bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐F‐T‐315: Comparative Studies of Planar Dose with Different Spatial Resolution for Head and Neck IMRT QA
Authors: Hwang T , Koo T
Published: July 2016

Purpose:

To quantitatively investigate the planar dose difference and the γ value between the reference fluence map with the 1 mm detector‐to‐detector distance and the other fluence maps with less spatial resolution for head and neck intensity modulated radiation (IMRT) therapy.

Methods:

For ten head and neck cancer patients, the IMRT quality assurance (QA) beams were generated using by the commercial radiation treatment planning system, Pinnacle3 (ver. 8.0.d Philips Medical System, Madison, WI). For each beam, ten fluence maps (detector‐to‐detector distance: 1 mm to 10 mm by 1 mm) were generated. The fluence maps with larger than 1 mm detector‐todetector distance were interpolated using MATLAB (R2014a, the Math Works,Natick, MA) by four different interpolation

Methods:

for the bilinear, the cubic spline, the bicubic, and the nearest neighbor interpolation, respectively. These interpolated fluence maps were compared with the reference one using the γ value (criteria: 3%, 3 mm) and the relative dose difference.

Results:

As the detector‐to‐detector distance increases, the dose difference between the two maps increases. For the fluence map with the same resolution, the cubic spline interpolation and the bicubic interpolation are almost equally best interpolation methods while the nearest neighbor interpolation is the worst.For example, for 5 mm distance fluence maps, γ≤1 are 98.12±2.28%, 99.48±0.66%, 99.45±0.65% and 82.23±0.48% for the bilinear, the cubic spline, the bicubic, and the nearest neighbor interpolation, respectively. For 7 mm distance fluence maps, γ≤1 are 90.87±5.91%, 90.22±6.95%, 91.79±5.97% and 71.93±4.92 for the bilinear, the cubic spline, the bicubic, and the nearest neighbor interpolation, respectively.

Conclusion:

We recommend that the 2‐dimensional detector array with high spatial resolution should be used as an IMRT QA tool and that the measured fluence maps should be interpolated using by the cubic spline interpolation or the bicubic interpolation for head and neck IMRT delivery.

This work was supported by Radiation Technology R&D program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (No. 2013M2A2A7038291)
SU‐F‐T‐322: A Comparison of Two Si Detectors for in Vivo Dosimetry
Authors: Talarico O , Krylova T , Lebedenko I
Published: July 2016

Purpose:

To compare two types of semiconductor detectors for in vivo dosimetry by their dependence from various parameters in different conditions.

Methods:

QED yellow (Sun Nuclear) and EDP (Scanditronix) Si detectors were radiated by a Varian Clinac 2300 ix with 6 and 18 MV energies. 10 cm thickness water equivalent phantom consisted of 30×30 cm2 squared plates was used for experiments. Dose dependencies for different beam angles (0 – 180°), field size (3–40 cm), dose (50 – 300 MU), and dose rates (50 – 300 MU/min) were obtained and calibrated with Standard Farmer chamber (PTW).

Results:

Reproducibility, linearity, dose rate, angular dependence, and field size dependence were obtained for QED and EDP. They show no dose‐rate dependence in available clinical dose rate range (100–600 MU/min). Both diodes have linear dependence with increasing the dose. Therefore even in case of high radiation therapy (including total body irradiation) it is not necessary to apply an additional correction during in vivo dosimetry. The diodes have different behavior for angular and field size dependencies. QED diode showed that dose value is stable for beam angles from 0 to 60°, for 60–180° correction factor has to be applied for each beam angle during in vivo measurements. For EDP diode dose value is sensitive to beam angle in whole range of angles.

Conclusion:

The study shows that QED diode is more suitable for in vivo dosimetry due to dose value independence from incident beam angle in the range 0–60°. There is no need in correction factors for increasing of dose and dose rate for both diodes. The next step will be to carry out measurements in non‐standard conditions of total body irradiation. After this modeling of these experiments with Monte Carlo simulation for comparison calculated and obtained data is planned.
SU‐F‐T‐326: Diode Array Transmission Detector Systems Evaluation
Authors: Hoffman D , Dyer B , Kumaran Nair C , Katuri Y , Rong Y , Benedict S
Published: July 2016

Purpose:

A new transmission detector, Delta4 Discover, developed by Scandidos (Uppsala, Sweden) was evaluated for external photon beam verification and quality assurance. The device is an array of 4040 diodes designed to be mounted on the linac accessory tray to measure photon field shape, position and fluence during patient treatment. Interfractional measurements are compared to a baseline measurement made during delivery quality assurance. The aim of this work is to evaluate the stability of the device and its effect on the shape and magnitude of the treatment beam.

Methods:

Beam profiles, percent depth dose, and beam attenuation was measured for 6, 10, and 15 MV photon beams with and without the device in place for 1×1 and 30×30 cm2 fields. Changes in profile and percent depth dose was quantified to evaluate the need to recommission the treatment beam, or account for the device with a tray factor. The stability of the radiation measurements was evaluated by measuring the deviation of each diode measurement during repeated prostate VMAT treatment delivery.

Results:

Photon beam profiles changed by < 1.25% in the nonpenumbra regions of the 30×30 cm2 beam. Percent depth dose curves show a 5–7% increased dose at depths < 2.5cm, but agreed within 1% at depths > 2.5cm. This indicates increased skin dose, similar to the use of a physical beam wedge. The device attenuated 6, 10, and 15 MV photon beams by 1.71±0.02%, 1.36±0.03%, and 1.17±0.03%, respectively. The diode array reproduced dosimetric measurements within 0.5% standard deviation for repeated prostate VMAT measurement.

