Biomedical subjects

G J Budgell

Publications and source records attributed to G J Budgell.

7 recordsLinked to original sources

PubMed2000

Verification of dynamic multileaf collimation using an electronic portal imaging device.

High standards of treatment verification are necessary where complex new delivery techniques, such as intensity modulated radiation therapy using dynamic multileaf collimation, are being developed. This paper describes the use of a fluoroscopic electronic portal imaging device (EPID) to provide real-time qualitative verification of leaf position during delivery of a dynamic MLC prescription in addition to off-line quantitative verification. A custom-built circuit triggers the EPID to capture a series of snap-shot images at equally spaced dose points during a dynamic MLC prescription. Real-time verification is achieved by overlaying a template of expected leaf positions onto the images as they are acquired. Quantitative off-line verification is achieved using a maximum gradient edge detection algorithm to measure individual leaf positions for comparison with required leaf positions. Investigations have been undertaken to optimize image acquisition and assess the edge detection algorithm for variations in machine dose rate, leaf velocity and beam attenuation. On-line verification enables the operator to monitor the progress of a dynamic delivery and has been used for independent confirmation of accurate dynamic delivery during intensity modulated treatments. Off-line verification allows measurement of leaf position with a precision of 1 mm although image acquisition times must be less than or equal to 140 ms to ensure coincidence of the maximum gradient in the image with the 50% dose level.

Algorithms↗

PubMed2000

Requirements for leaf position accuracy for dynamic multileaf collimation.

Intensity modulated radiation therapy can be achieved by driving the leaves of a multileaf collimator (MLC) across an x-ray therapy beam. Algorithms to generate the required leaf trajectories assume that the leaf positions are exactly known to the MLC controller. In practice, leaf positions depend upon calibration accuracy and stability and may vary within set tolerances. The purpose of this study was to determine the effects of potential leaf position inaccuracies on intensity modulated beams. Equations are derived which quantify the absolute error in delivered monitor units given a known error in leaf position. The equations have been verified by ionization chamber measurements in dynamically delivered flat fields, comparing deliveries in which known displacements have been applied to the defined leaf positions with deliveries without displacements applied. The equations are then applied to two clinical intensity modulations: an inverse planned prostate field and a breast compensating field. It is shown that leaf position accuracy is more critical for a highly modulated low-dose intensity profile than a moderately modulated high-dose intensity profile. Suggestions are given regarding the implications for quality control of dynamic MLC treatments.

Algorithms↗

PubMed2000

Leaf position verification during dynamic beam delivery: a comparison of three applications using electronic portal imaging.

The use of a dynamic multileaf collimator (MLC) to deliver intensity-modulated beams presents a problem for conventional verification techniques. The use of electronic portal imaging to track MLC leaves during beam delivery has been shown to provide a solution to this problem. An experimental comparison of three different verification systems, each using a different electronic portal imaging technology, is presented. Two of the systems presented are commercially available imagers with in-house modifications, with the third system being an in-house built experimental system. The random and systematic errors present in each of the verifications systems are measured and presented, together with the study of the effects of varying dose rate and leaf speed on verification system performance. The performance of the three systems is demonstrated to be very similar, with an overall accuracy in comparing measured and prescribed collimator trajectories of approximately +/-1.0 mm. Systematic errors in the percentage delivered dose signal provided by the accelerator are significant and must be corrected for good performance of the current systems. It is demonstrated that, with suitable modifications, commercially available portal imaging systems can be used to verify dynamic MLC beam delivery.

Algorithms↗

PubMed1999

Customised compensation using intensity modulated beams delivered by dynamic multileaf collimation.

BACKGROUND AND PURPOSE: This paper describes the development of customised compensation by intensity modulated radiation therapy (IMRT), delivered by dynamic application of a multileaf collimator (MLC), in order to improve dose homogeneity in treatments of the pelvic region. The introduction of this simple IMRT procedure will help facilitate the clinical implementation of more complex 3D conformal therapy techniques. MATERIALS AND METHOD: Computer software is used to generate profiles of the intensity modulated beams which are required to deliver a uniform dose in a plane, passing through the isocentre and normal to the beam axis, under an irregular surface contour. These profiles are then operated on by interpreter software which determines the leaf trajectories that are necessary to deliver these beam profiles using a single, unidirectional sweep of the MLC leaves. A full dose calculation based on the calculated leaf positions is subsequently performed, allowing further fine adjustments to the modulation where required. RESULTS AND CONCLUSION: The compensation procedure has been successfully tested using films placed under a test phantom. The effect of the compensation procedure on dose distributions in the transverse plane has been investigated using an anthropomorphic phantom. Overall dose homogeneity has been improved through the use of customised compensation delivered by dynamic multileaf collimation.

