Medical Physics
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Microdosimetry spectra of the Loma Linda proton beam and relative biological effectiveness comparisons Protons have long been recognized as low LET radiation in radiotherapy. However, a detailed account of LET (linear energy transfer) and RBE (relative biological effectiveness) changes with incident beam energy and depth in tissue is still unresolved. This issue is particularly important for treatment planning, where the physical dose prescription is calculated from a RBE using cobalt as the reference radiation. Any significant RBE changes with energy or depth will be important to incorporate in treatment planning. In this paper we present microdosimetry spectra for the proton beam at various energies and depths and compare the results to cell survival studies performed at Loma Linda. An empirically determined biological weighting function that depends on lineal energy is used to correlate the microdosimetry spectra with cell survival data. We conclude that the variations in measured RBE with beam energy and depth are small until the distal edge of the beam is reached. On the distal edge, protons achieve stopping powers as high as 100 keV/μm, which is reflected in the lineal energy spectra taken there. Lineal energy spectra 5 cm beyond the distal edge of the Bragg peak also show a high LET component but at a dose rate 600 times smaller than observed inside the proton field.
Medical Physics - Tập 24 Số 9 - Trang 1499-1506 - 1997
Integral dose conservation in radiotherapy Treatment planners frequently modify beam arrangements and use IMRT to improve target dose coverage while satisfying dose constraints on normal tissues. The authors herein analyze the limitations of these strategies and quantitatively assess the extent to which dose can be redistributed within the patient volume. Specifically, the authors hypothesize that (1) the normalized integral dose is constant across concentric shells of normal tissue surrounding the target (normalized to the average integral shell dose), (2) the normalized integral shell dose is constant across plans with different numbers and orientations of beams, and (3) the normalized integral shell dose is constant across plans when reducing the dose to a critical structure. Using the images of seven patients previously irradiated for cancer of brain or prostate cancer and one idealized scenario, competing three‐dimensional conformal and IMRT plans were generated using different beam configurations. Within a given plan and for competing plans with a constant mean target dose, the normalized integral doses within concentric “shells” of surrounding normal tissue were quantitatively compared. Within each patient, the normalized integral dose to shells of normal tissue surrounding the target was relatively constant (1). Similarly, for each clinical scenario, the normalized integral dose for a given shell was also relatively constant regardless of the number and orientation of beams (2) or degree of sparing of a critical structure (3). 3D and IMRT planning tools can redistribute, rather than eliminate dose to the surrounding normal tissues (intuitively known by planners). More specifically, dose cannot be moved between shells surrounding the target but only within a shell. This implies that there are limitations in the extent to which a critical structure can be spared based on the location and geometry of the critical structure relative to the target.
Medical Physics - Tập 36 Số 3 - Trang 734-740 - 2009
Online monitoring and error detection of real‐time tumor displacement prediction accuracy using control limits on respiratory surrogate statistics Purpose: To evaluate Hotelling'sT 2 statistic and the input variable squared prediction error (Q (X ) ) for detecting large respiratory surrogate‐based tumor displacement prediction errors without directly measuring the tumor's position. Methods: Tumor and external marker positions from a database of 188 Cyberknife Synchrony™ lung, liver, and pancreas treatment fractions were analyzed. The first ten measurements of tumor position in each fraction were used to create fraction‐specific models of tumor displacement using external surrogates as input; the models were used to predict tumor position from subsequent external marker measurements. A partial least squares (PLS) model with four scores was developed for each fraction to determineT 2 and Q (X ) confidence limits based on the first ten measurements in a fraction. The T 2 and Q (X ) statistics were then calculated for every set of external marker measurements. Correlations between model error and both T 2 and Q (X ) were determined. Receiver operating characteristic analysis was applied to evaluate sensitivities and specificities of T 2 , Q (X ) , and T 2 ∪Q (X ) for predicting real‐time tumor localization errors >3 mm over a range of T 2 and Q (X ) confidence limits. Results: Sensitivity and specificity of detecting errors >3 mm varied with confidence limit selection. At 95% sensitivity,T 2 ∪Q (X ) specificity was 15%, 2% higher than either T 2 or Q (X ) alone. The mean time to alarm for T 2 ∪Q (X ) at 95% sensitivity was 5.3 min but varied with a standard deviation of 8.2 min. Results did not differ significantly by tumor site. Conclusions: The results of this study establish the feasibility of respiratory surrogate‐based online monitoring of real‐time respiration‐induced tumor motion model accuracy for lung, liver, and pancreas tumors. TheT 2 and Q (X ) statistics were able to indicate whether inferential model errors exceeded 3 mm with high sensitivity. Modest improvements in specificity were achieved by combining T 2 and Q (X ) results.
