts were used to translate the planned dose distribution on the deformed planning CT. The structures of lungs, esophagus and heart were re‐delineated by a physician on each CBCT. The doses delivered to each OAR, reflecting changes in the position and shape variations, were recomputed. Resultant daily dose volume histograms (DVHs) for OARs were computed and compared to those from the planning CT.

Results:

Based on the findings of two patients and 24 CBCTs analyzed so far, higher doses are delivered to the lungs and esophagus compared to the treatment plan. The dose differences per fraction between the delivered doses and those in the treatment plan are: for patient 1, lung mean dose = 5.3±1.3cGy and esophagus mean dose = 3.4±3.5cGy. For patient 2, lung mean dose = 12.0±3.9cGy and esophagus mean dose = 34.2±7.5cGy. Regarding the maximum dose to heart, the results varied (−18.9±22.0cGy for patient1 and 53.0±62.2cGy for patient2).

Conclusion:

The dosimetric effects of inter‐fractional anatomical variations could be estimated using deformable image registration and manual organ segmentation for each CBCT. A considerable dose distribution variation between fractions was observed for the OARs. These changes are currently not taken into account while treating the patients and these may explain cases with severe side effects even when the treatment plan looks satisfactory. These results suggest the need for automated daily dose tracking and accumulation.
SU‐F‐T‐517: Determining the Tissue Equivalence of a Brass Mesh Bolus in a Reconstructed Chest Wall Irradiation
Authors: Shekel E , Epstein D , Levin D
Published: July 2016

Purpose:

To determine the tissue equivalence of a brass mesh bolus (RPD) in the setting of a reconstructed chest wall irradiation

Methods:

We measured breast skin dose delivered by a tangential field plan on an anthropomorphic phantom using Mosfet and nanoDot (Landauer) dosimeters in five different locations on the breast. We also measured skin dose using no bolus, 5mm and 10 mm superflab bolus. In the Eclipse treatment planning system (Varian, Palo Alto, CA) we calculated skin dose for different bolus thicknesses, ranging from 0 to 10 mm, in order to evaluate which calculation best matches the brass mesh measurements, as the brass mesh cannot be simulated due to artefacts.Finally, we measured depth dose behavior with the brass mesh bolus to verify that the bolus does not affect the dose to the breast itself beyond the build‐up region.

Results:

Mosfet and nanoDot measurements were consistent with each other.As expected, skin dose measurements with no bolus had the least agreement with Eclipse calculation, while measurements for 5 and 10 mm agreed well with the calculation despite the difficulty in conforming superflab bolus to the breast contour. For the brass mesh the best agreement was for 3 mm bolus Eclipse calculation. For Mosfets, the average measurement was 90.8% of the expected dose, and for nanoDots 88.33% compared to 83.34%, 88.64% and 93.94% (2,3 and 5 mm bolus calculation respectively).The brass mesh bolus increased skin dose by approximately 25% but there was no dose increase beyond the build‐up region.

Conclusion:

Brass mesh bolus is most equivalent to a 3 mm bolus, and does not affect the dose beyond the build‐up region. The brass mesh cannot be directly calculated in Eclipse, hence a 3mm bolus calculation is a good reflection of the dose response to the brass mesh bolus.
SU‐F‐T‐530: Characterization of a 60‐Leaf Motorized MLC Designed for Cobalt‐60 Units
Authors: Xu L , Smith L , Ciresianu A
Published: July 2016

Purpose:

In a continuing effort to improve conformal radiation therapy with Cobalt‐60 units, a 60‐leaf MLC was designed, manufactured, and released to market. This work describes the physics measurements taken to characterize the clinical performance of this MLC.

Methods:

A 60 leaf MLC was custom designed with tungsten leaves of 4.5 cm height, single focused, achieving field size of 30×30 cm^2 when mounted on a 100cm SAD Cobalt‐60 unit. Leakage and output factor measurements were performed using a single ion chamber in a solid water phantom. Penumbra and surface dose were measured using scanning chambers and diodes in a water phantom. Radiation‐light coincidence measurements were performed using radiographic films.

Results:

With MLC mounted, measured penumbras at all depths are smaller than with jaws only. Surface doses were not significantly affected by the presence of MLC, and remained below values recommended by regulatory bodies. Light‐radiation coincidences were found to be better than 3 mm for all field sizes. Leakage through the MLC was found to be strongly dependent on field size, increasing from 1.0 % for a 10×10 cm field to 2.0% for a 30×30 cm field. Such results meet the requirements of IEC 60601‐2‐11. The MLC was found to have significant influence on the output factor, when field size defined by MLC is significantly smaller than field size defined by jaws. Such effect is also observed on linear accelerators, but it is more pronounced on Cobalt‐60 units. A 10×10 “diamond” MLC shape inside a 14×14 cm jaw showed output factor that is 5.7% higher than 10×10 cm field defined by matching MLC and jaws.

Conclusion:

The MLC offers clinically acceptable performance in penumbra, surface dose, and light‐radiation coincidence. Several units of this MLC have recently been installed and used clinically. Validation of Cobalt‐60 based IMRT with this MLC is ongoing.

