D. Schmidhalter

Assessment of patient setup errors in IGRT in combination with a six degrees of freedom couch

PURPOSE The range of patient setup errors in six dimensions detected in clinical routine for cranial as well as for extracranial treatments, were analyzed while performing linear accelerator based stereotactic treatments with frameless patient setup systems. Additionally, the need for re-verification of the patient setup for situations where couch rotations are involved was analyzed for patients treated in the cranial region. METHODS AND MATERIALS A total of 2185 initial (i.e. after pre-positioning the patient with the infrared system but before image guidance) patient setup errors (1705 in the cranial and 480 in the extracranial region) obtained by using ExacTrac (BrainLAB AG, Feldkirchen, Germany) were analyzed. Additionally, the patient setup errors as a function of the couch rotation angle were obtained by analyzing 242 setup errors in the cranial region. Before the couch was rotated, the patient setup error was corrected at couch rotation angle 0° with the aid of image guidance and the six degrees of freedom (6DoF) couch. For both situations attainment rates for two different tolerances (tolerance A: ± 0.5mm, ± 0.5°; tolerance B: ± 1.0 mm, ± 1.0°) were calculated. RESULTS The mean (± one standard deviation) initial patient setup errors for the cranial cases were -0.24 ± 1.21°, -0.23 ± 0.91° and -0.03 ± 1.07° for the pitch, roll and couch rotation axes and 0.10 ± 1.17 mm, 0.10 ± 1.62 mm and 0.11 ± 1.29 mm for the lateral, longitudinal and vertical axes, respectively. Attainment rate (all six axes simultaneously) for tolerance A was 0.6% and 13.1% for tolerance B, respectively. For the extracranial cases the corresponding values were -0.21 ± 0.95°, -0.05 ± 1.08° and -0.14 ± 1.02° for the pitch, roll and couch rotation axes and 0.15 ± 1.77 mm, 0.62 ± 1.94 mm and -0.40 ± 2.15 mm for the lateral, longitudinal and vertical axes. Attainment rate (all six axes simultaneously) for tolerance A was 0.0% and 3.1% for tolerance B, respectively. After initial setup correction and rotation of the couch to treatment position a re-correction has to be performed in 77.4% of all cases to fulfill tolerance A and in 15.6% of all cases to fulfill tolerance B. CONCLUSION The analysis of the data shows that all six axes of a 6DoF couch are used extensively for patient setup in clinical routine. In order to fulfill high patient setup accuracies (e.g. for stereotactic treatments), a 6DoF couch is recommended. Moreover, re-verification of the patient setup after rotating the couch is required in clinical routine.

Comparison of monte carlo collimator transport methods for photon treatment planning in radiotherapy

PURPOSE The aim of this work was a Monte Carlo (MC) based investigation of the impact of different radiation transport methods in collimators of a linear accelerator on photon beam characteristics, dose distributions, and efficiency. Thereby it is investigated if it is possible to use different simplifications in the radiation transport for some clinical situations in order to save calculation time. METHODS Within the Swiss Monte Carlo Plan, a GUI-based framework for photon MC treatment planning, different MC methods are available for the radiation transport through the collimators [secondary jaws and multileaf collimator (MLC)]: EGSnrc (reference), VMC++, and Pin (an in-house developed MC code). Additional nonfull transport methods were implemented in order to provide different complexity levels for the MC simulation: Considering collimator attenuation only, considering Compton scatter only or just the firstCompton process, and considering the collimators as totally absorbing. Furthermore, either a simple or an exact geometry of the collimators can be selected for the absorbing or attenuation method. Phasespaces directly above and dose distributions in a water phantom are analyzed for academic and clinical treatment fields using 6 and 15 MV beams, including intensity modulated radiation therapy with dynamic MLC. RESULTS For all MC transport methods, differences in the radial mean energy and radial energy fluence are within 1% inside the geometric field. Below the collimators, the energy fluence is underestimated for nonfull MC transport methods ranging from 5% for Compton to 100% for Absorbing. Gamma analysis using EGSnrc calculated doses as reference shows that the percentage of voxels fulfilling a 1% /1 mm criterion is at least 98% when using VMC++, Compton, or firstCompton transport methods. When using the methods Pin, Transmission, Flat-Transmission, Flat-Absorbing or Absorbing, the mean value of points fulfilling this criterion over all tested cases is 97%, 88%, 74%, 68%, or 65%, respectively. However, compared to EGSnrc calculations, the gain in efficiency is a factor of up to 10 for VMC++ and up to 48 for the absorbing method. CONCLUSIONS The results of this investigation suggest that it is an option to use a simple transport technique in the initial treatment planning process and use more accurate transport methods for the final dose calculation accepting longer calculation times.

