A round-robin gamma stereotactic radiosurgery dosimetry interinstitution comparison of calibration protocols.

PURPOSE: Absorbed dose calibration for gamma stereotactic radiosurgery is challenging due to the unique geometric conditions, dosimetry characteristics, and nonstandard field size of these devices. Members of the American Association of Physicists in Medicine (AAPM) Task Group 178 on Gamma Stereotactic Radiosurgery Dosimetry and Quality Assurance have participated in a round-robin exchange of calibrated measurement instrumentation and phantoms exploring two approved and two proposed calibration protocols or formalisms on ten gamma radiosurgery units. The objectives of this study were to benchmark and compare new formalisms to existing calibration methods, while maintaining traceability to U.S. primary dosimetry calibration laboratory standards. METHODS: Nine institutions made measurements using ten gamma stereotactic radiosurgery units in three different 160 mm diameter spherical phantoms [acrylonitrile butadiene styrene (ABS) plastic, Solid Water, and liquid water] and in air using a positioning jig. Two calibrated miniature ionization chambers and one calibrated electrometer were circulated for all measurements. Reference dose-rates at the phantom center were determined using the well-established AAPM TG-21 or TG-51 dose calibration protocols and using two proposed dose calibration protocols/formalisms: an in-air protocol and a formalism proposed by the International Atomic Energy Agency (IAEA) working group for small and nonstandard radiation fields. Each institution's results were normalized to the dose-rate determined at that institution using the TG-21 protocol in the ABS phantom. RESULTS: Percentages of dose-rates within 1.5% of the reference dose-rate (TG-21+ABS phantom) for the eight chamber-protocol-phantom combinations were the following: 88% for TG-21, 70% for TG-51, 93% for the new IAEA nonstandard-field formalism, and 65% for the new in-air protocol. Averages and standard deviations for dose-rates over all measurements relative to the TG-21+ABS dose-rate were 0.999±0.009 (TG-21), 0.991±0.013 (TG-51), 1.000±0.009 (IAEA), and 1.009±0.012 (in-air). There were no statistically significant differences (i.e., p>0.05) between the two ionization chambers for the TG-21 protocol applied to all dosimetry phantoms. The mean results using the TG-51 protocol were notably lower than those for the other dosimetry protocols, with a standard deviation 2-3 times larger. The in-air protocol was not statistically different from TG-21 for the A16 chamber in the liquid water or ABS phantoms (p=0.300 and p=0.135) but was statistically different from TG-21 for the PTW chamber in all phantoms (p=0.006 for Solid Water, 0.014 for liquid water, and 0.020 for ABS). Results of IAEA formalism were statistically different from TG-21 results only for the combination of the A16 chamber with the liquid water phantom (p=0.017). In the latter case, dose-rates measured with the two protocols differed by only 0.4%. For other phantom-ionization-chamber combinations, the new IAEA formalism was not statistically different from TG-21. CONCLUSIONS: Although further investigation is needed to validate the new protocols for other ionization chambers, these results can serve as a reference to quantitatively compare different calibration protocols and ionization chambers if a particular method is chosen by a professional society to serve as a standardized calibration protocol.

Calibration of the Gamma Knife using a new phantom following the AAPM TG51 and TG21 protocols.