The authors are employees of Best Theratrnics Ltd.
SU‐F‐T‐533: Study of Dosimetric Properties of Cadmium Free Alloy Used in Compensator Based IMRT
Authors: Tyagi A , Kaushik S , Punia R , Singh M
Published: July 2016

Purpose:

To study the dosimetric properties of cadmium free alloy which is used in compensator based IMRT.

Methods:

A mixture of 30% of lead,52% of bismuth and 18% of tin was used to prepare alloy. We prepared slabs of different thicknesses ranging from 0.71 cm to 6.14 cm. Density of alloy was measured by Archimedes’ principle using SI‐234 Denver instrument and water as buoyant liquid. Transmission, linear attenuation coefficient (µ), tissue phantom ration (TPR), beam hardening, surface dose (Ds), percentage depth dose (PDD) and effect of scatter were measured and analyze for different field size and different thickness of compensator for 6 MV photon beam. Measurements were carried out at 100 cm SSD and 160 cm SSD.

Results:

Density of alloy was found to be 9.5456 gm/cm3. Melting point of alloy is 90–95 °C. For a field size of 10×10 cm2 µ was 0.4253 cm‐1 at 100 cm SSD. Calculated TPR was found to be within 3 % of measured TPR. Ds was found to be decreasing with increasing thickness of compensator. 1cm, 1.98 cm and 4.16 cm thick compensator slab decreased surface dose by 4.2%, 6.1% and 9.5% respectively for a field size of 10×10cm2 at 100 cm SSD. As field size increases Ds increases for a given compensator thickness. This is due to increase in amount of scattered dose from wider collimator opening. For smaller field size, PDDs are increased from 3.0% to 5.5% of open beam PDDs as compensator thickness increases from 1 cm to 6.14 cm at a depth of 10 cm in water. For larger field size variation in PDDs is not significant.

Conclusion:

High degree of modulation can be achieved from this compensator material, which is essential in compensator based IMRT. Dosimetric properties analyzed in this study establish this alloy as a reliable, cost effective, reusable compensator material.
SU‐F‐T‐534: Using Daily Images to Verify Dosimetric Advantage of Dual‐Isocentric Technique for High‐Risk Prostate Cancer Treated with Pelvic Lymph Node
Authors: Cui T , Shang Q , Li W , Vassil A , Xia P
Published: July 2016

Purpose:

To evaluate a dual‐isocentric technique to compensate for the daily independent movements of prostate and pelvic lymph nodes (PLN) of high‐risk prostate cancer patients.

Methods:

Three patients, who received 78 Gy to the prostate, 66 Gy to the seminal vesicle (SV), and 50.4 Gy to the PLN in 39 fractions under CT‐on‐rail (CTOR) imaging guidance, were retrospectively selected. Each patient was planned with volumetric modulated arc therapy (VMAT) techniques using (1) single isocenter with a 6 mm margin except 4 mm posteriorly (6/4 mm), to the prostate‐CTV and a uniform 5 mm margin to the PLN‐CTV, and (2) two isocenters, located at the centroids of the prostate or the PLN, with a 5/3 mm margin to the prostate‐CTV and a 5 mm uniform margin to the PLN‐CTV. Both plans met the established clinical guidelines, and were subsequently applied to the daily CTOR images, where prostate, SV, rectum and bladder were contoured by the same physician. On each of 117 CTOR images, two plans were created by registering only the bony anatomy for the single‐isocentric one, or by registering both the bony anatomy and the prostate for the dual‐isocentric one. These plans were then compared based on V78Gy of prostate‐CTV, V66Gy of SV‐CTV, and V50.4Gy of PLN‐CTV. The paired Student's t‐test was performed to test for significance.

Results:

The V78Gy of prostate‐CTV in the dual‐isocentric plans from 117 CTOR images was 99.32±0.89%, significantly (p<0.01) better than that in the single‐isocentric ones (98.29±2.11%). Both V66Gy of SV‐CTV and V50.4Gy of PLN‐CTV were increased from 99.15±3.28% and 99.38±0.74% in the dual‐isocentric plans to 99.31±2.02% and 99.48±0.90% in the single‐isocentric ones, respectively, but neither were statistically significant (p=0.47 and 0.34).

Conclusion:

By targeting prostate and PLN accordingly in the daily verification images, the dosimetric advantage of dual‐isocentric technique is verified even with a reduced margin.
SU‐F‐T‐537: Prone Breast Accelerated Partial Breast Irradiation Using Non‐Coplanar Volumetric Arc Therapy
Authors: Beninati G , Barbiere J , Godfrey L , Ndlovu A
Published: July 2016

Purpose:

To demonstrate that Volumetric Modulated Arc Therapy (VMAT) can be an alternative technique to Brachytherapy Accelerated Partial Breast Irradiation (APBI) for treating large breasted women. The non‐coplanar VMAT technique uses a commercially available couch and a small number of angles. This technique with the patient in the prone position can reduce high skin and critical structure doses in large breasted women, which are usually associated with Brachytherapy APBI.

