Varian Machine Research Articles

SU‐GG‐T‐196: Effect of Small Field Output Factors on Pinnacle IMRT Dose

Purpose: To evaluate how changes in the measured small field size output factors affect the computed and measured doses in Pinnacle IMRT treatment planning. Method and Materials: A simple five‐field IMRT plan was created using Pinnacle treatment planning system. The plan was optimized to a cylindrical target of 2 cm diameter and 2 cm length. Measured output factors in the planning system were decreased or increased in 5, 10, and 20% increments for field sizes of 2×2 and 3×3 cm2 for both an Elekta and a Varian linear accelerator. The treatment unit was re‐commissioned in the planning system after each modification of the output factors and treatment plan was re‐optimized. All plans were delivered to a solid water phantom and dose measurements were made using an ionization chamber. The percentage differences between the measured and computed doses were calculated. Results: For the Elekta unit, decreasing output factor values resulted in higher measured than computed doses of 3.0% for the 10% and 7.6% for the 20% decrease. Increasing output factors resulted in lower doses by 6.1% for 10% and 9.1% for the 20% increase. For the Varian machine we observed no systematic trends as in Elekta between measured and computed doses as the output factors were changed from −20 to +20%. Conclusion: The measurement accuracy of the small field output factors are of importance especially when the treatment plan consists of small segments as in IMRT. This method could be used to verify the accuracy of the measured small field output factors for certain linear accelerators. Differences in optimization methods between fixed jaw IMRT delivery (Varian) and conformai jaw IMRT delivery (Elekta) when Pinnacle treatment planning is employed may be significant.

SU-FF-T-172: Dosimetric Comparison of MapCheck with Monte Carlo Simulations

Purpose: This study is to verify IMRT QA MapCheck measurement using Monte Carlo simulations. Method and Materials: MapCheck (Sun Nuclear) contains 445 silicon diode detectors arranged in a 22×22-cm2 area, of which 221 are located in the 10×10 cm2 center. The diode spacing is 7.07 mm in the central portion and 14.14 outside of this area. The 2-cm water equivalent build-up is inherent in the device and additional 3-cm build-up is used for IMRT QA plan delivery at an SAD of 100 cm. The EGS4/MCSIM Monte Carlo package was used in this study. The MLC leaf sequence files for the IMRT plans were used for the Monte Carlo simulation. Results: Ten prostate and head-and-neck IMRT QA plans were generated on the Eclipse treatment planning system and delivered on Varian machines. The average field size for the prostate plans was 9×6 cm2, and 14.5×16.7 cm2 for the head-and-neck cases. For the prostate plans, the average dose at the reference point was 94.3 cGy by Mapcheck and 92.9 cGy by Monte Carlo. The difference was 1.5 %. For the head-and-neck plans, MapCheck measured 70.0 cGy while Monte Carlo predicted 69.5 cGy (a 0.7 % difference). The dose errors given by MapCheck and Monte Carlo were also compared. In the case of the prostate IMRT, MapCheck gave a dose error of 1.0 %, while Monte Carlo showed 0.5 %. In the case of head-and-neck, the 0.5 % of dose error by MapCheck was close to that (0.8 %) by Monte Carlo. Conclusions: MapCheck can perform accurate dose measurement for IMRT QA. Considering the limitations of the detectors and design such as low spatial resolution, however, the Monte Carlo simulation can be used to verify MapCheck systematically and help improve its functions in future design.