PURPOSE: To compare calibration of the Leksell Gamma Knife according to the American Association of Physicists in Medicine Task Groups 21 and 51 protocols. A new phantom was fabricated for this purpose. Its design, physical properties, and composition are described. MATERIALS AND METHODS: The Gamma Knife TG-51 calibration phantom is designed to be filled with water and support an ionization chamber positioned at its center. The phantom is thimble-shaped, with a 2 mm plastic wall to contain water. The phantom and chamber assembly was mounted in a Leksell stereotactic frame. The location of the chamber's sensitive volume was determined using computed tomography. The chamber-phantom assembly was attached to the 18 mm helmet in the Gamma Knife by the stereotactic frame. The phantom's geometry allowed radiation beams from each of the 201 Gamma Knife cobalt-60 sources to converge after an 8 cm path to the ionization chamber's sensitive volume. This is similar to the arrangement by which one calibrates the Gamma Knife using the manufacturer-supplied polystyrene phantom. RESULTS: The phantom was attached to the Gamma Knife so that the ionization chamber was reproducibly positioned at the convergence of the radiation beams. Because of the phantom's design, the phantom could be affixed to either trunnions or the automatic patient positioning system, once mounted in the Leksell stereotectic frame. Comparisons using different phantoms and protocols resulted in the following calibration ratios for TG-21 in the polystyrene sphere phantom, TG-21 in the water phantom, and TG-51 in the water phantom, respectively: 1.000, 1.008, 0.986, when corrected for transmission through the plastic water reservoir wall and using the same ionization chamber. Transmission measurements using a 1 cm thickness of the same material in the Co-60 beam determined that the phantom's 2 mm plastic wall resulted in a reduction in the measured the output by 0.5%. CONCLUSIONS: Calibration of the Gamma Knife can be performed in liquid water using the AAPM TG-51 protocol and this new phantom, thereby eliminating uncertainties with respect to the composition of the manufacturer's phantom. Perturbation of calibration measurements by nonwater materials was characterized and could be corrected. Calibration values for the Gamma Knife that were obtained using the three methods for our phantoms agree to within 1.4%. TG21 and TG51 calibration of the Gamma Knife using the water phantom agreed to within 2.2%.

A phase I-B trial of the radiosensitizer: etanidazole (SR-2508) with radiosurgery for the treatment of recurrent previously irradiated primary brain tumors or brain metastases (RTOG Study 95-02).

RTOG 95-02 assessed patient tolerance to hypoxic cell radiosensitizer, etanidazole (SR-2508), combined with radiosurgery. Patients had primary or metastatic brain tumors and previously localized or whole brain irradiation. The toxicity is reported in three groups of patients according to the tumor size. Etanidazole doses of 12g/m2 combined with radiosurgery were well tolerated.

Multimodality image registration quality assurance for conformal three-dimensional treatment planning.

PURPOSE: We present a quality assurance methodology to determine the accuracy of multimodality image registration and fusion for the purpose of conformal three-dimensional and intensity-modulated radiation therapy treatment planning. Registration and fusion accuracy between any combination of computed tomography (CT), magnetic resonance (MR), and positron emission computed tomography (PET) imaging studies can be evaluated. METHODS AND MATERIALS: A commercial anthropomorphic head phantom filled with water and containing CT, MR, and PET visible targets was modified to evaluate the accuracy of multimodality image registration and fusion software. For MR and PET imaging, the water inside the phantom was doped with CuNO(3) and 18F-fluorodeoxyglucose (18F-FDG), respectively. Targets consisting of plastic spheres and pins were distributed throughout the cranium section of the phantom. Each target sphere had a conical-shaped bore with its apex at the center of the sphere. The pins had a conical extension or indentation at the free end. The contours of the spheres, sphere centers, and pin tips were used as anatomic landmark models for image registration, which was performed using affine coordinate-transformation tools provided in a commercial multimodality image registration/fusion software package. Four sets of phantom image studies were obtained: primary CT, secondary CT with different phantom immobilization, MR, and PET study. A novel CT, MR, and PET external fiducial marking system was also tested. RESULTS: The registration of CT/CT, CT/MR, and CT/PET images allowed correlation of anatomic landmarks to within 2 mm, verifying the accuracy of the registration software and spatial fidelity of the four multimodality image sets. CONCLUSIONS: This straightforward phantom-based quality assurance of the image registration and fusion process can be used in a routine clinical setting or for providing a working image set for development of the image registration and fusion process and new software.

Angular measurement of the cobalt-60 emitted radiation spectrum from a radiosurgery irradiator.

