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A L Boyer

Publications and source records attributed to A L Boyer.

At least 37 records · Page 2Linked to original sources
PubMed1999

Dosimetric verification of a commercial inverse treatment planning system.

A commercial three-dimensional (3D) inverse treatment planning system, Corvus (Nomos Corporation, Sewickley, PA), was recently made available. This paper reports our preliminary results and experience with commissioning this system for clinical implementation. This system uses a simulated annealing inverse planning algorithm to calculate intensity-modulated fields. The intensity-modulated fields are divided into beam profiles that can be delivered by means of a sequence of leaf settings by a multileaf collimator (MLC). The treatments are delivered using a computer-controlled MLC. To test the dose calculation algorithm used by the Corvus software, the dose distributions for single rectangularly shaped fields were compared with water phantom scan data. The dose distributions predicted to be delivered by multiple fields were measured using an ion chamber that could be positioned in a rotatable cylindrical water phantom. Integrated charge collected by the ion chamber was used to check the absolute dose of single- and multifield intensity modulated treatments at various spatial points. The measured and predicted doses were found to agree to within 4% at all measurement points. Another set of measurements used a cubic polystyrene phantom with radiographic film to record the radiation dose distribution. The films were calibrated and scanned to yield two-dimensional isodose distributions. Finally, a beam imaging system (BIS) was used to measure the intensity-modulated x-ray beam patterns in the beam's-eye view. The BIS-measured images were then compared with a theoretical calculation based on the MLC leaf sequence files to verify that the treatment would be executed accurately and without machine faults. Excellent correlation (correlation coefficients > or = 0.96) was found for all cases. Treatment plans generated using intensity-modulated beams appear to be suitable for treatment of irregularly shaped tumours adjacent to critical structures. The results indicated that the system has potential for clinical radiation treatment planning and delivery and may in the future reduce treatment complexity.

Algorithms↗
PubMed1999

A comparison of Monte Carlo and analytic first scatter dose spread arrays.

We compare first scattered point dose spread arrays generated by Monte Carlo and an analytic method. The analytic method models energy deposition using Klein-Nishina cross sections for Compton scatter and approximations for electron transport. Assumptions in the analytic method are shown to be valid within a region of the point dose spread kernel in which meaningful comparisons can be made. Differences between the models are less than 10% for the forward scatter directions for radii greater than the electron range associated with the first scattered Compton photon. Differences in the backscatter region are discussed and indicate that the analytic model is useful for identifying large errors that might be present in numerically generated first scatter point dose spread arrays. The analytic method is simple and useful for validating first scatter kernels.

Monte Carlo Method↗
PubMed1999

Clinical implementation of a Monte Carlo treatment planning system.

The purpose of this study was to implement the Monte Carlo method for clinical radiotherapy dose calculations. We used the EGS4/BEAM code to obtain the phase-space data for 6-20 MeV electron beams and 4, 6, and 15 MV photon beams for Varian Clinac 1800, 2100C, and 2300CD accelerators. A multiple-source model was used to reconstruct the phase-space data for both electron and photon beams, which retained the accuracy of the Monte Carlo beam data. The multiple-source model reduced the phase-space data storage requirement by a factor of 1000 and the accelerator simulation time by a factor of 10 or more. Agreement within 2% was achieved between the Monte Carlo calculations and measurements of the dose distributions in homogeneous and heterogeneous phantoms for various field sizes, source-surface distances, and beam modulations. The Monte Carlo calculated electron output factors were within 2% of the measured values for various treatment fields while the heterogeneity correction factors for various lung and bone phantoms were within 1% for photon beams and within 2% for electron beams. The EGS4/DOSXYZ Monte Carlo code was used for phantom and patient dose calculations. The results were compared to the dose distributions produced by a conventional treatment planning system and an intensity-modulated radiotherapy inverse-planning system. Significant differences (>5% in dose and >5 mm shift in isodose lines) were found between Monte Carlo calculations and the analytical calculations implemented in the commercial systems. Treatment sites showing the largest dose differences were for head and neck, lung, and breast cases.

Algorithms↗
PubMed1999

Estimation theory and model parameter selection for therapeutic treatment plan optimization.

