Determination of prospective displacement-based gate threshold for respiratory-gated radiation delivery from retrospective phase-based gate threshold selected at 4D CT simulation.

Four-dimensional (4D) computed tomography (CT) imaging has found increasing importance in the localization of tumor and surrounding normal structures throughout the respiratory cycle. Based on such tumor motion information, it is possible to identify the appropriate phase interval for respiratory gated treatment planning and delivery. Such a gating phase interval is determined retrospectively based on tumor motion from internal tumor displacement. However, respiratory-gated treatment is delivered prospectively based on motion determined predominantly from an external monitor. Therefore, the simulation gate threshold determined from the retrospective phase interval selected for gating at 4D CT simulation may not correspond to the delivery gate threshold that is determined from the prospective external monitor displacement at treatment delivery. The purpose of the present work is to establish a relationship between the thresholds for respiratory gating determined at CT simulation and treatment delivery, respectively. One hundred fifty external respiratory motion traces, from 90 patients, with and without audio-visual biofeedback, are analyzed. Two respiratory phase intervals, 40%-60% and 30%-70%, are chosen for respiratory gating from the 4D CT-derived tumor motion trajectory. From residual tumor displacements within each such gating phase interval, a simulation gate threshold is defined based on (a) the average and (b) the maximum respiratory displacement within the phase interval. The duty cycle for prospective gated delivery is estimated from the proportion of external monitor displacement data points within both the selected phase interval and the simulation gate threshold. The delivery gate threshold is then determined iteratively to match the above determined duty cycle. The magnitude of the difference between such gate thresholds determined at simulation and treatment delivery is quantified in each case. Phantom motion tests yielded coincidence of simulation and delivery gate thresholds to within 0.3%. For patient data analysis, differences between simulation and delivery gate thresholds are reported as a fraction of the total respiratory motion range. For the smaller phase interval, the differences between simulation and delivery gate thresholds are 8 +/- 11% and 14 +/- 21% with and without audio-visual biofeedback, respectively, when the simulation gate threshold is determined based on the mean respiratory displacement within the 40%-60% gating phase interval. For the longer phase interval, corresponding differences are 4 +/- 7% and 8 +/- 15% with and without audiovisual biofeedback, respectively. Alternatively, when the simulation gate threshold is determined based on the maximum average respiratory displacement within the gating phase interval, greater differences between simulation and delivery gate thresholds are observed. A relationship between retrospective simulation gate threshold and prospective delivery gate threshold for respiratory gating is established and validated for regular and nonregular respiratory motion. Using this relationship, the delivery gate threshold can be reliably estimated at the time of 4D CT simulation, thereby improving the accuracy and efficiency of respiratory-gated radiation delivery.

Preliminary results of a randomized radiotherapy dose-escalation study comparing 70 Gy with 78 Gy for prostate cancer.

PURPOSE: To determine the effect of radiotherapy dose on prostate cancer patient outcome and biopsy positivity in a phase III trial. PATIENTS AND METHODS: A total of 305 stage T1 through T3 patients were randomized to receive 70 Gy or 78 Gy of external-beam radiotherapy between 1993 and 1998. Of these, 301 were assessable; stratification was based on pretreatment prostate-specific antigen level (PSA). Dose was prescribed to the isocenter at 2 Gy per fraction. All patients underwent planning pelvic computed tomography scan to confirm prostate position. Treatment failure was defined as an increasing PSA on three consecutive follow-up visits or the initiation of salvage treatment. Median follow-up was 40 months. RESULTS: One hundred fifty patients were randomized to the 70-Gy arm and 151 to the 78-Gy arm. The difference in freedom from biochemical and/or disease failure (FFF) rates of 69% and 79% for the 70-Gy and 78-Gy groups, respectively, at 5 years was marginally significant (log-rank P: =.058). Multiple-covariate Cox proportional hazards regression showed that the study randomization was an independent correlate of FFF, along with pretreatment PSA, Gleason score, and stage. The patients who benefited most from the 8-Gy dose escalation were those with a pretreatment PSA of more than 10 ng/mL; 5-year FFF rates were 48% and 75% (P: =.011) for the 70-Gy and 78-Gy arms, respectively. There was no difference between the arms ( approximately 80% 5-year FFF) when the pretreatment PSA was < or = 10 ng/mL. CONCLUSION: A modest dose increase of 8 Gy using conformal radiotherapy resulted in a substantial improvement in prostate cancer FFF rates for patients with a pretreatment PSA of more than 10 ng/mL. These findings document that local persistence of prostate cancer in intermediate- to high-risk patients is a major problem when doses of 70 Gy or less are used.