Evaluation of a new six degrees of freedom couch for radiation therapy

PURPOSE The aim of this work is to evaluate the geometric accuracy of a prerelease version of a new six degrees of freedom (6DoF) couch. Additionally, a quality assurance method for 6DoF couches is proposed. METHODS The main principle of the performance tests was to request a known shift for the 6DoF couch and to compare this requested shift with the actually applied shift by independently measuring the applied shift using different methods (graph paper, laser, inclinometer, and imaging system). The performance of each of the six axes was tested separately as well as in combination with the other axes. Functional cases as well as realistic clinical cases were analyzed. The tests were performed without a couch load and with a couch load of up to 200 kg and shifts in the range between -4 and +4 cm for the translational axes and between -3° and +3° for the rotational axes were applied. The quality assurance method of the new 6DoF couch was performed using a simple cube phantom and the imaging system. RESULTS The deviations (mean ± one standard deviation) accumulated over all performance tests between the requested shifts and the measurements of the applied shifts were -0.01 ± 0.02, 0.01 ± 0.02, and 0.01 ± 0.02 cm for the longitudinal, lateral, and vertical axes, respectively. The corresponding values for the three rotational axes couch rotation, pitch, and roll were 0.03° ± 0.06°, -0.04° ± 0.12°, and -0.01° ± 0.08°, respectively. There was no difference found between the tests with and without a couch load of up to 200 kg. CONCLUSIONS The new 6DoF couch is able to apply requested shifts with high accuracy. It has the potential to be used for treatment techniques with the highest demands in patient setup accuracy such as those needed in stereotactic treatments. Shifts can be applied efficiently and automatically. Daily quality assurance of the 6DoF couch can be performed in an easy and efficient way. Long-term stability has to be evaluated in further tests.

Validation of the Swiss Monte Carlo Plan for a static and dynamic 6 MV photon beam.

Monte Carlo (MC) based dose calculations can compute dose distributions with an accuracy surpassing that of conventional algorithms used in radiotherapy, especially in regions of tissue inhomogeneities and surface discontinuities. The Swiss Monte Carlo Plan (SMCP) is a GUI-based framework for photon MC treatment planning (MCTP) interfaced to the Eclipse treatment planning system (TPS). As for any dose calculation algorithm, also the MCTP needs to be commissioned and validated before using the algorithm for clinical cases. Aim of this study is the investigation of a 6 MV beam for clinical situations within the framework of the SMCP. In this respect, all parts i.e. open fields and all the clinically available beam modifiers have to be configured so that the calculated dose distributions match the corresponding measurements. Dose distributions for the 6 MV beam were simulated in a water phantom using a phase space source above the beam modifiers. The VMC++ code was used for the radiation transport through the beam modifiers (jaws, wedges, block and multileaf collimator (MLC)) as well as for the calculation of the dose distributions within the phantom. The voxel size of the dose distributions was 2mm in all directions. The statistical uncertainty of the calculated dose distributions was below 0.4%. Simulated depth dose curves and dose profiles in terms of [Gy/MU] for static and dynamic fields were compared with the corresponding measurements using dose difference and γ analysis. For the dose difference criterion of ±1% of D(max) and the distance to agreement criterion of ±1 mm, the γ analysis showed an excellent agreement between measurements and simulations for all static open and MLC fields. The tuning of the density and the thickness for all hard wedges lead to an agreement with the corresponding measurements within 1% or 1mm. Similar results have been achieved for the block. For the validation of the tuned hard wedges, a very good agreement between calculated and measured dose distributions was achieved using a 1%/1mm criteria for the γ analysis. The calculated dose distributions of the enhanced dynamic wedges (10°, 15°, 20°, 25°, 30°, 45° and 60°) met the criteria of 1%/1mm when compared with the measurements for all situations considered. For the IMRT fields all compared measured dose values agreed with the calculated dose values within a 2% dose difference or within 1 mm distance. The SMCP has been successfully validated for a static and dynamic 6 MV photon beam, thus resulting in accurate dose calculations suitable for applications in clinical cases.