Treatment optimization is usually formulated as an inverse problem, which starts with a prescribed dose distribution and obtains an optimized solution under the guidance of an objective function. The solution is a compromise between the conflicting requirements of the target and sensitive structures. In this paper, the treatment plan optimization is formulated as an estimation problem of a discrete and possibly nonconvex system. The concept of preference function is introduced. Instead of prescribing a dose to a structure (or a set of voxels), the approach prioritizes the doses with different preference levels and reduces the problem into selecting a solution with a suitable estimator. The preference function provides a foundation for statistical analysis of the system and allows us to apply various techniques developed in statistical analysis to plan optimization. It is shown that an optimization based on a quadratic objective function is a special case of the formalism. A general two-step method for using a computer to determine the values of the model parameters is proposed. The approach provides an efficient way to include prior knowledge into the optimization process. The method is illustrated using a simplified two-pixel system as well as two clinical cases. The generality of the approach, coupled with promising demonstrations, indicates that the method has broad implications for radiotherapy treatment plan optimization.

Algorithms↗
PubMed1999

Matching photon and electron fields with dynamic intensity modulation.

A technique was developed to reduce the size and magnitude of the hot and cold spots in the abutting regions of photon and electron fields. The photon and electron fields were set up such that the photon field extended approximately 2 cm into the electron field in the abutting region. The region of the photon beam that overlapped the electron field was modulated using a multileaf collimator, effectively broadening the photon penumbra to make it complimentary to the electron penumbra. The computer calculations were verified using film measurements for abutting a 6 MV photon beam with a 9 MeV electron beam. A uniform dose was achieved at a prespecified depth of 2 cm, and dose uniformity was improved at the specified depth and beyond compared with unmodulated photon beams. A slight increase in dose inhomogeneity was seen at shallower depths. The overall areas of the hot and cold spots were significantly reduced. The technique also reduced the sensitivity of dose homogeneity to setup errors such that the magnitudes of the hot and cold spots were about half of those produced with unmodulated photon beam when an overlap or gap of 4 mm was introduced. The technique was applied to the treatment of a head and neck cancer and a lymphoma involving the right pleura with markedly reduced dose inhomogeneity in the abutting regions.

Computer Simulation↗
PubMed1998

Using static MLC fields to replace partial transmission cerrobend blocks in treatment planning of rectal carcinoma cases.

A treatment planning technique has been developed for using static multileaf collimators to replace partial transmission blocks for treating rectal or cervix carcinoma. The static MLC fields were used to replace the partial transmission block of the anterior-posterior pelvis field for the so-called "thunderbird" technique. Treatment plans were developed and evaluated on a commercial three-dimensional treatment planning system (FOCUS, Computerized, Medical Systems, St. Louis, MO). The result of the treatment plan comparison indicates that the static MLC fields are capable of achieving the same target, inguinal and pelvic dose distribution as the partial transmission cerrobend blocked fields. The MLC fields are easy to modify particularly for the match line adjustments. In conclusion, it is efficient and effective to use static MLC fields to replace partial transmission blocks in the "thunderbird" technique for treating rectal or cervix carcinoma.

Humans↗
PubMed1998

Monte Carlo calculations of electron beam output factors for a medical linear accelerator.

The purpose of this study was to investigate the application of the Monte Carlo technique to the calculation and analysis of output factors for electron beams used in radiotherapy. The code EGS4/BEAM was used to obtain phase-space files for 6, 12 and 20 MeV clinical electron beams from a scattering-foil linac (Varian Clinac 2100C) for a clinically representative range of applicator and square or rectangular insert combinations. The source-to-surface distance used was 100 cm. The field sizes ranged from 1 x 1 cm2 to 20 x 20 cm2. These phase-space files were analysed to study the intrinsic beam characteristics and used as source input for relative dose and output factor computations in homogeneous water phantoms using the code EGS4/DOSXYZ. The calculated relative central-axis depth-dose and transverse dose profiles at various depths of clinical interest agreed with the corresponding measured dose profiles to within 2% of the maximum dose. Calculated output factors for the fields studied agreed with measured output factors to about 2%. This demonstrated that for the Varian Clinac 2100C linear accelerator, electron beam dose calculations in homogeneous water phantoms can be performed accurately at the 2% level using Monte Carlo simulations.

