Golden Beam Data Research Articles

Golden beam data for proton pencil-beam scanning

Proton, as well as other ion, beams applied by electro-magnetic deflection in pencil-beam scanning (PBS) are minimally perturbed and thus can be quantified a priori by their fundamental interactions in a medium. This a priori quantification permits an optimal reduction of characterizing measurements on a particular PBS delivery system. The combination of a priori quantification and measurements will then suffice to fully describe the physical interactions necessary for treatment planning purposes. We consider, for proton beams, these interactions and derive a ‘Golden’ beam data set. The Golden beam data set quantifies the pristine Bragg peak depth-dose distribution in terms of primary, multiple Coulomb scatter, and secondary, nuclear scatter, components. The set reduces the required measurements on a PBS delivery system to the measurement of energy spread and initial phase space as a function of energy. The depth doses are described in absolute units of Gy(RBE) mm2 Gp−1, where Gp equals 109 (giga) protons, thus providing a direct mapping from treatment planning parameters to integrated beam current. We used these Golden beam data on our PBS delivery systems and demonstrated that they yield absolute dosimetry well within clinical tolerance.

SU‐E‐T‐338: Percent Depth Dose Comparison Between Measurements from Three Varian Linear Accelerators and Varian AAA Golden Beam Data

Purpose: To validate the use of Varian Golden Beam Data Percentage Depth Dose (PDD) information for the Eclipse AAA algorithm for newly installed Varian Linear Accelerators Methods: Comparison was made between measured data and Varian Golden Beam Data which is often used in AAA beam models in Eclipse treatment planning systems. Open field and wedge field data were compared for 4 × 4 cm, 10×10 cm, 20 × 20 cm, and 30 × 30 cm field sizes for four physical wedges and for two photon energies. Results: Small differences on the order of 1% are observed when comparing measured open field PDD data and PDD data from the AAA Golden Beam Data set. In contrast, larger differences were observed in wedge field percentage depth dose for these data sets with the measured data being greater than AAA data. The difference increases with increasing depth and decreasing field size. For a 4 × 4 cm field with a 15 degree wedge the average difference is 6 % at a 25 cm depth for 6 MV photons. These differences are slightly smaller for 18 MV photon data for most field size and wedge combinations. Conclusions: The Varian Golden Beam wedged field PDD data for photon beams is compiled from older linear accelerators and shows variations from data collected from recently installed machines. This variation could be due to increased beam hardening effect from the wedges compared to previous machines. Therefore, caution should be applied when using the AAA Golden Beam model for wedged field calculations, particularly for field sizes smaller or equal to 20 × 20 cm and depths deeper than 15cm. It is recommended that carefully collected commissioning data be used for physical wedge PDDs in Eclipse.

SU-E-T-50: Updating a Familiar Clinical Tool: Including Structures in Gamma Index Calculations

Purpose: In order to improve the correlation between gamma index pass rates and plan quality, a new method was developed to incorporate DVH structures into the gamma index analysis. Methods: Five treatment machines were created in the Varian Eclipse platform. The original machine was created using Varian golden beam data. Deviations, representing clinical errors, were introduced in the other machines by modifying MLC transmission factors and smoothing beam penumbras. Doses for 9 Head and Neck dynamic IMRT plans were calculated, creating 36 modified and 9 original plans. The max spinal cord dose and dose to 95% of the PTV (PTV-95%) were calculated. The modified plans were compared to the original using a gamma analysis commonly used in IMRT QA. A new method of gamma analysis was performed by using the cord and PTV structures to vary the passing criteria (%/mm). The structures were extracted using Matlab and projected from a “beams eye view” onto the QA dose planes. To emphasize the importance of the dose to the structures, the passing criteria were tightened in proportion to the thickness of the structure that the beam traveled through. Pearson correlation coefficients(r-values) comparing the PTV-95%/spine dose differences and the gamma pass rates were calculated. Results: Pearson coefficients for the max cord dose were −0.717/−0.720/−.750 for traditional 3/2/1 (%/mm) passing criteria, respectively. This improved to −0.831 when using the structure dependent passing criteria. Pearson coefficients for PTV-95% were −0.729/−0.703/−.711 for the 3/2/1 (%/mm) criteria and −0.812 for the structures dependent criteria. Conclusions: Using structural information to vary gamma analysis passing criteria can result in improved correlation between gamma index pass rates and clinically relevant dose objectives. This enhances the ability of the gamma index, making it a more powerful IMRT QA tool.