Radiotherapy patterns of care study in lung carcinoma.

PURPOSE: For the first time, a lung Patterns of Care Study was conducted to determine the national patterns of radiation (RT) practice in patients treated for nonmetastatic lung cancer in 1998 to 1999. MATERIALS AND METHODS: A national survey of randomly selected RT institutions in the United States was conducted using two-stage cluster sampling, stratified by practice type. Patients with nonmetastatic lung cancer (Karnofsky performance score [KPS] > or = 60), who received RT as definitive or adjuvant therapy, were randomly selected. To determine national estimates, sample size was weighted by the relative number of institutions per strata and the number of patient records reviewed per the number of patients eligible. Accordingly, 42,335 patient records from 58 institutions were reviewed by trained research associates. The unweighted sample size (or number of patients) was 541. RESULTS: The histologies were small-cell lung cancer (SCLC) in 14.5% of patients versus non-small-cell lung cancer (NSCLC) in 85.5% of patients. The median age was 67 years (range, 29 to 92 years); 61% of patients were male, and 38% were current smokers. Bone scans and brain imaging were not obtained in 34% and 52% of clinical stage (CS) III NSCLC patients, respectively. Regarding treatment strategies, for SCLC and CS III NSCLC, chemotherapy plus RT was used significantly more than RT alone (P <.05); in CS I NSCLC, RT alone was the primary treatment (P <.05). Overall, 58% of patients received systemic therapy. On multivariate analysis, factors correlating with increased use of chemotherapy included younger age, histology (SCLC > NSCLC), increasing CS, increasing KPS, and lack of comorbidities. Only 3% of all patients were treated on prospective clinical trials. CONCLUSION: This study establishes the general patterns of care for lung carcinoma in RT facilities within the United States. As supported by clinical trials, patients with limited-stage SCLC and CS III NSCLC received chemotherapy plus RT more than they received RT alone. Further improvements in staging, smoking cessation, and increased accrual to clinical trials must be encouraged.

Use of a personal computer and spreadsheet software for treatment machine time calculations.

The application of spreadsheet software to aid in the calculation of treatment times for both rectangular and irregular radiation fields is presented. Specifications for the use of the spreadsheet software are discussed. An implementation using spreadsheet software on a personal computer is described.

A computer-aided instructional module for radiological physics.

A computer program and instructional module have been written to assist the physicist in radiation therapy physics instruction. The program, written in Pascal, presents the student with a screen of instructional material, followed by a multiple choice question pertaining to that material. A correct response allows the student to proceed to the next screen. If the answer is incorrect, another screen is presented which explains to the student why the answer is incorrect. Then the original screen and question are repeated. The program is general and can be used with any instructional module. It also is transportable, with a minimum of machine-dependent instructions.

An interactive system for point dose optimization.

An interactive system has been developed to aid in determining optimal photon and electron beams and beam weights for radiotherapy treatment planning. Dose constraints at various points are selected and an algorithm searches for a set of beams and weighting factors that satisfy these constraints. In the event that no combination of beam weights satisfies the choice of treatment modalities and dose constraints, the treatment modalities and dose constraints can be modified interactively. The goal of this procedure is different from that of more conventional optimization schemes in which optimal dose values are specified and the optimization algorithm determines the set of beam weights that yields a dose distribution closest to optimal.

Treatment planning using a dose-volume feasibility search algorithm.

PURPOSE: An approach to treatment plan optimization is presented that inputs dose--volume constraints and utilizes a feasibility search algorithm that seeks a set of beam weights so that the calculated dose distributions satisfy the dose--volume constraints. In contrast to a search for the "best" plan, this approach can quickly determine feasibility and point out the most restrictive of the predetermined constraints. METHODS AND MATERIALS: The cyclic subgradient projection (CSP) algorithm was modified to incorporate dose--volume constraints in a treatment plan optimization schema. The algorithm was applied to determine beam weights for several representative three-dimensional treatment plans. RESULTS: Using the modified CSP algorithm, we found that either a feasible solution to the dose--volume constraint problem was found or the program determined, after a predetermined set of iterations was performed, that no feasible solution existed for the particular set of dose--volume constraints. If no feasible solution existed, we relaxed several of the dose--volume constraints and were able to achieve a feasible solution. CONCLUSION: Feasibility search algorithms can be used in radiation treatment planning to generate a treatment plan that meets the dose--volume constraints established by the radiation oncologist. In the absence of a feasible solution, these algorithms can provide information to the radiation oncologist as to how the dose--volume constraints may be modified to achieve a feasible solution.

