Varian Eclipse Research Articles

Automatic measurement of air gap for proton therapy using orthogonal x-ray imaging with radiopaque wires.

PurposeThe main objective of this study was to develop a technique to accurately determine the air gap between the end of the proton beam compensator and the body of the patient in proton radiotherapy.MethodsOrthogonal x‐ray image‐based automatic coordinate reconstruction was used to determine the air gap between the patient body surface contour and the end of beam nozzle in proton radiotherapy. To be able to clearly identify the patient body surface contour on the orthogonal images, a radiopaque wire was placed on the skin surface of the patient as a surrogate. In order to validate this method, a Rando® head phantom was scanned and five proton plans were generated on a Mevion S250 Proton machine with various air gaps in Varian Eclipse Treatment Planning Systems (TPS). When setting up the phantom in a treatment room, a solder wire was placed on the surface of the phantom closest to the beam nozzle with the knowledge of the beam geometry in the plan. After the phantom positioning was verified using orthogonal kV imaging, the last pair of setup kV images was used to segment the solder wire and the in‐room coordinates of the wire were reconstructed using a back‐projection algorithm. Using the wire as a surrogate of the body surface, we calculated the air gaps by finding the minimum distance between the reconstructed wire and the end of the compensator. The methodology was also verified and validated on clinical cases.ResultsOn the phantom study, the air gap values derived with the automatic reconstruction method were found to be within 1.1 mm difference from the planned values for proton beams with air gaps of 85.0, 100.0, 150.0, 180.0, and 200.0 mm. The reconstruction technique determined air gaps for a patient in two clinical treatment sessions were 38.4 and 41.8 mm, respectively, for a 40 mm planned air gap, and confirmed by manual measurements. There was strong agreement between the calculated values and the automatically measured values, and between the automatically and manually measured values.ConclusionsAn image‐based automatic method has been developed to conveniently determine the air gap of a proton beam, directly using the orthogonal images for patient positioning without adding additional imaging dose to the patient. The method provides an objective, accurate, and efficient way to confirm the target depth at treatment to ensure desired target coverage and normal tissue sparing.

6 Tumor motion for SBRT and IMRT treatment of lung cancer

Introduction The aim of this study was to determinate breathing amplitude and amplitude of lung tumor motion for Stereotactic Body Radiation Therapy (SBRT) with small (5 cm) tumor volume. Method and materials In this study we had thirty male patients. The patients were separated in two groups depending on tumor volume and treatment technique. In the first group we had 15 patients with small volume who underwent SBRT treatment. While other group of 15 patients, with large volume, were treated with IMRT technique. All patients were scanned with GE LightSpeed scanner, IMRT patients one time (2.5 mm) and SBRT patients two times (1.25 mm) with interval of one week between two scans. The contouring was performed in all ten phases for all patients, in Varian Eclipse 13.7 treatment planning system (TPS). Results Amplitude of tumor motion was 5.9 mm (range from 2 to 10.9 mm) for tumor volume 3.8 cc (range from 0.7 to 11.2 cc) for SBRT patients. The IMRT patients had amplitude of tumor motion 2.9 mm (range from 2 to 4.8 mm) for tumor volume 120.4 cc (range from 44.4 to 416.2 cc). Breathing amplitude was 14.6 mm (range from 8.5 mm to 22.6 mm) for SBRT patients and 12.1 mm (range from 7.5 mm to 18.2 mm) for IMRT patients. Conclusion The mobility and direction of tumor motion depend of the localization in the lungs. While amplitude of breathing depends of general condition of patient, physiology of lungs, respiratory problems and pain.