SU‐GG‐T‐238: In‐Silico Verification of Rapid Arc Using Swiss Monte Carlo Plan

Purpose: Nowadays, patient specific quality assurance (QA) for RapidArc treatments is performed by pre‐treatment verification based on measured dose distribution within phantoms. This procedure is well‐established in clinical practice for IMRT and IMAT verification. However, the procedure is in principle a mixture of machine and patient specific QA and lacks of the fact that linac treatment time is needed. We believe that instead of performing the patient specific QA on the machine the QA could be realized using computers, which is referred to as in‐silico verification of RapidArd treatments. The aim of this work is to use the Monte Carlo(MC) method as an in‐silico verification of RapidARc treatments. Method and Materials: We extended and modified our in‐house developed Swiss Monte Carlo Plan (SMCP) such that dose distributions for RapidArc treatments on the Novalis TX can be calculated. Several methods were developed in order to calculate dose distributions within reasonable computing time. The validation of this approach is performed by comparing MC calculated dose distributions with measured dose distributions for open as well as RapidArc treatment fields. The usefulness of this verification approach is shown by comparing MC calculated dose distributions within the patient with corresponding dose distributions using Eclipse (Varian Medical Systems). Results: The measured and SMCP calculated dose distributions are in good agreement for open as well as for RapidArc treatment fields. In general, the agreement between SMCP and AAA calculated dose distributions is also good. However, due to the presence of inhomogeneities in the patient anatomy, the AAA typically overestimates the mean PTV dose and underestimates the dose for organs‐at‐risk. Conclusion: In this work, SMCP was used for in‐silico verification of clinical RApidArc treatment fields. The results demonstrate the usefulness and the efficiency of this approach. Conflict of Interest: This work was supported by Varian Medical Systems.

Evaluation of clinically applied treatment beams with respect to bunker shielding parameters for a Cyberknife M6

Abstract Compared to a conventional linear accelerator, the Cyberknife (CK) is a unique system with respect to radiation protection shielding and the variety and number of non‐coplanar beams are two key components regarding this aspect. In this work, a framework to assess the direction distribution and modulation factor (MF) of clinically applied treatment beams of a CyberKnife M6 is developed. Database filtering options allow studying the influence of different parameters such as collimator types, treatment sites or different bunker sizes. A distribution of monitor units (MU) is generated by projecting treatment beams onto the walls, floor and ceiling of the CyberKnife bunker. This distribution is found to be highly heterogeneous and depending, among other parameters, on the bunker size. For our bunker design, 10%–13% of the MUs are delivered to the right and left wall, each. The floor receives more than 64% of the applied MUs, while the wall behind the patients head is not hit by primary treatment beams. Between 0% and 5% of the total MUs are delivered to the wall at the patients feet. This number highly depends on the treatment site, e.g., for extracranial patients no beams hit that wall. Collimator choice was found to have minor influence on the distribution of MUs. On the other hand, the MF depends on the collimator type as well as on the treatment site. The MFs (delivered MU/prescribed dose) for all treatments, all MLC treatments, cranial and extracranial treatments are 8.3, 6.4, 7.7, and 9.9 MU/cGy, respectively. The developed framework allows assessing and monitoring important parameters regarding radiation protection of a CK‐M6 using the actually applied treatment beams. Furthermore, it enables evaluating different clinical and constructional situations using the filtering options.

Independent Monte-Carlo dose calculation for MLC based CyberKnife radiotherapy

This work aims to develop, implement and validate a Monte Carlo (MC)-based independent dose calculation (IDC) framework to perform patient-specific quality assurance (QA) for multi-leaf collimator (MLC)-based CyberKnife® (Accuray Inc., Sunnyvale, CA) treatment plans. The IDC framework uses an XML-format treatment plan as exported from the treatment planning system (TPS) and DICOM format patient CT data, an MC beam model using phase spaces, CyberKnife MLC beam modifier transport using the EGS++ class library, a beam sampling and coordinate transformation engine and dose scoring using DOSXYZnrc. The framework is validated against dose profiles and depth dose curves of single beams with varying field sizes in a water tank in units of cGy/Monitor Unit and against a 2D dose distribution of a full prostate treatment plan measured with Gafchromic EBT3 (Ashland Advanced Materials, Bridgewater, NJ) film in a homogeneous water-equivalent slab phantom. The film measurement is compared to IDC results by gamma analysis using 2% (global)/2 mm criteria. Further, the dose distribution of the clinical treatment plan in the patient CT is compared to TPS calculation by gamma analysis using the same criteria. Dose profiles from IDC calculation in a homogeneous water phantom agree within 2.3% of the global max dose or 1 mm distance to agreement to measurements for all except the smallest field size. Comparing the film measurement to calculated dose, 99.9% of all voxels pass gamma analysis, comparing dose calculated by the IDC framework to TPS calculated dose for the clinical prostate plan shows 99.0% passing rate. IDC calculated dose is found to be up to 5.6% lower than dose calculated by the TPS in this case near metal fiducial markers. An MC-based modular IDC framework was successfully developed, implemented and validated against measurements and is now available to perform patient-specific QA by IDC.

