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P M Evans

Publications and source records attributed to P M Evans.

At least 19 recordsLinked to original sources
PubMed2003

Polymer gel measurement of dose homogeneity in the breast: comparing MLC intensity modulation with standard wedged delivery.

Polymer gel dosimetry has been used to measure the radiotherapy dose homogeneity in a breast phantom for two different treatment methods. The first 'standard' method uses two tangential wedged fields while the second method has three static fields shaped by multileaf collimators (MLCs) in addition to the standard wedged fields to create intensity modulated fields. Gel dose distributions from the multileaf modulation treatment show an improved dose uniformity in comparison to the standard treatment with a decreased volume receiving doses over 105%.

Breast Neoplasms↗
PubMed2003

Combining dosimetry for targeted radionuclide and external beam therapies using the biologically effective dose.

It is not uncommon for a patient to receive both external beam and targeted radionuclide therapy during the course of a cancer treatment. The total dose received by the tumor and by normal tissues will therefore be subject to the contributions of both treatment modalities. However, the two treatments are generally applied independently of one another, with little attention paid to the combined effect. With the availability of patient-specific three-dimensional dosimetry for radionuclide therapies, it is pertinent now to consider the combined effect of the two treatments, and to investigate how dosimetry for this situation may be carried out. Methodology has been developed to allow a combination of dose information from the two types of therapy. The biologically effective dose (BED) has been employed to address the issue of inequivalence of biological effect of the two therapies. Dose distributions have been represented as distributions of BED, and the net effect resulting from the combination of these two therapies demonstrated through a combination of BED maps. Examples are presented of cases in which this analysis of a combined therapy provides a more favorable treatment than either therapy alone. For one patient the ratio of the mean spinal cord dose to the mean CTV dose was calculated for both an external beam therapy alone and for a combined therapy and was found to be 0.40 and 0.16, respectively.

Humans↗
PubMed2002

Optimization of accelerator target and detector for portal imaging using Monte Carlo simulation and experiment.

Megavoltage portal images suffer from poor quality compared to those produced with kilovoltage x-rays. Several authors have shown that the image quality can be improved by modifying the linear accelerator to generate more low-energy photons. This work addresses the problem of using Monte Carlo simulation and experiment to optimize the beam and detector combination to maximize image quality for a given patient thickness. A simple model of the whole imaging chain was developed for investigation of the effect of the target parameters on the quality of the image. The optimum targets (6 mm thick aluminium and 1.6 mm copper) were installed in an Elekta SL25 accelerator. The first beam will be referred to as A16 and the second as Cu1.6. A tissue-equivalent contrast phantom was imaged with the 6 MV standard photon beam and the experimental beams with standard radiotherapy and mammography film/screen systems. The arrangement with a thin Al target/mammography system improved the contrast from 1.4 cm bone in 5 cm water to 19% compared with 2% for the standard arrangement of a thick, high-Z target/radiotherapy verification system. The linac/phantom/detector system was simulated with the BEAM/EGS4 Monte Carlo code. Contrast calculated from the predicted images was in good agreement with the experiment (to within 2.5%). The use of MC techniques to predict images accurately, taking into account the whole imaging system, is a powerful new method for portal imaging system design optimization.

Bone and Bones↗
PubMed2002

An optimization algorithm that incorporates IMRT delivery constraints.

An intensity-modulated beam optimization algorithm is presented which incorporates the delivery constraints into the optimization cycle. The optimization algorithm is based on the quasi-Newton method of iteratively solving minimization problems. The developed algorithm iteratively corrects the incident, pencil-beam-like, fluence to incorporate the delivery constraints. In the present study, the goal of the optimization algorithm is to achieve the best deliverable radiotherapy plan, subject to the constraints of the delivery technique described by a leaf-sequencing algorithm being applied concurrently. In general, if they are applied after, rather than during, the optimization cycle, the delivery constraints associated with the IMRT technique can produce local variations up to 6% in the 'optimized' dose (i.e., distribution without applied constraints) and reduce the degree of conformity, of the dose, to the PTV region. The optimization method has been applied to three IMRT delivery techniques: dynamic multileaf (DMLC), multiple-static-field (MSF) and slice-by-slice tomotherapy (NOMOS MIMiC). The beam profiles were generated for a prostate tumour with organs at risk being the rectum, bladder and femoral heads. The optimization method described was shown to generate optimum and deliverable IMRT plans for these three delivery techniques. In the case of the DMLC and MSF the optimization converged within 3-5 iterations to a mean PTV dose of 69.60 +/- 1.34 Gy and 69.71 +/- 1.34 Gy, respectively, while for NOMOS MIMiC approximately 10 iterations were needed to obtain 69.68 +/- 1.55 Gy. In addition to this, the IMRT optimization also yielded optimum fluence profiles when clustering was performed concurrently with the leaf-sequencer. An optimum between 8 and 15 clusters of equal fluence 'intensity' was shown to establish the best compromise between the number of fluence levels and the PTV dose coverage.