Conclusion:

The device demonstrated stabile radiation measurements, while not changing the treatment beam shape in a clinically significantly manner. Use of this device can be accounted for with a tray factor, as opposed to recommissioning the treatment beam.
SU‐F‐T‐396: Impact of Shoulder Deformation for Head and Neck VMAT
Authors: Uchida Y , Tachibana H
Published: July 2016

Purpose:

For head and neck VMAT (HN‐VMAT), variations of position and deformation of patient's shoulders is a concern to affect inaccuracy of dose distribution. It has been reported that the setup error of the shoulders was variable from 5 mm – 1 cm. The beams of the HN‐VMAT pass through the shoulders. We assessed the impact of shoulder deformation to dose distribution for HN‐VMAT.

Methods:

One HN‐VMAT plan was generated using a patient's CT. The patient's CT was deformed using ImSimQA (Oncology Systems Limited, Shrewsbury, Shropshire, UK) to generate several patterns of the shoulders’ deformations when the right and left humeral heads were shifted with 3, 6, and 15 mm in the superior and inferior directions (SI), 3, 5, and 15 mm in the anterior and posterior directions (AP), and 5 and 15 mm in the right or left direction (LR). DVH comparison was performed in the different deformation patterns. The dosimetric parameters of D95% for CTV70Gy, CTV60Gy and CTV54Gy and dmax for Spinal cord were also measured. Gamma index evaluation (Criteria: 3%/2mm) was performed to exhibit clinically tolerable area in the comparison.

Results:

DVH comparison shows similar for all structures. As the comparison for the dosimetric parameters, the variations of D95% in the LR and AP were within 1%. There were larger variations in the SI than those in the other directions, however were within 1.5%. In gamma index evaluation, the small spots with higher gamma index values were appeared when the shift was 6 mm, however the pass ratio was 99.13%.

Conclusion:

HN‐VMAT should be robust for shoulder deformation and geometric accuracy within 6 mm from patient's setup and image‐guided radiotherapy may be clinically acceptable for target dose coverage or normal tissue dose sparing.
SU‐F‐T‐399: Migration of Treatment Planning Systems Without Beam Data Measurement
Authors: Tolakanahalli R , Tewatia D
Published: July 2016

Purpose:

Data acquisition for commissioning is steered by Treatment Planning System (TPS) requirements which can be cumbersome and time consuming involving significant clinic downtime. The purpose of this abstract is to answer if we could circumvent this by extracting data from existing TPS and speed up the process.

Methods:

Commissioning beam data was obtained from a clinically commissioned TPS (Pinnacle™) using Matlab™ generated Pinnacle™ executable scripts to commission a secondary 3D dose verification TPS (Eclipse™). Profiles and output factors for commissioning as required by Eclipse™ were computed on a 50 cm3 water phantom at a dose grid resolution of 2mm3. Verification doses were computed and compared to clinical TPS dose profiles as per TG‐106 guidelines. Standard patient plans from Pinnacle™ including IMRT and VMAT plans were re‐computed keeping the same monitor units (in order to perform true comparison) using Eclipse™. Computed dose was exported back to Pinnacle for comparison to original plans. This methodology enables us to alleviate all ambiguities that arise in such studies.

Results:

Profile analysis using in‐house software for 6x, showed that for all field sizes including small MLC generated fields, 100% of infield and penumbra data points of Eclipse™ match Pinnacle™ generated and measured profiles with 2%/2 mm gamma criteria. Excellent agreement was observed in the penumbra regions, with all data points passing DTA criteria for complex C‐shaped and S‐shaped profiles. Patient plan dose volume histograms (DVHs) and isodose lines agreed well to within a 1.5% for target coverage.

Conclusion:

Secondary 3D dose checking is of utmost importance with advanced techniques such as IMRT and VMAT. Migration of TPS is possible without compromising accuracy or enduring the cumbersome measurement of commissioning data. Economizing time for commissioning such a verification system or for migration of TPS can add great QA value and minimize downtime
SU‐F‐T‐402: The Effect of Extremely Narrow MLC Leaf Width On the Plan Quality of VMAT for Prostate Cancer
Authors: Kim J , Park S , Kim J , Choi C , Park J
Published: July 2016

Purpose:

To investigate the effect of multi‐leaf collimators (MLCs) with leaf width of 1.25 mm on the plan quality of volumetric modulated arc therapy (VMAT) for prostate cancer.

Methods:

A total of 20 patients with prostate cancer were retrospectively selected. Using a high definition MLC (HD MLC), primary and boost VMAT plans with two full arcs were generated for each patient (original plan). After that, by shifting patient CT images by 1.25 mm in the cranio‐caudal direction between the 1st and the 2nd arc, we simulated fluences made with MLCs with leaf width of 1.25 mm. After shifting, primary and boost plans were generated for each patient (shifted plan). A sum plan was generated by summation of the primary and boost plan for each patient. Dose‐volumetric parameters were calculated and compared.

Results:

Both homogeneity index (HI) and conformity index (CI) of the shifted plans were better than those of the original plans in primary plans (HI = 0.044 vs. 0.040 with p < 0.001 and CI = 1.056 vs. 1.044 with p = 0.006). Similarly, the shifted plans for boost target volume showed better homogeneity and conformity than did the original plans (HI = 0.042 vs. 0.037 with p = 0.006 and CI = 1.015 vs. 1.009 with p < 0.001). The total body volumes of the original plans irradiated by the prescription dose were larger than those of the shifted plans in sum plans (60.9 cc vs. 49.0 cc with p = 0.007).

Conclusion:

Use of extremely narrow MLCs could increase dose homogeneity and conformity of the target volume for prostate VMAT.