Evaluation and scoring of radiotherapy treatment plans using an artificial neural network.

PURPOSE: The objective of this work was to demonstrate the feasibility of using an artificial neural network to predict the clinical evaluation of radiotherapy treatment plans. METHODS AND MATERIALS: Approximately 150 treatment plans were developed for 16 patients who received external-beam radiotherapy for soft-tissue sarcomas of the lower extremity. Plans were assigned a figure of merit by a radiation oncologist using a five-point rating scale. Plan scoring was performed by a single physician to ensure consistency in rating. Dose-volume information extracted from a training set of 511 treatment plans on 14 patients was correlated to the physician-generated figure of merit using an artificial neural network. The neural network was tested with a test set of 19 treatment plans on two patients whose plans were not used in the training of the neural net. RESULTS: Physician scoring of treatment plans was consistent to within one point on the rating scale 88% of the time. The neural net reproduced the physician scores in the training set to within one point approximately 90% of the time. It reproduced the physician scores in the test set to within one point approximately 83% of the time. CONCLUSIONS: An artificial neural network can be trained to generate a score for a treatment plan that can be correlated to a clinically-based figure of merit. The accuracy of the neural net in scoring plans compares well with the reproducibility of the clinical scoring. The system of radiotherapy treatment plan evaluation using an artificial neural network demonstrates promise as a method for generating a clinically relevant figure of merit.

Dosimetric evaluation of total scalp irradiation using a lateral electron-photon technique.

PURPOSE: To evaluate the radiation dosimetry of a new technique for total scalp irradiation. METHODS AND MATERIALS: A treatment technique described by Akazawa (1989) has been studied. During each fraction, two electron and two photon fields are treated. While most of the lateral scalp is treated with the electron fields, a rind of scalp close to the midsagittal plane is irradiated by parallel-opposed lateral photon fields. A wax bolus is used to build up skin dose and to protect the brain from electron dose. The dose distribution and dose-volume histograms were evaluated for different field arrangements using a 3-dimensional treatment planning system. After modifying the technique, in-vivo thermoluminescent dosimetry were used to evaluate the dose distributions for the first two patients. RESULTS: To compensate for the lack of dose from the opposed photon field at the junction, the technique was modified using overlapped fields instead of abutting fields. A field overlap of 3 to 4 mm between the electron and photon fields was found optimal. When used with the field junction shift of 1 cm midway through the treatment, this scheme resulted in a dose uniformity of -5% to +15% of the prescribed dose in the region of abutment. Results of the 3-dimensional dose calculation were supported by in-vivo thermoluminescent dosimetry on two patients. CONCLUSION: On the basis of computer dose calculations and in-vivo dosimetry. Akazawa's technique for scalp irradiation can be improved by using a 3 to 4 mm overlap of electron and photon fields. This modified technique is practical and produces clinically acceptable dosimetry.

Computer-aided design and fabrication of an electron bolus for treatment of the paraspinal muscles.