171. Dosimetric validation for an automatic multiple brain-metastases planning system using a single isocenter and conformal arc

PurposeA commercial planning software using this technique, called Multiple Brain Mets SRS (MBM) Element from Brainlab AG (Feldkirchen, Germany), version 1.5, has been installed in our institution. It uses a preconfigured set of dynamic noncoplanar, conformal arcs to treat up to 15 brain-metastases with a single isocenter. It optimizes the weight of each arc to achieve the best conformity for all targets. Planning time for optimization and dose calculation is 2∼4 min, which improves the planning efficiency dramatically in our clinic. In this study we describe the method used in our clinic to measure the delivered dose and then compare to the calculated dose distribution. MethodsWe included in this study 10 patients whith multiple brain mets. For these 10 pz the number of lesions planned ranges from a minimum of two to a maximum of six. The verification plan is calculated by Brainlab Element for the QA and measured in Easycube phantom equipped with a CC01 ionization chamber and EBT film. Furthermore the comparison of the plans, computed by MBM Element and Varian Eclipse, version 13.0, is realised. ResultsThe percentage difference between calculated by MBM Element and measured is included in 5%.Treatment plans generated using MBM Element and Eclipse resulted in a good agreement, the difference of the maximun dose is lower 2%. The PTV mean dose values difference between MBM Element and Eclipse have resulted less 1,7%. ConclusionsSatisfactory absolute dose and distribution agreement were observed. We conclude that MBM Element was in clinical use after these validations.

120. Electron density calibration curve for Philips Brilliance Big Bore CT for radiotherapy: HU stability and comparison with Varian default curve

PurposeThe calibration of the newly installed Philips Brilliance Big Bore CT scanner, finding the most appropriate curve (HU vs electron densities) for Varian Eclipse AAA algorithm. FOV dimensions, CT voltage, object dimensions and reconstruction filters are the major parameters that can affect the response curve, depending on the particular scanner model. They’ve been studied, and differences respect Varian default curve were evaluated. Methods and materialsThe CIRS62M HEAD and BODY phantoms, equipped with low and high densities inserts, of known electron densities were used. Scans were made varying the position of inserts from the centre to the border of the test object, then varying separately CT voltage, FOV, test-object dimensions with respect to FOV, CT protocol (head vs body protocols), with particular attention to reconstruction filters. Test acquisitions regarded both allowed and not-allowed filters (Philips gives general indications about filters to be avoided for planning, cause their effect on HU is not negligible). ResultsCT voltage and FOV dimensions have shown not to influence the response curve. Variability among allowed reconstruction filters is acceptable even for high HU (<2% at 1000 HU), while the not-allowed filters move the HU values till 3% and 12% (at 1000 HU, body and head respectively). With allowed filters, the factor mainly influencing the HU curve has proved to be the object dimension: the head phantom scanned with a body protocol, independently of FOV, (e.g head-and-neck or arm/leg scans) gives differences till 9% at 1000 HU: the head curve should be used. ConclusionMeasures analysis suggests to create two different curves for planning purposes, with the average data of allowed filters: (1) head and (2) body curves. Varian default curve is much similar to the head one, while differences are not negligible with the body curve extrapolating at very high densities: titanium HU is evaluated 5200 against Varian 5600.▪

134. DynaLog file-based Procedure for a daily monitoring of IMRT dose delivery

PurposeThanks to the intensity modulated radiation therapy (IMRT) it is possible to achieve highly conformal dose distribution with an important shield to the adjacent organs at risk. Nevertheless, the complex nature of such treatments requires a comprehensive Quality Assurance (QA). The last decade witnessed a proliferation of QA method based on machine log files (DynaLogs Files) since their several advantages [1,2]. In this study we propose a DynaLog file-based method to ensure a daily monitoring of the correct dose delivery. MethodsThe delivery of one prostate IMRT treatment has been daily monitored thanks to the DynaLogs Files. These files are generated for each field delivery and record many useful information [1,2]. Herein the DynaLogs information about planned and actual cumulative dose fraction is taken into account. The prostate IMRT plan consists of 5 fixed field arrangements using a dynamic delivery technique (sliding window). It was calculated using the Varian Eclipse TPS (version 13.6; Varian, Palo Alto) with the analytical anisotropic algorithm (AAA) and it was delivered on a Varian Clinac2100CD. Target dose prescription is 80 Gy in 40 fractions. ResultsThe deviation between planned and actual dose fraction was calculated, for each field, as a function of both fraction numbers and delivery time. The mean and maximum dose deviations were 0.1 and 0.3 Monitor Units, respectively. ConclusionsUsually the IMRT QA is performed before the treatment beginning and it is assumed that no further variations will occur. The proposed method allows to daily assess the correct dose delivery thanks to DynaLogs Files in a no time-consuming way.

