This book on The Physics of Radiotherapy X-Rays and Electrons is an expanded and updated successor to The Physics of Radiotherapy X-Rays from Linear Accelerators by the same authors and published by Medical Physics Publishing in 1997. It is pleasing to see many coloured illustrations in this new edition that encompasses many of the advances made in radiotherapy in the last decade, including new sections on IMRT, IGRT and tomotherapy. It also includes a new chapter on electrons in radiotherapy which was one of the shortcomings of 1997 edition. The quality of paper used in the production of this book has also improved. Chapters 1–4 provide an outline of medical linear accelerators, interaction properties of X-rays and electron beams, dosimetric equipment and methods used for therapeutic X-rays and electrons, and properties of X-ray beams from linear accelerators. Properties of electron beams from linear accelerators are given in a new Chapter 5. It also contains a few brief comments on protons, including their production, beam delivery and a general report on clinical experience. Chapter 6 covers the treatment planning process including the use of traditional X-ray simulators and CT simulation in acquiring patient datasets. Description of MR and PET ultrasound imaging devices as treatment planning aids is also provided but it is very brief. The authors have given an excellent coverage of special radiotherapy procedures including IMRT, SRS, tomotherapy etc. in Chapter 7. However, I am surprised to note that total body photon irradiation, one of the important treatment techniques used prior to bone marrow transplantation, has not been covered in any section. Chapter 8 contains discussions on beam calibration protocols and current dosimetry practices. Chapters 9 and 10 contain topics on photon beam modelling and inhomogeneity correction methods including a discussion on Monte Carlo and Convolution methods. The topics of quality assurance including patient immobilisation and image guidance in radiotherapy are covered in Chapters 11 and 12. Radiation protection methods including the principles of accelerator room design are given in Chapter 13. Discussions on radiobiological modelling including TCP, NTCP and EUD are provided in Chapter 14. The graphic illustrations are generally very good, although some have inconsistent font sizes, and they make the learning process a refreshing experience. I believe The Physics of Radiotherapy X-rays and Electrons is an excellent source of knowledge in the field of radiation oncology medical physics for students as well as practicing professionals. I congratulate the authors for their significant contribution to the community of radiation oncology.
Abstract This study aimed to identify potential anatomical variation triggers using magnetic resonance imaging for plan adaption of cervical cancer patients to ensure dose requirements were met over an external beam radiotherapy course. Magnetic resonance images (MRIs) acquired before and during treatment were rigidly registered to a pre-treatment computerised tomography (CT) image for 11 retrospective cervix cancer datasets. Target volumes (TVs) and organs at risk (OARs) were delineated on both MRIs and propagated onto the CT. Treatment plans were generated based on the pre-treatment contours and applied to the mid-treatment contours. Anatomical and dosimetric changes between each timepoint were assessed. The anatomical changes included the change in centroid position and volume size. Dosimetric changes included the V30Gy and V40Gy for the OARs, and V95%, V100%, D95% and D98% for the TVs. Correlation with dosimetric and anatomical changes were assessed to determine potential replan triggers. Changes in the bowel volume and position in the superior-inferior direction, and the high-risk CTV anterior posterior position were highly correlated with a change in dose to the bowel and target, respectively. Hence changes in bowel and high-risk CTV could be used as a potential replan triggers.
A new multileaf collimator (MLC) model has been incorporated into version 7.4 of the Pinnacle radiotherapy treatment planning system (Philips Radiation Oncology Systems, Milpitas, CA). The MLC model allows for rounded MLC leaf-ends and provides separate parameters for inter-leaf transmission, intra-leaf transmission and the tongue width of the MLC leaf. In this report we detail the method followed to commission the MLC model for a Varian 120-leaf Millennium MLC (Varian Medical Systems, Palo Alto, CA, USA) for both 6 and 10 MV photons, and test the validity of the model for an IMRT field. Dose profiles in water were measured for a range of square MLC field sizes and compared to the Pinnacle computed dose profiles; in addition, the dose distribution for a series of adjacent MLC fields was measured to observe the model's behaviour along match-lines. Based on these results intra-leaf transmissions of 1.5% for 6 MV and 1.8% for 10 MV, leaf-tip radius of 12.0 cm, an inter-leaf transmission of 0.5%, and a tongue width of 0.1 cm were chosen. Using these values to compute the planar dose distribution for a 6 MV IMRT field, the new version of Pinnacle displayed improved dosimetric agreement with the dose-to-water EPID image and ion chamber measurements when compared to the old version of Pinnacle, particularly along the MLC tongue edge and across match-lines. Discrepancies of up to 5% were observed between calculated and measured doses along match-lines for both 6 MV and 10 MV photons; however, the new MLC model did predict the presence of match-lines and was a significant improvement on the previous model.