PURPOSE: Demonstrate the technology for the design, fabrication, and verification of an electron bolus used in the preoperative irradiation of a mesenchymal chondrosarcoma in the paraspinal muscle region (T8-T12), in which the target volume overlay a portion of the spinal cord, both lungs, and the right kidney. METHODS AND MATERIALS: An electron-bolus design algorithm implemented on a three dimensional (3D) radiotherapy treatment planning system designed the bolus to yield a dose distribution that met physician-specified clinical criteria. Electron doses were calculated using a 3D electron pencil-beam dose algorithm. A computer-driven milling machine fabricated the bolus from modeling wax, machining both the patient surface and the beam surface of the bolus. Verification of the bolus fabrication was achieved by repeating the patient's computed tomography (CT) scan with the fabricated bolus in place (directly on the posterior surface of the prone patient) and then recalculating the patient's dose distribution using the 3D radiotherapy treatment planning system. RESULTS: A treatment plan using a 17-MeV posterior electron field with a bolus delivered a superior dose distribution to the patient than did the same plan without a bolus. The bolus plan delivered a slightly increased dose to the target volume as a result of a slightly broader range of doses. There were significant reductions in dose to critical structures (cord, lungs, and kidney) in the bolus plan, as evidenced by dose-volume histograms (DVHs). The patient dose distribution, calculated using CT scan data with the fabricated bolus, showed no significant differences from the planned dose distribution. CONCLUSIONS: A bolus can provide considerable sparing of normal tissues when using a posterior electron beam to irradiate the paraspinal muscles. Bolus design and fabrication using the tools described in this paper are adequate for patient treatment. CT imaging of the patient with the bolus in place followed by calculation of the patient's dose distribution demonstrated a useful method for verification of the bolus design and fabrication process.

Water bolus for electron irradiation of the ear canal.

PURPOSE: To demonstrate that water bolus in the external ear can decrease the dose inhomogeneity caused by auricular surface irregularities when the ear is in an electron-beam field. METHODS AND MATERIALS: Three-dimensional (3D) dose distributions with and without water bolus in the external ear were calculated for a representative patient. The electron dose calculations were made using the Hogstrom pencil beam algorithm as implemented in 3D by Starkschall. To demonstrate the use of water bolus in the ear clinically, the case of a patient with squamous carcinoma of the concha who was treated with electrons is presented. RESULTS: Water bolus markedly lessens the dose heterogeneity caused by the surface irregularities of the ear and the air in the external auditory canal. In the test case, the maximum dose was reduced by 25% using this technique. CONCLUSION: When the ear is in an electron beam field, warm water should be placed in the external auditory canal and concha. This maneuver may reduce the incidence of auricular complications that occur after electron-beam therapy.

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.

Respiratory-driven lung tumor motion is independent of tumor size, tumor location, and pulmonary function.

PURPOSE: To determine whether superior-inferior lung tumor motion is predictable by tumor size or location, or pulmonary function test results. METHODS AND MATERIALS: Superior-inferior tumor motion was measured on orthogonal radiographs taken during simulation of 22 patients with inoperable lung cancer diagnosed by orthogonal radiographs. RESULTS: The tumor size averaged 5.5 +/- 3.1 cm (range 1.5-12 cm). Seven of 11 central tumors demonstrated some motion compared with 5 of 11 peripheral tumors. Four of 5 upper lobe tumors moved compared with 8 of 17 tumors that were either middle or lower lobe lesions. The mean fourth rib motion was 7.3 +/- 3.2 mm (range 2-15). The mean FeV(1) was 1.8 +/- 1.2 (range 0.55-5.33. The mean diffusing capacity of the lung for carbon monoxide was 14.0 +/- 6.5 (range 7.8-21.9). The mean total lung capacity was 6.5 +/- 1.2 (range 3.3-8.4). None of these parameters correlated with tumor motion. Although lateral tumor motion could not be consistently determined, 1 tumor moved 10 mm anterior-posteriorly. CONCLUSIONS: Lung tumors often move significantly during respiration. Tumor motion is not predictable by tumor size or location, or pulmonary function test results. Therefore, tumor motion must be measured in all patients. Measurement in three dimensions will likely be necessary to maximize the irradiated lung volumes or choose beam arrangements parallel to the major axis of motion.

Analytic evaluation of depths of dose calculation points for external beam radiation therapy treatment planning.

A method has been developed to calculate the depth of a point in a patient contour analytically. In this method, the patient contour is approximated by a Fourier series comprising 5-10 terms. The depth of a calculation point is the distance between the calculation point and the intersection of the ray line with the contour line. Depth calculations using this method are compared with calculations using a more conventional method in which the intersection is found by representing the contour as a set of straight-line segments between pairs of contour points. This method gives depth values comparable in accuracy to the more conventional method and computed approximately twice as fast.

A constrained least-squares optimization method for external beam radiation therapy treatment planning.

