On the practice of the clinical implementation of enhanced dynamic wedges

Enhanced dynamic wedge output factors for Varian 2300CD and the case for a reference database

Dose inhomogeneity in treatment planning can be compensated using physical wedges. Enhanced dynamic wedges (EDW) were introduced by Varian to overcome some of the shortcomings of physical wedges. The objectives of this study were to measure EDW output factors for 6 MV and 20 MV photon energies for a Varian 2300CD. Secondly, to review the literature in terms of published enhanced dynamic wedge output factors (EDWOF) for different Varian models and thereby add credence to the case of the validity of reference databases. The enhanced dynamic wedge output factors were measured for the Varian 2300CD for both 6MV and 20 MV photon energies. Twelve papers with published EDWOF for different Varian linac models were found in the literature. Comparing our results with the published mean, we found an excellent agreement for 6 MV EDWOF, with the percentage differences ranging from 0.01% to 0.57%, with a mean of 0.03%. The coefficient of variation of published EDWOF ranged from 0.17% to 0.85% and 0.1% to 0.9% for the for 6 MV and 18 MV photon energies, respectively. This paper provides the first published EDWOF for 20 MV photon energy. In addition, we have provided the first compendium of EDWOFs for different Varian linac models. The consistency of value across models and institution provide further support that a standard dataset of basic photon and electron dosimetry could be established as a guide for future commissioning, beam modeling, and quality assurance purposes.

Clinical implementation of enhanced dynamic wedges into the Pinnacle treatment planning system: Monte Carlo validation and patient-specific QA

The goal of this work is to present a systematic Monte Carlo validation study on the clinical implementation of the enhanced dynamic wedges (EDWs) into the Pinnacle(3) (Philips Medical Systems, Fitchburg, WI) treatment planning system (TPS) and QA procedures for patient plan verification treated with EDWs. Modeling of EDW beams in the Pinnacle(3) TPS, which employs a collapsed-cone convolution superposition (CCCS) dose model, was based on a combination of measured open-beam data and the 'Golden Segmented Treatment Table' (GSTT) provided by Varian for each photon beam energy. To validate EDW models, dose profiles of 6 and 10 MV photon beams from a Clinac 2100 C/D were measured in virtual water at depths from near-surface to 30 cm for a wide range of field sizes and wedge angles using the Profiler 2 (Sun Nuclear Corporation, Melbourne, FL) diode array system. The EDW output factors (EDWOFs) for square fields from 4 to 20 cm wide were measured in virtual water using a small-volume Farmer-type ionization chamber placed at a depth of 10 cm on the central axis. Furthermore, the 6 and 10 MV photon beams emerging from the treatment head of Clinac 2100 C/D were fully modeled and the central-axis depth doses, the off-axis dose profiles and the output factors in water for open and dynamically wedged fields were calculated using the Monte Carlo (MC) package EGS4. Our results have shown that (1) both the central-axis depth doses and the off-axis dose profiles of various EDWs computed with the CCCS dose model and MC simulations showed good agreement with the measurements to within 2%/2 mm; (2) measured EDWOFs used for monitor-unit calculation in Pinnacle(3) TPS agreed well with the CCCS and MC predictions within 2%; (3) all the EDW fields satisfied our validation criteria of 1% relative dose difference and 2 mm distance-to-agreement (DTA) with 99-100% passing rate in routine patient treatment plan verification using MapCheck 2D diode array.

The quality assurance of Varian enhanced dynamic wedges using Daily QA Check 2

Background: Clinical implementation of enhanced dynamic wedges (EDW) requires, as any other dynamic treatment, very serious quality assurance (QA) program. In this work, we presented the results of six-month evaluation of Varian enhanced dynamic wedges and detailed QA program for those wedges. Methods: The Sun Nuclear Daily QA Check 2 was used for QA purpose. The QA program included daily and monthly checks. The daily QA program included central axis dose and wedge angle measurements. Within the monthly QA program wedge factors and delivered STTs from the Dynalog files were checked. Results: Our daily QA measurements of dose and wedge angle showed the reproducibility error less than ?1.5%. On the other hand, monthly QA measurements of wedge factor the reproducibility error was less than ?0.5%. The monthly QA check of delivered STTs showed excellent agreement. Conclusion: During the six-month quality control we observed a good reproducibility in the delivery of EDW treatment. Also, the Sun Nuclear Daily QA Check 2 showed to be very satisfying in QA of the EDW treatment.