Purpose/Objective: The XRAD225Cx is a small animal radiotherapy device using a medium energy beam (225 kVp) and small circular fields.In addition to the half-value layers and the absolute dose rate, the commissioning of this equipment requires relative dose measurements such as percentage depth dose (PDD), Output Factor (OF) and Tissue Maximum Ratio (TMR).The aim of this study was to compare two media and four detectors to determine the optimal conditions to perform these relative measurements.Materials and Methods: RW3 material is known not to be waterequivalent at medium energy for absolute dose measurements.To evaluate the impact of this medium for relative dose measurements, PDDs were obtained in water and RW3 for a 10x10 cm 2 field with a plane-parallel ionization chamber and EBT2 Gafchromic films.Simulated PDDs were generated using a GATE Monte Carlo model of the irradiator.To study the influence of the detector, four dosimeters (an IBA SFD diode, a PTW PinPoint 31014 microchamber, EBT2 films and a PTW-23342 plane-parallel chamber) were compared for OFs, PDDs and TMRs in water and/or RW3 depending on the dosimeter sealing.Measurements were performed in small fields (20, 15, 10, 8, 5 and 2.5 mm in diameter).OFs, PDDs, and TMRs were also computed with the Monte Carlo model.Results: Measured and simulated PDDs were similar in water and RW3.Regardless of media and detectors, simulated and measured OFs showed no differences down to a diameter beam of 5 mm.For the smallest beam (2.5 mm),ionization chambers yielded large discrepancies (up to -22%) compared to SFD and EBT2 measurements and Monte Carlo simulations.This is due to the size of the sensitive volume of chambers compared to beam diameter.For PDDs and TMRs, measurement accuracy depends on spatial resolution in depth of the detector.Therefore, PinPoint chamber was not used.Plane ionization chamber and film measurements were closed to Monte Carlo computed results.SFD diode results showed significant discrepancies (up to 9%) due to the important variation in the relative energy response of the diode at 225 kVp.Conclusions: For relative measurements, RW3 can be used instead of water at 225 kVp for convenient considerations.For OFs, all studied detectors may be used down to a beam diameter of 5 mm.For smaller beams, measurements should be performed with the SFD diode or Gafchromic films.For PDDs and TMRs, plane ionization chamber can be used down to a beam diameter of 5 mm.Gafchromic films are suitable whatever the beam diameter.
Purpose: Accurate geometry is required for radiotherapy treatment planning (RTP). When considering the use of magnetic resonance imaging (MRI) for RTP, geometric distortions observed in the acquired images should be considered. While scanner technology and vendor supplied correction algorithms provide some correction, large distortions are still present in images, even when considering considerably smaller scan lengths than those typically acquired with CT in conventional RTP. This study investigates MRI acquisition with a moving table compared with static scans for potential geometric benefits for RTP. Methods: A full field of view (FOV) phantom (diameter 500 mm; length 513 mm) was developed for measuring geometric distortions in MR images over volumes pertinent to RTP. The phantom consisted of layers of refined plastic within which vitamin E capsules were inserted. The phantom was scanned on CT to provide the geometric gold standard and on MRI, with differences in capsule location determining the distortion. MRI images were acquired with two techniques. For the first method, standard static table acquisitions were considered. Both 2D and 3D acquisition techniques were investigated. With the second technique, images were acquired with a moving table. The same sequence was acquired with a static table and then with table speeds of 1.1 mm/s and 2 mm/s. All of the MR images acquired were registered to the CT dataset using a deformable B ‐spline registration with the resulting deformation fields providing the distortion information for each acquisition. Results: MR images acquired with the moving table enabled imaging of the whole phantom length while images acquired with a static table were only able to image 50%–70% of the phantom length of 513 mm. Maximum distortion values were reduced across a larger volume when imaging with a moving table. Increased table speed resulted in a larger contribution of distortion from gradient nonlinearities in the through‐plane direction and an increased blurring of capsule images, resulting in an apparent capsule volume increase by up to 170% in extreme axial FOV regions. Blurring increased with table speed and in the central regions of the phantom, geometric distortion was less for static table acquisitions compared to a table speed of 2 mm/s over the same volume. Overall, the best geometric accuracy was achieved with a table speed of 1.1 mm/s. Conclusions: The phantom designed enables full FOV imaging for distortion assessment for the purposes of RTP. MRI acquisition with a moving table extends the imaging volume in the z direction with reduced distortions which could be useful particularly if considering MR‐only planning. If utilizing MR images to provide additional soft tissue information to the planning CT, standard acquisition sequences over a smaller volume would avoid introducing additional blurring or distortions from the through‐plane table movement.
Photon beam dosimetry using Kodak extended dose range (EDR2) radiographic film can provide accurate and high spatial resolution information especially for areas such as IMRT dosimetry where a higher dose level (100–400 cGy) is often required to be delivered for verification. For such dosimetry checks, it may sometimes be useful to place the film in a tank filled with water during irradiation. The effects of water on the film when packaged and when removed from the packaging have been examined. Results have shown that the EDR2 film when supplied in the ready pack form is provided in water proof packages and no significant absorption effects are observed or measured on the film even after 48 h of soaking in a water bath. When the film is removed from the ready packs and exposed to water directly, various effects are seen. In the visible spectrum region, small variations (up to 3%) in recorded optical density (OD) are recorded using a fluorescent light densitometer. These effects become much larger in the infrared region (e.g. 7.5% at 900 nm and 12.5% at 1000 nm) and are wavelength dependent. The changes produced by the water are relatively independent of the exposure time to water from 5 s up to 1 h or whether the water exposure occurred.
When we decided to put this issue together, we called for contributions from CALL researchers and practitioners who were ‘‥concerned with the teaching of grammar in the post-communicative world…’. We were aware of the contentious nature of such a description – it suggests that the communicative approach is not, perhaps, a complete answer to the question of how to teach languages. Amongst the correspondence we entered into, subsequently, were some exchanges with colleagues who believed we were advocating a retrogressive step. It is only comparatively recently, they pointed out, that language learners have been freed from the shackles of piece-meal learning, drilling, correction and self-monitoring, and allowed to focus on ‘the challenge of communication’
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