The six-bank multi-leaf system : A large field size, high resolution collimator for advanced

A linear accelerator (linac) is the most commonly used device for treatment of patients with cancer in external beam radiotherapy (EBRT). The Linac delivers a high-energy ionizing radiation (photons or electrons) to the region of the patient's tumor. The absorption of radiation in the treated area damages the diseased cells. To minimize irradiation of healthy tissue beams should be shaped. Commonly, this is achieved by using a multi-leaf collimator (MLC). The motivation for this work was given by the fact that a conventional Linac/MLC, currently available on the market, was designed for conformal radiotherapy, but is nowadays also used for intensity-modulated radiotherapy (IMRT). In this thesis the first goal was to understand and to quantify the impact of Linac/MLC design parameters on IMRT treatment plans. The investigated parameters were: leaf width of the MLC, leaf transmission related to the thickness of the leaves, and penumbra related primarily to the source size. For this purpose, various head and neck IMRT plans were evaluated using the Plato and Pinnacle treatment planning systems. Lead by those findings, and a desire to obtain a collimator which could deliver large field size with high resolution field shaping, we present an alternative design of a multi-leaf collimator, called a six-bank MLC. The MLC system consists of three layers of two opposing leaf banks. The layers are rotated 60 degrees relative to each other. The leaves in each bank have a standard width of 1 cm projected at the isocentre. This is a novelty compared to the existing systems which can achieve either large field size with low resolution field shaping, or small field size with high resolution field shaping. For the six-bank MLC which would function as a multi-purpose collimator, suitable for all types of treatments, two methods for delivering IMRT were developed. In a low-resolution mode similar segments can be delivered as with a conventional two-bank MLC with a leaf width of 1 cm. The performance in high-resolution mode is comparable to that of a mini-MLC, but a trade-off had to be made between accuracy and number of segments. Finally, an analytical model of the optimal MLC leaves design was made, and was used to find the optimal leaf design parameters for a six-bank MLC. In conclusion, Linac/MLC design is of great importance for IMRT treatment of patients. By further improvement of image-guided radiotherapy (IGRT) and application of on-line position verification on a daily basis, additional reduction of margins will be possible. This would lead to a great benefit enhancement of a six-bank MLC over the conventional MLCs

IMAGE-GUIDED RADIOTHERAPY FOR BREAST CANCER PATIENTS: SURGICAL CLIPS AS SURROGATE FOR BREAST EXCISION CAVITY

Purpose: To determine the use of surgical clips as a surrogate for localization of the excision cavity and to quantify the stability of the clips' positions during the course of external beam radiotherapy for breast cancer patients, using cone beam computed tomography (CBCT) scans.Methods and Materials: Twenty-one breast cancer patients with surgical clips placed in the breast excision cavity were treated in a supine position with 28 daily fractions. CBCT scans were regularly acquired for a setup correction protocol. Retrospectively, the CBCT scans were registered to the planning CT scans, using gray-value registration of the excision cavity region and chamfer matching of the clips. Subsequently, residual setup errors (systematic [Sigma] and random [sigma]) of the excision cavity were estimated relative to the clips' registration. Finally, the stability of the clips' positions were quantified as the movement of each separate clip according to the center of gravity of the excision cavity.Results: When clips were used for online setup corrections, the residual errors of the excision cavity were Sigma(left-right) = 1.2, sigma(left-right) = 1.0; Sigma(cranial-caudal) = 1.3, sigma(cranial-caudal) = 1.2; and Sigma(anterior-posterior) = 0.7, sigma(anterior-posterior) = 0.9 mm. Furthermore, the average distance (over all patients) between the clips and centers of gravity of the excision cavities was 18.8 mm (on the planning CT) and was reduced to 17.4 mm (measured on the last CBCT scan).Conclusion: Clips move in the direction of the center of gravity of the excision cavity, on average, 1.4 mm. The clips are good surrogates for locating the excision cavity and providing small residual errors. (c) 2011 Elsevier Inc.Biological, physical and clinical aspects of cancer treatment with ionising radiatio

BREAST PATIENT SETUP ERROR ASSESSMENT: COMPARISON OF ELECTRONIC PORTAL IMAGE DEVICES AND CONE-BEAM COMPUTED TOMOGRAPHY MATCHING RESULTS

Purpose: To quantify the differences in setup errors measured with the cone-beam computed tomography (CBCT) and electronic portal image devices (EPID) in breast cancer patients.Methods and Materials: Repeat CBCT scan were acquired for routine offline setup verification in 20 breast cancer patients. During the CBCT imaging fractions, EPID images of the treatment beams were recorded. Registrations of the bony anatomy for CBCT to planning CT and EPID to digitally reconstructed-radiographs (DRRs) were compared. In addition, similar measurements of an anthropomorphic thorax phantom were acquired. Bland-Altman and linear regression analysis were performed for clinical and phantom registrations. Systematic and random setup errors were quantified for CBCT and EPID-driven correction protocols in the EPID coordinate system (U, V), with V parallel to the cranial-caudal axis and U perpendicular to V and the central beam axis.Results: Bland-Altman analysis of clinical EPID and CBCT registrations yielded 4 to 6-mm limits of agreement, indicating that both methods were not compatible. The EPID-based setup errors were smaller than the CBCT-based setup errors. Phantom measurements showed that CBCT accurately measures setup error whereas EPID underestimates setup errors in the cranial caudal direction. In the clinical measurements, the residual bony anatomy setup errors after offline CBCT-based corrections were Sigma(U) = 1.4 mm, Sigma(V) = 1.7 mm, and sigma(U) = 2.6 mm, sigma(V) = 3.1 mm. Residual setup errors of EPID driven corrections corrected for underestimation were estimated at Sigma(U) = 2.2mm, Sigma(V) = 3.3 mm, and sigma(U) = 2.9 mm, sigma(V) = 2.9 mm.Conclusion: EPID registration underestimated the actual bony anatomy setup error in breast cancer patients by 20% to 50%. Using CBCT decreased setup uncertainties significantly. (C) 2010 Elsevier Inc.Biological, physical and clinical aspects of cancer treatment with ionising radiatio

Beam modeling and VMAT performance with the Agility 160-leaf multileaf collimator

The Agility multileaf collimator (Elekta AB, Stockholm, Sweden) has 160 leaves of projected width 0.5 cm at the isocenter, with maximum leaf speed 3.5 cms(-1). These characteristics promise to facilitate fast and accurate delivery of radiotherapy, particularly volumetric-modulated arc therapy (VMAT). The aim of this study is therefore to create a beam model for the Pinnacle(3) treatment planning system (Philips Radiation Oncology Systems, Fitchburg, WI), and to use this beam model to explore the performance of the Agility MLC in delivery of VMAT. A 6 MV beam model was created and verified by measuring doses under irregularly shaped fields. VMAT treatment plans for five typical head-and-neck patients were created using the beam model and delivered using both binned and continuously variable dose rate (CVDR). Results were compared with those for an MLCi unit without CVDR. The beam model has similar parameters to those of an MLCi model, with interleaf leakage of only 0.2%. The verification of irregular fields shows a mean agreement between measured and planned dose of 1.3% (planned dose higher). The Agility VMAT head-and-neck plans show equivalent plan quality and delivery accuracy to those for an MLCi unit, with 95% of verification measurements within 3% and 3 mm of planned dose. Mean delivery time is 133 s with the Agility head and CVDR, 171 s without CVDR, and 282 s with an MLCi unit. Pinnacle(3) has therefore been shown to model the Agility MLC accurately, and to provide accurate VMAT treatment plans which can be delivered significantly faster with Agility than with an MLCi