SU‐GG‐T‐307: Multi‐Institution Comparison of Varian Linac Electron Commissioning Data: Application to Eclipse Generalized Gaussian Pencil Beam Algorithm Commissioning

Purpose: To point out clinically relevant issues in the commissioning process of the Eclipse Generalized Gaussian Pencil Beam (GGPB) algorithm and to post a multi‐institution comparison of Varian linac electron commissioning data. Research was performed to quantify the accuracy of commissioning a treatment planning system using either Varian “golden beam data” or common data for several linacs. Using such data may be a useful option for clinics installing or replacing a linac, with the need to make the transition with minimal down time. This work seeks to address whether or not such techniques are clinically acceptable. Method and Materials: Graphical techniques were used to compare the electron commissioning data from multiple independent Varian linacs. The performance of the Varian “golden beam data” for beam modeling using the Eclipse GGPB algorithm was examined by comparing calculations to measured dose distributions as well to calculations made with a fully commissioned pencil beam algorithm in CMS Xio. Results: Depth dose curves from the various independent linacs were typically found to be within a 3 mm distance to agreement, with the Varian “golden beam data” falling within an approximate 5 mm distance to agreement. Comparisons between CMS Xio and Eclipse showed that Eclipse depth dose profiles are less smooth and tend to contain calculation algorithm artifacts. Conclusion: Although good agreement was found between the commissioning data of the clinics in this study, special care should be taken when using data from another linac to commission a treatment planning system. With careful adjustment of the modeling parameters in the treatment planning systems it may be possible to achieve calculation accuracy that is within a clinically acceptable tolerance. However, if optimal accuracy is desired, a full set of commissioning data should be collected for the specific linac being modeled.

SU‐GG‐T‐303: Evaluation of the Sensitivity of the Anisotropic Analytical Algorithm (AAA) to the Commissioning Dataset

Purpose: To test the sensitivity of the Eclipse anisotropic analytical algorithm (AAA) to expanded commissioning data that includes small‐field data and data corrected for detector size. Method and Materials: A 0.01 cm3 ionization chamber was used to measure an extended commissioning data set for a 6 MV photon beam including open field profiles (OFP), percent depth dose (PDD), and output factors for field sizes ≥ 1×1 cm2. OFP were processed using mean‐value symmetry and adjusted for detector size effects. OFP were smoothed outside the penumbra region for noise reduction. PDD data were shifted to account for the effective point of measurement and then smoothed. Four unique beam models were commissioned to evaluate the impact of correcting the measured data for detector size as well as small‐field data inclusion. We compared the resulting AAA calculated models with the model based on Golden Beam Data (GBD) provided by Varian Medical Systems. Models were compared using a 1 mm calculation grid. Results: The various models proved to be insensitive to adjustments for detector size effects and also to the inclusion of small‐field data. All models, regardless of the penumbra of input profiles, exhibited nearly identical penumbra in the final calculated model. The algorithm narrowed the penumbra for the model commissioned with GBD i.e. with broad penumbra. Conversely, the algorithm broadened the penumbra for the model commissioned with measured data that were smoothed and corrected for volume averaging effects i.e. data with narrow penumbra. Conclusion: Results suggest that the TPS utilizes a generic model for the penumbra region in calculated models, thus ignoring measured data. The negligible impact of inclusion of small‐field data on the TPS's calculated model may be due to the large relative size of the penumbra region for those fields.

Validation of the Eclipse AAA algorithm at extended SSD

The accuracy of dose calculations at extended SSD is of significant importance in the dosimetric planning of total body irradiation (TBI). In a first step toward the implementation of electronic, multi‐leaf collimator compensation for dose inhomogeneities and surface contour in TBI, we have evaluated the ability of the Eclipse AAA to accurately predict dose distributions in water at extended SSD. For this purpose, we use the Eclipse AAA algorithm, commissioned with machine‐specific beam data for a 6 MV photon beam, at standard SSD (100 cm). The model was then used for dose distribution calculations at extended SSD (179.5 cm). Two sets of measurements were acquired for a 6 MV beam (from a Varian linear accelerator) in a water tank at extended SSD: i) open beam for 5 × 5, 10 × 10, 20 × 20 and 40×40 cm2 field sizes (defined at 179.5 cm SSD), and ii) identical field sizes but with a 1.3 cm thick acrylic spoiler placed 10 cm above the water surface. Dose profiles were acquired at 5 cm, 10 cm and 20 cm depths. Dose distributions for the two setups were calculated using the AAA algorithm in Eclipse. Confidence limits for comparisons between measured and calculated absolute depth dose curves and normalized dose profiles were determined as suggested by Venselaar et al. The confidence limits were within 2% and 2 mm for both setups. Extended SSD calculations were also performed using Eclipse AAA, commissioned with Varian Golden beam data at standard SSD. No significant difference between the custom commissioned and Golden Eclipse AAA was observed. In conclusion, Eclipse AAA commissioned at standard SSD can be used to accurately predict dose distributions in water at extended SSD for 6 MV open beams.PACS numbers: 87.53.Kn, 87.55.D‐, 87.55.de, 87.56.ng