Medical Physics - Tập 39 Số 4 - Trang 2042-2048 - 2012
Automatic on‐line electronic portal image analysis with a wavelet‐based edge detector A fully automatic method for on‐line electronic portal image analysis is proposed. The method uses multiscale edge detection with wavelets for both the field outline and the anatomical structures. An algorithm to extract and combine the information from different scales has been developed. The edges from the portal image are aligned with the edges from the reference image using chamfer matching. The reference is the first portal image of each treatment. The matching is applied first to the field and subsequently to the anatomy. The setup deviations are quantified as the displacement of the anatomical structures relative to the radiation beam boundaries. The performance of the algorithm was investigated for portal images with different contrast and noise level. The automatic analysis was used first to detect simulated displacements. Then the automatic procedure was tested on anterior–posterior and lateral portal images of a pelvic phantom. In both sets of tests the differences between the measured and the actual shifts were used to quantify the performance. Finally we applied the automatic procedure to clinical images of pelvic and lung regions. The output of the procedure was compared with the results of a manual match performed by a trained operator. The errors for the phantom tests were small: average standard deviation of 0.39 mm and 0.26 degrees and absolute mean error of 0.31 mm and 0.2 degrees were obtained. In the clinical cases average standard deviations of 1.32 mm and 0.6 degrees were found. The average absolute mean errors were 1.09 mm and 0.39 degrees. Failures were registered in 2% of the phantom tests and in 3% of the clinical cases. The algorithm execution is approximately 5 s on a 168 MHz Sun Ultra 2 workstation. The automatic analysis tool is considered to be a very useful tool for on‐line setup corrections.
Medical Physics - Tập 27 Số 2 - Trang 321-329 - 2000
Remote control for a stand‐alone freely movable treatment couch with limitation system One of our linear accelerators is equipped with a free‐movable treatment couch. An additional projects was to develop a system that first protects the free‐movable couch against collisions, secondly build a remote control for moving the couch from outside the treatment room and finally implement this remote control/limitation system in an automatic position algorithm using an electronic portal image. The latter has been the subject of an on‐going departmental investigation on intra‐fractional correction of set‐up errors. A few years ago, we developed a limitation system to protect both the table and the accelerator against collisions. In this paper we describe the second part of this project, the remote control system.
Medical Physics - Tập 28 Số 12 - Trang 2518-2521 - 2001
Target margins for random geometrical treatment uncertainties in conformal radiotherapy
Medical Physics - Tập 23 Số 9 - Trang 1537-1545 - 1996
Tomotherapy: A new concept for the delivery of dynamic conformal radiotherapy
Medical Physics - Tập 20 Số 6 - Trang 1709-1719 - 1993
A computerized remote table control for fast on‐line patient repositioning: Implementation and clinical feasibility A computerized remote control for a Siemens ZXT treatment couch was implemented and its characteristics were investigated to establish its feasibility for on‐line setup corrections, using portal imaging. Communication with the table was obtained by connecting it via a serial line to a work station. The treatment couch enables “goto” commands in the three main directions and around the isocenter. The accuracy of the movements after giving such a command was checked and the time for each movement was recorded. First, the movements into a single direction were studied (range of −4 to +4 cm and −4° to +4°). Each command was repeated four times. Second, the table was moved into the three main directions simultaneously. For this experiment a clinically relevant three‐dimensional (3‐D) normal distribution of shifts was used
Medical Physics - Tập 27 Số 2 - Trang 354-358 - 2000
Microprocessor controlled limitation system for a stand‐alone freely movable treatment couch Because of the capability of free movement in the treatment room, we recently introduced a Hercules treatment couch on one of our linear accelerators. One of the advantages of this couch is that it allows for a more flexible way of patient setup and that it can be moved entirely out of the way to enable treatment with a hospital bed. A disadvantage, however, is that the couch can hit a wall or a cover of the accelerator accidentally. A limitation system has been developed to protect both the table and the accelerator against such collisions.
Medical Physics - Tập 25 Số 6 - Trang 897-899 - 1998
A robust coregistration method for <i>in vivo</i> studies using a first generation simultaneous PET/MR scanner Purpose: Hybrid positron emission tomography (PET)/magnetic resonance (MR) imaging systems have recently been built that allow functional and anatomical information obtained from PET and MR to be acquired simultaneously. The authors have developed a robust coregistration scheme for a first generation small animal PET/MR imaging system and illustrated the potential of this system to study intratumoral heterogeneity in a mouse model. Methods: An alignment strategy to fuse simultaneously acquired PET and MR data, using the MR imaging gradient coordinate system as the reference basis, was developed. The fidelity of the alignment was evaluated over multiple study sessions. In order to explore its robustnessin vivo , the alignment strategy was applied to explore the heterogeneity of glucose metabolism in a xenograft tumor model, using Results: The alignment method consistently fused the PET/MR data sets with subvoxel accuracy (registration error Conclusions: The authors present an implementation of an efficient and robust coregistration scheme for multimodal noninvasive imaging using PET and MR. This setup allows time‐sensitive, multimodal studies of physiology to be conducted in an efficient manner.
Medical Physics - Tập 37 Số 5 - Trang 1995-2003 - 2010
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