Methods:

Philips Pinnacle treatment planning system with Smart Arc was used to plan a left sided laterally located excision cavity on a standard prone breast patient setup. Three thirty‐degree arcs entered from the lateral side at respective couch angles of 345, 0, and 15 degrees. A fourth thirty degree arc beam entered from the medial side at a couch angle of 0 degrees. The arcs were selected to avoid critical structures as much as possible. A test run was then performed to verify that the beams did not collide with the patient nor support structures. NSABP B‐39/RTOG 0413 protocol guidelines were used for dose prescription, normal tissue, and target definition.

Results:

Dose Volume Histogram analysis indicated that all parameters were equal or better than RTOG recommendations. Of particular note regarding the plan quality:1.(a) For a prescribed dose of 3850cGy the PTV‐EVAL target volume receiving 100 percent of the dose(V100) was 93; protocol recommendation is V90 > 90 percent. (b) Maximum dose was 110 percent versus the allowed 120 percent .2. Uninvolved percentage of normal breast V100 and V50 were 17 and 47 versus allowed 35 and 60 percent respectively.3. For the skin, V100 was 5.7cc and the max dose to 0.1 cc was 4190cGy.

Conclusion:

Prone Breast non‐coplanar VMAT APBI can achieve better skin cosmesis and lower critical structure doses than Brachytherapy APBI.
SU‐F‐T‐538: CyberKnife with MLC for Treatment of Large Volume Tumors: A Feasibility Study
Authors: Bichay T , Mayville A
Published: July 2016

Purpose:

CyberKnife is a well‐documented modality for SRS and SBRT treatments. Typical tumors are small and 1–5 fractions are usually used. We determined the feasibility of using CyberKnife, with an InCise multileaf collimator option, for larger tumors undergoing standard dose and fractionation. The intent was to understand the limitation of using this modality for other external beam radiation treatments.

Methods:

Five tumors from different anatomical sites with volumes from 127.8 cc to 1,320.5 cc were contoured and planned on a Multiplan V5.1 workstation. The target average diameter ranged from 7 cm to 13 cm. The dose fractionation was 1.8–2.0 Gy/fraction and 25–45 fractions for total doses of 45–81 Gy. The sites planned were: pancreas, head and neck, prostate, anal, and esophagus. The plans were optimized to meet conventional dose constraints based on various RTOG protocols for conventional fractionation.

Results:

The Multiplan treatment planning system successfully generated clinically acceptable plans for all sites studied. The resulting dose distributions achieved reasonable target coverage, all greater than 95%, and satisfactory normal tissue sparing. Treatment times ranged from 9 minutes to 38 minutes, the longest being a head and neck plan with dual targets receiving different doses and with multiple adjacent critical structures.

Conclusion:

CyberKnife, with the InCise multileaf collimation option, can achieve acceptable dose distributions in large volume tumors treated with conventional dose and fractionation. Although treatment times are greater than conventional accelerator time; target coverage and dose to critical structures can be kept within a clinically acceptable range. While time limitations exist, when necessary CyberKnife can provide an alternative to traditional treatment modalities for large volume tumors.
SU‐F‐T‐539: Dosimetric Comparison of Volumetric Modulated Arc Therapy and Intensity Modulated Radiation Therapy for Whole Brain Hippocampal Sparing Radiation Therapy Treatments
Authors: Kendall E , Higby C , Algan O , Ahmad S , Hossain S
Published: July 2016

Purpose:

To compare the treatment plan quality and dose gradient near the hippocampus between VMAT (RapidArc) and IMRT delivery techniques for whole brain radiation therapy.

Methods:

Fifteen patients were evaluated in this retrospective study. All treatments were planned on Varian Eclipse TPS, using 3‐Arc VMAT and 9‐Field IMRT, following NRG Oncology protocol NRG‐CC001 guidelines evaluated by a single radiation oncologist. Prescribed doses in all plans were 30 Gy delivered over 10 fractions normalized to a minimum of 100% of the dose covering 95% of the target volume. Identical contour sets and dose‐volume constraints following protocol guidelines were also applied in all plans. A paired t‐test analysis was used to compare VMAT and IMRT plans.

Results:

NRG‐CC001 protocol dose‐volume constraints were met for all VMAT and IMRT plans. For the planning target volume (PTV), the average values for D2% and D98% were 6% lower and 4% higher in VMAT than in IMRT, respectively. The average mean and maximum hippocampus doses in Gy for VMAT vs IMRT plans were (11.85±0.81 vs. 12.24±0.56, p=0.10) and (16.27±0.78 vs. 16.59±0.71, p=0.24), respectively. In VMAT, the average mean and maximum chiasm doses were 3% and 1% higher than in IMRT plans, respectively. For the left optic nerve, the average mean and maximum doses were 10% and 5% higher in VMAT than in IMRT plans, respectively. These values were 12% and 3% for the right optic nerve. The average percentage of dose gradient around the hippocampus in the 0–5mm and 5–10mm abutted regions for VMAT vs. IMRT were (4.42%±2.22% /mm vs. 3.95%±2.61% /mm, p=0.43) and (4.54%±1.50% /mm vs. 4.39%±1.28% /mm, p=0.73), respectively.