The photon energy spectrum emanating from a Leksell Gamma Knife, Model 23004B, was measured between 0.250 and 3.5 MeV with the sources exposed. Measurements were made using a 2x2 inch NaI detector enclosed in a lead-shielded apparatus having a 1/4 inch diameter measurement aperture, which reduced the amount of radiation received by the crystal. All measurements were made one meter above the floor within a quadrant toward one side of the Gamma Knife couch. The measured spectra displayed the expected 60Co doublet of photon peaks at energies of 1.17 and 1.33 MeV. These peaks appeared in spectra beginning at approximately 50 degrees, as one proceeds from a point directly lateral to the source enclosure (0 degrees) toward the foot of the couch (90 degrees). The average photon energy of the spectrum shifts to lower values as the doublet decreases in magnitude with increasing angle until almost vanishing at an angle equal to 90 degrees. Inserting a 16 cm diameter plastic sphere phantom, provided with the Gamma Knife, into the radiation beams increases the low energy photon emissions appearing in the spectrum, especially for measurements at the foot of the couch. Implications for the design of shielding a treatment room containing the Gamma Knife, Model B, and estimation of the radiation exposure to personnel during an emergency procedure in the treatment room with the sources exposed are discussed.

Stereotactic imaging quality assurance using an anthropomorphic phantom.

OBJECTIVE: A custom-designed anthropomorphic head phantom, containing computed tomography (CT) and magnetic resonance (MR) viewable targets, was used in the assessment of stereotactic localization accuracy. MATERIALS AND METHODS: The Brown-Roberts-Wells (BRW) or Leksell stereotactic ring was rigidly fixed to the phantom. CT and MR images were then obtained according to radiosurgery protocols with the corresponding localizer frame attached. Plastic spheres and rods appeared at various locations within the phantom, when filled with aqueous solution, and their images served as targets to compute stereotactic target coordinates using software compatible with each frame. Coordinates derived using CT and MR were compared with mechanical measurements obtained using the BRW or Leksell stereotactic arc systems. RESULTS: For the BRW stereotactic system, the average vector distance to agreement of image-derived coordinates with the mechanical measurements was 1.41 +/- 0.90 mm (CT) and 1.37 +/- 0.38 mm (MR). Similar results were obtained using the Leksell system: 0.78 +/- 0.33 mm (CT) and 1.45 +/- 0.86 mm (MR). The vector distance to agreement between CT and MR was 1.42 +/- 0.55 mm for the BRW and 1.31 +/- 0.60 mm for the Leksell systems. CONCLUSIONS: The data support the use of our anthropomorphic phantom, and present a methodology for assessing radiosurgery target localization and imaging accuracy.

A change in treatment process with a modern record and verify system.

PURPOSE: With the introduction of new treatment devices, such as a multileaf collimator (MLC) and dynamic wedge (DW), therapists have an increased responsibility to ensure correct treatment. Simultaneously, three-dimensional treatment planning (3DTP) has led to an increased number of portals and table movements. To counteract this challenge and maintain efficiency, a comprehensive record and verify (R&V) system is mandatory. We evaluated a commercial system (Varis) for reliability, ease of use, efficiency, and integration with our planning systems. METHODS AND MATERIALS: Some key elements of the Varis system are: integration of MLC and DW; auto setup for MLC, jaw, collimator, gantry, and limited table parameters; direct download of simulation beam data; and a regimented field scheduling system that prescribes all beam data for particular fractions. Evaluation of the system was driven by treatment time analysis, error rates, and an increased workload. These issues were governed by how we disseminated duties and how the system accommodated or changed our processes. RESULTS: Most data entry is performed by our dosimetry staff. Data can be downloaded from the simulator, but more patients now move from CT simulation and/or 3DTP to the treatment machine. Varis does not link to these systems. The physics staff confirms all entries to correct data entry errors. The workload for dosimetrists increased by an average of 8 minutes/patient entry; physics time increased by 7 minutes/patient entry; the weekly electronic chart check takes approximately 3 minutes/patient. Therapists who used Varis efficiently showed a slight decrease in treatment times, attributed to MLC integration and auto-setup. Some therapists experienced a decrease in efficiency, because of unfamiliarity and excess intervention. On a positive note, notable events have decreased by a factor of 10 since full initiation. Unfortunately, the remaining errors are often the result of a therapist relying on incorrect electronic information. CONCLUSION: The Varis R&V system has had an impact on our clinic's process and efficiency. Checking of all beam data and related field scheduling have helped reduce errors and misconceptions. We feel a dual-energy machine can be operated with two experienced therapists and an up-to-date R&V system more accurately and efficiently than with three therapists working without an integrated R&V. We anticipate future Varis releases will further promote efficiency and accuracy.

