State-of-the-art of external photon beam radiation treatment planning. Photon Treatment Planning Collaborative Working Group

Analysis of Irradiated Lung and Heart Volumes using Virtual Simulation in Postoperative Treatment of Stage I Breast Carcinoma

Aims and Background

The aim of the study was to assess the usefulness of virtual simulation in postoperative radiotherapy treatment planning of early-stage breast cancer and to evaluate its potential to reduce the volume of critical structures exposed compared to treatment plans produced by a conventional 2D system.

Methods and Study Design

Eighteen patients undergoing breast radiotherapy following conservative surgery for small breast carcinomas were studied. Scans from spiral CT equipment (with the patient in the treatment position) were transferred to a virtual simulator. From the screen images the operator contoured breast, lung and heart. Calculations were made of the extent to which the heart and lung were included in the irradiation fields (50% isodose line of tangential fields).

Results

Manual contouring was time-consuming, but when virtual simulation was used, the mean volume of the lung included in the radiation fields was significantly reduced compared to the 2D treatment plan (4.5% vs 5.4%, P = 0.034); in addition, a slight reduction was observed for the heart (0.5% to 1.2%), but this was not statistically significant.

Conclusions

With a 3D system we obtained optimal target coverage and a reduction of the dose to critical structures (statistically significant only for the lung). From a clinical point of view, this 0.9% reduction in the mean irradiated lung volume is probably not significant, as the percentage irradiated with a 2D system is considerably below the recommended value. Furthermore, our analysis was performed in a relatively small group of patients; for a reliable estimate larger series would be required. Consequently, the 3D system should not be considered in routine treatment after breast conserving surgery for early stage carcinomas; for the time being it should be reserved for selected cases.

Radiotherapy-induced malignancies: review of clinical features, pathobiology, and evolving approaches for mitigating risk

One of the most significant effects of radiation therapy on normal tissues is mutagenesis, which is the basis for radiation-induced malignancies. Radiation-induced malignancies are late complications arising after radiotherapy, increasing in frequency among survivors of both pediatric and adult cancers. Genetic backgrounds harboring germline mutations in tumor suppressor genes are recognized risk factors. Some success has been found with using genome wide association studies to identify germline polymorphisms associated with susceptibility. The insights generated by genetics, epidemiology, and the development of experimental models are defining potential strategies to offer to individuals at risk for radiation-induced malignancies. Concurrent technological efforts are developing novel radiotherapy delivery to reduce irradiation of normal tissues, and thereby, to mitigate the risk of radiation-induced malignancies. The goal of this review is to discuss epidemiologic, modeling, and radiotherapy delivery data, where these lines of research intersect and their potential impact on patient care.

Radiobiological indices that consider volume: a review

Understanding and predicting the impact of any radiotherapy treatment is critical if patients are to receive treatment with a high likelihood of eliminating the tumour and low likelihood of complications. One of the major contributing factors in determining these effects is the volume treated. This review assesses the current use and accuracy of a series of models which consider volume, building on a previous review which investigated the impact of fractionation particularly with respect to the linear quadratic model. Volume is particularly important in assessing the overall effect with respect to destroying the clonogenic cells and preventing damage to the normal tissues. Dose volume histograms are one of the simplest and most useful forms of representing volume information, however it is difficult to correlate plans based only on DVHs. For this reason various reduction schemes have been introduced and tumour control probability and normal tissues complication probability models adjusted to use this information. Many of these models have proved quite useful in the clinic although they are limited by the available radiobiological data.

[Foundations of the Monte Carlo method for dose calculation in radiotherapy]

Monte Carlo (MC) methods applied in dose calculation are based on fundamental principles of radiation interaction with matter. In contrast to other methods, the accuracy of dose calculation achievable with MC depends only on the determination of the beam quality and the interaction coefficients. Using MC techniques it is possible to predict the dose for clinical photon and electron beams with an accuracy of > +/- 2%. Especially for inhomogeneous regions like head, neck, and lung, the MC technique can significantly improve the accuracy compared to conventional algorithms. Therefore, in the present paper the basic features of the MC method are reviewed in the context of treatment planning in radiation therapy. The main shortcoming in the past, i.e., that MC algorithms are too slow to be acceptable for clinical purposes, could be solved by using faster computers and by introducing new variance reduction (VR) techniques. These techniques decrease the statistical fluctuations without increasing the number of particle histories. Therefore, MC calculation times in the order of a few minutes are possible. A brief overview of VR methods is provided.