Conclusion:

VMAT plans can achieve higher hippocampus sparing with a faster dose fall‐off than IMRT plans. Though statistically insignificant, VMAT offers better PTV coverage with slightly higher doses to OARs.
SU‐F‐T‐541: Impact of VMAT Dose Calculations with Respiratory Movements in Lung
Authors: Komatsu K , Araki F , Ohno T , Shimohigashi Y , Onizuka R
Published: July 2016

Purpose:

To evaluate dose impact due to respiratory movements for volumetric modulated arc therapy (VMAT) in lung, using deformation technique.

Methods:

The dose impact due to respiratory movements was evaluated for VMAT in lung. VMAT dose distributions were calculated with 4D‐CT images for a 3‐D dynamic lung phantom and seven patients. First, one‐arc VMAT dose distributions were calculated at the reference phase (the maximum of expiratory phase) using the Varian Eclipse (ver.10) treatment planning system. All VMAT were planned with 48Gy for 4 fractions. The VMAT dose distributions were divided into 88 control points (CPs) and were assigned to each one of ten phases divided through one respiratory cycle time. Next, the dose distributions were recalculated at each phase and were deformed at the reference phase using DIRART ver.1.0a and Velocity AI ver.3.1 software. The deformed dose distributions were summed up. Finally, VMAT dose distributions with respiratory movements were evaluated from dose volume histograms (DVHs).

Results:

VMAT dose distributions deformed by DIRART and the Velocity AI in the phantom study agreed within 2% with that of the reference phase for 95% of GTV (D95). In the patient study, deformed VMAT dose distributions decreased up to 4.15% and 3.46% for DIRART and Velocity AI, respectively. The maximum dose of GTV (Dmax) tended to be higher with increasing tumor movements. In constant, D95 for DVHs decreased with increasing tumor movements. The discrepancy in the VMAT dose distributions deformed by two software depended on specific properties of respective algorithms and parameters.

Conclusion:

Respiratory movements impact on the dose distributions for GTV. The Dmax may relate with amplitude of tumor movements.
SU‐F‐T‐542: Using Calypso Electromagnetic Beacons to Determine When to Modulate and When Not to Modulate SBRT Treatment Plans When Breathing Motion Is Present
Authors: Saleh H , Howlin T , Pokhrel D
Published: July 2016

Purpose:

To use the Calypso system to conduct a comprehensive study to determine what is the maximum breathing amplitude and length of breathing cycle allowed to accurately deliver IMRT or VMAT plans for liver tumors treated with SBRT when breathing motion is present.

Methods:

A phantom that simulates breathing motion was constructed using solid water. The phantom contains a 5 cm diameter insert of low density material to simulate a tumor, 3 calypso beacons to track breathing motion and a micro ion chamber for point dose measurements. Radiochromic films were used to measure spatial dose distribution. Gamma 3%, 3 mm metrics were used for plan evaluation. With phantom placed on a respiratory gating platform, 4DCT data sets were acquired for breathing amplitudes of 5, 10, 15, 20, 25, 30, and 40 mm respectively. Breathing cycles of 2, 4, and 6‐seconds‐long were used for each case. Minimum intensity projection (MINIP) datasets were generated for each case. Eclipse treatment planning system was used to generate IMRT and VMAT plans. Calypso system was used to gate all treatment deliveries.

Results:

Point dose measurement did not show significant difference for different breathing amplitude. This is because the ion chamber is placed at isocenter, and film analysis revealed most dose variations occur on the edges of the tumor. Gamma analysis shows significant reduction in passing rate for plans with motion due to MLC interplay effect and dose blurring. With 15 mm breathing amplitude, gamma passing rate dropped by 10% and 25% for IMRT and VMAT respectively.

Conclusion:

With a breathing motion management, it is safe to modulate SBRT plans when breathing amplitude is less than 10 mm and 15 mm for IMRT and VMAT respectively. Longer breathing cycles lead to less accurate dose delivery. More analysis is being performed for breathing amplitudes greater than 15 mm.
SU‐F‐T‐543: The Impact of Density Overrides in IMRT and VMAT Planning for Lung Cancer Patients Treated SBRT
Authors: Qin Y , Chetty I
Published: July 2016

Purpose:

To evaluate the dosimetric impact of target volume overrides on the quality of IMRT and VMAT lung, SBRT treatment plans.