Radiation pneumonitis as a function of mean lung dose: an analysis of pooled data of 540 patients.

PURPOSE: To determine the relation between the incidence of radiation pneumonitis and the three-dimensional dose distribution in the lung. METHODS AND MATERIALS: In five institutions, the incidence of radiation pneumonitis was evaluated in 540 patients. The patients were divided into two groups: a Lung group, consisting of 399 patients with lung cancer and 1 esophagus cancer patient and a Lymph./Breast group with 78 patients treated for malignant lymphoma, 59 for breast cancer, and 3 for other tumor types. The dose per fraction varied between 1.0 and 2.7 Gy and the prescribed total dose between 20 and 92 Gy. Three-dimensional dose calculations were performed with tissue density inhomogeneity correction. The physical dose distribution was converted into the biologically equivalent dose distribution given in fractions of 2 Gy, the normalized total dose (NTD) distribution, by using the linear quadratic model with an alpha/beta ratio of 2.5 and 3.0 Gy. Dose-volume histograms (DVHs) were calculated considering both lungs as one organ and from these DVHs the mean (biological) lung dose, NTDmean, was obtained. Radiation pneumonitis was scored as a complication when the pneumonitis grade was grade 2 (steroids needed for medical treatment) or higher. For statistical analysis the conventional normal tissue complication probability (NTCP) model of Lyman (with n=1) was applied along with an institutional-dependent offset parameter to account for systematic differences in scoring patients at different institutions. RESULTS: The mean lung dose, NTDmean, ranged from 0 to 34 Gy and 73 of the 540 patients experienced pneumonitis, grade 2 or higher. In all centers, an increasing pneumonitis rate was observed with increasing NTDmean. The data were fitted to the Lyman model with NTD50=31.8 Gy and m=0.43, assuming that for all patients the same parameter values could be used. However, in the low dose range at an NTDmean between 4 and 16 Gy, the observed pneumonitis incidence in the Lung group (10%) was significantly (p=0.02) higher than in the Lymph./Breast group (1.4%). Moreover, between the Lung groups of different institutions, also significant (p=0.04) differences were present: for centers 2, 3, and 4, the pneumonitis incidence was about 13%, whereas for center 5 only 3%. Explicitly accounting for these differences by adding center-dependent offset values for the Lung group, improved the data fit significantly (p < 10(-5)) with NTD50=30.5+/-1.4 Gy and m=0.30+/-0.02 (+/-1 SE) for all patients, and an offset of 0-11% for the Lung group, depending on the center. CONCLUSIONS: The mean lung dose, NTDmean, is relatively easy to calculate, and is a useful predictor of the risk of radiation pneumonitis. The observed dose-effect relation between the NTDmean and the incidence of radiation pneumonitis, based on a large clinical data set, might be of value in dose-escalating studies for lung cancer. The validity of the obtained dose-effect relation will have to be tested in future studies, regarding the influence of confounding factors and dose distributions different from the ones in this study.

Radiation-induced apoptosis in dorsal root ganglion neurons.