Computer Simulation↗
PubMed1998

Application of a video-optical beam imaging system for quality assurance of medical accelerators.

Method validation techniques were developed and experiments were carried out using a beam imaging system (BIS, Wellhöfer Dosimetrie, Schwarzenbruck, Germany) for routine quality assurance of medical accelerators. The routine quality assurance tasks include x-ray beam flatness and symmetry check, light/radiation field congruence test, beam energy constancy for electrons and mechanical checks for couch and collimator rotations. Comparisons were made between the BIS application and conventional quality assurance methods that use radiographic films or detector arrays. In this work, we have demonstrated efficiency and accuracy of the BIS to perform some of the routine quality assurance tasks for medical linear accelerators.

Electrons↗
PubMed1998

An optimized leaf-setting algorithm for beam intensity modulation using dynamic multileaf collimators.

A leaf-setting algorithm is developed for generating arbitrary beam intensity profiles in discrete levels using dynamic multileaf collimators (DMLCs). The algorithm starts with the algebraic expression for the area under the beam profile. It is shown that the coefficients in this expression can be transformed into the specifications for the leaf-setting sequence. It is proven that the algorithm optimizes beam delivery time and total monitor units for the DMLC leaf setting for intensity modulated radiotherapy (IMRT). The algorithm is demonstrated to be applicable to both the 'step-and-shoot' and 'dynamic' type of beam delivery. The graphical interpretation and numerical implementation scheme of the algorithm is illustrated using a simplified example.

Algorithms↗
PubMed1998

Fast iterative algorithms for three-dimensional inverse treatment planning.

Three types of iterative algorithms, algebraic inverse treatment planning (AITP), simultaneous iterative inverse treatment planning (SIITP), and iterative least-square inverse treatment planning (ILSITP), differentiated according to their updating sequences, were generalized to three dimension with true beam geometry and dose model. A rapid ray-tracing approach was developed to optimize the primary beam components. Instead of recalculating the dose matrix at each iteration, the dose distribution was generated by scaling up or down the dose matrix elements of the previous iteration. This significantly increased the calculation speed. The iterative algorithms started with an initial intensity profile for each beam, specified by a two-dimensional pixel beam map of M elements. The calculation volume was divided into N voxels, and the calculation was done by repeatedly comparing the calculated and desired doses and adjusting the values of the beam map elements to minimize an objective function. In AITP, the iteration is performed voxel by voxel. For each voxel, the dose discrepancy was evaluated and the contributing pencil beams were updated. In ILSITP and SIITP, the iteration proceeded pencil beam by pencil beam instead of voxel by voxel. In all cases, the iteration procedure was repeated until the best possible dose distribution was achieved. The algorithms were applied to two examples and the results showed that the iterative techniques were able to produce superior isodose distributions.

Algorithms↗
PubMed1997

Modulated beam conformal therapy for head and neck tumors.

PURPOSE: The goal of modulated-beam conformal therapy is to reduce the dose to healthy tissue and sensitive structures around a uniformly irradiated target volume. Multiple intensity-modulated fields offer improved tissue-sparing dose distributions. New computer-based systems for planning and delivering such treatments may soon be available from different commercial sources that will make the formulation of an intensity-modulated treatment plan and its execution widely available at any treatment facility that has the resources to acquire the necessary equipment. This work reports on a study of the integration of two such systems. METHODS AND MATERIALS: Treatment planning was done using a commercially available inverse planning algorithm based on simulated annealing. The plans arbitrarily assumed nine coplanar x-ray beams at nonopposed gantry angles. Intensity modulation was computed for each beam. The modulated field at each gantry angle was broken down into a series of uniform (nonmodulated) subfields, which could be delivered as a sequence to produce the desired dose distribution. Because a large number of subfields was delivered, a multileaf collimator (MLC) was used for field shaping. This allowed rapid and accurate field shaping for treatments made up of several hundred subfields. Computer control of the MLC and linear accelerator allowed delivery of doses less than .01 Gy per subfield. Treatment was delivered on a prototype, computer-controlled accelerator and MLC system. Resulting dose distributions were analyzed using film and an anatomically specific, homogeneous phantom. RESULTS: The treatment plans were evaluated using dose-volume histogram analysis. The plans provided acceptably uniform irradiation of the target volume without exceeding dose tolerances for nearby critical structures. The plans were successfully delivered by a prototype dynamic MLC. The time needed to deliver a sequence of subfields at one gantry angle ranged from 0.7 to 2.0 min. Isodoses from film agreed reasonably well with planned isodose distributions. CONCLUSIONS: It is feasible to plan and deliver fixed gantry, modulated-beam conformal therapy for head and neck tumors with systems being developed commercially. The planned dose distributions exhibit significant potential for sparing closely spaced normal tissue structures in the head and neck.