Dose calibration uncertainty and plan-specific dose calibration for IMRT QA

Purpose: This study was to investigate IMRT QA dose calibration uncertainty that has not been realized before, and develop plan-specific calibration methods to reduce such uncertainty. Methods: IMRT plans were created on the Varian Eclipse treatment planning system. IMRT QA used a PTW Octavius phantom with an Octavius 2D array ionization chamber detector and was also tested for a solid water cube with a Farmer chamber. Dose calibration for a solid phantom was analyzed using the AAPM TG-51/TG-21 formulism. The reference fields used for IMRT dose calibration included squared fields and IMRT plan-specific fields that included geometric equivalent fields, and dose-equivalent fields defined by the 50% isodose distributions. Results: Thirteen IMRT QA cases including three VMAT cases were used for this study. For the single field calibration on the PTW Octavius device, the calibration factors varied with field size by up to 5.6%, and varied with gantry angle by up to 4.9%. Those variations changed IMRT QA passing rates by up to 16.2%. Using the plan-specific calibration, the Gamma passing rate was increased by up to 27.7%. The VMAT passing rates varied by up to 30%, and the equivalent field dose calibration gave the highest passing rate. For absolute point dose measurement using a Farmer chamber, calibration factors varied by up to 3.7% with different field sizes and different gantry angles. The dose equivalent field calibration achieved the best IMRT QA results. Conclusion: Different fields can have different dose calibration factors. Conventional dose calibration methods for IMRT QA using simple reference fields include significant uncertainty that would affect IMRT QA results. Dose calibration uncertainty is caused by the complex composition of phantom materials and complex design and characteristics of the detectors. The plan-specific dose calibration can reduce dose calibration uncertainty and improve IMRT QA accuracy.

137. Monitor Units constraints for SBRT plans and prostate cancer

PurposeIn this work the correlation between total monitor units (MUs) constraints, dosimetric findings and pre-treatment verifications has been studied for prostate Stereotactic Body Radiation Therapy (SBRT) VMAT plans. MethodsA study on 5 SBRT prostate treatments has been performed.Plans were been accomplished with Varian Eclipse TPS on TrueBeam™ 2.5. The so called MU Objective tool implemented in VMAT optimization engine was used. This tool allows setting the range and the relative priority to the minimum and maximum number of MU employed for plan optimization.Dose prescription was 35 Gy in 5 fractions, using 10MV-FFF photon beams and plans were calculated using Acuros XB algorithm. All plans were calculated with and without constraints on MUs.For each case the 2 plans were compared in terms of DVH, MUs, modulation complexity coefficient (MU/Gy) and pre-treatment verifications performed with EPID and Epiqa software. ResultsAverage MUs were 973 with a modulation coefficient of 277.5 MU/Gy and 1120 with 320 MU/Gy, respectively for plans with and without forced MUs.Differences in DVHs have been found. In fact, for 3 of the 5 cases, use of “MU objective” involve a better PTV coverage and a reduced dose to OARs. Pre-treatments verifications showed a better agreement between calculated and delivered dose distribution for all SBRT plans obtained forcing MUs (see Table 1).All plans were acceptable with a GAI > 95%. ConclusionsImportant discrepancies have been found in terms of total MUs and modulation complexity coefficient, that are considerably reduced for plans obtained forcing the MUs.SBRT prostate plans obtained using forced MUs reveals an increasing of GAI value associated with MUs reduction.In our opinion, for SBRT treatments further studies are necessaries on a wider sample of cases to understand if minimizing modulation (i.e. decreasing total MUs) could improve SBRT plans quality.