SU-F-T-586: Pre-Treatment QA of InCise2 MLC Plans On a Cyberknife-M6 Using the Delta4 System in SBRT

PURPOSE Performing pre-treatment quality assurance (QA) with the Delta4 system (ScandiDos Inc., Madison, WI) is well established for linac-based radiotherapy. This is not true when using a Cyberknife (Accuray Inc., Sunnyvale, CA) where, typically film-based QA is applied. The goal of this work was to test the feasibility to use the Delta4 system for pre-treatment QA for stereotactic body radiation therapy (SBRT) using a Cyberknife-M6 equipped with the InCise2 multileaf collimator (MLC). METHODS In order to perform measurements without accelerator pulse signal, the Tomotherapy option within the Delta4 software was used. Absolute calibration of the Delta4 phantom was performed using a 10×10 cm2 field shaped by the InCise2 MLC of the Cyberknife-M6. Five fiducials were attached to the Delta4 phantom in order to be able to track the phantom before and during measurements. For eight SBRT treatment plans (two liver, two prostate, one lung, three bone metastases) additional verification plans were recalculated on the Delta4 phantom using MultiPlan. Dicom data was exported from MultiPlan and was adapted in order to be compatible with the Delta4 software. The measured and calculated dose distributions were compared using the gamma analysis of the Delta4 system. RESULTS All eight SBRT plans were successfully measured with the aid of the Delta4 system. In the mean, 98.0±1.9%, 95.8±4.1% and 88.40±11.4% of measured dose points passed the gamma analysis using a global dose deviation criterion of 3% (100% corresponds to the dose maximum) and a distance-to-agreement criterion of 3 mm, 2 mm and 1 mm, respectively, and a threshold of 20%. CONCLUSION Pre-treatment QA of SBRT plans using the Delta4 system on a Cyberknife-M6 is feasible. Measured dose distributions of SBRT plans showed clinically acceptable agreement with the corresponding calculated dose distributions.

SU‐GG‐T‐332: Monte Carlo Collimator Transport Methods for Photon Treatment Planning

Purpose:Monte Carlo(MC) based investigation of the impact of different radiation transport methods in collimators on beam characteristics,dose distributions and efficiency. Method and Materials: Within the Swiss Monte Carlo Plan (SMCP) — a GUI‐based framework for photonMCtreatment planning — different MC methods are available for the radiation transport through the collimators (secondary jaws and MLC): EGSnrc, VMC++ and Pin (an in‐house developed MC code). Additional MC methods were implemented in order to provide different complexity levels for the MC simulation: Considering the collimators as totally absorbing, considering attenuation only, considering first order or all Compton scatter. Furthermore, either a simple or an exact geometry can be selected for the absorbing or attenuation method. Phase spaces above and dose distributions in a water phantom are analyzed for artificial and clinical treatment fields using 6 and 15 MV beams. Results: For all MC methods, differences in the radial mean energy and radial energy fluence are within 1% inside the geometric field. Below the collimators the energy fluence is underestimated for non‐full MC transport methods ranging from 5% for all Compton to 100% for absorbing. Gamma analysis using EGSnrc calculated doses as reference shows that the percentage of voxels fulfilling a 1%/1mm‐criterion decreases from 99% when using VMC++ to 80% (non‐IMRT) and 40% (IMRT) when the absorbing method in a simple geometry is used. However, compared with EGSnrc calculations, the gain in efficiency is a factor of 10 for VMC++ and 90 for the absorbing method. Conclusion: The results of this investigation suggest that it might be suitable to use a simple transport technique in the initial treatment planning process and use more accurate transport methods towards the end of the planning process accepting longer calculation time. Conflict of Interest: This work was supported in part by Varian Medical Systems.