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1C1A1A02036331).
SU‐F‐T‐408: On the Determination of Equivalent Squares for Rectangular Small MV Photon Fields
Authors: Sauer OA , Wegener S , Exner F
Published: July 2016

Purpose:

It is common practice to tabulate dosimetric data like output factors, scatter factors and detector signal correction factors for a set of square fields. In order to get the data for an arbitrary field, it is mapped to an equivalent square, having the same scatter as the field of interest. For rectangular fields both, tabulated data and empiric formula exist. We tested the applicability of such rules for very small fields.

Methods:

Using the Monte‐Carlo method (EGSnrc‐doseRZ), the dose to a point in 10cm depth in water was calculated for cylindrical impinging fluence distributions. Radii were from 0.5mm to 11.5mm with 1mm thickness of the rings. Different photon energies were investigated. With these data a matrix was constructed assigning the amount of dose to the field center to each matrix element. By summing up the elements belonging to a certain field, the dose for an arbitrary point in 10cm depth could be determined. This was done for rectangles up to 21mm side length. Comparing the dose to square field results, equivalent squares could be assigned. The results were compared to using the geometrical mean and the 4Xperimeter/area rule.

Results:

For side length differences less than 2mm, the difference between all methods was in general less than 0.2mm. For more elongated fields, relevant differences of more than 1mm and up to 3mm for the fields investigated occurred. The mean square side length calculated from both empiric formulas fitted much better, deviating hardly more than 1mm and for the very elongated fields only.

Conclusion:

For small rectangular photon fields, deviating only moderately from square both investigated empiric methods are sufficiently accurate. As the deviations often differ regarding their sign, using the mean improves the accuracy and the useable elongation range. For ratios larger than 2, Monte‐Carlo generated data are recommended.

SW is funded by Deutsche Forschungsgemeinschaft (SA481/10‐1)
SU‐F‐T‐409: Modelling of the Magnetic Port in Temporary Breast Tissue Expanders for a Treatment Planning System
Authors: Yoon J , Heins D , Zhang R
Published: July 2016

Purpose:

To model the magnetic port in the temporary breast tissue expanders and to improve accuracy of dose calculation in Pinnacle, a commercial treatment planning system (TPS).

Methods:

A magnetic port in the tissue expander was modeled with a radiological measurement‐basis; we have determined the dimension and the density of the model by film images and ion chamber measurement under the magnetic port, respectively. The model was then evaluated for various field sizes and photon energies by comparing depth dose values calculated by TPS (using our new model) and ion chamber measurement in a water tank. Also, the model was further evaluated by using a simplified anthropomorphic phantom with realistic geometry by placing thermoluminescent dosimeters (TLD)s around the magnetic port. Dose perturbations in a real patient's treatment plan from the new model and a current clinical model, which is based on the subjective contouring created by the dosimetrist, were also compared.

Results:

Dose calculations based on our model showed less than 1% difference from ion chamber measurements for various field sizes and energies under the magnetic port when the magnetic port was placed parallel to the phantom surface. When it was placed perpendicular to the phantom surface, the maximum difference was 3.5%, while average differences were less than 3.1% for all cases. For the simplified anthropomorphic phantom, the calculated point doses agreed with TLD measurements within 5.2%. By comparing with the current model which is being used in clinic by TPS, it was found that current clinical model overestimates the effect from the magnetic port.

Conclusion:

Our new model showed good agreement with measurement for all cases. It could potentially improve the accuracy of dose delivery to the breast cancer patients.
SU‐F‐T‐413: Calculation Accuracy of AAA and Acuros Using Cerrobend Blocks for TBI at 400cm
Authors: Lamichhane N , Studenski M
Published: July 2016

Purpose:

It is essential to assess the lung dose during TBI to reduce toxicity. Here we characterize the accuracy of the AAA and Acuros algorithms when using cerrobend lung shielding blocks at an extended distance for TBI.

Methods:

We positioned a 30×30×30 cm3 solid water slab phantom at 400 cm SSD and measured PDDs (Exradin A12 and PTW parallel plate ion chambers). A 2 cm thick, 10×10 cm2 cerrobend block was hung 2 cm in front of the phantom. This geometry was reproduced in the planning system for both AAA and Acuros. In AAA, the mass density of the cerrobend block was forced to 9.38 g/cm3 and in Acuros it was forced to 8.0 g/cm3 (limited to selecting stainless steel). Three different relative electron densities (RED) were tested for each algorithm; 4.97, 6.97, and 8.97.

Results:

PDDs from both Acuros and AAA underestimated the delivered dose. AAA calculated that depth dose was higher for RED of 4.97 as compared to 6.97 and 8.97 but still lower than measured. There was no change in the percent depth dose with changing relative electron densities for Acuros.

Conclusion:

Care should be taken before using AAA or Acuros with cerrobend blocks as the planning system underestimates dose. Acuros limits the ability to modify RED when compared to AAA.
SU‐F‐T‐417: Dosimetric Evaluation of Dose‐To‐Medium Vs. Dose‐To‐Water Using Acuros XB for Lung, Spine and Head‐And‐Neck Plans
Authors: Yu S , Rosenzweig D , Pacella M
Published: July 2016

Purpose:

This study evaluates the dosimetric differences between dose‐to‐medium (Dm) and conventional dose‐to‐water (Dw) reporting using Varian's Acuros XB (AXB) dose calculation algorithm with actual patient treatment plans.

Methods:

Twelve patients were selected: four head‐and‐neck (HN) (2 Gy × 33–35 fractions), four lung SBRT (10–12 Gy × 5 fractions) and four spine SBRT (6–8 Gy × 5 fractions) cases. All treatment plans were optimized using the Varian Eclipse Progressive Resolution Optimizer (version 13.6) to meet objectives of institutional guidelines. After the optimization process, each plan was calculated using both Dw and Dm options within AXB. The calculated treatment plans were normalized such that the prescribed dose covered at least 95% of the PTV. The dose‐volume histograms were evaluated for various dosimetric parameters. The percent difference between the two options is calculated as (AXB_Dw − AXB_Dm)/AXB_Dm *100.