Comparison of measured and calculated doses in a Rando phantom with a realistic lung radiotherapy treatment plan including heterogeneities

In this work, dose measurements were performed to evaluate an external radiotherapy treatment plan and, particularly, to validate dose calculations for a lung lesion case. Doses were calculated by the Varian Eclipse treatment planning system using the AAA anisotropic analytical algorithm. The measurements were performed using a Rando anthropomorphic phantom and TLD700 thermoluminescent dosimeters. The comparison between doses calculated and doses measured by means of thermoluminescence (TL) shows compatibility except for a few points, due to the limitations in the heterogeneity correction used for the case studied here. The deviation between the calculated and measured doses is about 6.5% for low (< 0.5Gy) doses and about 1% for higher doses (> 0.5Gy).The deviation between AAA-calculated and TL-measured doses was also found to be higher in proximity to heterogeneous tissue interfaces.

Accuracy and efficiency of graphics processing unit (GPU) based Acuros XB dose calculation within the Varian Eclipse treatment planning system

To evaluate, in terms of dosimetric accuracy and calculation efficiency, the implementation of a graphic processing unit (GPU)-based Acuros XB dose calculation engine within version 15.5 of the Varian Eclipse treatment planning system. Initial phantom based calculations and a range of 101 clinical cases were analyzed on a dedicated test system. Dosimetric differences, based on dose-volume histrogram parameters and plan comparison, were compared between central processing unit (CPU) and GPU based calculation. Calculation times were also compared between CPU and GPU, as well as PLAN and FIELD modes. No dosimetric differences were found between CPU and GPU. CPU based calculations ranged from 25 to 533 seconds per plan, reducing to 13 to 70 seconds for GPU. GPU was 4.4 times more efficient than CPU. FIELD mode was up to 1.3 times more efficient than PLAN mode. For the clinical cases and version of Eclipse used, no dosimetric differences were found between CPU and GPU. Based on this, GPU architecture has been safely implemented and is ready for clinical use. GPU based calculation times were superior to CPU, being on average, 4.4 times faster.

Experimental validatıon of peripheral dose distribution of electron beams for eclipse electron Monte Carlo algorithm

AbstractAimThe accuracy of two calculation algorithms of the Varian Eclipse treatment planning system (TPS), the electron Monte Carlo algorithm (eMC) and general Gaussian pencil beam algorithm (GGPB) for calculating peripheral dose distribution of electron beams was investigated.MethodsPeripheral dose measurements were carried out for 6, 9, 12, 15, 18 and 22 MeV electron beams using parallel plate ionisation chamber and EBT3 film in the slab phantom. Measurements were performed for 6×6, 10×10 and 25×25 cm2 cone sizes at dmax of each energy up to 20 cm beyond the field edges. The measured and TPS calculated data were compared.ResultsThe TPS underestimated the out-of-field doses. The difference between measured and calculated doses increase with the cone size. For ionisation chamber measurement, the largest deviation between calculated and measured doses is &lt;4·29% using the eMC, but can increase up to 8·72% of the distribution using GGPB. For film measurement, the minimum gamma analysis passing rates between measured and calculated dose distributions for all field sizes and energies used in this study were 91·2 and 74·7% for eMC and GGPB, respectively.FindingsThe use of GGPB for planning large field treatments with 6 MeV could lead to inaccuracies of clinical significance.

Estimating the uncertainty of calculated out-of-field organ dose from a commercial treatment planning system.