Methods:

Ninety one patients previously treated with lung SBRT at our institution were retrospectively studied. IMRT and VMAT plans were created on 5 image sets for each patient: free breathing scans(FB), average scans(AVE), average scan with internal target volume overridden to tumor density(ITVoverride), average scan with planning target volume overridden(PTVoverride), and a hybrid method where the internal target volume was set to tumor density and the set‐up margin was set to an intermediate density between lung and tumor(HBoverride). All plans were optimized with the same constraints as the original clinical plan AVE. Dose fractionation scheme included 12Gyx4, 10Gyx4, 8Gyx4, 12Gyx3, and 9Gyx5. EQD2 was computed for various target metrics (mean, D1%, D99%) and normal lung metrics (V5Gy, V20Gy, mean lung dose). Plan quality was investigated via conformity index (CI) and gradient index (GI).

Results:

The ITVoverride plans achieved the highest mean target EQD2(0.38±0.69Gy over AVE) and the highest D1% hotspot(0.73±1.17Gy over AVE) in comparison to all other plans. PTVoverride and HBoverride also demonstrated higher mean target EQD2 than AVE, but lower hotspots. Compared to AVE, PTVoverride and HBoverride resulted in lower coldspots of −9.95±3.78Gy and −9.91±3.68Gy respectively. All override plans achieved lower lung metrics than the AVE plans, with the largest reductions in PTVoverride (−0.31±0.30Gy for V20Gy, −0.44±0.44Gy for V5Gy, −0.23±0.18Gy for mean lung dose). Comparing FB (largest CI,GI) to PTVoverride (smallest CI,GI), CI and GI improved from 1.05 to 1.00, and 4.81 to 4.48 respectively. The differences in these metrics did not correlate with tumor motion or target volume.

Conclusion:

Despite the improvements in CI and GI, density overrides in target volumes for 91 lung SBRT patients showed limited dosimetric advantages over clinical plans on the average CT.
SU‐F‐T‐545: Dosimetric and Radiobiological Evaluation of Dose Calculation Algorithms On Prostate Stereotactic Body Radiotherapy Using Conventional Flattened and Flattening‐Filter‐Free Beam
Authors: Kang S , Chung J , Eom K , Lee J , Suh T
Published: July 2016

Purpose:

The purpose of this study is to evaluate the dosimetric and radiobiological impact of Acuros XB (AXB) and Anisotropic Analytic Algorithm (AAA) dose calculation algorithms on prostate stereotactic body radiation therapy plans with both conventional flattened (FF) and flattening‐filter free (FFF) modes.

Methods:

For thirteen patients with prostate cancer, SBRT planning was performed using 10‐MV photon beam with FF and FFF modes. The total dose prescribed to the PTV was 42.7 Gy in 7 fractions. All plans were initially calculated using AAA algorithm in Eclipse treatment planning system (11.0.34), and then were re‐calculated using AXB with the same MUs and MLC files. The four types of plans for different algorithms and beam energies were compared in terms of homogeneity and conformity. To evaluate the radiobiological impact, the tumor control probability (TCP) and normal tissue complication probability (NTCP) calculations were performed.

Results:

For PTV, both calculation algorithms and beam modes lead to comparable homogeneity and conformity. However, the averaged TCP values in AXB plans were always lower than in AAA plans with an average difference of 5.3% and 6.1% for 10‐MV FFF and FF beam, respectively. In addition, the averaged NTCP values for organs at risk (OARs) were comparable.

Conclusion:

This study showed that prostate SBRT plan were comparable dosimetric results with different dose calculation algorithms as well as delivery beam modes. For biological results, even though NTCP values for both calculation algorithms and beam modes were similar, AXB plans produced slightly lower TCP compared to the AAA plans.
SU‐F‐T‐546: A Radiobiological Comparative Study of Robotic and LINAC‐Based Lung SBRT
Authors: Roth T , Rankine L , Schreiber E , Das S , Mavroidis P
Published: July 2016

Purpose:

To expand the dosimetric comparison of CyberKnife and Elekta Versa HD regarding their capability to perform SBRT in lung cases by including radiobiological measures, which can estimate the clinical impact of the observed differences.

Methods:

This study analyzes treatment data of 10 patients, who were treated with SBRT on Elekta Versa HD and replanned using CyberKnife. The original structure sets from the RayStation treatment plans were transferred to MultiPlan and were used for planning, with the exception of the PTV. Special attention was given to the following structures during optimization in MultiPlan: PTV (expanded off GTV instead of ITV), ipsilateral and contralateral lungs, and chest wall. For each patient, the same dosimetric constraints were applied to both plans. The dose volume histograms (DVH) of the targets and organs at risk (OAR) were computed and used to compare the dosimetric differences between the two modalities. Additionally, the tumor control probability (TCP), normal tissue complication probability (NTCP) and complication‐free tumor control probability (P+) radiobiological indices were calculated and compared.

Results:

The dosimetric indices show a negligible differences (Versa – CK) for the PTV (mean dose difference of 1.7 Gy and average minimum dose difference of −0.3 Gy). For the OARs, the average differences in mean and maximum doses, which varied between 0.4 to 1.7 Gy and 1.1 to 4.7Gy, respectively. The average differences in the radiobiological indices were also similar TCP = 0.3±0.6%, Composite NTCP = 2.4±2.9%, and P+ = −.1±3.3%, respectively.

