Juergen Meyer

Characteristics of Gafchromic® XR-RV2 radiochromic film

Gafchromic® XR-RV2 is a revised version of the obsolete Gafchromic® XR-R-type radiochromic film. This article investigates the dose response, energy response, postexposure growth, and polarizing effects of this film after exposure to ionizing radiation in the diagnostic energy range. The effect of bit depth on scanning was also investigated. Films were scanned using an Epson Expression 10000XL document scanner or an X-Rite model 301 spot densitometer. Color channel analysis was performed. The film showed usable response in the air kerma range of 1-1000 cGy, although by 1500 cGy the film appeared saturated when using the red color channel on a document scanner. The film response varied by 11% between 60 and 96 kVp and 3.5% between 96 and 125 kVp for doses above 1 Gy. Postexposure growth was found to be approximately logarithmic and fairly stable after 24 h. Films stored under office lighting exhibited around twice the density growth compared with film stored in a dark environment. The film showed strong orientation dependence when scanned using a polarized light source. A 48 bit scan provided no increase in sensitivity over 24 bits. Gafchromic XR-RV2 film is a radiochromic film ideally suited for measurement of wide dose ranges at diagnostic energies. The energy dependence of this film limits its accuracy for dosimetry of unknown energy beams. For the document scanners used in this study a 24 bit scan was more than sufficient compared to a 48 bit scan. This is likely to be the case for most document scanners where electrical noise prevents higher bit depths from increasing the sensitivity of measurements.

A RapidArc planning strategy for prostate with simultaneous integrated boost

Since the clinical implementation of novel rotational forms of intensity‐modulated radiotherapy, a variety of planning studies have been published that reinforce the major selling points of the technique. Namely, comparable or even improved dose distributions with a reduction in both monitor units and treatment times, when compared with static gantry intensity‐modulated radiotherapy. Although the data are promising, a rigorous approach to produce these plans has yet to be established. As a result, this study outlines a robust and streamlined planning strategy with a concentration on RapidArc class solutions for prostate with a simultaneous integrated boost. This planning strategy outlines the field setup, recommended starting objectives, required user interactions to be made throughout optimization and post‐optimization adjustments. A comparative planning study, with static gantry IMRT, is then presented as justification for the planning strategy itself. A variety of parameters are evaluated relating to both the planning itself (optimization and calculation time) and the plans that result. Results of this comparative study are in line with previously published data, and the planning process is streamlined to a point where the RapidArc optimization time takes 15±1.3 minutes. Application of this planning strategy reduces the dependence of the produced plan on the experience of the planner, and has the potential to streamline the planning process within radiotherapy departments. PACS numbers: 87.55.x, 87.55.D, 87.55.de, 87.55.dk

Influence of increased target dose inhomogeneity on margins for breathing motion compensation in conformal stereotactic body radiotherapy.

BackgroundBreathing motion should be considered for stereotactic body radiotherapy (SBRT) of lung tumors. Four-dimensional computer tomography (4D-CT) offers detailed information of tumor motion. The aim of this work is to evaluate the influence of inhomogeneous dose distributions in the presence of breathing induced target motion and to calculate margins for motion compensation.MethodsBased on 4D-CT examinations, the probability density function of pulmonary tumors was generated for ten patients. The time-accumulated dose to the tumor was calculated using one-dimensional (1D) convolution simulations of a static dose distribution and target probability density function (PDF). In analogy to stereotactic body radiotherapy (SBRT), different degrees of dose inhomogeneity were allowed in the target volume: minimum doses of 100% were prescribed to the edge of the target and maximum doses varied between 102% (P102) and 150% (P150). The dose loss due to breathing motion was quantified and margins were added until this loss was completely compensated.ResultsWith the time-weighted mean tumor position as the isocentre, a close correlation with a quadratic relationship between the standard deviation of the PDF and the margin size was observed. Increased dose inhomogeneity in the target volume required smaller margins for motion compensation: margins of 2.5 mm, 2.4 mm and 1.3 mm were sufficient for compensation of 11.5 mm motion range and standard deviation of 3.9 mm in P105, P125 and P150, respectively. This effect of smaller margins for increased dose inhomogeneity was observed for all patients. Optimal sparing of the organ-at-risk surrounding the target was achieved for dose prescriptions P105 to P118. The internal target volume concept over-compensated breathing motion with higher than planned doses to the target and increased doses to the surrounding normal tissue.ConclusionTreatment planning with inhomogeneous dose distributions in the target volume required smaller margins for compensation of breathing induced target motion with the consequence of lower doses to the surrounding organs-at-risk.