Fast Monte Carlo dose calculation for photon beams based on the VMC electron algorithm

A new Monte Carlo algorithm for 3D photon dose calculation in radiation therapy is presented, which is based on the previously developed Voxel Monte Carlo (VMC) for electron beams. The main result is that this new version of VMC (now called XVMC) is more efficient than EGS4/PRESTA photon dose calculation by a factor of 15-20. Therefore, a standard treatment plan for photons can be calculated by Monte Carlo in about 20 min. on a "normal" personal computer. The improvement is caused mainly by the fast electron transport algorithm and ray tracing technique, and an initial ray tracing method to calculate the number of electrons created in each voxel by the primary photon beam. The model was tested in comparison to calculations by EGS4 using several fictive phantoms. In most cases a good coincidence has been found between both codes. Only within lung substitute dose differences have been observed.

Review and evaluation of MRI nonuniformity corrections for brain tumor response measurements

Current MRI nonuniformity correction techniques are reviewed and investigated. Many approaches are used to remedy this artifact, but it is not clear which method is the most appropriate in a given situation, as the applications have been with different MRI coils and different clinical applications. In this work four widely used nonuniformity correction techniques are investigated in order to assess the effect on tumor response measurements (change in tumor volume over time): a phantom correction method, an image smoothing technique, homomorphic filtering, and surface fitting approach. Six brain tumor cases with baseline and follow-up MRIs after treatment with varying degrees of difficulty of segmentation were analyzed without and with each of the nonuniformity corrections. Different methods give significantly different correction images, indicating that rf nonuniformity correction is not yet well understood. No improvement in tumor segmentation or in tumor growth/shrinkage assessment was achieved using any of the evaluated corrections.

[Virtual simulation: means and methodology]

State of the art imaging and computer systems coupled with the development of superior visualisation and multiplane reconstruction software have opened new perspectives in treatment planning. "Virtual simulation" of the irradiation technique allows definition in 3D of the optimised geometrical characteristics of the treatment beams with respect to anatomical structures of the patient. One can then visualise the beam direction and target volume coverage in the "virtual patient", leading to very precise radiotherapy. However, this "three dimensional" approach requires not only technical means, but also a methodology and an appropriate system of quality assurance for each step of the treatment planning, as well as significant multidisciplinary input.

A test of the claim that plan rankings are determined by relative complication and tumor-control probabilities

PURPOSE

This study tests an accepted claim regarding tumor control (TCP) and normal tissue complication (NTCP) probability functions. The claim is that treatment plans can be ranked using relative probabilities, even when the absolute probabilities are unknown. The assumption supports the use of probability models for plan optimization and the comparison of treatment techniques.

METHODS

The claim was tested using a hypothetical model consisting of two tissues, and illustrated with clinical data. Plans were scored using the probability of uncomplicated tumor control. The scores of different plans were compared by fixing their relative risks for an individual tissue complication, but adjusting the absolute probability levels up or down. The tested claim is that the plan rankings should not change.

RESULTS

In the two-tissue model, the rankings of competing plans were reversed by doubling all the probabilities. The preference ordering of lung cancer plans changed after the risk of pulmonary complication was reduced by 3-fold. In another site, the ranking of plans by overall complication-free probability was disturbed by errors that preserved the ordering of plans with respect to any individual complication. An adjustment of +/- 2.5% in the initial NTCP values for two tissues changed the direction in which a plan score moved in response to a fixed tradeoff in complication risk in an optimization search.

CONCLUSIONS

Contrary to claims, plan rankings are not determined by the relative probabilities of adverse events. The effect on plan scores of trading one complication for another depends on the absolute levels of risk. Absolute errors in NTCP and TCP functions result in the wrong ranking of plans, even when relative probabilities are correct. An optimization routine based on TCP and NTCP calculations may be forced in the wrong direction by small errors in the probability estimates.