Algorithms↗
PubMed1997

Geometric analysis of light-field position of a multileaf collimator with curved ends.

The purpose of this work was to analyze the difference between the light-field edge and the radiation field edge produced by multileaf collimator (MLC) leaves that are constructed with rounded ends. A formula was derived using three-dimensional geometry predicting the location of the ideal point-source light-field edge relative to the geometric projection of the tip of the rounded leaf end. The theoretical edge of the radiation field for a point source was also calculated along the projection of a chord whose length was one Half-Value Thickness (HVT). Analytic solutions were found for the radiation-field edge and the transmission penumbra. The displacement of the light-field edge relative to the projection of the leaf tip varied from 0 mm for the leaf tip projected onto the central axis to approximately 3.2 mm for the leaf tip projected to 20 cm off the central axis. The light-field edge was always displaced into the unblocked area. The location of the projection of the 1 HVT chord was displaced from the light-field edge by a small and essentially constant distance (approximately 0.3 mm) for all field sizes. The transmission penumbra (80%-20%) was about 1.2 mm for all leaf positions. The calibration of the leaf position must account for the nonlinear relationship between the leaf-tip projection and the light-field (or radiation-field) edge.

Evaluation Studies as Topic↗
PubMed1997

Quality assurance for dynamic multileaf collimator modulated fields using a fast beam imaging system.

A quality assurance procedure was developed for x-ray beam intensity modulated conformal radiotherapy (IMCRT) using dynamic multileaf collimators (MLC). The procedure verifies a prescribed intensity modulated x-ray beam pattern in the beam eye's view (BEV) before the treatment procedure is applied to a patient. It verifies that (a) the leaf sequencing computer files were transferred correctly to the linac control computer; (b) the treatment can be correctly executed without machine faults. A fast beam imaging system (BIS) consisting of a Gd2O2S scintillation screen, a charge-coupled device (CCD) camera, and a portable personal computer (Wellhöfer Dosimetrie, Schwarzenbruck, Germany) was commissioned for this purpose. Measurements for the BIS performance are presented in this work. Reference images were derived from MLC leaf sequencing files that were used to drive a dynamic MLC system (Varian Oncology Systems, Palo Alto, CA). A correlation method was developed to compare the BIS measurements with the calculated reference images. A correlation coefficient calculated using 26 correct intensity modulated fields was shown to be a reliable threshold to identify inaccurate treatment delivery files. The study has demonstrated the feasibility of using the BIS and the correlation method to carry out on-line quality assurance tasks for IMCRT treatment fields in the BEV.

Biophysical Phenomena↗
PubMed1997

Breast-conserving radiation therapy: potential of inverse planning with intensity modulation.

PURPOSE: To evaluate inverse planning with beam-intensity modulation in breast-conserving radiation therapy. MATERIALS AND METHODS: A prototype inverse treatment-planning system was used in five patients with early-stage breast tumors. The prescription for the breast was 5,040 cGy in 180-cGy fractions, with the primary tumor site receiving 6,000 cGy in 214-cGy fractions. Isodose distributions from the prototype were compared section by section with those from conventional three-dimensional planning. RESULTS: The inverse planning system showed dose variation of 84%-98% to 111%-113% in the primary tumor site. For whole-breast irradiation, 95% of the target volume received at least 4,000 cGy. High-dose areas were adjacent to the primary site. Lower-dose areas were in the most medial part of the breast. Compared with the conventional system, target doses were similar. The volume of lung and heart that received high doses was smaller; however, larger lung and heart volumes received doses of less than 1,200 cGy. CONCLUSION: Intensity modulation may accomplish "concomitant boost" treatment. Acceptable dose gradients are achieved in the target volumes, with lower volumes of high-dose treatment in normal tissue. Plans that required comprehensive nodal radiation, in particular, were improved relative to conventional plans. Optimization of the planning system is needed to minimize the lung and heart volumes that receive low-dose radiation.