Algorithms↗
PubMed2002

Dose-position and dose-volume histogram analysis of standard wedged and intensity modulated treatments in breast radiotherapy.

The aim of this work was to evaluate the positional distribution of dose in a concise manner and to analyse dose-histogram results in tangential breast radiotherapy in 300 patients, randomized to standard wedged or intensity modulated radiotherapy (IMRT), for future correlation with clinical outcome data. A simple method for analysing the dose-position relationship in the treatment volume was used to compare the spatial distribution of dose in patients. The breast was divided into equal thirds (upper, middle and lower) and dose was assessed using three dose bands; 95-105%, >105-110% and >110% of the prescription dose. The effect of using IMRT on the dosimetry was assessed from dose-volume histogram data using the following parameters: percentage of the target volume receiving a dose less than 95%, greater than 105%, either less than 95% or greater than 105% of that prescribed; the mean dose; and the maximum dose. Doses greater than 105% were predominantly in the upper and lower regions of the breast in the standard wedged treatment. 96% of these patients received doses greater than 105% in the upper region of the breast and 70% received doses greater than 105% in the lower breast. Only 4% of patients allocated IMRT received doses greater than 105% in either region. Analysis of dose-volume histogram data showed that IMRT reduced the volume receiving a dose greater than 105% by a mean of 10.7% (p= or <0.001); the mean change in the volume receiving a dose less than 95% was 0.2% (p=0.63). Average mean plan dose was 101.6% for standard treatment and 99.6% for IMRT (p<0.001 for each compared with 100.0% ideal). The mean value of maximum dose was reduced from 111% to 106% in the group of patients randomized to IMRT. A simple method for describing the relationship between dose and position in the breast, which is helpful for the effective correlation of dosimetry and clinical effects, is reported. Further, application of IMRT to the tangential field irradiation of the breast has been demonstrated to reduce high dose regions in both volume and dose level without compromising either minimum dose coverage or mean dose delivered to the breast.

Breast Neoplasms↗
PubMed2001

The dosimetric consequences of inter-fractional patient movement on three classes of intensity-modulated delivery techniques in breast radiotherapy.

BACKGROUND AND PURPOSE: A comparison between three classes of intensity-modulated delivery techniques was undertaken to examine the dosimetric consequences of using a multileaf collimator (MLC) reshaped on each imaged fraction as opposed to compensators designed on the first day of treatment potentially giving a treatment technique whose accuracy is thus degraded by movement. MATERIALS AND METHODS: The effects of inter-fractional patient movement for a cohort of six breast patients were studied. Five treatment techniques were evaluated, two using a compensator, two using multiple static fields (MSF) and one using a dynamic multileaf collimator (DMLC). The compensated techniques consisted of (i) the use of compensators designed on day 1 only and used each fraction thereafter and (ii) the use of a compensator redesigned for each imaged fraction. The two MSF techniques were (i) a four-field-component design and (ii) a method where the fluence interval between the MLC field components was set so they were equivalent to the compensator ('quantized' MSF-MLC). The final technique investigated was the DMLC. Plans were produced for each of the five methods and a paired t-test was used to assess the reduction in the breast volume outside the dose range 95-105% between sets of pairs of techniques. An on-line correction strategy was simulated to determine the number of treatments that required intervention. The action levels were calculated using the difference between the volume outside the dose range 95-105% calculated for treatments where the DMLC was designed on day 1 only and for each imaged fraction. Differences of greater than 2%, greater than 5% and greater than 10% were investigated. RESULTS: Thirty-five plans were evaluated for each technique. Results showed that a statistically significant mean reduction in the volume of the breast outside the dose range 95-105% could be achieved if the compensators were designed on each imaged fraction rather than on day 1 only (P=0.0045). When the comparison was made between the 'quantized' MSF-MLC and the technique where the compensators were designed on day 1 only, a statistically significant mean reduction in the volume of the breast tissue outside the dose range 95-105% was not achieved (P=0.21). Comparison of the DMLC technique to the technique where the compensators were designed on day 1 only results in a statistically significant mean reduction in the volume outside the dose range 95-105% (P=0.024). This corresponds to a mean reduction in the volume outside 95-105% dose of 1.94%. The 2% action level showed the greatest reduction in the volume outside 95-105% dose and intervention was only required in approximately one-third of the treatments investigated. CONCLUSIONS: Redesigning MSFs for each imaged fraction did not provide a statistically significant mean reduction in the volume outside the dose range 95-105%. However, using the DMLC technique creates a statistically significant mean reduction in the volume outside the dose range 95-105%.