The use of medical images in planning and delivery of radiation therapy

The authors provide a survey of how images are used in radiation therapy to improve the precision of radiation therapy plans, and delivery of radiation treatment. In contrast to diagnostic radiology, where the focus is on interpretation of the images to decide if disease is present, radiation therapy quantifies the extent of the region to be treated, and relates it to the proposed treatment using a quantitative modeling system called a radiation treatment planning (RTP) system. This necessitates several requirements of image display and manipulation in radiation therapy that are not usually important in diagnosis. The images must have uniform spatial fidelity: i.e., the pixel size must be known and consistent throughout individual images, and between spatially related sets. The exact spatial relation of images in a set must be known. Radiation oncologists draw on images to define target volumes; dosimetrists use RTP systems to superimpose quantitative models of radiation beams and radiation dose distributions on the images and on the sets of organ and target contours derived from them. While this mainly uses transverse cross-sectional images, projected images are also important, both those produced by the radiation treatment simulator and the treatment machines, and so-called "digital reconstructed radiographs," computed from spatially related sets of cross-sectional images. These requirements are not typically met by software produced for radiologists but are addressed by RTP systems. This review briefly summarizes ongoing work on software development in this area at the University of Washington Department of Radiation Oncology.

Dose-volume complication analysis for visual pathway structures of patients with advanced paranasal sinus tumors

PURPOSE

The purpose of the present work was to relate dose and volume information to complication data for visual pathway structures in patients with advanced paranasal sinus tumors.

METHODS AND MATERIALS

Three-dimensional (3D) dose distributions for chiasm, optic nerve, and retina were calculated and analyzed for 20 patients with advanced paranasal sinus malignant tumors. 3D treatment planning with beam's eye view capability was used to design beam and block arrangements, striving to spare the contralateral orbit (to lessen the chance of unilateral blindness) and frequently the ipsilateral orbit (to help prevent bilateral blindness). Point doses, dose-volume histogram analysis, and normal tissue complication probability (NTCP) calculations were performed. Published tolerance doses that indicate significant risk of complications were used as guidelines for analysis of the 3D dose distributions.

RESULTS

Point doses, percent volume exceeding a specified published tolerance dose, and NTCP calculations are given in detail for patients with complications versus patients without complications. Two optic nerves receiving maximum doses below the published tolerance dose sustained damage (mild vision loss). Three patients (of 13) without optic nerve sparing and/or chiasm sparing had moderate or severe vision loss. Complication data, including individual patient analysis to estimate overall risk for loss of vision, are given.

CONCLUSION

3D treatment planning techniques were used successfully to provide bilateral sparing of the globe for most patients. It was more difficult to spare the optic nerves, especially on the ipsilateral side, when prescription dose exceeded the normal tissue tolerance doses. NTCP calculations may be useful in assessing complication risk better than point dose tolerance criteria for the chiasm, optic nerve, and retina. It is important to assess the overall risk of blindness for the patient in addition to the risk for individual visual pathway structures.

The conceptual design of a radiation oncology planning system

The conceptual design of a three-dimensional, radiation oncology planning system is described. To assure that clinical needs were met, the working routines in two major Swedish radiation oncology departments were analysed in detail. Generic work flow was identified and mapped and compared to those in other institutions. The flow was partitioned into a number of nodes that together formed a basis for the design of the system handling logistics. The design criteria of this system emphasised accommodation of current clinical practice and traditional treatment modalities, and facilitated means to validate the computational techniques. The system should also allow for new procedures and was based on the analysis of current practice and a synthetic idea of how 3D treatment planning should be done. The final product supports the treatment planning work in its entirety. It is believed that the techniques followed are of interest to those engaged in computer systems of similar purposes and complexities.

Prism: a new approach to radiotherapy planning software

PURPOSE

We describe the capabilities and performance of Prism, an innovative new radiotherapy planning system with unusual features and design. The design and implementation strategies are intended to assure high quality and clinical acceptability. The features include Artificial Intelligence tools and special support for multileaf collimator (MLC) systems. The design provides unusual flexibility of operation and ease of expansion.

