Janos Szanto

A prospective comparison of three systems of patient immobilization for prostate radiotherapy

PURPOSEnThe study compared the setup reliability of 3 patient immobilization systems, a rubber leg cushion, the alpha cradle, and the thermoplastic Hipfix device, in 77 patients with cT1-T3, N0, M0 prostate cancer receiving conformal radiotherapy.nnnMETHODS AND MATERIALSnPort films were analyzed and compared to simulation films to estimate the setup errors in the three coordinate axes (anterior-posterior, cranial-caudal, medial-lateral). A total vector error was calculated from these shifts.nnnRESULTSnThe Hipfix was found significantly superior to the other two devices in reducing mean setup errors in all axes (p < 0.005). The average field-positioning error with the Hipfix ranged from 1.9 mm to 2.6 mm for all axes, whereas the deviation for the other two systems ranged from 2.7 to 3. 4 mm. Errors greater than 10 mm were virtually eliminated with the Hipfix system. There was a reduction in the mean total vector error in the alpha cradle and Hipfix patient cohorts over time, reflecting improved efficacy as a result of experience.nnnCONCLUSIONnThere was a significant difference in the performance of each immobilization device. The Hipfix was consistently more reliable in reducing setup errors than the other devices.

Effects of urethrography on prostate position: considerations for radiotherapy treatment planning of prostate carcinoma

PURPOSEnRetrograde urethrography is commonly used to define the prostate apex at simulation. This study evaluated the hypothesis that urethrography causes prostate displacement, resulting in an error in treatment planning.nnnMETHODS AND MATERIALSnForty-five patients with carcinoma of the prostate were evaluated. Gold seeds were placed in the apex, posterior wall, and base of the gland. In the first 20 patients, the position of the seed-defined apex was compared at simulation (with urethrogram) and on day 1 of treatment (without urethrogram). In the second cohort of 25 patients, the effects of urethrography on prostate position were evaluated directly at simulation by comparing the position of apex pre- and post-urethrography. An analysis was performed to estimate the possible impact of urethrogram-induced prostate motion on target coverage.nnnRESULTSnThe mean superior displacement in the first and second cohort was 5.2 mm and 6.8 mm, respectively (combined mean shift 6.1 mm). With a 10-mm field margin below the tip of the urethrogram cone, 56% of patients in this study would have inadequate planning target volume (PTV) coverage.nnnCONCLUSIONnRetrograde urethrography causes a significant superior shift of the prostate. Strict reliance on urethrography in determining the inferior field margin could result in inadequate treatment.

Development and clinical application of a patient-position monitoring system

We have developed and clinically tested a computer vision system capable of real time monitoring of the position of an oncology (cancer) patient undergoing radiation therapy. The system is able to report variations in patient setup from day to day, as well as patient motion during an individual treatment. The system consists of two CCD cameras mounted in the treatment room and focused on the treatment unit isocenter. The cameras are interfaced to a PC via a two channel video board. Special targets, placed on the patient surface are automatically recognized and extracted by our 3D vision software. The three coordinates of each target are determined using a triangulation algorithm. System accuracy, stability and reproducibility were tested in the laboratory as well as in the radiation therapy room. Beside accuracy, the system must ensure the highest reliability and safety in the actual application environment. In this paper we also report on the results of clinical testing performed on a total of 23 patients having various treatment sites and techniques. The system in its present configuration is capable of measuring multiple targets placed on the patient surface during radiation therapy. In the clinical environment the system has an accuracy and repeatability of better than 0.5 mm in Cartesian space over extended periods (> 1 month). The system can measure and report patient position in less than 5 seconds. Clinically we have found that the system can easily and accurately detect patient motion during treatment as well as variations in patient setup from day to day. A brief description of the system and detailed analysis of its performance in the laboratory and in the clinic are presented.

