Abel Cheng

A critical evaluation of the planning target volume for 3-D conformal radiotherapy of prostate cancer

PURPOSE To determine an adequate planning target volume (PTV) margin for three-dimensional conformal radiotherapy (3D CRT) of prostate cancer, the uncertainties in the internal positions of the prostate and seminal vesicles (SV) and in the treatment setups were measured. METHODS AND MATERIALS Weekly computed tomography (CT) scans of the pelvis (n=51) and daily electronic portal images (n=1630) were reviewed for eight patients who received seven-field 3D CRT for prostate cancer. The CT scans were registered in three dimensions to the original planning CT scan using commercially available software to measure the center-of volume (COV) motion of the prostate and SV. The daily portal images were registered to the corresponding simulation films to measure the setup displacements. The standard deviation (SD) of the internal organ motions was added to the SD of the setups in quadrature to determine the total uncertainty. Positive directions were left, anterior, and superior. Rotations necessary to register the CT scans and portal images were minimal and not further analyzed. RESULTS The mean motion for the COV of the prostate+/-the SD was 0+/-0.9 mm in the left-right (LR), 0.5+/-2.6 mm in the anterior-posterior (AP), and 1.5+/-3.9 mm in the superior-inferior (SI) directions. The mean motion for the COV of the SV+/-the SD was 0.3+/-1.7 mm in the LR, 0.7+/-3.8 mm in the AP, and 0.9+/-3.5 mm in the SI directions. For all patients the mean isocenter displacement+/-the SD was 0+/-3.1 mm in the LR, 1.4+/-3.0 mm in the AP, and -0.4+/-2.1 mm in the SI directions. The total uncertainty for the prostate was 3.2 mm, 4.0 mm, and 4.4 mm in the LR, AP, and SI directions, respectively. For the SV, the total uncertainty was 3.5, 4.8, and 4.1 mm in the LR, AP, and SI directions, respectively. CONCLUSIONS PTV margins of 10 to 16 mm are required to encompass all (99%) possible positions of the prostate or SV during 3D CRT. PTV margins of 7 to 11 mm will encompass the measured uncertainties with a 95% probability. PTV margins of 5 mm may not adequately cover the intended volume.

IS WEEKLY PORT FILMING ADEQUATE FOR VERIFYING PATIENT POSITION IN MODERN RADIATION THERAPY

PURPOSE The objective of this study is to use daily electronic portal imaging to evaluate weekly port filming in detecting patient set-up position. METHODS AND MATERIALS A computer-based portal alignment method was used to quantify the field displacements on 191 digitized weekly port films and 848 daily electronic portal images in 21 radiation therapy patients. An electronic portal image data set as a control for actual daily treatment position was used to evaluate weekly port films with respect to same-day field displacement, rate of field placement error detection, and prediction of subsequent daily field displacements. RESULTS The field displacements measured on a port film frequently deviated from the corresponding field displacements on the electronic portal image obtained in the same treatment set-up. A linear regression analysis showed that the curves fitted to the same-day field displacements had slopes that differed significantly from unity (p < 0.001). Overall, the respective frequencies of field placement error, beyond clinical tolerance limits of 5, 7, and 10 mm (corresponding to head and neck, thoracic, and pelvic sites) for port filming and electronic portal imaging were 11% and 14% (p = 0.4) in the X-direction (lateral or anteroposterior) and 24% and 13% (p = .0001) in the Y-direction (caphalad-caudad). When the data were broken down by anatomical region, this discrepancy was found to be mainly due to the differences in the thorax, and head and neck image data sets. For thoracic fields, error in Y-shifts was 28% by port filming, but only 9% by portal imaging (p = 0.01). In the head and neck region, 18% of the port films exceeded tolerance, whereas only 6% of the electronic portal images did (p = 0.0001). Field displacements on the treatment set-ups between the acquisition of port films were not predicted by those films. CONCLUSION There are discrepancies between the field displacements and field placement errors detected by weekly port films and daily electronic portal images. This study suggests that improved methods of treatment verification may be necessary in modern radiation therapy.