METHODS AND MATERIALS

We have implemented Prism, a three-dimensional (3D) radiotherapy treatment-planning system on standard commercial workstations with the widely available X window system. The design and implementation use ideas taken from recent software engineering research, for example, the use of behavioral entity-relationship modeling and the "Mediator Method" instead of ad-hoc programming. The Prism system includes the usual features of a 3D planning system, including Beam's Eye View and the ability to simulate any treatment geometry possible with any standard radiotherapy accelerator. It includes a rule-based expert system for automated generation of the planning target volume as defined in ICRU Report 50. In addition, it provides special support for planning treatments with a multileaf collimator (MLC). We also implemented a Radiotherapy Treatment Planning Tools Foundation for Prism, so that we are able to use software tools form other institutions without any source code modification.

RESULTS

The Prism system has been in clinical operation at the University of Washington since July 1994 and has been installed at several other clinics. The system is run simultaneously by several users, each with their own workstation operating from a common networked database and software. In addition to the dosimetrists, the system is used by radiation oncologists to define tumor and target volumes and by radiation therapists to select treatment setups to load into a computer controlled accelerator.

CONCLUSIONS

Experience with the installation and operation has shown the design to be effective as both a clinical and research tool. Integration of software tools has eased the development and significantly enhanced the clinical usability of the system. The design has been shown to be a sound basis for further innovation in radiation treatment planning software and for research in the treatment planning process.

Uncertainties in CT-based radiation therapy treatment planning associated with patient breathing

PURPOSE

To evaluate uncertainties associated with treatment-planning computed tomography (CT) data obtained with the patient breathing freely.

METHODS AND MATERIALS

Patients with thoracic or abdominal tumors underwent a standard treatment-planning CT study while breathing quietly and freely, followed by CT scans while holding their breath at normal inhalation and normal exhalation. Identical treatment plans on all three CT data sets for each patient pointed out differences in: (a) radiation path lengths; (b) positions of the organs; (c) physical volumes of the lung, liver, and kidneys; (d) the interpretation of plan evaluation tools such as dose-volume histograms and normal tissue complication probability (NTCP) models; and (e) how well the planning CT data set represented the average of the inhalation and exhalation studies.

RESULTS

Inhalation and exhalation data differ in terms of radiation path length (nearly one quarter of the cases had path-length differences > 1 cm), although the free breathing and average path lengths do not exhibit large differences (0-9 mm). Liver and kidney movements averaged 2 cm, whereas differences between the free breathing and average positions averaged 0.6 cm. The physical volume of the liver between the free breathing and static studies varied by as much as 12%. The NTCP calculations on exhale and inhale studies varied from 3 to 43% for doses that resulted in a 15% NTCP on the free-breathing studies.

CONCLUSION

Free-breathing CT studies may improperly estimate the position and volume of critical structures, and thus may mislead evaluation of plans based on such volume dependent criteria such as dose-volume histograms and NTCP calculations.

Criteria for the evaluation of treatment planning systems

Radiation treatment planning systems (RTPS) are evolving on a rapid and continual basis. After the evaluation of several commercial systems, we have developed a list of features we consider desirable in a product. The goal in the compilation of these criteria was a comprehensive worksheet which categorized the characteristics of RTPS into hardware (computer and peripheral devices), 2-D planning tools, 3-D planning tools, irregular field planning tools, and brachytherapy planning. With these distinctions, one can evaluate a system conforming to the specific planning needs, e.g., conformal therapy, dynamic therapy capabilities, or optimized remote afterloading brachytherapy, of a department. The rationales of the special requirements are provided for justification.