Conclusion:

Both modalities managed to meet the same dosimetric goals equally well. The similarities between the dosimetric and radiobiological indices indicate that the two treatment modalities are comparable. A larger patient cohort involving patients of different complexities would give a better insight for this comparison.
SU‐F‐T‐550: Radiochromic Plastic Thin Sheet Dosimeter: Initial Performance
Authors: Jordan K , Adamovics J
Published: July 2016

Purpose:

Thin sheets, of a high sensitivity formulation of radiochromic dosimeter, Presage were prepared and evaluated for optical readout.

Methods:

Sheets of radiochromic polyurethane, 12 cm long, 10 cm wide and 0.2 cm thick were prepared with leuco crystal violet as the reporter molecule. Sample transmission was evaluated at a wavelength of 590 nm with in‐house constructed instruments: optical cone beam laser CT scanner, fixed and scanning spot densitometers. Sample sequential irradiations to a total dose of 40 Gy were conducted with a modified, Theratron 60, cobalt radiotherapy machine at dose rates of 1 or 0.25 Gy per minute. Exposure to ambient and readout light was minimized to limit background photochromic signals. Samples were stored at 4°C. Optical activity was assessed from linearly polarized transmission images. Comparison sensitivity measurements with EBT3 film were conducted.

Results:

Samples were transparent, smooth and pale purple before irradiation. Radiochromic reaction was completed in less than 5 minutes. A linear dose response with a sensitivity of 0.5 cm‐1Gy‐1 was observed. Micrometer measurements found sheet thickness variations up to 20%. Uniform dose, 2 Gy attenuation images, correlated with local sheet thicknesses. Comparable measurements with EBT3 film were 3 times more sensitive at 1 Gy but above 15 Gy, EBT3 film had lower sensitivity than 0.2 cm thick Presage sheet dosimeter due to its non‐linear response.

Conclusion:

Dose sensitivity provided a 10% decrease in transmission for a 1 Gy dose. Improvements in mold design are expected to allow production of sheets with less than 5% variation in thickness. Above, 10 Gy, Presage sheet dosimeter performance expected to exceed EBT3 film based on linearity, sensitivity, transparency and smoothness of samples.

J Adamovics is owner of Heuris Inc
SU‐F‐T‐552: A One‐Year Evaluation of the QABeamChecker+ for Use with the CyberKnife System
Authors: Gersh J
Published: July 2016

Purpose:

By attaching an adapter plate with fiducial markers to the QA BeamChecker+ (Standard Imaging, Inc., Middleton, WI), the output of the CyberKnife can be accurately, efficiently, and consistently evaluated. The adapter plate, known as the Cutting Board, allows for automated alignment of the QABC+ using the CK's stereoscopic kV image‐based treatment localization system (TLS). Described herein is an evaluation of the system following a year of clinical utilization.

Methods:

Based on a CT scan of the QABC+ and CB, a treatment plan is generated which delivers a beam to each of the 5 plane‐parallel ionization chambers. Following absolute calibration of the CK, the QA plan is delivered, and baseline measurements are acquired (and automatically corrected for temperature and pressure). This test was performed at the beginning of each treatment day for a year. A calibration evaluation (using a water‐equivalent slab and short thimble chamber) is performed every four weeks, or whenever the QABC+ detects a deviation of more than 1.0%.

Results:

During baseline evaluation, repeat measurements (n=10) were performed, with an average output of 0.25% with an SD of 0.11%. As a test of the reposition of the QABC+ and CB, ten additional measurements were performed where between each acquisition, the entire system was removed and re‐positioned using the TLS. The average output deviation was 0.30% with a SD of 0.13%. During the course of the year, 187 QABC+ measurements and 13 slab‐based measurements were performed. The output measurements of the QABC+ correlated well with slab‐based measurements (R2=0.909).

Conclusion:

By using the QABC+ and CB, daily output was evaluated accurately, efficiently, and consistently. From setup to break‐down (including analysis), this test required 5 minutes instead of approximately 15 using traditional techniques (collimator‐mounted ionization chambers). Additionally, by automatically saving resultant output deviation to a database, trend analysis was simplified.

Spectrum Medical Physics, LLC of Greenville, SC has a consulting contract with Standard Imaging of Middleton, WI
SU‐F‐T‐556: A Potential Real Time AQA for Cyberknife Cones and MLC Based Treatments
Authors: Shang C , Gibbard G , Cole J , Schramm A , Leventouri T , Khanal S
Published: July 2016

Purpose:

To investigate the potential use of the Raven™ (LAP of America Laser Applications) for real time AQA of Cyberknife™ with InCise2 MLC (Accuray™ Medical).

Methods:

At setup, the Raven was rotated 45° on which an Accuray™ AQA cube was positioned. Three different AQA plans for fixed cone, InCise2MLC, and a modified MLC plan were delivered repeatedly ten times. The additional shapes in modified AQA plan enable additional reproducibility checks for all the MLC pairs. During the test, the cube was aligned by imbedded fiducials and irradiated. The two angled radiation beams aimed center tungsten ball of the cube and projected 45° to phosphor screen and registered by a CCD camera of the Raven device. The centricity of the metal ball in the irradiated field was then analyzed using Matlab codes.