SU-FF-T-148: Developing a Convenient and Effective IMRT QA Procedure Using MLC Dynalog Files

Purpose: To develop a convenient and effective patient‐specific IMRTquality assurance (QA) procedure utilizing MLC dynalog files. Method and Materials:IMRT plans are delivered with multi‐leaf collimators(MLC). The complexity of MLC motion and patterns may cause leaf positioning and M errors that lead to IMRT dose delivery errors. MLC dynalog log files record actual MLC leaf motion and MU fractions. MLC log files are taken for the IMRT plan delivery. In‐house software is used to analyze the log files. MLC leaf sequence files are rebuilt from MLC log files and used for Monte Carlo simulation to calculate plan delivery error. MU‐weighted position‐MU (WPM) is defined to quantify the dose error caused by MLC leaf position errors and MU errors. Mapcheck is used to verify the dose error calculation. Results: Ten IMRT plans for prostate and head‐and‐neck cancers with different field sizes were included in this study. The plans were generated as real IMRT QA plans and delivered in both sliding‐window and step‐and‐shoot modes on three Varian machines. The average field size for the prostate plans was 9×6 cm2, and 14.5×16.7 cm2 for the head‐and‐neck cases. WPM averaged over prostate plans was −0.2 % for both sliding‐window and step‐and‐shoot modes, while Monte Carlo showed a 0.5 % dose difference between the plan and measurement, and Mapcheck measurement gave a dose error of 1.0 %. For the head‐and‐neck plans, WPM was 1.74 % in average, and the dose error from Mapcheck was 0.5 %, which was comparable to 0.8 % from Monte Carlo. The variations of WPM between three linacs were less than 0.08 %. Conclusions: IMRT QA can be simplified utilizing MLC log files. WPM can be directly used to estimate the IMRT dose delivery error caused by MLC motion and MU errors.

SU-FF-T-308: Monte Carlo Simulation for Evaluating the Matchline Effect of IMRT Technique

Purpose: To examine and diminish the IMRT matchline effect caused by curved leaf end design of MLC using a Monte Carlo simulation. Method and Materials: The procedure is divided into four phases. The first is validating the Monte Carlo simulation using BEAMnrc, DOSXYZnrc, and EGSnrc code to reproduce a 6 MV beam of Varian machine with 120 leaves for a conventional technique of a standard field size 10×10 cm2. The results were compared to the measurement by IC 0.01 cm3. The second phase is testing the simulation for the geometry of MLC specifically a curved leaf end design compared with Kodak XO mat V film measurement. The third phase is applying the matchline situation which generated by Pinnacle treatment planning system (step & shoot technique) into the Monte Carlo simulation. The fourth phase is applying the overlapped values to the MLC files of the Monte Carlo simulation to decrease the matchline effect of IMRT dose distribution. Results: The agreement between simulation and measurement for conventional technique was within 2% for depth dose and beam profile. The test of the geometry of MLC was done by comparison of closing leaf end of dose distribution. The result showed that the agreement was within 2%. The overlapped values were applied to the MLC leaf files and found that 0.09 cm was selected for the optimal overlapped value with some certain segment weights to compromise among matchlines. Conclusion: The Monte Carlo simulation is able to investigate the matchline effect of IMRT dose distribution. The distance of the overlapped region of MLC 0.09 cm can reduce the IMRT matchline effect. However this value may possibly depend on the weighting dose of a segmented field of IMRT technique, further investigation should be required.

WE‐C‐224A‐04: Measurements of Secondary Radiation for Electron and Proton Accelerators