Breast Neoplasms↗
PubMed2001

Analysis of the effects of the delivery technique on an IMRT plan: comparison for multiple static field, dynamic and NOMOS MIMiC collimation.

The process of delivering an IMRT treatment may involve various beam-modifying techniques such as multileaf collimators (MLCs), the NOMOS MIMiC, blocks, wedges, etc. In the case of the MLC, the spatial/temporal variation of the position of the leaves and diaphragms in the beam allows the delivery of modulated beam profiles either by the multiple-static-field (MSF) method or by the dynamic multileaf collimator (DMLC) method. The constraints associated with the IMRT delivery technique are usually neglected in the process of obtaining the 'optimal' inverse treatment plan. Consequently, dose optimization may be significantly reduced when the 'optimal' beam profiles are converted to leaf/diaphragm positions via a leaf-sequencing interpreter. The paper presented here assesses the effects on the optimum treatment plan of the following leaf-sequencing algorithms: MSF, DMLC and NOMOS MIMiC. The results obtained suggest that the delivery of an 'optimum' plan produces an overdosage of the PTV region due to various factors such as leaf/diaphragm transmission effects, head-scatter and phantom-scatter contributions. The overdosage observed for a cohort of ten patients was 2.5, 3.7 and 5.7%, respectively, for the DMLC, MSF and NOMOS MIMiC, after normalizing the delivered fluence to account for IMRT effects (using the method of Convery et al (Convery D J, Cogrove V P and Webb S 2000 Proc. 13th Int. Conf. on Computers in Radiotherapy (Heidelberg, 2000)) such as to obtain 70 Gy at the isocentre. The IMRT techniques DMLC, MIMiC and MSF were compared for the organs at risk: rectum, bladder, and left and right femoral heads.

Algorithms↗
PubMed2001

A simulation of the effects of set-up error and changes in breast volume on conventional and intensity-modulated treatments in breast radiotherapy.

The effect of interfractional patient movement on dosimetry has been investigated for breast radiotherapy. Errors in patient set-up and changes in breast volume were simulated individually to determine how each contributes to the total dosimetric error. Two treatment techniques were investigated: a conventional treatment and an intensity-modulated treatment delivered using compensators. Six patients were investigated and anterior-posterior (AP) and superior-inferior (SI) displacements were simulated by displacing the isocentre in both directions by 2, 5 and 10 mm. A model of the breast was developed from the six patients to simulate changes in breast volume. In this model, the breast was described as a set of semi-ellipses. The volume of the breast was changed by varying the magnitude of the semi-major and semi-minor axes. Anisotropic changes in breast volume were also investigated. The dosimetric error was evaluated for each dose plan by calculating the volume outside the 95-105% dose range resulting from the simulations. A number of parameters describing the size and shape of the breast were also investigated to determine whether a susceptibility of outline sets to interfractional patient movement could be predicted. A parameter describing the increase in the breast volume outside the 95-105% dose range was calculated for AP a

Breast↗
PubMed2001

An intercomparison of IMRT delivery techniques: a case study for breast treatment.