A spring–dashpot system for modelling lung tumour motion in radiotherapy

A 3D system of springs and dashpots is presented to model the motion of a lung tumour during respiration. The main guiding factor in configuring the system is the spatial relationship between abdominal and lung tumour motion. A coupled, non-dimensional triple of ordinary differential equations models the tumour motion when driven by a 3D breathing signal. Asymptotic analysis is used to reduce the system to a single equation driven by a 3D signal, in the limit of small lateral and transverse tumour motions. A numerical scheme is introduced to solve this equation, and tested over wide parameter ranges. Real clinical data is used as input to the model, and the tumour motion output is in excellent agreement with that obtained by a prototype tumour tracking system, with model parameters obtained by optimization. The fully 3D model has the potential to accurately model the motion of any lung tumour given an abdominal signal as input, with model parameters obtained from an internal optimization routine.

A patient position guidance system in radiation therapy using augmented reality

With the increased precision in dose delivery provided by highly conformal radiotherapy techniques such as intensity modulated radiotherapy (IMRT), the requirement for accuracy in patient positioning for treatment is emphasised. This system uses augmented reality (AR) to allow the radiation therapist to visually guide the patient during positioning for treatment. It superimposes three-dimensional scan data acquired from the planning CT over a real-time view of the patient, which is obtained from a conventional webcam. The 3D data is positioned relative to AR tracking markers visible to the camera. Throughout development, the system was tested on a 30 cm wooden phantom in place of the patient. A CT scan was performed on the phantom to obtain 3D data, and a small scale test couch was set up to perform registration tests. Modifications to the position of the phantom on the test couch on the order of a millimetre have been visible.

A Method for Patient Set-up Guidance in Radiotherapy Using Augmented Reality

A system for patient set-up in external beam radiotherapy was developed using Augmented Reality (AR). Live images of the linac treatment couch and patient were obtained with video cameras and displayed on a nearby monitor. A 3D model of the patient’s external contour was obtained from planning CT data, and AR tracking software was used to superimpose the model onto the video images in the correct position for treatment. Throughout set-up and treatment, the user can view the monitor and visually confirm that the patient is positioned correctly. To ensure that the virtual contour was displayed in the correct position, a process was devised to register the coordinates of the linac with the camera images. A cube with AR tracking markers attached to its faces was constructed for alignment with the isocentre using room lasers or conebeam CT. The performance of the system was investigated in a clinical environment by using it to position an anthropomorphic phantom without the aid of additional set-up methods. The positioning errors were determined by means of CBCT and image registration. The translational set-up errors were found to be less than 2.4 mm and the rotational errors less than 0.3°. This proof-of-principle study has demonstrated the feasibility of using AR for patient position and pose guidance.