Therapeutic radiation to cancer patients is accompanied by unintended radiation to organs outside the treatment field. It is known that the model‐based dose algorithm has limitation in calculating the out‐of‐field doses. This study evaluated the out‐of‐field dose calculated by the Varian Eclipse treatment planning system (v.11 with AAA algorithm) in realistic treatment plans with the goal of estimating the uncertainties of calculated organ doses. Photon beam phase‐space files for TrueBeam linear accelerator were provided by Varian. These were used as incident sources in EGSnrc Monte Carlo simulations of radiation transport through the downstream jaws and MLC. Dynamic movements of the MLC leaves were fully modeled based on treatment plans using IMRT or VMAT techniques. The Monte Carlo calculated out‐of‐field doses were then compared with those calculated by Eclipse. The dose comparisons were performed for different beam energies and treatment sites, including head‐and‐neck, lung, and pelvis. For 6 MV (FF/FFF), 10 MV (FF/FFF), and 15 MV (FF) beams, Eclipse underestimated out‐of‐field local doses by 30%–50% compared with Monte Carlo calculations when the local dose was <1% of prescribed dose. The accuracy of out‐of‐field dose calculations using Eclipse is improved when collimator jaws were set at the smallest possible aperture for MLC openings. The Eclipse system consistently underestimates out‐of‐field dose by a factor of 2 for all beam energies studied at the local dose level of less than 1% of prescribed dose. These findings are useful in providing information on the uncertainties of out‐of‐field organ doses calculated by Eclipse treatment planning system.

Evaluation of target dose inhomogeneity in breast cancer treatment due to tissue elemental differences

BackgroundMonte Carlo simulations were run to estimate the dose variations generated by thedifference arising from the chemical composition of the tissues.MethodsCT datasets of five breast cancer patients were selected. Mammary gland was delineated as clinical target volume CTV, as well as CTV_lob and CTV_fat, being the lobular and fat fractions of the entire mammary gland. Patients were planned for volumetric modulated arc therapy technique, optimized in the Varian Eclipse treatment planning system. CT, structures and plans were imported in PRIMO, based on Monte Carlo code Penelope, to run three simulations: AdiMus, where the adipose and muscle tissues were automatically assigned to fat and lobular fractions of the breast; Adi and Mus, where adipose and muscle, respectively were assigned to the whole mammary gland. The specific tissue density was kept identical from the CT dataset. Differences in mean doses in the CTV_lob and CTV_fat structures were evaluated for the different tissue assignments. Differences generated by the tissue composition and estimated by Acuros dose calculations in Eclipse were also analysed.ResultsFrom Monte Carlo simulations, the dose in the lobular fraction of the breast, when adipose tissue is assigned in place of muscle, is overestimated by 1.25 ± 0.45%; the dose in the fat fraction of the breast with muscle tissue assignment is underestimated by 1.14 ± 0.51%. Acuros showed an overestimation of 0.98 ± 0.06% and an underestimation of 0.21 ± 0.14% in the lobular and fat portions, respectively. Reason of this dissimilarity resides in the fact that the two calculations, Monte Carlo and Acuros, differently manage the range of CT numbers and the material assignments, having Acuros an overlapping range, where two tissues are both present in defined proportions.ConclusionAlthough not clinically significant, the dose deposition difference in the lobular and connective fat fraction of the breast tissue lead to an improved knowledge of the possible dose distribution and homogeneity in the breast radiation treatment.

Clinical implementation of AXB from AAA for breast: Plan quality and subvolume analysis.

PurposeTwo dose calculation algorithms are available in Varian Eclipse software: Anisotropic Analytical Algorithm (AAA) and Acuros External Beam (AXB). Many Varian Eclipse‐based centers have access to AXB; however, a thorough understanding of how it will affect plan characteristics and, subsequently, clinical practice is necessary prior to implementation. We characterized the difference in breast plan quality between AXB and AAA for dissemination to clinicians during implementation.MethodsLocoregional irradiation plans were created with AAA for 30 breast cancer patients with a prescription dose of 50 Gy to the breast and 45 Gy to the regional node, in 25 fractions. The internal mammary chain (IMCCTV) nodes were covered by 80% of the breast dose. AXB, both dose‐to‐water and dose‐to‐medium reporting, was used to recalculate plans while maintaining constant monitor units. Target coverage and organ‐at‐risk doses were compared between the two algorithms using dose–volume parameters. An analysis to assess location‐specific changes was performed by dividing the breast into nine subvolumes in the superior–inferior and left–right directions.ResultsThere were minimal differences found between the AXB and AAA calculated plans. The median difference between AXB and AAA for breastCTV V 95%, was <2.5%. For IMCCTV, the median differences V 95%, and V 80% were <5% and 0%, respectively; indicating IMCCTV coverage only decreased when marginally covered. Mean superficial dose increased by a median of 3.2 Gy. In the subvolume analysis, the medial subvolumes were “hotter” when recalculated with AXB and the lateral subvolumes “cooler” with AXB; however, all differences were within 2 Gy.ConclusionWe observed minimal difference in magnitude and spatial distribution of dose when comparing the two algorithms. The largest observable differences occurred in superficial dose regions. Therefore, clinical implementation of AXB from AAA for breast radiotherapy is not expected to result in changes in clinical practice for prescribing or planning breast radiotherapy.