Results:

For AP images, the average offsets of X, Y, and radial directions are 0.24 ± 0.04 mm, 0.25 ± 0.02 mm and 0.35 ±± 0.03 mm respectively for the cone; 0.34 ± 0.02 mm, 0.49 ± 0.04 mm and 0.60 ± 0.04 mm respectively for the MLC. For lateral images, they are 0.63 ± 0.05 mm, 0.11 ± 0.02 mm and 0.64 ± 0.04 mm respectively for the cone, 0.79 ± 0.08 mm, −0.23 ± 0.06 mm and 0.82 ± 0.09 mm respectively for the MLC. No inconsistent MLC shapes were found in the modified AQA group.

Conclusion:

The results are consistent with clinically acceptable values (≤1mm from baseline). The results suggest the potential of replacement of the standard AQA test with the novel real‐time Raven device for Cyberknife daily QA. The modified MLC based AQA provides a more comprehensive MLC daily QA capability. Further improvements in its resolution and automatic analyzing capability are warranted.
SU‐F‐T‐557: Evaluation of Detector Response in Rectangular Small Field Dosimetry
Authors: Qureshi A , Tanny S , Parsai E , Sperling N
Published: July 2016

Purpose:

As stereotactic treatment modalities grow towards becoming the standard of care, the need for accurate dose computation in small fields is becoming increasingly essential. The purpose of this study is to evaluate the response of different detectors, intended for small field dosimetry, in jaw defined small rectangular fields by analyzing output factors from a stereotactic clinical accelerator.

Methods:

Two Dosimeters, the Exradin A26 Microionization Chamber (Standard Imaging) and Edge Diode Detector (Sun Nuclear) were used to measure output factors taken on the Varian Edge Stereotactic Linear accelerator. Measurements were taken at 6MV and 6FFF at 10cm depth, 100cm SSD in a 48×48×40cm3 Welhoffer BluePhantom2 (IBA) with X and Y jaws set from 0.6 to 2.0cm. Output factors were normalized to a 5×5cm2 machine‐specific reference field. Measurements were made in the vertical orientation for the A26 and horizontal orientation for both the A26 and Edge. Output factors were measured as: OFFS = MFS/Mref where MFS and Mref are the measured signals for the clinical field and the reference field, respectively. Measured output factors were then analyzed to establish relative responses of the detectors in small fields.

Results:

At 6MV the Edge detector exhibited a variation in output factors dependent on jaw positioning (X‐by‐Y vs Y‐by‐X) of 5.7% of the 5×5cm reference output and a variation of 3.33% at 6FFF. The A26 exhibited variation of output factor dependent on jaw positioning of upto 7.7% of the 5×5cm reference field at 6MV and upto 5.33% at 6FFF.

Conclusion:

Both the Edge detector and A26 responded as expected at small fields however a dependence on the jaw positioning was noted. At 6MV and 6FFF the detector response showed an increased dependence on the positioning of the X jaws as compared to the positioning of the Y jaws.
SU‐F‐T‐558: ArcCheck for Patient Specific QA in Stereotactic AblativeRadiotherapy
Authors: Ramachandran P , Tajaldeen A , Taylor D , Wanigaratne D , Roozen K , Esen N , Geso M , Kron T
Published: July 2016

Purpose:

Stereotactic Ablative Radiotherapy (SABR) is one of the most preferred treatment techniques for early stage lung cancer. This technique has been extended to other treatment sites like Spine, Liver, Scapula, Sternum etc., This has resulted in increased physics QA time on machine. In this study, we've tested the feasibility of using ArcCheck as an alternative method to replace film dosimetry.

Methods:

Twelve patients with varied diagnosis of Lung, Liver, scapula, sternum and Spine undergoing SABR were selected for this study. Pre‐treatment QA was performed for all the patients which include ionization chamber and film dosimetry. The required gamma criteria for each SABR plan to pass QA and proceed to treatment is 95% (3%,1mm). In addition to this routine process, the treatment plans were exported on to an ArcCheck phantom. The planned and measured dose from the ArcCheck device were compared using four different gamma criteria: 2%,2 mm, 3%,2 mm, 3%,1 mm and 3%, 3 mm. In addition to this, we've also introduced errors to gantry, collimator and couch angle to assess sensitivity of the ArcCheck with potential delivery errors.

Results:

The ArcCheck mean passing rates for all twelve cases were 76.1%±9.7% for gamma criteria 3%,1 mm, 89.5%±5.3% for 2%,2 mm, 92.6%±4.2% for 3%,2 mm, and 97.6%±2.4% for 3%,3 mm gamma criteria. When SABR spine cases are excluded, we observe ArcCheck passing rates higher than 95% for all the studied cases with 3%, 3mm, and ArcCheck results in acceptable agreement with the film gamma results.