Results:

The largest difference between AXB_Dw and AXB_Dm was seen in the mandible structure for HN cases, where AXB_Dw was up to 10% higher than AXB_Dm results (5.3%–10.2%). AXB_Dw was also higher for PTV maximum dose in these cases, ranging from 2.5%–5.7%. Among all other dosimetric parameters in HN, Spine and lung SBRT plans; the differences between AXB_Dw and AXB_Dm were within 5%. We also noted that AXB_Dw was consistently higher (1.8–3.8%) for cord in lung SBRT, and was consistently lower for esophagus/bowel in spine SBRT (−5% − −2.9%).

Conclusion:

The preliminary dosimetric results from our clinical study show that the selection of either Dm or Dw in AXB is not likely to produce significant dosimetric differences in most clinical cases. However, Dw can be different by up to 10% in bony anatomy compared to Dm. Therefore, Dm is the preferred option to achieve the greatest accuracy in both target and critical structure, providing a smooth transition from conventional to state‐of‐the‐art treatment planning.
SU‐F‐T‐420: Dosimetry Comparison of Advanced External Beam Radiation Treatment Modalities to Brachytherapy Treatments in Patients with Cervical Cancer
Authors: Mwidu U , Devic S , Shehadeh M , AlKafi M , Mahmood R , Moftah B
Published: July 2016

Purpose:

A retrospective comparison of dose distributions achievable by High dose rate brachytherapy (HDRBT), Helical TomoTherapy (TOMO), CyberKnife (CK) and RapidArc (RA) in locally advanced inoperable cervical cancer patients is presented.

Methods:

Five patients with advanced stage cervical carcinoma were selected for this study after a full course of external beam radiotherapy (EBRT), chemotherapy and HDR Brachytherapy. To highlight any significant similarities/differences in dose distributions, high‐risk clinical target volume (HRCTV) coverage, organs at risk (OAR) sparing, and machine specific delivery limitations, we used D90 (dose received by 90% of the volume) as the parameter for HRCTV coverage as recommended by the GEC‐ESTRO Working Group. We also compared both integral and differential dose volume histograms (DVH) between different dose distributions treatment modalities for HRCTV and OAR.

Results:

TOMO and RA provided the most conformal dose distributions to HRCTV. Median doses (in Gy) to organs at risk were; for rectal wall: 1.7±0.6, 2.5±0.6,1.2±0.3, and 1.5±0.6, and for bladder wall: 1.6±0.1, 2.4±0.4, 0.8±0.6, and 1.5±0.5, for HDRBT, TOMO, CK, and RA, respectively.

Conclusion:

Contemporary EBRT modalities might be able to replace brachytherapy treatments for cervix cancer. While brachytherapy dose distributions feature high dose gradients, EBRT modalities provide highly conformal dose distributions to the target. However, it is still not clear whether a highly conformal dose or high gradient dose is more clinically relevant for the HRCTV in cervix cancer patients.
SU‐F‐T‐422: Detection of Optimal Tangential Partial Arc Span for VMAT Planning in IntactLeft‐Breast Treatment
Authors: Giri U , Sarkar B , Munshi A , Kaur H , Jassal K , Rathinamuthu S , Kumar S , Ganesh T , Mohanti B
Published: July 2016

Purpose:

This study was designed to investigate an appropriate arc span for intact partial Left breast irradiation by VMAT planning.

Methods:

Four cases of carcinoma left intact breast was chosen randomly for this study. Both medial tangential and left‐lateral tangential arc (G20°, G25°, G30°, G35°, G40°) were used having the same length and bilaterally symmetric. For each patient base plan was generated for 30° arc and rest of other arc plans were generated by keeping all plan parameters same, only arc span were changed. All patient plans were generated on treatment planning system Monaco (V 5.00.02) for 50 Gy dose in 25 fractions. PTV contours were clipped 3 mm from skin (patient). All plans were normalized in such a way that 95 % of prescription dose would cover 96 % of PTV volume.

Results:

Mean MU for 20°, 25°, 30°, 35° and 40° were 509 ± 18.8, 529.1 ± 20.2, 544.4 ± 20.8, 579.1 ±51.8, 607.2 ± 40.2 similarly mean hot spot (volume covered by 105% of prescription dose) were 2.9 ± 1.2, 3.7 ± 3.0, 1.5 ± 1.7, 1.3±0.6, 0.4 ± 0.4, mean contralateral breast dose (cGy) were 180.4 ± 242.3, 71.5 ± 52.7, 76.2 ± 58.8, 85.9 ± 70.5, 90.7 ± 70.1, mean heart dose (cGy) were 285.8 ± 87.2, 221.2 ± 62.8, 274.5 ± 95.5, 234.8 ± 73.8, 263.2 ± 81.6, V20 for ipsilateral lung were 15.4 ± 5.3, 14.3 ± 3.6, 15.3 ± 2.9, 14.2 ± 3.9, 14.7 ± 3.2 and V5 for ipsilateral lung were 33.9 ± 8.2, 31.0 ± 3.5, 42.6 ±15.6, 36.4 ± 12.9, 37.0 ± 7.5.

