Arthur L. Boyer

X-RAY FIELD COMPENSATION WITH MULTILEAF COLLIMATORS

PURPOSE It has been proposed that conformal therapy can be carried out with static ports that are each individually compensated to deliver an optimal total dose distribution. If this proposal is to be implemented, one must have a means of compensating or modulating the fluence distributions within the boundaries of individual treatment fields. A theory was developed and implemented to achieve this goal. METHODS AND MATERIALS The theory allowed creation of a leaf-setting sequence for a desired level of field-modulation precision. This method of beam modulation was experimentally verified using radiographic film to integrate the dose delivered by the sequence of discrete static multileaf collimator-defined subfields. RESULTS Beam profiles were generated that matched the planned beam profiles to within the specified degree of precision. CONCLUSION This methodology is a candidate for implementation of inverse planning for conformal therapy.

A review of electronic portal imaging devices (EPIDs)

On-line electronic portal imaging devices are beginning to come into clinical service in support of radiotherapy. A variety of technologies are being explored to provide real-time or near real-time images of patient anatomy within x-ray fields during treatment on linear accelerators. The availability of these devices makes it feasible to verify treatment portals with much greater frequency and clarity than with film. This article reviews the physics of high-energy imaging and describes the operation principles of the electronic portal imaging devices that are under development or are beginning to be used clinically.

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.

Estimates of whole-body dose equivalent produced by beam intensity modulated conformal therapy

PURPOSE To estimate the dose delivered to patients by photons and neutrons outside the radiation fields when beam intensity modulation conformal radiotherapy is given. These estimates are then used to compute the risk of secondary cancers as a sequela of the radiation therapy. MATERIALS AND METHODS The x-ray and neutron leakage accompanying two beam-intensity modulation techniques delivered by currently available linear accelerators were estimated for 6-MV, 18-MV, and 25-MV x-ray energies. Estimates of whole-body dose equivalents were determined using leakage measurements reported in the literature and treatment parameters derived for two modulated beam-intensity conformal therapy techniques. Risk values recommended by the National Council on Radiation Protection and Measurements (NCRP) were used to estimate the resulting risk of fatal radiation-induced cancer for 70.00 Gy prescribed tumor doses. RESULTS The computed worst-case risks of secondary cancers increased in the range from 1.00% for 6-MV x-rays to 24.4% for 25-MV x-rays. CONCLUSIONS Careful consideration should be made of the risks associated with secondary whole-body radiation before implementation of beam intensity modulated conformal therapy at x-ray energies greater than 10 MV.

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.

Realization and verification of three-dimensional conformal radiotherapy with modulated fields

PURPOSE We describe the experimental demonstration of the delivery of a three-dimensional conformal radiotherapy dose distribution using in-field modulation of nine fixed-gantry fields. METHODS AND MATERIALS Two-dimensional in-field modulation profiles, varying from field to field, were realized by quasi-dynamic multileaf collimation using the prototype of a commercially available multileaf collimator installed on a medical linear accelerator. The profiles were calculated to deliver an optimal dose distribution for a patient with a prostate carcinoma. The target volume surface was invaginated and bifurcated. The calculated dose distribution was delivered to a homogeneous polystyrene phantom consisting of 1 cm thick slices that were cut to match the patients outer contour. Seven therapy verification films were placed between the phantom slices. RESULTS Analysis of the films revealed a degree of conformation of the high-dose region to the target shape that would not be possible with unmodulated conformal therapy. However, small observed spatial displacements of the dose distribution confirm the need for very accurate positioning. CONCLUSIONS It is feasible to deliver clinically relevant, three-dimensional dose distributions that conform to invaginated and bifurcated target volumes using fields modulated by multileaf collimators.

The potential and limitations of the inverse radiotherapy technique

The objective of the work presented in this paper is to explore the scope of the applicability of the inverse radiotherapy technique for designing optimized intensity distributions to achieve a desired dose distribution. A specified desired uniform dose to the target volume is inverted, subject to constraints on the surrounding normal tissue dose, to produce optimum intensity distributions in a set of beams arranged around the target volume. We employed the inverse technique and software developed by Bortfeld and evaluated results both qualitatively and quantitatively using dose distribution displays, dose-volume histograms and biological indices including tumor control probability and normal tissue complication probabilities. So far we have applied this methodology to prostate and lung treatment plans. For prostate the inverse technique produces satisfactory approximations of the desired dose distributions. However, for lung its performance is considerably inferior. Our investigations point to a number of factors for this difference, the primary ones being differences in the tolerance doses of neighboring normal tissues, magnitudes of volume effect, tissue architectures, and the achievability of the specified desired dose distributions. We conclude that, for certain clinical situations, it is not sufficient to specify the objectives of optimization purely in terms of the desired pattern of the dose. The objectives must also include dose-volume effects and biological indices. Furthermore, the mathematics of optimization must be able to incorporate these factors into the process. We find that the inverse technique is not suitable for situations where dose-volume considerations and biological indices are important and that other methods of optimization of intensity distributions should be explored.