Ionizing radiation (IR) results in apoptosis in a number of actively proliferating or immature cell types. The effect of IR on rat dorsal root ganglion (DRG) neurons was examined in dissociated cell cultures. After exposure to IR, embryonic DRG neurons, established in cell culture for six days, underwent cell death in a manner that was dose-dependent, requiring a minimum of 8 to 16 Gy. Twenty-five per cent cell loss occurred in embryonic day 15 (E-15) neurons, grown in cell culture for 6 days ('immature'), and then treated with 24 Gy IR. In contrast, only 2% cell loss occurred in E-15 neurons maintained in culture for 21 days ('mature') and then treated with 24 Gy IR. Staining with a fluorescent DNA-binding dye demonstrated clumping of the nuclear chromatin typical of apoptosis. Initiation of the apoptosis occurred within 24 h after exposure to IR. Apoptosis was prevented by inhibition of protein synthesis with cycloheximide. Apoptosis induced by IR occurred more frequently in immature than in mature neurons. Immature DRG neurons have a lower concentration of intracellular calcium ([Ca2+]i) than mature neurons. Elevation of [Ca2+]i by exposure to a high extracellular potassium ion concentration (35 microM) depolarizes the cell membrane with a resultant influx of calcium ions. The activation of programmed cell death after nerve growth factor (NGF) withdrawal is inversely correlated with [Ca2+]i in immature DRG neurons. When treated with high extracellular potassium, these immature neurons were resistant to IR exposure in a manner similar to that observed in mature neurons. These data suggest that [Ca2+]i modulates the apoptotic response of neurons after exposure to IR in a similar manner to that proposed by the "Ca2+ setpoint hypothesis" for control of NGF withdrawal-induced apoptosis.

Evaluation of an objective plan-evaluation model in the three dimensional treatment of nonsmall cell lung cancer.

PURPOSE: Evaluation of three dimensional (3D) radiotherapy plans is difficult because it requires the review of vast amounts of data. Selecting the optimal plan from a set of competing plans involves making trade-offs among the doses delivered to the target volumes and normal tissues. The purpose of this study was to test an objective plan-evaluation model and evaluate its clinical usefulness in 3D treatment planning for nonsmall cell lung cancer. METHODS AND MATERIALS: Twenty patients with inoperable nonsmall cell lung cancer treated with definitive radiotherapy were studied using full 3D techniques for treatment design and implementation. For each patient, the evaluator (the treating radiation oncologist) initially ranked three plans using room-view dose-surface displays and dose-volume histograms, and identified the issues that needed to be improved. The three plans were then ranked by the objective plan-evaluation model. A figure of merit (FOM) was computed for each plan by combining the numerical score (utility in decision-theoretic terms) for each clinical issue. The utility was computed from a probability of occurrence of the issue and a physician-specific weight indicating its clinical relevance. The FOM was used to rank the competing plans for a patient, and the utility was used to identify issues that needed to be improved. These were compared with the initial evaluations of the physician and discrepancies were analyzed. The issues identified in the best treatment plan were then used to attempt further manual optimization of this plan. RESULTS: For the 20 patients (60 plans) in the study, the final plan ranking produced by the plan-evaluation model had an initial 73% agreement with the ranking provided by the evaluator. After discrepant cases were reviewed by the physician, the model was usually judged more objective or "correct." In most cases the model was also able to correctly identify the issues that needed improvement in each plan. Subsequent replanning confirmed that further manual plan optimization could be achieved in 17 patients. CONCLUSION: The objective plan-evaluation model was able to rank lung cancer radiotherapy plans from best to worst. It was useful in improving plans and may be useful to physicians in defining goals for patients based on the ability to effectively and safely treat their tumors.

Minimization of target positioning error in accelerator-based radiosurgery.

The stereotactic radiosurgery system used at the Mallinckrodt Institute of Radiology is patterned after that developed at the Joint Center for Radiation Therapy (Brigham & Women's Hospital, Boston, MA) and uses the Brown-Roberts-Wells computed tomography (CT) stereotactic system. The patient's head is attached to a stand that rotates with the treatment couch. The irradiation is conducted using a set of converging arcs of irradiation. Because of mechanical limitations, no accelerator or treatment couch is capable of placing the center of the radiation beam at precisely the same point for all gantry and couch angles and a compromise must be made when locating the nominal isocenter. The stand settings are checked by placing a radiopaque QA sphere at the desired target location. The QA sphere is imaged using a series of eight films exposed at a set of couch and gantry angles that encompass the treatment angles. The distances between the QA sphere image and the center of the radiation field indicate if the correct coordinates were set on the stand and if the radiation beam converges to a sufficiently small region (< 0.1-cm diameter) for treatment. A mathematical procedure has been developed to use the film-measured position errors to determine a stand offset that will minimize the distance between the accelerator isocenter and the target. The technique is capable of reducing the average placement error, as measured by imaging the QA sphere, to 0.035 cm with a maximum deviation of 0.07 cm.