Conclusion:

The study concluded that appropriate arc span used for tangential intact breast treatment was optimally 30° because larger arc span were giving lower isodose spill in ipsilateral lung and smaller arc were giving heterogeneous dose distribution in PTV.
SU‐F‐T‐423: Automating Treatment Planning for Cervical Cancer in Low‐ and Middle‐ Income Countries
Authors: Kisling K , Zhang L , Yang J , Jhingran A , Balter P , McCarroll R , Beadle B , Howell R , Schmeler K , Court L
Published: July 2016

Purpose:

To develop and test two independent algorithms that automatically create the photon treatment fields for a four‐field box beam arrangement, a common treatment technique for cervical cancer in low‐ and middle‐income countries.

Methods:

Two algorithms were developed and integrated into Eclipse using its Advanced Programming Interface:3D

Method:

We automatically segment bony anatomy on CT using an in‐house multi‐atlas contouring tool and project the structures into the beam's‐eye‐view. We identify anatomical landmarks on the projections to define the field apertures. 2D

Method:

We generate DRRs for all four beams. An atlas of DRRs for six standard patients with corresponding field apertures are deformably registered to the test patient DRRs. The set of deformed atlas apertures are fitted to an expected shape to define the final apertures. Both algorithms were tested on 39 patient CTs, and the resulting treatment fields were scored by a radiation oncologist. We also investigated the feasibility of using one algorithm as an independent check of the other algorithm.

Results:

96% of the 3D‐Method‐generated fields and 79% of the 2D‐method‐generated fields were scored acceptable for treatment (“Per Protocol” or “Acceptable Variation”). The 3D Method generated more fields scored “Per Protocol” than the 2D Method (62% versus 17%). The 4% of the 3D‐Method‐generated fields that were scored “Unacceptable Deviation” were all due to an improper L5 vertebra contour resulting in an unacceptable superior jaw position. When these same patients were planned with the 2D method, the superior jaw was acceptable, suggesting that the 2D method can be used to independently check the 3D method.

Conclusion:

Our results show that our 3D Method is feasible for automatically generating cervical treatment fields. Furthermore, the 2D Method can serve as an automatic, independent check of the automatically‐generated treatment fields. These algorithms will be implemented for fully automated cervical treatment planning.
SU‐F‐T‐425: Head‐Scatter Off‐Axis for FFF Megavoltage Photon Beams
Authors: Zhu T , Penjweini R , Dimofte A
Published: July 2016

Purpose:

Head‐scatter photons inside a clinical accelerator cause the output to change with collimator setting. On the central‐axis, this contribution is well‐described by output factor in air (or head‐scatter factor). However, a few studies have examined this component at off‐axis points.

Methods:

We define the head‐scatter off‐axis ratio, HOA, as the ratio of the water kerma in air due to head‐scatter photons at the off‐axis position x to the water kerma from direct primary photons on the central axis: HOA(cx,cy,xy) = (Q(cx,cy,x) – QP(cx,cy,x))/QP(cx,cy,0), where Q(cx, cy,x), QP(cx,cy,x) are charges measured by an ionization chamber in a miniphantom for collimator setting cx × cy and cx × 3 cm, respectively, at off‐axis point x. “Direct primary” is those photons that come from the source without interactions in the intervening structures.

Results:

We measured HOA for two energies (6XFFF and 6X) along X and Y jaw directions for various collimator settings. The shape of HOA has well defined penumbra for collimator setting larger than 10 × 10 cm2.

Conclusion:

The narrow gaussian component is interpreted as the source of photons scattered in the flattening filter and the primary collimator. The broad component is attributed to photons scattered in the secondary (variable) collimators. By a direct comparison between 6X and 6XFFF beams, we can confirm that the second component is indeed coming from collimator jaws and identify the parts coming from the primary collimator and the flattening filter, respectively.
SU‐F‐T‐428: An Optimization‐Based Commissioning Tool for Finite Size Pencil Beam Dose Calculations
Authors: Li Y , Tian Z , Song T , Jia X , Gu X , Jiang S
Published: July 2016

Purpose:

Finite size pencil beam (FSPB) algorithms are commonly used to pre‐calculate the beamlet dose distribution for IMRT treatment planning. FSPB commissioning, which usually requires fine tuning of the FSPB kernel parameters, is crucial to the dose calculation accuracy and hence the plan quality. Yet due to the large number of beamlets, FSPB commissioning could be very tedious. This abstract reports an optimization‐based FSPB commissioning tool we have developed in MatLab to facilitate the commissioning.

Methods:

A FSPB dose kernel generally contains two types of parameters: the profile parameters determining the dose kernel shape, and a 2D scaling factors accounting for the longitudinal and off‐axis corrections. The former were fitted using the penumbra of a reference broad beam's dose profile with Levenberg‐Marquardt algorithm. Since the dose distribution of a broad beam is simply a linear superposition of the dose kernel of each beamlet calculated with the fitted profile parameters and scaled using the scaling factors, these factors could be determined by solving an optimization problem which minimizes the discrepancies between the calculated dose of broad beams and the reference dose.

Results:

We have commissioned a FSPB algorithm for three linac photon beams (6MV, 15MV and 6MVFFF). Dose of four field sizes (6*6cm2, 10*10cm2, 15*15cm2 and 20*20cm2) were calculated and compared with the reference dose exported from Eclipse TPS system. For depth dose curves, the differences are less than 1% of maximum dose after maximum dose depth for most cases. For lateral dose profiles, the differences are less than 2% of central dose at inner‐beam regions. The differences of the output factors are within 1% for all the three beams.