Use of a multileaf collimator to increase the field width achievable with a dynamic wedge

A method is proposed for generating dynamic wedges spanning the entire field width, defined as the collimator opening in the wedged direction, without changes to existing hardware. The technique approximates the fluence pattern of a dynamic wedge by sequentially closing the leaves of a 120‐leaf multileaf collimator (MLC). Closure times for the individual leaves were derived by extending the segmented treatment table of the dynamic wedge provided by the manufacturer of the linear accelerator. Using film dosimetry, beam properties of MLC wedges were compared to those of conventional dynamic and mechanical wedges. Profiles and isodose lines of the MLC wedge were almost identical to those of the dynamic wedge, and differed only modestly from the mechanical counterparts. Dose inhomogeneity due to the individually closing leaves was not significant. The high‐dose region at the junction between opposing MLC leaves, unavoidable when the field length (i.e., the opening of the collimator in the nonwedged direction) exceeds the maximum leaf extension of 15 cm, was feathered by moving leaf pairs after their closure for the remainder of the irradiation time. Combining the MLC wedge with a regular dynamic wedge to reduce the line of high dose under the leaf junction is under consideration.

PACS number: 87.53.Mr

Performance evaluation and quality assurance of Varian enhanced dynamic wedges

Dynamic wedges have been used in clinical practice for many years. Obvious superiority of dynamic over physical wedges is accompanied by the increased overhead involved in verifying the accuracy and reliability of their use. Contrary to very limited QA required to ensure proper functioning of the physical wedges, dynamic wedges, like any other dynamic treatment, require a robust QA program. This work expands upon previous suggestions and describes a comprehensive QA program for Varian enhanced dynamic wedges (EDWs) and presents the results of an 18‐month evaluation of these wedges. The QA program includes daily, monthly, and yearly tests and individual treatment QA at the onset of use of the EDWs. The results of the 18‐month evaluation show reproducibility in the wedge factors of better than 1% and in dose profiles of better than 2% on a monthly basis. Daily output measurements are generally within 2% of expected values.

PACS numbers: 87.53.Xd, 87.56.Fc

Analytical representation for varian EDW factors at off-center points

The purpose of this study is to describe and evaluate a new analytical model for Varian enhanced dynamic wedge factors at off-center points. The new model was verified by comparing measured and calculated wedge factors for the standard set of wedge angles (i.e., 15 degrees, 30 degrees, 45 degrees and 60 degrees), different symmetric and asymmetric fields, and two different photon energies. The maximum difference between calculated and measured wedge factors is less than 2%. The average absolute difference is within 1%. The obtained results indicate that the suggested model can be useful for independent dose calculation with enhanced dynamic wedges.

Enhanced dynamic wedge and independent monitor unit verification

Some serious radiation accidents have occurred around the world during the delivery of radiotherapy treatment. The regrettable incident in Panama clearly indicated the need for independent monitor unit (MU) verification. Indeed the International Atomic Energy Agency (IAEA), after investigating the incident, made specific recommendations for radiotherapy centres which included an independent monitor unit check for all treatments. Independent monitor unit verification is practiced in many radiotherapy centres in developed countries around the world. It is mandatory in USA but not yet in Australia. This paper describes development of an independent MU program, concentrating on the implementation of the Enhanced Dynamic Wedge (EDW) component. The difficult case of non centre of field (COF) calculation points under the EDW was studied in some detail. Results of a survey of Australasian centres regarding the use of independent MU check systems is also presented. The system was developed with reference to MU calculations made by Pinnacle 3D Radiotherapy Treatment Planning (RTP) system (ADAC - Philips) for 4MV, 6MV and 18MV X-ray beams used at the Newcastle Mater Misericordiae Hospital (NMMH) in the clinical environment. A small systematic error was detected in the equation used for the EDW calculations. Results indicate that COF equations may be used in the non COF situation with similar accuracy to that achieved with profile corrected methods. Further collaborative work with other centres is planned to extend these findings.

A new analytical model for Varian enhanced dynamic wedge factors

Dynamic and physical (hard) wedges are used in 3D conformal radiotherapy in order to improve dose distribution in patients. Unlike wedge factors for physical wedges that depend on wedge material and thickness, wedge factors for Varian dynamic wedges depend on the relationship between the position of the moving jaw and the number of delivered monitor units. In this study, we describe a new analytical model for dynamic wedge factors. We also review the existing analytical models and compare calculated and measured wedge factors. The comparison is performed for different wedge angles, symmetric and asymmetric fields and two different photon energies. The obtained results indicate that the new dynamic wedge model provides the best overall agreement (within 1%) with the measured wedge factors.