Head and neck cancer

This synthesis of the literature on radiotherapy for head and neck cancer is based on 424 scientific articles, including 3 meta-analyses, 38 randomized studies, 45 prospective studies, and 246 retrospective studies. These studies involve 79174 patients. The literature review shows that radiotherapy, either alone or in combination with surgery, plays an essential role in treating head and neck cancers. When tumors are localized, many tumor patients can be cured by radiotherapy alone and thereby maintain full organ function (1, 2). Current technical advancements in radiotherapy offer the potential for better local tumor control with lower morbidity (3). This, however, will require more sophisticated dose planning resources. To further improve treatment results for advanced tumors, other fractionation schedules, mainly hyperfractionation, should be introduced (5). This mainly increases the demands on staff resources for radiotherapy. The combination of radiotherapy and chemotherapy should be subjected to further controlled studies involving a sufficiently large number of patients (4, 5). Interstitial treatment (in the hands of experienced radiotherapists) yields good results for selected cancers. The method should be more generally accessible in Sweden. Intraoperative radiotherapy should be targeted for further study and development.

Multisectional planning for external beam radiotherapy: a "poor person's" alternative to three-dimensional treatment planning

Recent technical advances in the field of computers have led to the use of three-dimensional dose computation for optimizing a radiation therapy plan. However, in centers which lack such a state-of-the-art technology, one could explore the use of multisectional planning to obtain information about the dose profiles all along the target volume. This article highlights the utility of multisectional planning as an alternative to three-dimensional treatment planning systems for external beam radiation therapy.

Field displacement during external radiotherapy in prostatic adenocarcinoma treated with radioactive 198Au implants and external irradiation

The purpose of this work was to study displacement error and internal movements of the prostate during external beam radiotherapy. Verification films in the frontal (n = 194) and lateral (n = 64) portals were investigated in 14 patients treated with radioactive 198Au implants. Displacement errors of two implants were investigated. In seven patients, filling of the rectum and the bladder with contrast medium or isotonic saline was performed during CT investigation for planning purposes to detect movements of the prostate. Most (95%) of the displacement errors were less than 10 mm in the frontal portal and less than 15 mm in the lateral portals. No correlation to the patient's weight was found. The displacement errors were randomly distributed. The spatial relations between the implants were not altered during the treatments. Small movements of the prostate were observed. To conclude, the positioning system employed at present (laser) can be sufficient for the margins used (2 cm). In lateral portals, however, the system did not have the ability to detect a possible systematic displacement error from simulator to accelerator. The intention is to decrease the margins to 1 cm, which will necessitate a better positioning system.

Imaging of head and neck tumors with positron emission tomography and [11C]methionine

PURPOSE

To evaluate the value of positron emission tomography and [11C]methionine in imaging of malignant tumors of the head and neck region.

METHODS AND MATERIALS

Forty-seven tumors of the head and neck were investigated with 11C-labeled methionine and positron emission tomography before treatment. Because of the resolution limits of the positron emission tomography scanner, all tumors selected for the study were larger than 1 cm in diameter.

RESULTS

Forty-two (91%) of the 46 malignant tumors were clearly visible in the positron emission tomography image (squamous cell carcinoma, n = 26; lymphoma, n = 9; adenocystic carcinoma, n = 2; lymphoepithelioma, n = 1; adenocarcinoma, n = 1; transitional cell carcinoma, n = 1; esthesioneuroblastoma, n = 1; plasmocytoma, n = 1), while three (7%) squamous cell carcinomas were visible, but less easy to detect due to physiological accumulation of the tracer in the area under observation. Only one (2%) squamous cell carcinoma could not be delineated from the positron emission tomography image, and there was no uptake of [11C]methionine in a benign pleomorphic adenoma. No correlation was found between the uptake of [11C]methionine and the histological grade in the subset of squamous cell carcinoma (n = 30). High physiological uptake of [11C]methionine was observed in the salivary glands and the bone marrow.

CONCLUSIONS

Malignant head and neck tumors can be effectively imaged with positron emission tomography using [11C]methionine as the tracer.

Dose-volume histogram and 3-D treatment planning evaluation of patients with pneumonitis

PURPOSE

Tolerance of normal lung to inhomogeneous irradiation of partial volumes is not well understood. This retrospective study analyzes three-dimensional (3-D) dose distributions and dose-volume histograms for 63 patients who have had normal lung irradiated in two types of treatment situations.