Intensity-modulated radiotherapy beams can be delivered using a multileaf collimator by one of two methods: either by superposition of a series of multiple-static fields, or by moving the collimators while the beam is on to produce 'dynamically' modulated beams. The leaf trajectories in this dynamic mode are given by a series of linear steps between control points defining each collimator position at known intervals throughout an exposure. The complexity of the resulting modulation is limited in the first case by the number of fields superposed and in the second case by the number of control points defined. Results are presented for an experimental study that investigates the effect of changing both the number of fields for the multiple-static technique, and the number of control points for a dynamic 'close-in' technique. All deliveries studied are clinical intensity-modulated breast fields. The effect of using a universal wedge in conjunction with the multileaf collimator is also studied, together with a comparison of the relative efficiency, time taken and the absolute dosimetric accuracy of the various delivery options. It is shown that all delivery techniques produce equivalent dose distributions when using 15 control points, with 10 control points being sufficient to produce an adequate breast compensator distribution. Except for the case of a four-control-point dynamic delivery, the universal wedge makes no significant difference to the dose distribution. However, it makes the delivery less efficient. The close-in interpreter consistently produces deliveries that are more efficient than the more conventional sliding-window technique and faster than the multiple-static-field technique. Finally the close-in technique is compared to the more 'standard' leaf-sweep technique and shown to be equivalent.

Breast Neoplasms↗
PubMed2001

Sampling considerations for intensity modulated radiotherapy verification using electronic portal imaging.

A model has been developed to describe the sampling process that occurs when intensity modulated radiotherapy treatments (delivered with a multileaf collimator) are imaged with an electronic portal imaging device that acquires a set of frames with a finite dead-time between them. The effects of the imaging duty cycle and frame rate on the accuracy of dosimetric verification have been studied. A frame interval of 1 s with 25%, 50% and 75% duty cycle, and a 50% duty cycle with frame intervals of 1, 2, 4, 8, and 16 s have been studied for a smoothly varying hemispherical intensity profile, and a 50% duty cycle with frame intervals of 1, 2, 4, and 8 s for a pixellated distribution. In addition an intensity modulated beam for breast radiotherapy has been modeled and imaged for 0.33 s frame time and 1, 2, and 3 s frame separation. The results show that under sparse temporal sampling conditions, errors of the order of 10% may ensue and occur with an oscillatory pattern. For the beams studied, imaging with a 1 or 2 s frame interval resulted in small errors at the 1%-2% level, for all duty cycles shown.

Breast↗
PubMed2000

Evaluation of compensation in breast radiotherapy: a planning study using multiple static fields.

PURPOSE: A method that uses electronic portal imaging to design intensity-modulated beams for compensation in breast radiotherapy was implemented using multiple static fields in a planning study. We present the results of the study to verify the algorithm, and to assess improvements to the dosimetry. METHODS AND MATERIALS: Fourteen patients were imaged with computed tomography (CT) and on a treatment unit using an electronic portal imager. The portal imaging data were used to design intensity-modulated beams to give an ideal dose distribution in the breast. These beams were implemented as multiple static fields added to standard wedged tangential fields. Planning of these treatments was performed on a commercial treatment planning system (Target 2, IGE Medical Systems, Slough, U.K.) using the CT data for each patient. Dose-volume histogram (DVH) analysis of the plans with and without multileaf collimator (MLC) compensation was carried out. This work has been used as the basis for a randomized clinical trial investigating whether improvements in dosimetry are correlated with the reduction of long-term side effects from breast radiotherapy. RESULTS: The planning analysis showed a mean increase in target volume receiving 95-105% of prescribed dose of 7.5% (range -0.8% to 15.9%) when additional MLC compensation was applied. There was no change to the minimum dose for all 14 patient data sets. The change in the volume of breast tissue receiving over 105% of prescribed dose, when applying MLC compensation, was between -1.4% and 11.9%, with positive numbers indicating an improvement. These effects showed a correlation with breast size; the larger the breast the greater the amount of improvement. CONCLUSIONS: The method for designing compensation for breast treatments using an electronic portal imager has been verified using planning on CT data for 14 patients. An improvement was seen in planning when applying MLC compensation and this effect was greater the larger the breast size.

Algorithms↗
PubMed2000

The delivery of intensity modulated radiotherapy to the breast using multiple static fields.