ROLE OF BEAM ORIENTATION OPTIMIZATION IN INTENSITY- MODULATED RADIATION THERAPY

PURPOSE To investigate the role of beam orientation optimization in intensity-modulated radiation therapy (IMRT) and to examine the potential benefits of noncoplanar intensity-modulated beams. METHODS AND MATERIALS A beam orientation optimization algorithm was implemented. For this purpose, system variables were divided into two groups: beam position (gantry and table angles) and beam profile (beamlet weights). Simulated annealing was used for beam orientation optimization and the simultaneous iterative inverse treatment planning algorithm (SIITP) for beam intensity profile optimization. Three clinical cases were studied: a localized prostate cancer, a nasopharyngeal cancer, and a paraspinal tumor. Nine fields were used for all treatments. For each case, 3 types of treatment plan optimization were performed: (1) beam intensity profiles were optimized for 9 equiangular spaced coplanar beams; (2) orientations and intensity profiles were optimized for 9 coplanar beams; (3) orientations and intensity profiles were optimized for 9 noncoplanar beams. RESULTS For the localized prostate case, all 3 types of optimization described above resulted in dose distributions of a similar quality. For the nasopharynx case, optimized noncoplanar beams provided a significant gain in the gross tumor volume coverage. For the paraspinal case, orientation optimization using noncoplanar beams resulted in better kidney sparing and improved gross tumor volume coverage. CONCLUSION The sensitivity of an IMRT treatment plan with respect to the selection of beam orientations varies from site to site. For some cases, the choice of beam orientations is important even when the number of beams is as large as 9. Noncoplanar beams provide an additional degree of freedom for IMRT treatment optimization and may allow for notable improvement in the quality of some complicated plans.

Monte Carlo verification of IMRT dose distributions from a commercial treatment planning optimization system

The purpose of this work was to use Monte Carlo simulations to verify the accuracy of the dose distributions from a commercial treatment planning optimization system (Corvus, Nomos Corp., Sewickley, PA) for intensity-modulated radiotherapy (IMRT). A Monte Carlo treatment planning system has been implemented clinically to improve and verify the accuracy of radiotherapy dose calculations. Further modifications to the system were made to compute the dose in a patient for multiple fixed-gantry IMRT fields. The dose distributions in the experimental phantoms and in the patients were calculated and used to verify the optimized treatment plans generated by the Corvus system. The Monte Carlo calculated IMRT dose distributions agreed with the measurements to within 2% of the maximum dose for all the beam energies and field sizes for both the homogeneous and heterogeneous phantoms. The dose distributions predicted by the Corvus system, which employs a finite-size pencil beam (FSPB) algorithm, agreed with the Monte Carlo simulations and measurements to within 4% in a cylindrical water phantom with various hypothetical target shapes. Discrepancies of more than 5% (relative to the prescribed target dose) in the target region and over 20% in the critical structures were found in some IMRT patient calculations. The FSPB algorithm as implemented in the Corvus system is adequate for homogeneous phantoms (such as prostate) but may result in significant under or over-estimation of the dose in some cases involving heterogeneities such as the air-tissue, lung-tissue and tissue-bone interfaces.

Clinical dosimetry for implementation of a multileaf collimator

In order to initiate the use of a multileaf collimator (MLC) in the clinic, a set of technical procedures needs to be available sufficient to create MLC leaf settings and to deliver an accurate dose of radiation through the MLC-shaped field. Dosimetry data for clinical use of the MLC were measured. Dosimetric characteristics included central axis percent depth dose, output factors, and penumbra. In this paper, it has been concluded that a dose control monitor unit calculation procedure that has been applied to the use of conventional secondary field-shaping blocks can be applied to the multileaf collimator dosimetry. The multileaf collimator penumbra (20% to 80%) is only slightly wider (1-3 mm) than the penumbra of the conventional collimator jaws. Beams-eye-view comparisons made between the isodose curves in fields shaped by conventional Cerrobend blocks and isodose curves in fields shaped by the multileaf collimator demonstrated that the 50% isodose line at 10-cm depth exhibited the discrete steps of the multileaf collimator leaves, but that the 90% and 10% isodose curves of the multileaf were close to those shaped by Cerrobend blocks.