METHODS AND MATERIALS

3-D treatment plans were examined for 21 patients with Hodgkin's disease and 42 patients with nonsmall-cell lung cancer. All patients were treated with conventional fractionation, with a dose of 67 Gy (corrected) or higher for the lung cancer patients. A normal tissue complication probability description and a dose-volume histogram reduction scheme were used to assess the data. Mean dose to lung was also calculated.

RESULTS

Five Hodgkin's disease patients and nine lung cancer patients developed pneumonitis. Data were analyzed for each individual independent lung and for the total lung tissue (lung as a paired organ). Comparisons of averages of mean lung dose and normal tissue complication probabilities show a difference between patients with and without complications. Averages of calculated normal tissue complication probabilities for groups of patients show that empirical model parameters correlate with actual complication rates for the Hodgkin's patients, but not as well for the individual lungs of the lung cancer patients treated to larger volumes of normal lung and high doses.

CONCLUSION

This retrospective study of the 3-D dose distributions for normal lung for two types of treatment situations for patients with irradiated normal lung gives useful data for the characterization of the dose-volume relationship and the development of pneumonitis. These data can be used to help set up a dose escalation protocol for the treatment of nonsmall-cell lung cancer.

The value of three-dimensional radiotherapy planning in advanced carcinoma of the urinary bladder based on computed tomography

Since 1987, radiotherapy planning of advanced bladder cancer T3-T4 has been based on computed tomography (CT), and since 1989 we have used a three-dimensional (3-D) planning system. The treatment plans of 110 patients referred to our department in 1989-1991 have been evaluated. The field sizes used after CT-based treatment planning were compared with the field sizes that would have been used after conventional planning alone (cystogram). The changes of the treatment plans are described. In 62 (60%) cases the treatment plans were altered because of the CT-scans, mainly as an enlargement of the fields. The field enlargements were mostly in the dorsal and caudal direction. In 10 (10%) cases the CT-scans provided diagnostic information which contra-indicated the proposed radical irradiation. The 3-D dose planning system has been of great value, especially the beams-eye view presentation which is an efficient tool for adjusting the field size.

Advances in 3-dimensional radiation treatment planning systems: room-view display with real time interactivity

PURPOSE

We describe our 3-dimensional (3-D) radiation treatment planning system for external photon and electron beam 3-D treatment planning which provides high performance computational speed and a real-time display which we have named "room-view" in which the simulated target volumes, critical structures, skin surfaces, radiation beams and/or dose surfaces can be viewed on the display monitor from any arbitrary viewing position.

METHODS AND MATERIALS

We have implemented the 3-D planning system on a graphics superworkstation with parallel processing. Patient's anatomical features are extracted from contiguous computed tomography scan images and are displayed as wireloops or solid surfaces. Radiation beams are displayed as a set of diverging rays plus the polygons formed by the intersection of these rays with planes perpendicular to the beam axis. Controls are provided for each treatment machine motion function. Photon dose calculations are performed using an effective pathlength algorithm modified to accommodate 3-D off-center ratios. Electron dose calculations are performed using a 3-D pencil beam model.

RESULTS

Dose distribution information can be displayed as 3-D dose surfaces, dose-volume histograms, or as isodoses superimposed on 2-D gray scale images of the patient's anatomy. Tumor-control-probabilities, normal-tissue-complication probabilities and a figure-of-merit score function are generated to aid in plan evaluation. A split-screen display provides a beam's-eye-view for beam positioning and design of patient shielding block apertures and a concurrent "room-view" display of the patient and beam icon for viewing multiple beam set-ups, beam positioning, and plan evaluation. Both views are simultaneously interactive.

CONCLUSION

The development of an interactive 3-D radiation treatment planning system with a real-time room-view display has been accomplished. The concurrent real-time beam's-eye-view and room-view display significantly improves the efficacy of the 3-D planning process.

Use of Veff and iso-NTCP in the implementation of dose escalation protocols

PURPOSE

This report investigates the use of a normal tissue complication probability (NTCP) model, 3-D dose distributions, and a dose volume histogram reduction scheme in the design and implementation of dose escalation protocols for irradiation of sites that are primarily limited by the dose to a normal tissue which exhibits a strong volume effect (e.g., lung, liver).