As new treatment modalities in Radiation Oncology such as Intensity Modulated Radiation Therapy (IMRT) and Proton Therapy (PT) become more widely used, improved target coverage and lower doses to surrounding normal tissues are achieved at the expense of higher out‐of‐field doses to distant normal tissues. These higher out‐of‐field normal tissue doses are the result of increased X ray leakage radiation from longer beam‐on times associated with IMRT and neutron leakage radiation associated with high energy X ray beams (>10 MV) and protons. IMRT beam‐on times can be 4–6 times that of conventional 3D treatments and neutrons produced by the high‐energy X rays and protons striking a patient have a high relative biological effectiveness (RBE). Measurement of the increased X ray and neutron leakage radiation is crucial if one is to assess any additional risks to the patient that might result from the higher out‐of‐field radiation doses from these new treatment modalities. A variety of dosimeters, large volume ion chambers, diodes, survey meters, small volume ion chambers and thermoluminescent dosimeters (TLD), have been used to measure the X ray leakage in the treatment room and in the patient plane. TLD, because of its spatial resolution and ability to precisely measure low doses associated with leakage radiation, appears to be the preferable dosimeter. Neutrons are measured with bubble detectors, neutron meters utilizing a BF3 proportional counter, 197Au foil based activation either with a Bonner Sphere system/moderator or LiI dosimeters in Bonner Spheres. The gold activation technique has a calibration directly traceable to NIST and is the detector of choice for X ray produced neutrons. Due to the limited amount of neutron leakage measurement data around proton machines the ideal dosimeter has yet to be determined for this type of measurement. The measured dose‐equivalent values in an anthropomorphic phantom for X ray leakage from an X ray accelerator decrease with increased distance from central axis (CAX) in the patient plane by a factor of 3 to 10, depending on the delivery mode and accelerator type. IMRT treatments on a Siemens machine results in higher dose‐equivalents than the same energies on a Varian machine. Neutron measurements in the patient plane were approximately the same despite the distance from the CAX. Overall, the total dose‐equivalent, X ray and neutron dose, decrease with distance from CAX with the Varian IMRT having the greatest decrease. Measured neutron dose‐equivalents from proton therapy strongly depend on the delivery apparatus and decrease with distance from the nozzle. This lecture will provide an overview of the measurement techniques for X ray and neutron leakage radiation from both X ray and proton therapy machines and the resulting measured dose‐equivalent values from which the risk of secondary cancers can be estimated.Educational Objectives:1. Understand the various techniques used to accurately measure X ray and neutron leakage radiation in the patient plane.2. Understand the magnitude of the dose‐equivalent in the patient plane from the various X ray accelerators and from proton therapy.

SU‐FF‐T‐103: Application of the Strydom Analytical Model to the Remote Monitoring of Electron Beam Dosimetry Parameters by Thermoluminescent Dosimetry

Purpose: To provide a practical methodology and a reference percentage depth dose (PDD) data set for streamlining the Radiation Dosimetry Services (RDS) TLD verification of electron beam output and PDD. Methods and Materials: PDD data from 29 Siemens Primus and 19 Varian Clinac 21EX accelerators were retrieved from the databases of RDS and the Radiological Physics Center (RPC) and were simultaneously fitted to the nonlinear Strydom analytical model. The NLIN (SAS 9.1) procedure was used to fit the models. The PDD data sets used in the models were independently measured by many physicists from many institutions and verified by TLD and RPC onsite measurements. The resulting reference PDD data sets were incorporated into the RDS TLD analysis software for electron beam monitoring. Results: The parameters of the model were estimated for 5 to 21 MeV electron beams for Siemens and Varian machines. Individual PDD data fell within ± 1.5 mm of the reference PDD data for all depths for all beams analyzed. The curves exhihited similar shapes and were found to agree within ± 1.0 mm at all depths when all the curves for a particular energy were made to coincide at the depth of 50%. The validated reference model's predictions shows good agreement with other published results. Conclusions: The shape of the PDD curves was found to depend only on beam energy, and was independent of model for newer machines. These results allowed RDS to streamline its process of evaluating TLD results for electron beams and to acquire additional information about an institution's PDD data to facilitate the identification of problems. The reference PDD data sets can serve as a resource to the medical physics community. This work was supported by PHS grant CA 10953 awarded by the NCI, DHHS and Radiation Dosimetry Services.