On the Use of a Hexapod Table to Improve Tumour Targeting in Radiation Therapy

Following the discovery of x-rays by Rontgen at the end of the 19th century it did not take long before ionising radiation was used to treat cancer (Del Regato, 2000). Early treatments were not very accurate in terms of targeting the tumour and sparing surrounding healthy tissue. Geometrical accuracy was in the range of centimetres rather than millimetres. Since those pioneering days considerable improvements facilitated by several technological advances and treatment strategies have been made (Abrams, 1992; Schlegel et al., 2006; Webb, 2001). Today treatment with high energy ionising radiation is one of the three traditional forms of medical treatment used to treat cancer and for palliation of symptoms. It may be used alone or in conjunction with surgery or chemotherapy. It is unrivalled as a treatment in cases where surgical removal of the cancer is impossible or might debilitate the patient, e.g. tumours that are infiltrative or located close to a critical organ such as the spinal cord. The more precisely the tumour can be localised the better it can be targeted. Improvements over the last decades in both anatomical and functional imaging as well as detector technology have made it possible for tumours to be more accurately located (Apisarnthanarax & Chao, 2005; Grosu et al., 2005; Jaffray & Siewerdsen, 2000; Ling et al., 2000). Improved localization has the potential to reduce safety margins around the tumour volume leading to more patient-specific but also more complex shaped target volumes reflecting the demarcation of the tumour in the medical images. Delivering the radiation dose to these irregular target volumes requires a great deal of technological but also human effort. It is against this background that radiotherapy has become a discipline with a quest for precision and sub-millimetre accuracy (Guckenberger et al., 2006a; Meyer et al., 2007; Murphy, 1997; Solberg et al., 2004; Yu et al., 2004). Considering the diversity and elasticity of a human body, its temporal biological variations and various sources of organ motion this is a challenging pursuit requiring sophisticated technology. The use of robots in radiotherapy is beginning to play an increasingly important role in achieving this goal. The challenge is to integrate and utilize robotic technology in a judicious and safe way. The aim is to be able to perform treatments, which were previously unattainable, less accurate and/or reliable or dependent on the skills and experience of the medical team performing the treatment. For a detailed description and review of the fundamentals of radiotherapy the reader is referred to other sources (Khan, 2003; Podgorsak, 2005). However, a brief introduction is O pe n A cc es s D at ab as e w w w .ite ch on lin e. co m

Two-step intensity modulated arc therapy (2-step IMAT) with segment weight and width optimization

Background2-step intensity modulated arc therapy (IMAT) is a simplified IMAT technique which delivers the treatment over typically two continuous gantry rotations. The aim of this work was to implement the technique into a computerized treatment planning system and to develop an approach to optimize the segment weights and widths.Methods2-step IMAT was implemented into the Prism treatment planning system. A graphical user interface was developed to generate the plan segments automatically based on the anatomy in the beams-eye-view. The segment weights and widths of 2-step IMAT plans were subsequently determined in Matlab using a dose-volume based optimization process. The implementation was tested on a geometric phantom with a horseshoe shaped target volume and then applied to a clinical paraspinal tumour case.ResultsThe phantom study verified the correctness of the implementation and showed a considerable improvement over a non-modulated arc. Further improvements in the target dose uniformity after the optimization of 2-step IMAT plans were observed for both the phantom and clinical cases. For the clinical case, optimizing the segment weights and widths reduced the maximum dose from 114% of the prescribed dose to 107% and increased the minimum dose from 87% to 97%. This resulted in an improvement in the homogeneity index of the target dose for the clinical case from 1.31 to 1.11. Additionally, the high dose volume V105 was reduced from 57% to 7% while the maximum dose in the organ-at-risk was decreased by 2%.ConclusionsThe intuitive and automatic planning process implemented in this study increases the prospect of the practical use of 2-step IMAT. This work has shown that 2-step IMAT is a viable technique able to achieve highly conformal plans for concave target volumes with the optimization of the segment weights and widths. Future work will include planning comparisons of the 2-step IMAT implementation with fixed gantry intensity modulated radiotherapy (IMRT) and commercial IMAT implementations.

A General Model of Lung Tumour Motion

A limiting factor for the effective delivery of radiotherapy to lung tumours is the tumour motion as the patient breathes. If the tumour position is known at all times then treatment parameters may be adjusted accordingly. We formulate a general approach to model the spatial relationship between an external respiratory signal and the tumour position. The model treats the tumour as a point mass attached to a spring-dashpot system driven by abdominal motion. We present the model and show results of numerical computations based on clinical data.

An Augmented Reality Application for Patient Positioning and Monitoring in Radiotherapy

A system for visual image guidance in patient set-up for external-beam radiotherapy procedures was developed using augmented reality. The system uses video cameras to obtain views of the linear accelerator, and the live images are displayed on a monitor in the treatment room. 3D models of the patient’s external surface, obtained from planning CT data, are superimposed onto the treatment couch in the camera images. The real patient can then be aligned with the virtual contour guided by the augmented monitor.