METHODS AND MATERIALS

Plots containing iso-NTCP contours are generated as a function of dose and partial volume using a parameterization of a NTCP description. Single step dose volume histograms are generated from 3-D dose distributions using the effective-volume (Veff) reduction scheme. In this scheme, the value of Veff for each dose volume histogram is independent of dose units (Gy, %). Thus, relative dose distributions (%) may be used to segregate patients by Veff into bins containing different ranges of Veff values before the assignment of prescription doses (Gy). The doses for each bin of Veff values can then be independently escalated between estimated complication levels (iso-NTCP contours).

RESULTS AND CONCLUSION

Given that for the site under study, an investigator believes that the NTCP parameterization and the Veff methodology at least describe the general trend of clinical expectations, the concepts discussed allow the use of patient specific 3-D dose/volume information in the design and implementation of dose escalation studies. The result is a scheme with which useful prospective tolerance data may be systematically obtained for testing the different NTCP parameterizations and models.

Three-dimensional display in planning radiation therapy: a clinical perspective. Photon Treatment Planning Collaborative Working Group

We describe the experience with 3-D display capabilities of four institutions engaged in a comparative three-dimensional treatment planning study. A high degree of interactivity was found to be desirable, which puts great demands on the hardware. Displays which were found particularly helpful included: display of multiple 2-D sections (transverse, sagittal, coronal and/or oblique) of one or more imaging studies; superimposition of outlines of structures of interest; superimposition of dose distributions displayed as isodose lines or as a color-wash overlay; beam's eye view displays of: structures displayed as contour-stacks or as shaded surfaces, dose distributions on the surface of structures, interactively viewed isodose surface displays, and digitally reconstructed radiographs. Dose-volume histograms provided valuable summarizations of dose data.

The role of uncertainty analysis in treatment planning

The role of uncertainty analysis in 3-D treatment planning systems was addressed by four institutions which contracted with NCI to evaluate high energy photon external beam treatment planning. Treatment plans were developed at eight disease sites and the effects of uncertainties assessed in a number of experiments. Uncertainties which are patient-site specific included variations in the delineation of target volumes and normal tissues and the effects of positional uncertainties due to physiological motion and setup nonreproducibility. These were found to have a potentially major impact on the doses to the target volumes and to critical normal tissues which could result in significantly altered probabilities of tumor control and normal tissue complications. Other uncertainties, such as the conversion of CT data to electron densities, heterogeneities and dose calculation algorithms' weaknesses, are related to physical processes. The latter was noted to have the greatest potential contribution to uncertainty in some sites. A third category of uncertainty related to the treatment machine, the consequences of compensator misregistration, are exclusive to the site and the treatment portal. Because conventional treatment planning systems have not incorporated uncertainty analysis, tools and techniques had to be devised for this work; further development in this area is needed. Many of the analyses could not have been done without full 3-D capabilities of the planning systems, and it can be anticipated that the availability of uncertainty analysis in these systems which allow nontraditional beam arrangements will be of great value.

Numerical scoring of treatment plans

This is a report on numerical scoring techniques developed for the evaluation of treatment plans as part of a four-institution study of the role of 3-D planning in high energy external beam photon therapy. A formal evaluation process was developed in which plans were assessed by a clinician who displayed dose distributions in transverse, sagittal, coronal, and arbitrary oblique planes, viewed dose-volume histograms which summarized dose distributions to target volumes and the normal tissues of interest, and reviewed dose statistics which characterized the volume dose distribution for each plan. In addition, tumor control probabilities were calculated for each biological target volume and normal tissue complication probabilities were calculated for each normal tissue defined in the agreed-upon protocols. To score a plan, the physician assigned a score for each normal tissue to reflect possible complications; for each target volume two separate scores were assigned, one representing the adequacy of tumor coverage, the second the likelihood of a complication. After scoring each target and normal tissue individually, two summary scores were given, one for target coverage, the second reflecting the impact on all normal tissues. Finally, each plan was given an overall rating (which could include a downgrading of the plan if the treatment was judged to be overly complex).