Experimental validation of the Eclipse AAA algorithm

The present study evaluates the performance of a newly released photon‐beam dose calculation algorithm that is incorporated into an established treatment planning system (TPS). We compared the analytical anisotropic algorithm (AAA) factory‐commissioned with “golden beam data” for Varian linear accelerators with measurements performed at two institutions using 6‐MV and 15‐MV beams. The TG‐53 evaluation regions and criteria were used to evaluate profiles measured in a water phantom for a wide variety of clinically relevant beam geometries. The total scatter factor (TSF) for each of these geometries was also measured and compared against the results from the AAA.At one institute, TLD measurements were performed at several points in the neck and thoracic regions of a Rando phantom; at the other institution, ion chamber measurements were performed in a CIRS inhomogeneous phantom. The phantoms were both imaged using computed tomography (CT), and the dose was calculated using the AAA at corresponding detector locations. Evaluation of measured relative dose profiles revealed that 97%, 99%, 97%, and 100% of points at one institute and 96%, 88%, 89%, and 100% of points at the other institution passed TG‐53 evaluation criteria in the outer beam, penumbra, inner beam, and buildup regions respectively. Poorer results in the inner beam regions at one institute are attributed to the mismatch of the measured profiles at shallow depths with the “golden beam data.”For validation of monitor unit (MU) calculations, the mean difference between measured and calculated TSFs was less than 0.5%; test cases involving physical wedges had, in general, differences of more than 1%. The mean difference between point measurements performed in inhomogeneous phantoms and Eclipse was 2.1% (5.3% maximum) and all differences were within TG‐53 guidelines of 7%. By intent, the methods and evaluation techniques were similar to those in a previous investigation involving another convolution–superposition photon‐beam dose calculation algorithm in another TPS, so that the current work permitted an independent comparison between the two algorithms for which results have been provided.PACS number: 87.53.Dq

Po‐Thur Eve General‐06: Experimental Validation of the Eclipse AAA Algorithm

The Medical Physics departments of the Tom Baker Cancer Center (TBCC) and the Cross Cancer Institute (CCI) independently performed preliminary evaluation of the new Analytical Anisotropic Algorithm (AAA) implemented in Varian's Eclispe (v. 6.0) treatment planning system (TPS). The TPS was pre‐commissioned with “Golden Beam Data” from the vendor. We measured central and off‐axis profiles in several beam configurations including: open square, rectangular and asymmetric (half‐blocked) beams; wedged square and half‐blocked beams; square fields at three SSDs; open and wedged oblique beams; irregular field defined by MLC and cerrobend blocks. All measurements were performed on Varian 2100EX linear accelerators. Measurements were made to assess the dose in heterogeneous media at both the CCI (CIRS Thorax IMRT phantom) and at the TBCC (TLDs in a Rando phantom). Profiles were evaluated in the buildup, penumbra, inner and outer beam regions as per AAPM Task Group 53.Measured and calculated profiles agreement was very good in all regions except for the inner beam region at the CCI, attributed a difference in interpolation schemes at the two institutions and the large volume ion chamber used for measurements. The AAA penumbra was also found to be steeper than measured penumbra since AAA was pre‐commissioned using diode measurements. Total scatter factors for most measurements differed by less than 2% from the calculated ones except for the hard wedges where differences up to 4% were found. Anthropomorphic phantoms measurements differed from AAA by as much as 5.6%.Funding provided by Varian.

SU‐FF‐T‐137: Commissioning the Eclipse AAA Algorithm with Golden Beam Data

Purpose: Evaluation of the accuracy of Eclipse AAA TPS when commissioned with Golden Beam or measured data. Method and Materials: Two cancer centers independently performed preliminary evaluation of the Analytical Anisotropic Algorithm (AAA) implemented in Eclipse TPS. The AAA photon algorithm was commissioned with vendor supplied “Golden Beam Data”(GBD). We measured central and off‐axis profiles in several beam configurations including: open square, rectangular and asymmetric (half‐blocked) beam; wedged square and half‐blocked beam; square fields at three different source to surface distances; open and wedged beam at oblique incidence; beams shaped using cerrobend blocks and MLC. The measurements were performed on the Varian 2100 EX linear accelerators installed at the two institutions. After the initial tests, Eclipse was recommissioned with measured data from one of the machines. Results: The evaluation of the profiles was performed in the buildup, penumbra, inner and outer beam regions as per AAPM TG53. With the GBD the agreement of the measured and calculated profiles at the two institutions was very good in all regions except for the inner beam region on one machine. The tests that had a significant number of failures in the inner portion of the beam were mainly those cases where the TG53 tolerances are very tight. In these cases a significant number of points were just beyond the tolerance, and some of the off axis scans had 100% fail. On one particular test case where 16 profiles were measured for a particular geometry, 59% of points passed in the inner section with GBD, while 91% of points in that region passed once the beam was commissioned. Conclusion: When Eclipse is commissioned with GBD it was quite accurate, however, commissioning with measured data can improve the overall match. Conflict of Interest: Funding provided by Varian.