Stereotactic external beam irradiation using a linear accelerator: the Washington University experience.

From February 1989 to December 1993, 139 patients with a variety of brain lesions were treated with stereotactic external beam irradiation using a 6MV linear accelerator. The largest group consisted of patients with recurrent brain metastases (n = 46). Twenty seven patients had malignant gliomas, most of which were recurrent. Several benign conditions were treated, including arteriovenous malformations (n = 27), acoustic neuromas (n = 9), meningiomas (n = 7), and pituitary adenomas (n = 2). Durable responses were seen in the majority of patients with transient, mild, side effects. This experience suggests that stereotactic external beam irradiation is a safe, reliable, and effective method for non-invasive treatment of selected patients with small, localized brain lesions.

Integrated software tools for the evaluation of radiotherapy treatment plans.

PURPOSE: This article announces the availability of a convenient and useful software environment for the evaluation of three-dimensional (3D) radiotherapy treatment plans. MATERIALS AND METHODS: Using standards such as American National Standards for Information Systems C and the X Window System allowed us to bring the computation and display of dose-volume histograms, dose statistics, tumor control probabilities, normal tissue complication probabilities, and a figure of merit together under one user interface. These plan evaluation tools are not stand alone, but must interact with a 3D radiation therapy planning system to obtain the required dose matrices and patient anatomical contours. Installation of the software involves a programmer who writes a software bridge between the radiation therapy planning system and the tools, thereby providing access to local data files. This design strategy confines portability issues to one area of the software. RESULTS: Access to the other tools is through the Graphical Plan Evaluation Tool (GPET). GPET coordinates the use of each of the tools and provides graphical facilities for display of their results. Importantly, GPET assures that the displayed results of each tool have been computed with the same input specifications for all treatment plans being compared. For added convenience, the user can rearrange the resultant data to be reviewed in various ways on the video screen. The software design also allows incorporation of customized algorithms and input data for computing tumor control probability and normal tissue complication probabilities, since those currently available are controversial. CONCLUSION: The Graphical Plan Evaluation Tool unifies the simultaneous computation for several analytical tools and graphical display of their results. Within the constraints of the X Window System environment, this assemblage of software tools provides a portable, flexible, and convenient method for the quantitative evaluation of several radiotherapy treatment plans.

Assurance of high quality linac-based stereotactic radiosurgery.

PURPOSE: Stereotactic radiosurgery is generally a single, high-dose radiation treatment for the brain requiring targeting accuracy on the order of a millimeter. From the initial implementation of radiosurgery, therefore, quality assurance is an ongoing process of paramount importance. In this paper, we outline the basic elements of a quality assurance program for our linear accelerator that has been in use at Washington University Medical Center over the past 2 years. METHODS AND MATERIALS: Various devices and procedures have been developed to verify the accuracy and safety of the stereotactic radiosurgery regimen. Specifically, we present methods for assessing the attainment of spatially correct patient images, the reliability of the computerized treatment planning system, achieving physical safety for the patient, as well as the proper operation of the radiation treatment device. RESULTS: Our procedures have allowed us to assure quality patient treatments and, additionally, has permitted monitoring our performance for continual improvement. For example, a plot of targeting accuracy with the number of patients shows an asymptotic approach to a value within 0.6 mm of that ideally expected. CONCLUSION: To maintain high-quality patient care, one must review critical aspects of the treatment regimen on a periodic basis. Providing for the appropriate level of staff training, periodic reviews of procedures and maintenance of forms are also very important.

Objective evaluation of 3-D radiation treatment plans: a decision-analytic tool incorporating treatment preferences of radiation oncologists.