Breast↗
PubMed1996

Conventional vs. conformal radiotherapy for prostate cancer: preliminary results of dosimetry and acute toxicity.

PURPOSE: To compare conformal radiotherapy using three dimensional treatment planning (3D-CRT) to conventional radiotherapy (Conven-RT) for patients with Stages T2-T4 adenocarcinoma of the prostate. METHODS AND MATERIALS: A Phase III randomized study was activated in May 1993, to compare treatment toxicity and patient outcome after 78 Gy in 39 fractions using 3D-CRT to that after 70 Gy in 35 fractions using Conven-RT. The first 46 Gy were administered using the same nonconformal field arrangement (four field) in both arms. The boost was given nonconformally using four fields in the Conven-RT arm and conformally using six fields in the 3D-CRT arm. The dose was specific to the isocenter. The first 60 patients, 29 in the 3D-CRT arm and 31 in the Conven-RT arm, are the subject of this preliminary analysis. RESULTS: The two treatment arms were first compared in terms of dosimetry by dose-volume histogram analysis. Using a subgroup of patients in the 3D-CRT arm (n=15), both Conven-RT and 3D-CRT plans were generated and the dose-volume histogram data compared. The mean volumes treated to doses above 60 Gy for the bladder and rectum were 28 and 36% for the 3D-CRT plans, and 43 and 38% for the Conven-RT plans, respectively (p < 0.05 for the bladder volumes). The mean clinical target volume (prostate and seminal vesicles) treated to 95% of the prescribed dose was 97.5% for the 3D-CRT arm, and 95.6% for the Conven-RT arm (p < 0.05). There were no significant differences in the acute reactions between the two arms, with the majority experiencing Grade 2 or less toxicity (92%). Moreover, no relationship was seen between acute toxicity and the volume of bladder and rectum receiving in excess of 60 Gy for those in the 3D-CRT arm. There was also no difference between the groups in terms of early biochemical response. Prostate-specific antigen levels at 3 and 6 months after completion of radiotherapy were similar in the two treatment arms. There was only one biochemical failure in the study population at the time of the analysis. CONCLUSIONS: Comparison of the Conven-RT and 3D-RT treatment plans revealed that significantly less bladder was in the high dose volume in the 3D-CRT plans, while the volume of rectum receiving doses over 60 Gy was equivalent. There were no differences between the two treatment arms in terms of acute toxicity or early biochemical response. Longer follow-up is needed to determine the impact of 3D-CRT on long-term patient outcome and late reactions.

Adenocarcinoma↗
PubMed1996

A portal image alignment and patient setup verification procedure using moments and correlation techniques.

The objective of this study was to develop an integrated field shape and patient setup verification procedure for portal images. The proposed procedure used one of the approved portal images as the reference image for automated comparison with subsequent portal images. The first step of the procedure used a moments method to align treatment field boundaries. This step was necessary to compensate for the repositioning error of an image detector and to create a common frame of reference for comparing anatomical shifts relative to the field boundary. At the end of the moments alignment, a moments figure of merit was computed and compared with a pre-established threshold. This verified whether there was a potential shape change in the treatment field. To measure anatomical misalignment, the last step in the procedure was to use a grey-scale image correlation method to align translations and in-plane rotations relative to the anatomy of the reference image. The procedure was shown in phantom studies to err by less than 1 mm when detecting translational shifts and less than 1 degree when detection in-plane rotations. The moments verification method showed a sensitivity of detecting a placement error of 6 mm for a single leaf in a controlled experiment where a multileaf collimator was used for field shaping. The alignment procedure was fast and could be done in less than 12 s on an IBM-compatible 486 personal computer.

Humans↗