Conclusion:

We have developed an optimization‐based commissioning tool for FSPB algorithms to facilitate the commissioning, providing sufficient accuracy of beamlet dose calculation for IMRT optimization.
SU‐F‐T‐437: 3 Field VMAT Technique for Irradiation of Large Pelvic Tumors
Authors: Stakhursky V
Published: July 2016

Purpose:

VMAT treatment planning for large pelvic volume irradiation could be suboptimal due to inability of Varian linac to split MLC carriage during VMAT delivery for fields larger than 14.5cm in X direction (direction of leaf motion). We compare the dosimetry between 3 VMAT planning techniques, two 2‐arc field techniques and a 3‐arc field technique: a) two small in X direction (less than 14.5cm) arc fields, complementing each other to cover the whole lateral extent of target during gantry rotation, b) two large arc fields, each covering the targets completely during the rotation, c) a 3 field technique with 2 small in X direction arcs and 1 large field covering whole target.

Methods:

5 GYN cancer patients were selected to evaluate the 3 VMAT planning techniques. Treatment plans were generated using Varian Eclipse (ver. 11) TPS. Dose painting technique was used to deliver 5300 cGy to primary target and 4500 cGy to pelvic/abdominal node target. All the plans were normalized so that the prescription dose of 5300 cGy covered 95% of primary target volume. PTV and critical structures DVH curves were compared to evaluate all 3 planning techniques.

Results:

The dosimetric differences between the two 2‐arc techniques were minor. The small field 2‐arc technique showed a colder hot spot (0.4% averaged), while variations in maximum doses to critical structures were statistically nonsignificant (under 1.3%). In comparison, the 3‐field technique demonstrated a colder hot spot (1.1% less, 105.8% averaged), and better sparing of critical structures. The maximum doses to larger bowel, small bowel and gluteal fold were 3% less, cord/cauda sparing was 4.2% better, and bladder maximum dose was 4.6% less. The differences in maximum doses to stomach and rectum were statistically nonsignificant.

Conclusion:

3‐arc VMAT technique for large field irradiation of pelvis demonstrates dosimetric advantages compared to 2‐arc VMAT techniques.
SU‐F‐T‐439: Proposal of An Index That Identifies Left‐Sided Breast/chest‐Wall Patients Who Benefit From VMAT Planning
Authors: Xu H
Published: July 2016

Purpose:

To propose an index called field‐heart‐overlap‐index (FHOI) that can be used to determine if a left‐sided breast (or chest‐wall) patient will benefit from VMAT planning; Calculation of FHOI does not need creation of either VMAT or field‐in‐field (FinF) plans.

Methods:

Four indices were defined: 1) total heart volume; 2) total PTV volume; 3) breast/chest‐wall separation; and 4) field‐heart‐overlap‐index (FHOI). FHOI is defined as the ratio of the heart volume inside the FOV to the total heart volume. VMAT plans were deemed superior to FinF plans when V10Gy for heart is substantially lower while PTV coverage was not compromised. A point biserial correlation coefficient (r_pb) is used to quantitatively describe the correlation between a dichotomous variable (Y_p) and a continuous variable (an index under investigation); with 0 being completely uncorrelated and 1 being completely correlated. If VMAT planning is superior to FinF planning for a patient, Y_p=1, otherwise Y_p=0. r_pb will be calculated for four potential correlations: FHOI and Y_p, PTV volume and Y_p, heart volume and Y_p, separation and Y_p. For each of the four indices, the correlation relation was retrospectively analyzed for twelve breast patients and twelve chest‐wall patients.

Results:

Calculation of the four indices for 12 breast patients and 12 chest‐wall patients indicates that FHOI was strongly correlated with the choice of VMAT or FinF techniques, while the other three indices were poorly correlated the choice of the techniques. r_pb for FHOI and Y_p is 0.93 for breast patients and 0.99 for chest‐wall patients, respectively. r_pb for PTV volume and Y_p, heart volume and Y_p, separation and Y_p are 0.17, 0.55, 0.06, respectively.

Conclusion:

This study shows that FHOI is an appropriate to determine left‐sided breast/chestwall patients who will benefit from VMAT. Patients with high FHOI tend to benefit from VMAT as heart dose is significantly decreased.
SU‐F‐T‐440: The Feasibility Research of Checking Cervical Cancer IMRT Pre‐ Treatment Dose Verification by Automated Treatment Planning Verification System
Authors: Liu X , Yin Y , Lin X
Published: July 2016

Purpose:

To assess the preliminary feasibility of automated treatment planning verification system in cervical cancer IMRT pre‐treatment dose verification.

Methods:

The study selected randomly clinical IMRT treatment planning data for twenty patients with cervical cancer, all IMRT plans were divided into 7 fields to meet the dosimetric goals using a commercial treatment planning system(PianncleVersion 9.2and the EclipseVersion 13.5). The plans were exported to the Mobius 3D (M3D)server percentage differences of volume of a region of interest (ROI) and dose calculation of target region and organ at risk were evaluated, in order to validate the accuracy automated treatment planning verification system.

Results:

The difference of volume for Pinnacle to M3D was less than results for Eclipse to M3D in ROI, the biggest difference was 0.22± 0.69%, 3.5±1.89% for Pinnacle and Eclipse respectively. M3D showed slightly better agreement in dose of target and organ at risk compared with TPS. But after recalculating plans by M3D, dose difference for Pinnacle was less than Eclipse on average, results were within 3%.

Conclusion:

The method of utilizing the automated treatment planning system to validate the accuracy of plans is convenientbut the scope of differences still need more clinical patient cases to determine. At present, it should be used as a secondary check tool to improve safety in the clinical treatment planning.
SU‐F‐T‐442: Dose Distribution Comparison for Post‐Laryngectomy Stoma Area Between Conventional AP and VMAT Plans with Or Without Bolus
Authors: Lee B , Zhang J , Leu M , Cho‐Lim J , Inouye W , Lorentz W , Lee S
Published: July 2016

Purpose:

To compare dose distributions of conventional AP vs. VMAT treatment plans with or without bolus around post‐laryngectomy stoma.