A METHOD TO ANALYZE 2-DIMENSIONAL DAILY RADIOTHERAPY PORTAL IMAGES FROM AN ON-LINE FIBER-OPTIC IMAGING SYSTEM

On-line radiotherapy imaging systems allow convenient treatment verification and generate a wealth of data. Quantitative analysis of data will provide important information about the nature of treatment variations. Using an inhouse fiber-optic imaging system to acquire daily portal images for five patients, we have developed a method to analyze the cumulative positional variation of blocks in the 2-dimensional images. For each beam arrangement used to treat a particular patient, a reference portal image was established. All other images for that patient were registered with respect to the anatomical landmarks visible on the reference image. Two-dimensional frequency distributions describing the overlap of the blocks during the course of treatment were then calculated and superimposed on the reference image. Results of the analysis show positional and quantitative information about the daily variation in block placement, and appeared to be site-dependent. Long term verification studies using on-line imaging systems will be important in the understanding of treatment uncertainties.

Systematic verification of a three-dimensional electron beam dose calculation algorithm.

A three-dimensional electron beam dose calculation algorithm implemented on a commercial radiotherapy treatment planning system is described. The calculation is based on the M. D. Anderson Hospital (M.D.A.H.) pencil beam model, which uses the Fermi-Eyges theory of thick-target multiple Coulomb scattering. To establish the calculation algorithms accuracy as well as its limitations, it was systematically and extensively tested and evaluated against a set of benchmark measurements. Various levels of dose and spatial tolerances were used to validate the calculation quantitatively. Results are presented in terms of the percentage of data points meeting a specific tolerance level. The algorithms ability to accurately simulate commonly used clinical setup geometries, including standard or extended SSDs, blocked fields, irregular surfaces, and heterogeneities, is demonstrated. Regions of disagreement between calculations and measurements are also shown. The clinical implication of such disagreements is addressed, and the algorithmic assumptions involved are discussed.

An evaluation of two methods of anatomical alignment of radiotherapy portal images

PURPOSE Two techniques have been developed at our institution to allow anatomical registration of digitized portal images to a simulation film. Accuracy of the portal image alignment methods is tested and single intrauser and multiple interuser variation is examined using each technique. METHODS AND MATERIALS Method one requires the identification of anatomical fiducial points on a simulation image and its corresponding portal image. The parameters required to align the corresponding points are calculated by a least squares fit algorithm. Method two uses an anatomical template generated from the simulation image and superimposing it upon a portal image. The template is then adjusted by a computer mouse to obtain the best subjective anatomical fit on the portal image. Megavoltage portal images of a skull phantom with various known shifts and eight clinical image files were aligned by each method. Each data set was aligned several times by both a single user and multiple users. RESULTS Alignment of the anatomical phantom portal images demonstrates an accuracy of less than 0.8 +/- 0.9 mm and 0.7 +/- 1.0 degrees with either method. As out of plane rotation increased from 0 to 5 degrees, simulating out of plane malpositioning, alignment orthogonal to the plane of rotation worsened to 1.5 +/- 1.1 mm with the point method and 2.4 +/- 1.6 mm with the template method. Alignment parallel to the axis of the gantry rotation was insensitive to this change and remained constant as did the rotational alignment parameters. For the clinical image files the magnitude of variation for a single user is typically less than +/- 1 mm or +/- 1 degree. The magnitude of variation of alignment increased when multiple users aligned the same image files. The variation was dependent upon anatomical site and to a lesser degree the method of alignment used. The root mean square deviation of translational shifts range from +/- 0.68 mm when using the template method in the pelvis to as high as +/- 2.94 mm with the template method to align abdominal portal images. In the thorax and pelvis translational alignments along the horizontal axis were more precise than along the vertical axis. Multiple user variability was in part due to poor image quality, user experience, non rigidity of the anatomical features, and the difficulty in locating an exact point on a continuous anatomical structure. CONCLUSION In well controlled phantom studies both the fiducial point and template method provide similar and adequate results. The phantom studies show that alignment error and variance increase with distortion in anatomical features secondary to out of plane rotations. In clinical situations intrauser variation is small, however, multiple interuser variation is larger. The magnitude of variation is dependent upon the anatomical site aligned.