SU‐FF‐T‐306: Is It Safe to Switch Patients Between Different Linacs

Purpose: Due to the limited machine availability, a clinical decision has to be made as whether a patient can be treated on a different machine of the same nominal energies. This work investigates the feasibility of switching patients between machines for IMRT. Method and Materials: We have performed Monte Carlo simulations of photon beam from different models and different vendors' clinical accelerators. Treatment plans of Siemens Primus, Primart and Varian‐21Ex accelerators and tumor sizes are studied including prostate and head and neck tumors. We have compared the maximum, minimum and mean dose to the target and to the critical structures, and the tumor‐control‐probability to determine the possibility to switch a patient or cancel the treatment. Results: In FCCC, three attuned Primus linacs can be used one set of beam data for commissioning the treatment planning systems, while Primart and two Varain‐21Ex linacs have different beam data. Our results show that the two Siemens machine models share similar features. The dose difference at the minimum dose point for Primus is 2% higher than Primart. Owing to a larger beam penumbra for Varian machines, it appeared a cold spot at the target, but more volume at higher dosages to the surrounding structures. To reduce the cold spot, a factor of 1.042 can be used to increase the MUs. The TCP results show that treating a patient with slightly less accurate dose is generally better than canceling the treatment. Conclusions: Patients can be safely treated using same nominal energy photon beams on linacs of the same model and vendor; small differences are found between different vendors, which may be compensated by adjusting MUs to ensure target coverage. It is generally more therapeutically beneficial to switch a patient to a different machine than to cancel a treatment.

SU‐FF‐T‐100: Aperture Effects in Dynamic Fields

Purpose: We have measured collimator and phantom scatter factors (Sc, Sp) as a function of field size for a variety of moving slit widths and compared them with static field factors. Dynamic treatment plans were devised to compare the ability of two planning systems to model the aperture effect on dose delivery, especially for small targets. Method and Materials: An 80 leaf Varian 2300cd and a 120 leaf Varian 21ex accelerator were used with 6 MV photons. 0.6 cm3 ionization chambers were used, for both Sc and Scp measurements. 1–10mm leaf gaps were dynamically scanned across a range of field sizes (4×4–14×40). To mimic the small aperture effect in a treatment plan, we defined a series of cylindrical targets, 1–20 mm diameter by 3cm length and an IMRT plan using Eclipse or Pinnacle3 was developed to optimally treat them. EDR2 films were taken and used to compare delivered with planned doses. Results: The dynamic measurements of Sc and Scp were very similar on the two Varian machines and clearly demonstrated an aperture effect in the Sc measurements of as much as 30% (1mm gap, 4×4 field) which smoothly converged to the static field distribution as the gap was increased. Derived Sp values were approximately independent of gap width, essentially matching the static field cases. Planning system intercomparison of small target doses seem to indicate the Pinnacle3 system to be slightly better at correctly including aperture effect. Conclusions: Conventionally obtained output factors, using the secondary collimators, even extended down to 1–2 cm2 still do not describe the same head scatter contribution as is delivered by a dynamic aperture. Consequently, dose uncertainty may be amplified in the treatment of single or multiple small lesions using IMRT techniques.

TU‐C‐T‐6E‐05: Dosimetry Acccracy Of Delivery Of Gold Beam Data IMRT Plans On Similar Linear Accelerators With Same Vendor

Purpose: Under certain circumstances, IMRT patients need to be transferred from one Linac to another. Since a same set of commercial Gold Beam Data (GBD) have been used to model the treatment planning systems for multiple similar types of Linacs at our centers, the dosimetric accuracy needs to be investigated to determine the necessity of replanning when the treatment of an IMRT patient is moved from one machine to another. In this study, we investigated the dosimetric accuracies of IMRT plans delivered on different similar Varian accelerators. Method and Materials: In our institution, GBD has been used to model the Eclipse™ treatment planning systems for certain types of Varian machines (23 EX). IMRT treatments were planned using the Eclipse™ systems with 6 and 10 MV photon beams for different anatomical sites. Corresponding QA plans were also generated. The QA plans were used in the validation measurements, and the same measurements (same plans, same MU & dMLC files and phantom) were repeated on five of those Linacs. The measurements were performed using both ion chambers and films. Results: For plans using 6 MV photon beams, the mean ratios of planned to measured dose on the five machines for breast, prostate, tonsil, pelvis and brain plans were 0.984(SD0.0078), 0.998(SD0.0029), 0.979(SD0.0121), 1.006(SD0.0132) and 1.012(SD0.0054), respectively. For plans using 10 MV photon beams, the mean ratios of planned to measured dose on two of the machines for breast and pelvis were 0.979(SD0.00) and 0.949(SD0.0042), respectively. In the film analysis, a good agreement was found between films and plan isodoses on a coronal plane (for high dose gradients ± 2 mm and low dose gradients ± 4 mm). Conclusion: This study demonstrated that the IMRT patients could be transferred from one machine to another similar Varian machine with same energy without replanning.