EP-1879: Implementation of GPU technology into the Varian Eclipse TPS for Fast Dose Calculation

EP-1879: Implementation of GPU technology into the Varian Eclipse TPS for Fast Dose Calculation

A simple technique to improve calculated skin dose accuracy in a commercial treatment planning system.

Radiation dermatitis during radiotherapy is correlated with skin dose and is a common clinical problem for head and neck and thoracic cancer patients. Therefore, accurate prediction of skin dose during treatment planning is clinically important. The objective of this study is to evaluate the accuracy of skin dose calculated by a commercial treatment planning system (TPS). We evaluated the accuracy of skin dose calculations by the anisotropic analytical algorithm (AAA) implemented in Varian Eclipse (V.11) system. Skin dose is calculated as mean dose to a contoured structure of 0.5 cm thickness from the surface. The EGSnrc Monte Carlo (MC) simulations are utilized for the evaluation. The 6, 10 and 15 MV photon beams investigated are from a Varian TrueBeam linear accelerator. The accuracy of the MC dose calculations was validated by phantom measurements with optically stimulated luminescence detectors. The calculation accuracy of patient skin doses is studied by using CT based radiotherapy treatment plans including 3D conformal, static gantry IMRT, and VMAT treatment techniques. Results show the Varian Eclipse system underestimates skin doses by up to 14% of prescription dose for the patients studied when external body contour starts at the patient's skin. The external body contour is used in a treatment planning system to calculate dose distributions. The calculation accuracy of skin dose with Eclipse can be considerably improved to within 4% of target dose by extending the external body contour by 1 to 2 cm from the patient's skin. Dose delivered to deeper target volumes or organs at risk are not affected. Although Eclipse treatment planning system has its limitations in predicting patient skin dose, this study shows the calculation accuracy can be considerably improved to an acceptable level by extending the external body contour without affecting the dose calculation accuracy to the treatment target and internal organs at risk. This is achieved by moving the calculation entry point away from the skin.

Evaluation of dose calculation accuracy of treatment planning systems in the presence of tissue heterogeneities

Background: Accuracy of five dose calculation algorithms within three treatment planning systems (TPS): BrainLAB iPlan 4.2 (BL: pencil beam and Monte Carlo), Philips Pinnacle (PL: Collapsed Cone Calculation, CCC), and Varian Eclipse (VR: AAA and Acuros XB) is investigated in this multi-institutional study. Methods: A Monte-Carlo based TPS (BL) was first validated against benchmark measurements in heterogeneous-slab phantoms consisting of tissue-equivalent plastic, lung-equivalent cork, and bone density materials. Ion chamber/EDR film measurements of depth-dose and dose-profiles for 6MV enface photons in a range of field-sizes (12 mm × 12 mm – 60 mm × 60 mm) were performed. A common CT dataset for each phantom was sent to four participating institutions. Measured versus calculated dose differences for all dose algorithms considered in this study were quantified using two dose-profile indices: D diff within the central 80% of photon field, and D spill for dose outside the field-edge (50–10% dose). Results: BrainLAB TPS BL : MC and measured doses agreed well ( D diff <3%) for all field-sizes and phantom depths investigated. The agreement improved with increasing field size (2.6% for 12 mm × 12 mm vs. 1.1% for 60 mm × 60 mm at 60 mm depth in lung phantom). In contrast, for lung phantom, pencil beam (PB) calculations significantly over-predicted the measured dose (34% and 6.7% respectively). In general, PB vs. measured dose differences increased with decreasing field-size, decreasing phantom density and increasing depth within heterogeneity. Pinnacle TPS PL : pinnacle showed the best agreement with measured data in the presence of tissue heterogeneities and modeled dose changes at the tissue-lung interface better than AAA and PB. Varian TPS VR : AAA over-predicted measured results and was unable to replicate dose variation near heterogeneities. In contrast, Acuros XB showed good agreement with measurements for heterogeneous media as well as superior agreement in the interface region. Conclusions: Based on the results of this multi-institutional study, appropriate corrections may be applied to lung stereotactic body radiotherapy (SBRT) plans for patients enrolled in RTOG protocols.