PURPOSE: Selecting the optimal radiation treatment plan from a set of competing plans involves making trade-offs among the doses delivered to the target volumes and normal tissues by the competing plans. Evaluation of 3-dimensional radiation treatment plans is difficult because it requires the review of vast amount of graphical and numerical data. We have developed an objective plan-ranking model based on the concepts of decision analysis. METHODS AND MATERIALS: Our model ranks a set of tentative radiation treatment plans from best to worst. A figure of merit is computed for each plan based on probabilities of possible clinical complications such as non-eradication of the tumor and radiation induced damage to the nearby healthy normal tissues, and weights which indicate their clinical relevance. This figure of merit is used to rank the plans. Key issues addressed by the model include the incorporation of individual treatment preferences of the radiation oncologist and clinical features of the patient. RESULTS: A methodology has been established for eliciting the treatment preferences of radiation oncologists. Results of this elicitation, and examples of several plan evaluations are presented. An interactive computer-based tool has been developed as one of a set of tools to assist in the evaluation of 3-dimensional radiation treatment plans. CONCLUSION: The paper presents a decision-analytic model incorporating radiation oncologists' treatment preferences and an interactive computer-based tool for objectively ranking competing radiation treatment plans. The tool can be used by radiation oncologists for the evaluation of competing plans, or as part of a system which tries to automatically generate optimal treatment plans using mathematical or symbolic techniques.

Use of transputers for real time dose calculation and presentation for three-dimensional radiation treatment planning.

PURPOSE: Real-time 3-dimensional dose calculation will allow display of isodose contours and other metrics for a planner to assess plan effectiveness during plan development, facilitating optimization. METHODS AND MATERIALS: Parallel processing provides an effective means to calculate 3-dimensional dose distribution in real-time while plan parameters are being chosen and adjusted. An array of 20 transputers and a high performance graphics workstation have demonstrated the feasibility of real-time 3-dimensional beam parameter specification, dose calculation, and dose-distribution presentation for evaluation. A mesh connected set of processors using surface processors to generate and terminate rays, and ray processors to calculate ray attenuation and dose distribution has been developed to efficiently utilize large numbers of processors and provide good load sharing, even for small beams that intersect only a small part of the volume. RESULTS: Our feasibility study has calculated dose distribution by the Effective Path Length method in about one second per beam for a treatment volume of 56,400 voxels. We expect to reduce the total time for computation, communication, and display, with even larger volumes, to less than one second. The number of processors can easily be increased for larger treatment volumes or more accurate and computation-intensive dose-calculation algorithms. Transputers provide an elegant and economical method for harnessing up to hundreds of powerful general-purpose processors for computational tasks including dose calculation and isodose contour generation. The same distributed-memory parallel-processing configuration is also suitable for calculation of isodose contours and dose-volume histograms for plan evaluation, automatic calculation of apertures and filters as beam parameters are manipulated, and more accurate dose calculation algorithms that incorporate the effects of scatter. CONCLUSION: Parallel processors can efficiently provide real-time calculation of the information necessary to evaluate treatment plans as they are developed allowing the planner to optimize the plan based on dose distribution and its effects on tumor control and complications.

Three-dimensional photon treatment planning of the intact breast.

Three-dimensional treatment planning for the intact breast was performed on two patients who had undergone CT scanning. A total of 38 treatment plans were evaluated. Multiple plans were evaluated for each patient including plans with and without inhomogeneity corrections, plans using varying photon energies of 60Co, 4 MV, 6 MV, 10 MV, and 15 MV, and three-dimensionally unconstrained plans. Increased hot spots were appreciated in the central axis plane when lung inhomogeneity corrections were used. Additional hot spots were appreciated in off-axis planes towards the cephalad and caudad aspects of the target volume because of lung inhomogeneity corrections and changes in the breast contour. The use of 60Co was associated with an increase in the magnitude and volume of hot spots, whereas the use of higher energy photons such as 10 MV and 15 MV was associated with an unacceptable target coverage at shallow depths. Therefore, for the two patients studied, the use of a medium energy photon beam (such as from a 6 MV linear accelerator) appeared to be the energy of choice for treatment of the intact breast. The three-dimensionally unconstrained plans were able to improve slightly upon the standard plans, particularly with relationship of dose to normal tissue structures. Areas for future research were identified, including the use of tissue compensators.