Asymmetric ARC technique for posterior pharyngeal wall and retropharyngeal space tumors

PURPOSEnTumors of the posterior pharyngeal wall and nasopharyngeal cancer with retropharyngeal extension can partly encircle the cervical vertebrae. Treating the patient within spinal cord tolerance can cause a geographic miss. A simple technique has been developed to avoid this problem.nnnMETHODS AND MATERIALSnThe standard fields for posterior pharyngeal wall and nasopharyngeal tumors are used up to 36-40 Gy. A planning computed tomography (CT) scan is taken during the second or third week of treatment with the patient fitted in a new shell ensuring that the cord is straight and parallel to the treatment couch. The asymmetric arc technique consists of two posterior arcs with closure of one jaw beyond the central axis. Each arc delivers the total dose to each ipsilateral side, while the median region of the U-shaped volume is treated by the summation of both arcs.nnnRESULTSnWe have treated 10 patients using asymmetric arcs in the last 3 years. This technique proved to be a versatile way of treating targets wrapped around the spine. The technique allows better individualization for target volume irregularities than the partial rotation with a central bar.

Using multileaf collimator interleaf leakage to extract absolute spatial information from electronic portal imaging device images

Electronic portal imaging devices (EPIDs) are potentially valuable tools for linear accelerator quality assurance and for measuring and analyzing geometric variations in radiation treatment delivery. Geometric analysis is more robust if referenced against an absolute position such as the isocenter (collimator axis of rotation), allowing the observer to discriminate between various setup errors and jaw or multileaf collimator (MLC) calibration errors. Unfortunately, mechanical instabilities in EPIDs make such analysis difficult. In the present work, we describe how MLC interleaf radiation leakage, hidden in the background of portal images, can be extracted and analyzed to find the field isocenter perpendicular to leaf travel direction. The signal from the interleaf radiation leakage is extracted to provide a precise and accurate determination of the isocenter location in the direction perpendicular to MLC leaf travel. In the direction of leaf travel, the minimization of residuals between planned and measured leaf positions is used to determine the isocenter. This method assumes that leaf positioning errors are randomly distributed. The validity of the method for determining the angular deviation between EPID image grid lines and collimator angle and for determining the known isocenter position is experimentally established. PACs numbers: 87.53.Oq, 87.53.Xd, 87.57.NK

Treatment planning aids in prostate cancer: friend or foe?

BACKGROUNDnRectal barium is commonly used as a treatment planning aid for prostate cancer to delineate the anterior rectal wall. Previous research at the Ottawa Regional Cancer Centre demonstrated that retrograde urethrography results in a systematic shift of the prostate. We postulated that rectal barium could also cause prostate motion.nnnPURPOSEnThe study was designed to evaluate the effects of rectal barium on prostate position.nnnMETHODS AND MATERIALSnThirty patients with cT1-T3 prostate cancer were evaluated. Three fiducial markers were placed in the prostate. During simulation, baseline posterior-anterior and lateral films were taken. Repeat films were taken after rectal barium opacification. The prostate position (identified by the fiducials) relative to bony landmarks was compared before and after rectal barium. Films were analyzed using PIPsPro software.nnnRESULTSnThe rectal barium procedure resulted in a significant displacement of the prostate in the anterior and superior direction. The mean displacement of the prostate measured on the lateral films was 3.8 mm (SD: 4.4 mm) in the superior direction and 3.0 mm (SD: 3.1) in the anterior direction.nnnCONCLUSIONSnRectal barium opacification results in a systematic shift of the prostate. This error could result in a geographic miss of the target; therefore, alternate methods of normal tissue definition should be used.

Dynamic CT Angiography for Cyberknife Radiosurgery Planning of Intracranial Arteriovenous Malformations: A Technical/ Feasibility Report

Abstract Background. Successful radiosurgery for arteriovenous malformations (AVMs) requires accurate nidus delineation in the 3D treatment planning system (TPS). The catheter biplane digital subtraction angiogram (DSA) has traditionally been the gold standard for evaluation of the AVM nidus, but its 2D nature limits its value for contouring and it cannot be imported into the Cyberknife TPS. We describe a technique for acquisition and integration of 3D dynamic CT angiograms (dCTA) into the Cyberknife TPS for intracranial AVMs and review the feasibility of using this technique in the first patient cohort. Patients and methods. Dynamic continuous whole brain CT images were acquired in a Toshiba 320 volume CT scanner with data reconstruction every 0.5 sec. This multi-time-point acquisition enabled us to choose the CT dataset with the clearest nidus without significant enhancement of surrounding blood vessels. This was imported to the Cyberknife TPS and co-registered with planning CT and T2 MRI (2D DSA adjacent for reference). The feasibility of using dCTA was evaluated in the first thirteen patients with outcome evaluation from patient records. Results. dCTA data was accurately co-registered in the Cyberknife TPS and appeared to assist in nidus contouring for all patients. Imaging modalities were complementary. 85% of patients had complete (6/13) or continuing partial nidus obliteration (5/13) at 37 months median follow-up. Conclusions. dCTA is a promising imaging technique that can be successfully imported into the Cyberknife TPS and appears to assist in radiosurgery nidus definition. Further study to validate its role is warranted.