Methods:

Radiation dose coverage for post‐laryngectomy stoma was analyzed using a set of real‐case CT‐simulation images. After meticulous contouring of the catheter cuff, stoma lumen, peri‐stoma skin and subclinical tumor bed at the larynx, the resulting dosimetry plans were analyzed with or without a 5 mm bolus placement. Wet gauze was used to minimize the effect of any air gap. Four plans were generated: AP superclavicular (SCV) plan with or without bolus, and VMAT plan with or without bolus. A dose of 60Gy in 30 fractions was prescribed at 3 cm depth for AP SCV plan, and to 95% of the PTV volume for VMAT plan.

Results:

For the conventional AP SCV plan, the peri‐stoma skin dose is sensitive to bolus placement as well as air gap compensation by wetted gauze (V95% of 20.7%, 33.0% and 94.8% for no bolus, bolus without and with air gap compensation, respectively). For stoma lumen, the dose drops off rapidly in depth. The catheter cuff may have certain dose‐buildup effect, but air gap around it and under the bolus placed can pose a more serious problem. The dose distributions of the two VMAT plans are moderately different for peri‐stoma skin (V95% of 95.0% with bolus and air gap compensation, and 82.3% without bolus), but nearly identical for stoma lumen (V95% of 91.5% and 92.0%, respectively). VMAT allows beamlets with different angles of incidence that helped achieve such dose distribution around the stoma even without bolus placement.

Conclusion:

Overall, the dose coverage around the stoma in the VMAT plan is better than the conventional AP SCV plan. To achieve optimal dose distribution, it is still recommended to place physical bolus and reduce the air gaps.
SU‐F‐T‐443: Quantification of Dosimetric Effects of Dental Metallic Implant On VMAT Plans
Authors: Lin C , Feng Y , Huang Z , Jiang W
Published: July 2016

Purpose:

To evaluate the dosimetric impact of metallic implant that correlates with the size of targets and metallic implants and distance in between on volumetric‐modulated arc therapy (VMAT) plans for head and neck (H&N) cancer patients with dental metallic implant.

Methods:

CT images of H&N cancer patients with dental metallic implant were used. Target volumes with different sizes and locations were contoured. Metal artifact regions excluding surrounding critical organs were outlined and assigned with CT numbers close to water (0HU). VMAT plans with half‐arc, one‐full‐arc and two‐full‐arcs were constructed and same plans were applied to structure sets with and without CT number assignment of metal artifact regions and compared. D95% was utilized to investigate PTV dose coverage and SNC Patient− Software was used for the analysis of dose distribution difference slice by slice.

Results:

For different targets sizes, variation of PTV dose coverage (Delta_D95%) with and without CT number replacement reduced with larger target volume for all half‐arc, one‐arc and two‐arc VMAT plans even though there were no clinically significant differences. Additionally, there were no significant variations of the maximum percent difference (max.%diff) of dose distribution. With regard to the target location, Delta_D95% and max. %diff dropped with increasing distance between target and metallic implant. Furthermore, half‐arc plans showed greater impact than one‐arc plans, and two‐arc plans had smallest influence for PTV dose coverage and dose distribution.

Conclusion:

The target size has less correlation of doseimetric impact than the target location relative to metallic implants. Plans with more arcs alleviate the dosimetric effect of metal artifact because of less contribution to the target dose from beams going through the regions with metallic artifacts. Incorrect CT number causes inaccurate dose distribution, therefore appropriately overwriting metallic artifact regions with reasonable CT numbers is recommended. More patient data are collected and under further analysis.
SU‐F‐T‐447: The Impact of Treatment Planning Methods On RapidPlan Modeling for Rectum Cancer
Authors: Lu S , Peng J , Li K , Wang J , Hu W
Published: July 2016

Purpose:

To investigate the dose volume histogram (DVH) prediction varieties based on intensity modulate radiotherapy (IMRT) plan or volume arc modulate radiotherapy (VMAT) plan models on the RapidPlan.

Methods:

Two DVH prediction models were generated in this study, including an IMRT model trained from 83 IMRT rectum plans and a VMAT model trained from 60 VMAT rectum plans. In the internal validation, 20 plans from each training database were selected to verify the clinical feasibility of the model. Then, 10 IMRT plans (PIMRT_by_IMRT_model) generated from IMRT model and 10 IMRT plans generated from VMAT model (PIMRT_by_VMAT_model) were compared on the dose to organs at risk (OAR), which included bladder, left and right femoral heads. The similar comparison was also performed on the VMAT plans generated from IMRT model (PVMAT_by_IMRT_model) and VMAT plans generated from VMAT (PVMAT_by_VMAT_model) model.

Results:

For the internal validation, all plans from IMRT or VMAT model shows significantly improvement on OAR sparing compared with the corresponded clinical ones. Compared to the PIMRT_by_VMAT_model, the PIMRT_by_IMRT_model has a reduction of 6.90±3.87%(p<0.001) on V40 6.63±3.62%(p<0.001) on V45 and 4.74±2.26%(p<0.001) on V50 in bladder; and a mean dose reduction of 2.12±1.75Gy(p=0.004) and 2.84±1.53Gy(p<0.001) in right and left femoral head, respectively. There was no significant difference on OAR sparing between PVMAT_by_IMRT_model and PVMAT_by_VMAT_model.

Conclusion:

The IMRT model for the rectal cancer in the RapidPlan can be applied to for VMAT planning. However, the VMAT model is not suggested to use in the IMRT planning. Cautions should be taken that the planning model based on some technique may not feasible to other planning techniques.
SU‐F‐T‐450: The Investigation of Radiotherapy Quality Assurance and Automatic Treatment Planning Based On the Kernel Density Estimation Method
Authors: Fan J , Fan J , Hu W , Wang J
Published: July 2016

Purpose:

To develop a fast automatic algorithm based on the two dimensional kernel density estimation (2D KDE) to predict the dose‐volume histogram (DVH) which can be employed for the investigation of radiotherapy quality assurance and automatic treatment planning.