A method has been developed to optimize the selection of beam weights for external beam radiation therapy treatment planning. This method uses a matrix transformation method to achieve a least-squares fit to doses at preselected points in a target volume. This method is more general than the conventional least-squares method in that it allows one to incorporate as constraints maximum allowable doses delivered to critical organs, as well as the requirement that all beam weights be non-negative. A computer algorithm DOSEOPT has been written which takes a matrix of doses to various points from beams whose field sizes and orientations have been predetermined, and performs the constrained least-squares fit to desired doses in the target volume and maximum doses to critical organs. Implementation of this algorithm and its application to several cases are discussed.

Analytic evaluation of heterogeneity locations for external beam treatment planning.

A method has been developed to calculate depths of calculation points analytically in patient contours containing internal heterogeneities. In this method, each contour is approximated by a Fourier series comprising five to ten terms. Using this approximation, the intersections of a ray line with each contour are found as the solution of a transcendental equation. Once these intersections have been located, effective depths used in heterogeneity calculations can readily be calculated.

Extension of Fourier methods to the calculation of effective depths in heterogeneous media of arbitrary contour.

The description of patient contours and internal structures by means of truncated Fourier series can be extended to continuous contours of arbitrary shape and location by expressing the x and z Cartesian coordinates of the contour as independent Fourier series in a parameter t. An analytic equation for the intersection of the contour and a ray line is then written as an equation in the parameter t. The equation can be solved using numerical methods yielding the Cartesian coordinates of the intersection point directly.

A convolution method for constructing primary beam profiles in the presence of beam modifiers.

Empirical functions that describe primary beam profiles for radiotherapy treatment planning systems generally account for finite source size only on beams unmodified by blocks, wedges, or compensating filters. To incorporate the effects of extended sources on such modified beams and to treat the effect of an extended source consistent with the manner in which the unmodified beam is treated, the unmodified beam profile can be written as a convolution of an unknown source function with a collimator transmission profile. Using Fourier transforms, one can then solve for the source function. This source function is then convolved with a beam transmission function that has been modified by blocks, wedges, or compensating filters to obtain a primary beam profile. A number of examples are presented that demonstrate the calculations of the effects of beam modifiers on primary beam profiles.

An interactive beam-weight optimization tool for three-dimensional radiotherapy treatment planning.

A computer software tool has been developed to aid the treatment planner in selecting beam weights for three-dimensional radiotherapy treatment planning. The program consists of a feasibility search algorithm embedded in an interactive, user-friendly driving program. The feasibility search algorithm is based on the iterative relaxation algorithm of Cimmino [La Ricerca Scientifica, Vol. I, pp. 326-333 (1938)] as applied to the radiotherapy inverse problem by Altschuler et al. [Med. Phys. 13, 590 (1986)]. Relative importances of structures based upon clinical considerations can be incorporated into the algorithm. In order to speed convergence, the relaxation parameter is made to vary, with its value based upon a measure of deviation from feasibility. The interactive driving program is designed so that the treatment planner can make reasonable judgments regarding the acceptability of a plan in the event that the dose constraints yield no feasible solution. An example of the use of this program applied to a problem in three-dimensional radiotherapy treatment planning is illustrated.

Electron pencil-beam redefinition algorithm dose calculations in the presence of heterogeneities.

The electron pencil-beam redefinition algorithm (PBRA) is currently being refined and evaluated for clinical use. The purpose of this work was to evaluate the accuracy of PBRA-calculated dose in the presence of heterogeneities and to benchmark PBRA dose accuracy for future improvements to the algorithm. The PBRA was evaluated using a measured electron beam dose algorithm verification data set developed at The University of Texas M. D. Anderson Cancer Center. The data set consists of measurements made using 9 and 20 MeV beams in a water phantom with air gaps, internal air and bone heterogeneities, and irregular surfaces. Refinements to the PBRA have enhanced the speed of the dose calculations by a factor of approximately 7 compared to speeds previously reported in published data; a 20 MeV, 15 x 15 cm2 field electron-beam dose distribution took approximately 10 minutes to calculate. The PBRA showed better than 4% accuracy in most experiments. However, experiments involving the low-energy (9 MeV) electron beam and irregular surfaces showed dose differences as great as 22%, in albeit a small fractional region. The geometries used in this study, particularly those in the irregular surface experiments, were extreme in the sense that they are not seen clinically. A more appropriate clinical evaluation in the future will involve comparisons to Monte Carlo generated patient dose distributions using actual computed tomography scan data. The present data also serve as a benchmark against which future enhancements to the PBRA can be evaluated.