Investigation of the accuracy of MV radiation isocentre calculations in the Elekta cone-beam CT software XVI.

Accurate determination of the megavoltage (MV) radiation isocentre of a linear accelerator (linac) is an important task in radiotherapy. The localization of the MV radiation isocentre is crucial for correct calibration of the in-room lasers and the cone-beam CT scanner used for patient positioning prior to treatment. Linac manufacturers offer tools for MV radiation isocentre localization. As a user, there is no access to the documentation for the underlying method and calculation algorithm used in the commercial software. The idea of this work was to evaluate the accuracy of the software tool for MV radiation isocentre calculation as delivered by Elekta using independent software. The image acquisition was based on the scheme designed by the manufacturer. Eight MV images were acquired in each series of a ball-bearing (BB) phantom attached to the treatment couch. The images were recorded at cardinal angles of the gantry using the electronic portal imaging device (EPID). Eight Elekta linacs with three different types of multileaf collimators (MLCs) were included in the test. The influence of MLC orientation, x-ray energy, and phantom modifications were examined. The acquired images were analysed using the Elekta x-ray volume imaging (XVI) software and in-house developed (IHD) MATLAB code. Results from the two different software were compared. A discrepancy in the longitudinal direction of the isocentre localization was found averaging 0.23 mm up to a maximum of 0.75 mm. The MLC orientation or the phantom asymmetry in the longitudinal direction do not appear to cause the discrepancy. The main cause of the differences could not be clearly identified. However, it is our opinion that the commercial software delivered by the linac manufacturer should be improved to reach better stability and precise results in the MV radiation isocentre calculations.

Investigation of the mechanical performance of Siemens linacs components during arc: gantry, MLC, and electronic portal imaging device.

BACKGROUND: In radiotherapy treatments, it is crucial to monitor the performance of linac components including gantry, collimation system, and electronic portal imaging device (EPID) during arc deliveries. In this study, a simple EPID-based measurement method is suggested in conjunction with an algorithm to investigate the stability of these systems at various gantry angles with the aim of evaluating machine-related errors in treatments. METHODS: The EPID sag, gantry sag, changes in source-to-detector distance (SDD), EPID and collimator skewness, EPID tilt, and the sag in leaf bank assembly due to linac rotation were separately investigated by acquisition of 37 EPID images of a simple phantom with five ball bearings at various gantry angles. A fast and robust software package was developed for automated analysis of image data. Three Siemens linacs were investigated. RESULTS: The average EPID sag was within 1 mm for all tested linacs. Two machines showed >1 mm gantry sag. Changes in the SDD values were within 7.5 mm. EPID skewness and tilt values were <1° in all machines. The maximum sag in leaf bank assembly was <1 mm. CONCLUSION: The method and software developed in this study provide a simple tool for effective investigation of the behavior of Siemens linac components with gantry rotation. Such a comprehensive study has been performed for the first time on Siemens machines.

Gantry and isocenter displacements of a linear accelerator caused by an add-on micromultileaf collimator.

PURPOSE: The delivery of high quality stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) treatments to the patient requires knowledge of the position of the isocenter to submillimeter accuracy. To meet the requirements the deviation between the radiation and mechanical isocenters must be less than 1 mm. The use of add-on micromultileaf collimators (µMLCs) in SRS and SRT is an additional challenge to the anticipated high-level geometric and dosimetric accuracy of the treatment. The aim of this work was to quantify the gantry excursions during rotation with and without an add-on µMLC attached to the gantry head. In addition, the shift in the position of the isocenter and its correlation to the kV beam center of the cone-beam CT system was included in the study. METHODS: The quantification of the gantry rotational performance was done using a pointer supported by an in-house made rigid holder attached to the gantry head of the accelerator. The pointer positions were measured using a digital theodolite. To quantify the effect of an µMLC of 50 kg, the measurements were repeated with the µMLC attached to the gantry head. The displacement of the isocenter due to an add-on µMLC of 50 kg was also investigated. In case of the pointer measurement the µMLC was simulated by weights attached to the gantry head. A method of least squares was applied to determine the position and displacement of the mechanical isocenter. Additionally, the displacement of the radiation isocenter was measured using a ball-bearing phantom and the electronic portal image device system. These measurements were based on 8 MV photon beams irradiated onto the ball from the four cardinal angles and two opposed collimator angles. The measurements and analysis of the data were carried out automatically using software delivered by the manufacturer. RESULTS: The displacement of the mechanical isocenter caused by a 50 kg heavy µMLC was found to be (-0.01 ± 0.05, -0.10 ± 0.03, -0.26 ± 0.05) mm in lateral, longitudinal, and vertical direction, respectively. Similarly, the displacement of the radiation isocenter was found to be (0.00 ± 0.03, -0.08 ± 0.06, -0.32 ± 0.02) mm. Good agreement was found between the displacement of the two isocenters. A displacement of the kV cone-beam CT beam center due to the attached weight of 50 kg could not be detected. CONCLUSIONS: General characteristics of the gantry arm excursions and displacements caused by an add-on µMLC have been reported. A 50 kg heavy add-on µMLC results in a isocenter displacement downward of 0.26-0.32 mm. The authors recommend that the beam center of the kV cone-beam CT image system should be matched to the isocenter related to the weight of the µMLC. Consequently, the imperfections in isocenter localizations are transferred to the conventional radiotherapy where the clinical consequences of uncertainties in the submillimeter regime are negligible.