Methods:

We propose a machine learning method that uses previous treatment plans to predict the DVH. The key to the approach is the framing of DVH in a probabilistic setting. The training consists of estimating, from the patients in the training set, the joint probability distribution of the dose and the predictive features. The joint distribution provides an estimation of the conditional probability of the dose given the values of the predictive features. For the new patient, the prediction consists of estimating the distribution of the predictive features and marginalizing the conditional probability from the training over this. Integrating the resulting probability distribution for the dose yields an estimation of the DVH. The 2D KDE is implemented to predict the joint probability distribution of the training set and the distribution of the predictive features for the new patient. Two variables, including the signed minimal distance from each OAR (organs at risk) voxel to the target boundary and its opening angle with respect to the origin of voxel coordinate, are considered as the predictive features to represent the OAR‐target spatial relationship. The feasibility of our method has been demonstrated with the rectum, breast and head‐and‐neck cancer cases by comparing the predicted DVHs with the planned ones.

Results:

The consistent result has been found between these two DVHs for each cancer and the average of relative point‐wise differences is about 5% within the clinical acceptable extent.

Conclusion:

According to the result of this study, our method can be used to predict the clinical acceptable DVH and has ability to evaluate the quality and consistency of the treatment planning.
SU‐F‐T‐453: Improved Head and Neck SBRT Treatment Planning Using PlanIQ
Authors: Wang H , Wang C , Phan J , Tung S , Chi P
Published: July 2016

Purpose:

Treatment planning for Head and Neck(HN) re‐irradiation is a challenge because of ablative doses to target volume and strict critical structure constraints. PlanIQ(Sun Nuclear Corporation) can assess the feasibility of clinical goals and quantitatively measure plan quality. Here, we assess whether incorporation of PlanIQ in our SBRT treatment planning process can improve plan quality and planning efficiency.

Methods:

From 2013–2015, 35 patients (29 retrospective, 6 prospective) with recurrent HN tumors were treated with SBRT using VMAT treatment plans. The median prescription dose was 45 Gy in 5 fractions. We retrospectively reviewed the treatment plans and physician directives of our first 29 patients and generated score functions of the dosimetric goals used in our practice and obtained a baseline histogram. We then re‐optimized 12 plans that had potential to further reduce organs‐at‐risk (OAR) doses according to PlanIQ feasibility DVH and plan quality analysis and compared them to the original plans. We applied our new PlanIQ‐assisted planning process for our 6 most recently treated patients and evaluated the plan quality and planning efficiency.

Results:

The mean plan quality metric(PQM) and feasibility adjusted PQM(APQM) scores of our initial 29 treatment plans were 77.1±13.1 and 88.7±11.9, respectively (0–100 scale). The PQM and APQM scores for the 12 optimized plans improved from 75.9±11.0 and 85.1±10.2 to 80.7±9.3 and 90.2±8.0, respectively (p<0.005). Using our newly developed PlanIQ‐assisted planning process, the PQM and APQM scores for the 6 most recently treated patients were 93.6±6.5 and 99.1±0.6, respectively. The planning goals were more straightforward to minimize OAR doses during optimization, thus less planning and revision time were used than before.

Conclusion:

PlanIQ has the potential to provide achievable planning goals and also improve plan quality and planning efficiency.
SU‐F‐T‐454: Dose‐Mass‐Histogram Sensitivity to Anatomical Changes During Radiotherapy for HNSCC
Authors: De Ornelas‐Couto M , Bossart E , Elsayyad N , Samuels M , Takita C , Mihaylov I
Published: July 2016

Purpose:

To determine the sensitivity of dose‐mass‐histogram (DMH) due to anatomical changes of head‐and‐neck squamous cell carcinoma (HNSCC) radiotherapy (RT).

Methods:

Eight patients undergoing RT treatment for HNSCC were scanned during the third and sixth week of RT. These second (CT2) and third (CT3) CTs were co‐registered to the planning CT (CT1). Contours were propagated via deformable registration from CT1 and doses were re‐calculated. DMHs were extracted for each CT set. DMH sensitivity was assessed by dose‐mass indices (DMIs), which represent the dose delivered to a certain mass of and anatomical structure. DMIs included: dose to 98%, 95% and 2% of the target masses (PTV1, PTV2, and PTV3) and organs‐at‐risk (OARs): cord DMI2%, brainstem DMI2%, left_ and right_parotid DMI2% and DMI50%, and mandible DMI2%. A two‐tailed paired t‐test was used to compare changes to DMIs in CT2 and CT3 with respect to CT1 (CT2/CT1 and CT3/CT1).

Results:

Changes to DMHs were found for all OARs and PTVs, but they were significant only for the PTVs. Maximum dose to PTVs increased significantly for CT2/CT1 in all three PTVs, but CT3/CT1 changes were only significantly different for PTV1 and PTV2. Dose coverage to the three PTVs was also significantly different, DMI98% was lower for both CT2/CT1 and CT3/CT1. DMI95% was significantly lower for PTV1 for CT2/CT1, PTV2 for CT2/CT1 and CT3/CT1, and PTV3 for CT3/CT1.

Conclusion:

Changes in anatomy significantly change dose‐mass coverage for the planning targets, making it necessary to re‐plan in order to maintain the therapeutic goals. Maximum dose to the PTVs increase significantly as RT progresses, which may not be problematic as long as the high dose remains in the gross tumor volume. Doses to OARs were minimally affected and the differences were not significant.