Humans↗

PubMed1999

Temporal resolution requirements for intensity modulated radiation therapy delivered by multileaf collimators.

Intensity modulated radiation therapy may be delivered via dynamic control of a multileaf collimator by defining dynamic leaf trajectories at a series of control points which are spaced at arbitrary intervals throughout an exposure. Leaves move linearly between the positions defined at the control points and hence leaf motion is a linear approximation to the planned trajectories. When many control points are used, very complex modulations can be defined accurately. If too few are used, dose errors are introduced which depend on the complexity of the modulation and the number of control points chosen. Proposed intensity modulations, of varying complexity, have been converted to trajectory plans where the number of equally spaced control points varied between 11 and 99. In each case the fluence map generated by the resulting leaf motions was computed, and this was quantitatively compared with the desired modulation by calculating the root mean square (RMS) error. It is shown that in no case can the RMS error be significantly reduced by increasing the number of control points beyond 50. Guidelines are derived such that compensators and simple modulations may be very accurately delivered with about 20 control points, moderately complex modulations require 30-35 control points and very complex modulations 50 control points. A similar analysis of the step and shoot technique suggests that very accurate deliveries are always achieved with a maximum of 25 equally weighted static fields and reasonable accuracy with 15 fields.

Calibration↗

PubMed1998

Rectangular edge synchronization for intensity modulated radiation therapy with dynamic multileaf collimation.

The implementation of intensity modulated radiotherapy by dynamic multileaf collimator control involves the use of interpreter software which creates leaf trajectory plans for each leaf pair. Interpreter software for use with an Elekta SL15 linear accelerator and dedicated multileaf collimator has been written and tested. In practice the ideal trajectory plans often predict contact between leaves from opposing leaf banks, but this is prohibited by control software on the Elekta system as it could lead to mechanical damage. If the modulation within the geometric limits of a shaped field is not to be compromised then strategies to avoid leaf contact result in additional unwanted doses outside the geometric edge. The magnitude of any such additional dose can be reduced to acceptable levels by a technique which we have called rectangular edge synchronization. The performance of interpreter software which incorporates rectangular edge synchronization has been compared with that of potentially more efficient software which does not. The option containing the rectangular edge synchronization algorithm was shown to work consistently well at high monitor unit rates, and without incurring leaf contacts, under a wide range of test conditions. It therefore provides a sound basis for using intensity modulation to replace mechanical wedges, to simulate customized patient shape compensators, or to implement the results of inverse treatment planning processes that require superimposed intensity modulated beams.

Algorithms↗

PubMed1998

Quality assurance of the dose delivered by small radiation segments.

The use of intensity modulation with multiple static fields has been suggested by many authors as a way to achieve highly conformal fields in radiotherapy. However, quality assurance of linear accelerators is generally done only for beam segments of 100 MU or higher, and by measuring beam profiles once the beam has stabilized. We propose a set of measurements to check the stability of dose delivery in small segments, and present measured data from three radiotherapy centres. The dose delivered per monitor unit, MU, was measured for various numbers of MU segments. The field flatness and symmetry were measured using either photographic films that are subsequently scanned by a densitometer, or by using a diode array. We performed the set of measurements at the three radiotherapy centres on a set of five different Philips SL accelerators with energies of 6 MV, 8 MV, 10 MV and 18 MV. The dose per monitor unit over the range of 1 to 100 MU was found to be accurate to within +/-5% of the nominal dose per monitor unit as defined for the delivery of 100 MU for all the energies. For four out of the five accelerators the dose per monitor unit over the same range was even found to be accurate to within +/-2%. The flatness and symmetry were in some cases found to be larger for small segments by a maximum of 9% of the flatness/symmetry for large segments. The result of this study provides the dosimetric evidence that the delivery of small segment doses as top-up fields for beam intensity modulation is feasible. However, it should be stressed that linear accelerators have different characteristics for the delivery of small segments, hence this type of measurement should be performed for each machine before the delivery of small dose segments is approved. In some cases it may be advisable to use a low pulse repetition frequency (PRF) to obtain more accurate dose delivery of small segments.

Biophysical Phenomena↗