Prospective clinical evaluation of an electronic portal imaging device

PURPOSE To determine whether the clinical implementation of an electronic portal imaging device can improve the precision of daily external beam radiotherapy. METHODS AND MATERIALS In 1991, an electronic portal imaging device was installed on a dual energy linear accelerator in our clinic. After training the radiotherapy technologists in the acquisition and evaluation of portal images, we performed a randomized study to determine whether online observation, interruption, and intervention would result in more precise daily setup. The patients were randomized to one of two groups: those whose treatments were actively monitored by the radiotherapy technologists and those that were imaged but not monitored. The treating technologists were instructed to correct the following treatment errors: (a) field placement error (FPE) > 1 cm; (b) incorrect block; (c) incorrect collimator setting; (d) absent customized block. Time of treatment delivery was recorded by our patient tracking and billing computers and compared to a matched set of patients not participating in the study. After the patients radiation therapy course was completed, an offline analysis of the patient setup error was planned. RESULTS Thirty-two patients were treated to 34 anatomical sites in this study. In 893 treatment sessions, 1,873 fields were treated (1,089 fields monitored and 794 fields unmonitored). Ninety percent of the treated fields had at least one image stored for offline analysis. Eighty-seven percent of these images were analyzed offline. Of the 1,011 fields imaged in the monitored arm, only 14 (1.4%) had an intervention recorded by the technologist. Despite infrequent online intervention, offline analysis demonstrated that the incidence of FPE > 10 mm in the monitored and unmonitored groups was 56 out of 881 (6.1%) and 95 out of 595 (11.2%), respectively; p < 0.01. A significant reduction in the incidence of FPE > 10 mm was confined to the pelvic fields. The time to treat patients in this study was 10.78 min (monitored) and 10.10 min (unmonitored). Features that were identified that prevented the technologists from recognizing more errors online include poor image quality inherent to the portal imaging device used in this study, artifacts on the portal images related to table supports, and small field size lacking sufficient anatomical detail to detect FPEs. Furthermore, tools to objectively evaluate a portal image for the presence of field placement error were lacking. These include magnification factor corrections between the simulation of portal image, online measurement tools, image enhancement tools, and image registration algorithms. CONCLUSION The use of an electronic portal imaging device in our clinic has been implemented without a significant increase in patient treatment time. Online intervention and correction of patient positioning occurred rarely, despite FPEs of > 10 mm being present in more than 10% of the treated fields. A significant reduction in FPEs exceeding 10 mm was made in the group of patients receiving pelvic radiotherapy. It is likely that this improvement was made secondarily to a decrease in systematic error and not because of online interventions. More significant improvements in portal image quality and the availability of online image registration tools are required before substantial improvements can be made in patient positioning with online portal imaging.

Development of a second-generation fiber-optic on-line image verification system☆

PURPOSE We have previously reported the development of a fiber-optic fluoroscopic system for on-line imaging on radiation therapy machines with beam-stops because of space limitation. While the images were adequate for clinical purposes in most cases, an undesirable grid artifact existed and distracted visualization. The resolving power of the system, limited by the 1.6 mm x 1.6 mm dimension of the input fibers, appeared insufficient in some cases. This work identifies solutions to reduce grid artifact and to improve the resolution of the system. METHODS AND MATERIALS In the clinical system, it was found that the scanning mechanism of the newvicon camera was deflected differently at various gantry positions because of the different orientation of the earths magnetic field. The small image misregistration produced grid artifact during image normalization, particularly near boundaries of the fiber bundles. One approach taken to reduce magnetic field effects was to shield the camera with mu-metal. Alternatively, a charged-coupled-device camera was used instead of the newvicon camera. As for improving spatial resolution, fibers with smaller input dimension were used. A 20 cm x 20 cm high resolution fiber-optic prototype consisting of 250 x 250 fibers, each with an input dimension of 0.8 mm x 0.8 mm was constructed. Its performance was tested using several phantoms studies. RESULTS Both shielding the newvicon camera with mu-metal or replacing it with a charge-coupled-device camera reduced grid artifact. However, optimal shielding could not be made for our clinical system because of the space limitation of its housing. High contrast resolution was improved, the 30% value of the modulation transfer function occurred at 0.3 linepairs per mm for the clinical system and at 0.7 linepairs per mm for the high-resolution prototype. However, because of the larger degree of transmission non-uniformity of the prototype, it was less effective using the current setup in detecting low contrast objects. CONCLUSIONS The results are encouraging and demonstrate successful reduction of grid artifact and improvement of high contrast spatial resolution using the proposed methods. The less effective low contrast detection was related to reduced light collection efficiency due to use of prototype fibers whose productions were not closely monitored. The findings are being considered in our construction of a second generation clinical fiber-optic on-line image verification system.