Three-dimensional treatment planning for para-aortic node irradiation in patients with cervical cancer.

Three-dimensional treatment planning has been used by four cooperating centers to prepare and analyze multiple treatment plans on two cervix cancer patients. One patient had biopsy-proven and CT-demonstrable metastasis to the para-aortic nodes, while the other was at high risk for metastatic involvement of para-aortic nodes. Volume dose distributions were analyzed, and an attempt was made to define the role of 3-D treatment planning to the para-aortic region, where moderate to high doses (50-66 Gy) are required to sterilize microscopic and gross metastasis. Plans were prepared using the 3-D capabilities for tailoring fields to the target volumes, but using standard field arrangements (3-D standard), and with full utilization of the 3-D capabilities (3-D unconstrained). In some but not all 3-D unconstrained plans, higher doses were delivered to the large nodal volume and to the volume containing gross nodal disease than in plans analyzed but not prepared with full 3-D capability (3-D standard). The small bowel was the major dose limiting organ. Its tolerance would have been exceeded in all plans which prescribed 66 Gy to the gross nodal mass, although some reduction in small bowel near-maximum dose was achieved in the 3-D unconstrained plans. All plans were able to limit doses to other normal organs to tolerance levels or less, with significant reductions seen in doses to spinal cord, kidneys, and large bowel in the 3-D unconstrained plans, as compared to the 3-D standard plans. A high probability of small bowel injury was detected in one of four 3-D standard plans prescribed to receive 50 Gy to the large para-aortic nodal volume; the small bowel dose was reduced to an acceptable level in the corresponding 3-D unconstrained plan. An optimum beam energy for treating this site was not identified, with plans using 4, 6, 10, 15, 18, and 25 MV photons all being equally acceptable. Attempts to deliver moderate or high doses (50-66 Gy) to this region should be made only after careful analysis of the plan with techniques similar to those employed in this study.

Three-dimensional treatment planning for postoperative treatment of rectal carcinoma.

The role of three-dimensional (3-D) treatment planning for postoperative radiation therapy was evaluated for rectal carcinoma as part of an NCI contract awarded to four institutions. It was found that the most important contribution of 3-D planning for this site was the ability to plan and localize target and normal tissues at all levels of the treatment volume, rather than using the traditional method of planning with only a single central transverse slice and simulation films. There was also a slight additional improvement when there were no constraints on the types of plans (i.e., when noncoplanar beams were used). Inhomogeneity considerations were not important at this site under the conditions of planning, i.e., with energies greater than 4 MV and multiple fields. Higher beam energies (15-25 MV) were preferred by a small margin over lower energies (down to 4 MV). The beam's eye view and dose-volume histograms were found quite useful as planning tools, but it was clear that work should continue on better 3-D displays and improved means of translating such plans to the treatment area.

Dose-volume histograms.

A plot of a cumulative dose-volume frequency distribution, commonly known as a dose-volume histogram (DVH), graphically summarizes the simulated radiation distribution within a volume of interest of a patient which would result from a proposed radiation treatment plan. DVHs show promise as tools for comparing rival treatment plans for a specific patient by clearly presenting the uniformity of dose in the target volume and any hot spots in adjacent normal organs or tissues. However, because of the loss of positional information in the volume(s) under consideration, it should not be the sole criterion for plan evaluation. DVHs can also be used as input data to estimate tumor control probability (TCP) and normal tissue complication probability (NTCP). The sensitivity of TCP and NTCP calculations to small changes in the DVH shape points to the need for an accurate method for computing DVHs. We present a discussion of the methodology for generating and plotting the DVHs, some caveats, limitations on their use and the general experience of four hospitals using DVHs.