Isocentric rotational performance of the Elekta Precise Table studied using a USB-microscope.

The isocentric three-dimensional performance of the Elekta Precise Table was investigated. A pointer was attached to the radiation head of the accelerator and positioned at the geometric rotational axis of the head. A USB-microscope was mounted on the treatment tabletop to measure the table position relative to the pointer tip. The table performance was mapped in terms of USB-microscope images of the pointer tip at different table angles and load configurations. The USB-microscope was used as a detector to measure the pointer tip positions with a resolution down to 0.01 mm. A new elastic model of the treatment table was developed. This model describes the performance of the treatment table quite well except from some deviations due to backlash effects. Geometric and elastic features are described through six parameters. These parameters are calculated using the linear least squares fitting technique. A new method to ensure optimal positioning of the table relative to the accelerator is presented. This method cannot eliminate systematic errors completely. To eliminate systematic errors we suggest that geometric and elastic models of the table and accelerator gantry arm are incorporated in the dose planning system.

Elekta Precise Table characteristics of IGRT remote table positioning.

INTRODUCTION: Cone beam CT is a powerful tool to ensure an optimum patient positioning in radiotherapy. When cone beam CT scan of a patient is acquired, scan data of the patient are compared and evaluated against a reference image set and patient position offset is calculated. Via the linac control system, the patient is moved to correct for position offset and treatment starts. This procedure requires a reliable system for movement of patient. In this work we present a new method to characterize the reproducibility, linearity and accuracy in table positioning. The method applies to all treatment tables used in radiotherapy. MATERIAL AND METHODS: The table characteristics are investigated on our two recent Elekta Synergy Platforms equipped with Precise Table installed in a shallow pit concrete cavity. Remote positioning of the table uses the auto set-up (ASU) feature in the linac control system software Desktop Pro R6.1. The ASU is used clinically to correct for patient positioning offset calculated via cone beam CT (XVI)-software. High precision steel rulers and a USB-microscope has been used to detect the relative table position in vertical, lateral and longitudinal direction. The effect of patient is simulated by applying external load on the iBEAM table top. For each table position an image is exposed of the ruler and display values of actual table position in the linac control system is read out. The table is moved in full range in lateral direction (50 cm) and longitudinal direction (100 cm) while in vertical direction a limited range is used (40 cm). RESULTS AND DISCUSSION: Our results show a linear relation between linac control system read out and measured position. Effects of imperfect calibration are seen. A reproducibility within a standard deviation of 0.22 mm in lateral and longitudinal directions while within 0.43 mm in vertical direction has been observed. The usage of XVI requires knowledge of the characteristics of remote table positioning. It is our opinion that the method presented meets the requirements in high precision IGRT.

Post-mastectomy radiotherapy in Denmark: from 2D to 3D treatment planning guidelines of The Danish Breast Cancer Cooperative Group.

This paper describes the procedure of changing from 2D to 3D treatment planning guidelines for post-mastectomy radiotherapy in Denmark. The aim of introducing 3D planning for post-mastectomy radiotherapy was to optimize the target coverage and minimize the dose to the normal tissues. Initially, it was investigated whether it was possible to find a treatment technique alternative to the one recommended by the Danish Breast Cancer Cooperative Group (DBCG). A dosimetric comparison of a combined photon/electron 3-field technique (3F) and a partial wide tangent technique (PWT) was carried out on individual planning CT-scans from seven patients selected to represent a wide range of sizes and shapes of chest walls. The heart dose was lower for PWT than for 3F, however, for both techniques the dose was within the accepted constraints. The lung dose was higher but acceptable for six of the seven patients with PWT. The dose to the internal mammary nodes (IMN) was not satisfactory for five of the seven patients for 3F, whereas only two of the seven patients had a minimum dose lower than 95% of the prescribed dose with PWT. Finally, the dose to the contralateral breast was increased when using PWT compared to 3F. It was concluded that PWT was an appropriate choice of technique for future radiation treatment of post-mastectomy patients. A working group was formed and guidelines for 3D planning were developed during a series of workshops where radiation oncologists and physicists from all radiotherapy centres participated. This work also included a definition of the tissue structures needed to be outlined on the planning CT-scan. The work was initiated in 2003 and the guidelines were approved by the DBCG Radiotherapy Committee in 2006. The first of January 2007 the 3D guidelines had been fully implemented in five of the seven radiotherapy centres.