Experimental Assessment of Proton Dose Calculation Accuracy in Small-Field Delivery Using a Mevion S250 Proton Therapy System.

Purpose:Dose calculation accuracy of the Varian Eclipse treatment planning system (TPS) is empirically assessed for small-aperture fields using a Mevion S250 double scattering proton therapy system.Materials and Methods:Five spherical pseudotumors were modeled in a RANDO head phantom. Plans were generated for the targets with apertures of 1, 2, 3, 4, or 5 cm diameter using one, two, and three beams. Depth-dose curves and lateral profiles of the beams were taken with the planned blocks and compared to Eclipse calculations. Dose distributions measured with EBT3 films in the phantom were also compared to Eclipse calculations. Film quenching effect was simulated and considered.Results:Depth-dose scans in water showed a range pullback (up to 2.0 mm), modulation widening (up to 9.5 mm), and dose escalation in proximal end and sub-peak region (up to 15.5%) when compared to the Eclipse calculations for small fields. Measured full width at half maximum and penumbrae for lateral profiles differed <1.0 mm from calculations for most comparisons. In the phantom study, Eclipse TPS underestimated sub-peak dose. Gamma passing rates improved with each beam added to the plans. Greater range pullback and modulation degradation versus water scans were observed due to film quenching, which became more noticeable as target size increased.Conclusions:Eclipse TPS generates acceptable target coverage for small targets with carefully arranged multiple beams despite relatively large dose discrepancy for each beam. Surface doses higher than Eclipse calculations can be mitigated with multiple beams. When using EBT3 film, the quenching effect should be considered.

Evaluation of Hybrid Arc and Volumetric-Modulated Arc Therapy Treatment Plans for Fractionated Stereotactic Intracranial Radiotherapy.

Purpose:The study was aimed to compare hybrid arc and volumetric-modulated arc therapy treatment plans for fractionated stereotactic radiotherapy of brain tumors.Methods:Treatment plans of 22 patients were studied. Hybrid arc and volumetric-modulated arc therapy plans were generated using Brainlab iPlanDose and Varian Eclipse treatment planning systems, respectively, with 6 MV photon beams on a Varian TrueBeam STx linear accelerator (Palo Alto, CA). Prescription dose was 54 Gy. The fractionation was 1.8 Gy per fraction and 30 fractions in total, or 2 Gy per fraction and 27 fractions in total. Planning target volume ranged from 2.4 to 28.6 cm3. Dose conformity index, gradient index, homogeneity index, and maximum doses in organs at risk were compared. Wilcoxon signed rank test was used to determine statistical significance in paired comparison.Results:Conformity indexes of hybrid arc and volumetric-modulated arc therapy plans are 1.10 ± 0.10 and 1.14 ± 0.07, respectively (P = .4); gradient indexes are 5.02 ± 1.20 and 5.64 ± 1.28, respectively (P = .0001); homogeneity indexes are 1.02 ± 0.01 and 1.05 ± 0.01, respectively (P = .0001); brainstem maximum doses are 53.87 ± 1.63 Gy and 54.06 ± 3.17 Gy, respectively (P = .1); and optic chiasm maximum doses are 53.86 ± 1.28 Gy and 53.95 ± 1.81, respectively (P = .4). The monitor unit efficiencies of hybrid arc and volumetric-modulated arc therapy plans are 2.57 ± 0.25 MU/cGy and 2.68 ± 0.24 MU/cGy, respectively (P = .2). The differences of conformity index, gradient index, and homogeneity index between hybrid arc and volumetric-modulated arc therapy plans are small: 0.08 ± 0.05, 0.65 ± 0.46, and 0.02 ± 0.01, respectively. The maximum doses in organs at risks are similar between hybrid arc and volumetric-modulated arc therapy plans. Hybrid arc and volumetric-modulated arc therapy plans, which have similar monitor unit efficiencies, present similar dosimetric results in the fractionated intracranial radiotherapy.