BACKGROUND AND PURPOSE: To develop a method of using a multileaf collimator (MLC) to deliver intensity modulated radiotherapy (IMRT) for tangential breast fields, using an MLC to deliver a set of multiple static fields (MSFs). MATERIALS AND METHODS: An electronic portal imaging device (EPID) is used to obtain thickness maps of medial and lateral tangential breast fields. From these IMRT deliveries are designed to minimize the volume of breast above 105% of prescribed dose. The deliveries are universally-wedged beams augmented with a set of low dose shaped irradiations. Dosimetric and planning QA of this method has been compared with the standard, wedged treatment and the corresponding treatment using physical compensators. Several options for delivering the MSF treatment are presented. RESULTS: The MSF technique was found to be superior to the standard technique (P value=0.002) and comparable with the compensated technique. Both IMRT methods reduced the volume of breast above 105% dose from a mean value of 12.0% of the total breast volume to approximately 2.8% of the total breast volume. CONCLUSIONS: This MSF method may be used to reduce the high dose volume in tangential breast irradiation significantly. This may have consequences for long-term side effects, particularly cosmesis.

Breast Neoplasms↗
PubMed2000

The dosimetric consequences of inter-fractional patient movement on conventional and intensity-modulated breast radiotherapy treatments.

BACKGROUND AND PURPOSE: A method has been developed to enable a comparison to be made between the effects of movement on conventional tangential breast treatments and intensity-modulated treatments delivered using compensators. MATERIALS AND METHODS: The effects of set-up error and organ motion were studied for a set of six patients. Images were taken of these patients over the course of their treatment and conventional wedged and compensated treatment plans were designed for each. Dose-volume statistics were used to evaluate each of the treatment plans by examining the volume outside the dose range 95-105%. To assess the effects of movement alone, the volume change from day 1 was also calculated. RESULTS: Thirty-six estimated CT-sets were available for evaluation. Measurements of breast volume showed the volume to increase to a peak between fraction 4 and 8 and then decrease back below the initial volume. The standard treatment was found to yield 29/36 plans (81%) with greater than 5% volume outside the dose range 95-105%. For the compensated plans this dropped to 11/36 plans (31%). The analysis of the volume changes from day 1 showed that for both standard and compensated treatments 7/30 plans (23%) had an increase in volume outside the dose range 95-105% of greater than 5% of the total planning target volume. CONCLUSIONS: The compensated treatment is more susceptible to patient movement. However, the actual volume of tissue outside 95-105% dose is less when compared to standard treatment implying the compensated treatment is still superior.

Artifacts↗
PubMed2000

IMRT verification with a camera-based electronic portal imaging system.

An evaluation of the capabilities of a commercially available camera-based electronic portal imaging system for intensity-modulated radiotherapy verification is presented. Two modifications to the system are demonstrated which use a novel method to tag each image acquired with the delivered dose measured by the linac monitor chamber and reduce optical cross-talk in the imager. A detailed performance assessment is presented, including measurements of the optical decay characteristics of the system. The overall geometric accuracy of the system is determined to be +/-2.0 mm, with a dosimetric accuracy of +/-1.25 MU. Finally a clinical breast IMRT treatment, delivered by dynamic multileaf collimation, is successfully verified both by tracking the position of each leaf during beam delivery and recording the integrated intensity observed over the entire beam.

Breast Neoplasms↗
PubMed2000

A method of improving the spatial resolution of treatments that involve a multileaf collimator.

In this paper we present a novel method of reducing the dosimetric effects of the finite leaf width of a multileaf collimator (MLC) in conformal and intensity modulated radiotherapy (IMRT). This is achieved by decomposing the required high-resolution fluence distribution into two orthogonal components, which are delivered with two leaf sweeps with head-twists differing by 90 degrees. Before the decomposition stage, a filter is applied to the required beam to force it to have a constant gradient in the two delivery directions. The component deliveries were found to be very spiky in nature, resulting in very inefficient delivery with the scanning leaves of our MLC. This method was evaluated using film dosimetry of four idealized beams: a 45 degree edge, a circle, a hemispherical intensity modulated beam (IMB) and a sine-like IMB. The measurements showed that this method had significantly reduced the effects of the 1 cm leaf width of our MLC at the 50% isodose level, but there was significant overdosage at the edge of the field and immediately inside the held edge. This method shows promise but further work is required before it may find clinical utility.

Algorithms↗