SU-FF-T-217: Reference Electron Beam Dosimetry Data Set: A Preliminary Analysis

Purpose: To investigate the feasibility of developing a set of standard percent depth dose (PDD) curves for electron beams to be used when providing quality assurance (QA) checks on machines. Method and Materials: A database of PDD data collected on more than 900 machines was used as a source of central axis PDD data. The database consists of curves from Varian (62%), Siemens (26%), Elekta/Philips (8%), Mitsubishi (2%) and GE/CGR (1%) machines. For this study the data was restricted to modern Varian and Siemens machines with data for 5–6 electron energies each. PDD data for each beam was extracted for 10×10 or 15×15 cm cones. The data were fitted to a six‐order polynomial to predict PDD from depth of 0.2 cm to approximately 20% of maximum dose providing excellent fits for all data (R2 >−0.99). The curves were compared for shape and location on the depth scale for each beam for each machine model. Selected PDD data were validated by TLD QA measurements at 2 depths in a fixed geometry system. Results: The curves were stratified by machine model and nominal energy and agreement was found to be ± 1 mm of the mean curve for all depths between 100% and 20% of maximum dose. Average curves developed were found to depend little on manufacturer. Conclusion: With the large database of PDD data measured independently by many physicists on many machines the information is available to develop standards for common machine models with a high degree of accuracy. The standards can be used to review PDD data for QA and to provide information on curve shape when data provided by physicists for QA is sparse. This work was supported by PHS grant CA 10953 awarded by the NCI, DHHS and Radiation Dosimetry Services.

SU‐FF‐T‐173: A Comparison of Techniques for Effective SSD Measurements and Evaluation of Data for Accelerators From Different Manufactures

Purpose: Investigating and comparing the effective SSDs of electron beams measured by ionization chamber and electron diode for Elekta linear accelerators. Method and Materials: The effective SSDs for different energies and field sizes are measured for three Elekta linear accelerators. In order to measure effective SSD (SSDeff), two methods are used. The first method is placing a Farmer ionization chamber in a solid water phantom. The second method is using an electron diode placed on top surface of a solid water phantom. Results: The data shows that effective SSDs are energy and insert field size dependent. For both ionization chamber and electron diode measurements, the effective SSDs of three Eleckta linear accelerators, 1) increase with increasing of energy and insert field sizes which can be explained by less lateral scattering with higher energy and more lateral scattering equilibrium with lager field sizes, 2) agree well between the ionization chamber and diode measurements except for under the combinations of low energy and small field size. In most cases, the diode measurements are about 8 to 12 percent lower than ion chamber measurements for the same energy and insert field, except low energy and small insert combinations. Interestingly, for small field size, the effective SSD of diode measurement is larger than that of ion chamber measurement. Conclusion: The effective SSDs of three Elekta Linear accelerators agree well under most conditions except low energy and small insert field size combinations. The data also indicate that ion chamber measurements give a more accurate measurement over diode measurement and we recommend that ionization chamber should be used to measure effective SSD. The data also show that effective SSDs for Eleckta accelerators are different but comparable to that of Varian machines.