Conclusion:

Our ArcCheck results at 3%, 3 mm were found to correlate well with our non‐SABR spine routine patient specific QA results (3%,1 mm). We observed significant reduction in QA time on using ArcCheck for SABR QA. This study shows that ArcCheck could replace film dosimetry for all sites except SABR spine.
SU‐F‐T‐560: Measurement of Dose Blurring Effect Due to Respiratory Motion for Lung Stereotactic Body Radiation Therapy (SBRT) Using Monte Carlo Based Calculation Algorithm
Authors: Badkul R , Pokhrel D , Ramanjappa T , Jiang H , Lominska C , Wang F
Published: July 2016

Purpose:

Intra‐fractional tumor motion due to respiration may potentially compromise dose delivery for SBRT of lung tumors. Even sufficient margins are used to ensure there is no geometric miss of target volume, there is potential dose blurring effect may present due to motion and could impact the tumor coverage if motions are larger. In this study we investigated dose blurring effect of open fields as well as Lung SBRT patients planned using 2 non‐coplanar dynamic conformal arcs(NCDCA) and few conformal beams(CB) calculated with Monte Carlo (MC) based algorithm utilizing phantom with 2D‐diode array(MapCheck) and ion‐chamber.

Methods:

SBRT lung patients were planned on Brainlab‐iPlan system using 4D‐CT scan and ITV were contoured on MIP image set and verified on all breathing phase image sets to account for breathing motion and then 5mm margin was applied to generate PTV. Plans were created using two NCDCA and 4‐5 CB 6MV photon calculated using XVMC MC‐algorithm. 3 SBRT patients plans were transferred to phantom with MapCheck and 0.125cc ion‐chamber inserted in the middle of phantom to calculate dose. Also open field 3×3, 5×5 and 10×10 were calculated on this phantom. Phantom was placed on motion platform with varying motion from 5, 10, 20 and 30 mm with duty cycle of 4 second. Measurements were carried out for open fields as well 3 patients plans at static and various degree of motions. MapCheck planar dose and ion‐chamber reading were collected and compared with static measurements and computed values to evaluate the dosimetric effect on tumor coverage due to motion.

Results:

To eliminate complexity of patients plan 3 simple open fields were also measured to see the dose blurring effect with the introduction of motion. All motion measured ionchamber values were normalized to corresponding static value. For open fields 5×5 and 10×10 normalized central axis ion‐chamber values were 1.00 for all motions but for 3×3 they were 1 up to 10mm motion and 0.97 and 0.87 for 20 and 30mm motion respectively. For SBRT plans central axis dose values were within 1% upto 10mm motions but decreased to average of 5% for 20mm and 8% for 30mm motion. Mapcheck comparison with static showed penumbra enlargement due to motion blurring at the edges of the field for 3×3,5×5,10×10 pass rates were 88% to 12%, 100% to 43% and 100% to 63% respectively as motion increased from 5 to 30mm. For SBRT plans MapCheck mean pass rate were decreased from 73.8% to 39.5% as motion increased from 5mm to 30mm.

Conclusion:

Dose blurring effect has been seen in open fields as well as SBRT lung plans using NCDCA with CB which worsens with increasing respiratory motion and decreasing field size(tumor size). To reduce this effect larger margins and appropriate motion reduction techniques should be utilized.
SU‐F‐T‐561: Energy Dependence of a Scintillation Detector
Authors: Volotskova O , Xu A , Jozsef G
Published: July 2016

Purpose:

To investigate the response and dose rate dependence of a scintillation detector over a wide energy range.

Methods:

The energy dependence of W1 scintillation detector was tested with: 1) 50–225 keV beams generated by an animal irradiator, 2) a Leksell Gamma Knife Perfexion Co‐60 source, 3) 6MV, 6FFF, 10FFF and 15MV photon beams, and 4) 6–20MeV electron beams from a linac. Calibrated linac beams were used to deliver 100 cGy to the detector at dmax in water under reference conditions. The gamma‐knife measurement was performed in solid water (100 cGy with 16mm collimator). The low energy beams were calibrated with an ion chamber in air (TG‐61), and the scintillation detector was placed at the same location as the ionization chamber during calibration. For the linac photon and electron beams, dose rate dependence was tested for 100–2400 and 100–800 MU/min.

Results:

The scintillation detector demonstrated strong energy dependence in the range of 50–225keV. The measured values were lower than the delivered dose and increased as the energy increased. Therapeutic photon beams showed energy independence with variations less than 1%. Therapeutic electron beams displayed the same sensitivity of ∼2–3% at their corresponding dmax depths. The change in dose‐rate of photon and electron beams within the therapeutic energy range did not affect detector output (<0.5%). Measurements acquired with the gamma knife showed that the output data agreed with the delivered dose up to 3%.

Conclusion:

W1 scintillation detector output has a strong energy dependence in the diagnostic and orthovoltage energy range. Therapeutic photon beams exhibited energy independence with no observable dose‐rate dependence. This study may aid in the implementation of a scintillation detector in QA programs by providing energy calibration factors.