Po‐Poster ‐ 16: Correcting geometric distortion of EPID images

The use of Camera based Electronic Portal Image Devices (EPIDs) for online patient positioning is limited by their performance characteristics including image quality, mechanic stability and both geometric and intensity distortion. Geometric distortion of the EPIDimages arises in the imaging chain from lens or mirror distortions or from misalignment of the mirror. Correcting this geometric distortion requires mapping the EPIDimages into flat Cartesian space. In this work, we describe a simple method to measure this image distortion and to develop the appropriate image‐to‐physical space transforms using the properties of the MLC. An well characterized geometric pattern is created in the image plane by the superposition of two images. The individual images are created having every second MLC leaf pair fully closed while the adjacent pair is fully open. A second image is produced using exactly the same leaf configuration, but with the collimator rotated 90 degrees. The composite image is the sum of the two images and produces a well‐defined geometric pattern. A threshold function is applied to the composite image to produce a binary grid and the centroid of each dark/light square was determined. An affine transformation matrix is then generated to map the image of grid back to the virtual grid (physical space), thus correcting the spatial distortion. The mapping function was tested at various gantry angles and demonstrated to be robust.

Sci‐PM Fri ‐ 09: Image analysis of inter‐leaf radiation leakage, a new approach to the correction of EPID mechanic inconsistencies

Electronic portal imaging devices are valuable for analyzingpatient treatment data for geometric errors. The analysis can be more robust if an absolute reference position such as the isocentre (collimator axis of rotation) is used, allowing discrimination between setup errors and Jaw/MLC calibration errors. Conventionally, investigators have extracted field edges through global thresholding, and compared the treated to the planned field placement using techniques such as the method of moments and normalized cross‐correlation. The accuracy of these methods depends on the accuracy and consistency of the MLC and Jaw calibration as well as on EPID performance such as intensity uniformity. In this work we describe how the MLC inter‐leaf radiation leakage, hidden in the background of portal images, can be extracted and analyzed to find the field isocentre. The peak locations of inter‐leaf radiation leakage in a portal image are extracted providing a very precise and accurate determination of the isocentre location in the direction perpendicular to the MLC leaf travel. Clearly orthogonal portal images can provide a very accurate determination of beam isocentre in both directions. The orientation of the interleaf leakage can also be used to determine mechanical inconsistencies in the EPID structure as a function of gantry angle. The image analysis of the inter‐leaf radiation leakage for determining imager translation and orientation is described.

The development and clinical application of a patient position monitoring system

We have developed and clinically tested a computer vision system capable of real time monitoring of the position of an oncology (cancer) patient undergoing radiation therapy. The system is able to report variations in patient setup from day to day, as well as patient motion during an individual treatment. The system consists of two CCD cameras mounted in the treatment room and focused on the treatment unit isocentre. The cameras are interfaced to a PC via a two channel video board. Special targets, placed on the patient surface are automatically recognized and extracted by our 3-D vision software. The three coordinates of each target are determined using a triangulation algorithm. System accuracy, stability and reproducibility were tested in the laboratory as well as in the radiation therapy room. Beside accuracy, the system must ensure the highest reliability and safety in the actual application environment. In this paper we also report on the results of clinical testing performed on a total of 23 patients having various treatment sites and techniques. The system in its present configuration is capable of measuring multiple targets placed on the patient surface during radiation therapy. In the clinical environment the system has an accuracy and repeatability of better than 0.5 mm in Cartesian space over extended periods (> 1 month). The system can measure and report patient position in less than 5 seconds. Clinically we have found that the system can easily and accurately detect patient motion during treatment as well as variations in patient setup from day to day. A brief description of the system and detailed analysis of its performance in the laboratory and in the clinic are presented.