Comparison of build-up dose between Elekta and Varian linear accelerators for high-energy photon beams using radiochromic film and clinical implications for IMRT head and neck treatments

Skin toxicity has been reported for IMRT of head and neck cancer. The purpose of this study was to investigate the dose in the build-up region delivered by a 6 MV treatment plan for which important skin toxicity was observed. We also investigated if the different designs of the treatment head of an Elekta and a Varian linear accelerator, especially the lower position of the Varian multi-leaf collimator, give rise to different build-up doses. For regular square open beams, the build-up dose along the central beam axis is higher for the Varian machine than for the Elekta machine, both for 6 MV and 18 MV. At the Elekta machine at 18 MV, the superficial dose of a diamond shaped 10 × 10 cm2 field is 3.6% lower than the superficial dose of a regular 10 × 10 cm2 field. This effect is not seen at 6 MV. At the Varian machine, the superficial dose of the diamond shaped field is respectively 3.5 and 14.2% higher than the superficial dose of the regular 10 × 10 cm2 field for 6 MV and 18 MV. Despite the differences measured in build-up dose for single beams between the Elekta and the Varian linear accelerator, there were no measurable differences in superficial dose when a typical IMRT dose plan of 6 MV for a head and neck tumour is executed at the two machines.

Preclinical studies with the photon radiosurgery system (PRS)

Purpose: Determine the radiobiological effectiveness (RBE) for low-energy X-rays (average energy of 23 KeV) produced by the Photon Radiosurgery System (PRS). Methods and Materials: RBE values were assessed by comparison with survival data obtained for cells irradiated with either low-energy X-rays from a GE Maxitron 100 machine or high-energy photons from a clinically used Varian 6 MV LINAC. The output of the GE and PRS sources was determined using Baldwin-Farmer and Markus thin window ionization chambers calibrated with 50 kVp X-rays and cross-checked against figures supplied by Photoelectron Corporation. The dose-rate for the PRS was 1.2 Gy/min at a distance of 35 mm with a field flatness of ±2%. Results: The RBE for the PRS low-energy X-ray source (at 1-mm depth) was greater than either the GE or Varian machines and varied with cell survival. For Chinese hamster ovary (CHO) cells, the PRS was 1.25 and 3.3 times more effective than 90 kVp X-rays and 6 MeV photons at 0.5% cell survival, respectively; by comparison, the PRS was 1.2 and 1.9 times more effective at 0.05% cell survival, respectively. Similar RBE values of 1.4 and 1.2 were obtained for human U373 and T98 glioblastoma cells grown in vitro irradiated with the PRS or GE sources, respectively. Other studies showed that the RBE for the PRS low-energy X-ray source increased with depth. The RBEs for the PRS source at 1-mm and 4-mm depth were 1.2 and 2.5 (0.5% survival) and 1.2 and 1.9 (0.05% survival). Conclusions: The biological and physical properties of the PRS low-energy X-rays offer, under the right conditions, a significant advantage for patient treatment over conventional external beam, stereotactic, or brachytherapy treatment.

Synthesis and1H and31P NMR spectral characteristics of 2-substituted and 2,2-disubstituted 5-phenyl-5-thio-1,3,5-dioxaphosphorinanes

The authors have studied the reactions of phenylbis(hydroxymethyl)phosphine sulfide (PHPS) with acetals and ketals. /sup 31/P NMR Spectra were recorded at 10.2 HMz on a KGU-4 spectrometer with proton noise decoupling at 25.2 MHz. Chemical shifts are given relative to 85% H/sub 3/PO/sub 4/ (negative values to higher field). PMR Spectra were measured on a Varian T60 machine at 34.5/sup 0/C. Treatment of phenylbis(hydroxymethyl)phosphine sulfide with acetals and ketals gives the corresponding 2-substituted and 2,2-disubstituted 5-phenyl-5-thio-1,3,5-dioxaphosphorinanes in which the P-Phenyl substituent occupies an axial position.