20. Pre-treatment quality assurance for flattening filter free dynamic arcs: A detector comparison

Introduction Pre-treatment quality assurance (QA) with flattening filter free (FFF) beams is challenging because of the high dose rate and the required spatial resolution as small field are usually used. Our current clinical practice is to perform pre-treatment QA with a diode array in a rotational phantom. This study investigates the replacement of this process by an electronic portal imager (EPI) measurement without a phantom in place in order to shorten the QA time on the machine. Methods Pre-treatment QA was performed with PTW Octavius 4D phantom equipped with SRS1000 diode array and with Varian aS1000 EPI. Diode array measurements were compared with the dose calculated in Octavius phantom with Varian Eclipse 13.7. EPI measurements was converted to water measurement and compared with water calculation performed in Dosisoft EPIBEAM (ThinkQA 1.0.2.2ALPHA). QA results were compared for Varian Truebeam 10 FFF dynamic arcs of 10 liver stereotactic radiotherapy plans and for plans with dose and MLC errors voluntarily introduced during beam delivery. Results Clinical plans covered a wide range of PTV size (average = 129.5 cm3, minimum = 9.8 cm3, maximum = 327.5 cm3) and dose prescription (average = 11.8 Gy, minimum = 6 Gy, maximum = 18 Gy) resulting in 1866 MU on average (minimum = 806 MU, maximum = 3072). Diode array and portal imager QA process gave both gamma values higher than 97% for 2%-2 mm criteria (10% threshold). Analysis of plans with errors is pending. Conclusions EPI QA process for FFF beams is achievable. It is much faster than diode array process as there is no need for a phantom setup and it provides similar results.

On the gEUD biological optimization objective for organs at risk in Photon Optimizer of Eclipse treatment planning system.

Inverse planning optimization using biologically based objectives is becoming part of the intensity modulated optimization process. The performances and efficacy of the biologically based gEUD (generalized Equivalent Uniform Dose) objective implemented in the Photon Optimizer (PO) of Varian Eclipse treatment planning system have been here analyzed. gEUD is associated with the parameter a that accounts for the seriality of a structure, being higher for more serial organs. The PO was used to optimize volumetric modulated arc therapy (VMAT) plans on a virtual homogeneous cylindrical phantom presenting a target and an organ at risk (OAR). The OAR was placed at 4 mm, 1 and 2 cm distance, or cropped at 0, 2 and 4 mm from the target. Homogeneous target dose of 60 Gy in 20 fractions was requested with physical dose‐volume objectives, while OAR dose was minimized with the upper gEUD objective. The gEUD specific a parameter was varied from 0.1 to 40 to assess its impact to OAR sparing and target coverage. Actual head and neck and prostate cases, with one parotid and the rectum as test OAR, were also analyzed to translate the results in the more complex clinical environment. Increasing the a parameter value in the gEUD objective, the optimization achieved lower volumes of the OAR which received the highest dose levels. The maximum dose in the OAR was minimized well with a values up to 20, while further increase of a to 40 did not further improve the result. The OAR mean dose was reduced for the OAR located at 1 and 2 cm distance from the target, enforced with increasing a. For cropped OARs, a mean dose reduction was achieved for a values up to 3–5, but mean dose increased for higher a values. The optimal choice of the parameter a depends on the mutual OAR and target position, and seriality of the organ. Today no significant compendium of clinical and biological specific a and gEUD values are available for a wide range of OARs.