[en] Optimized intensity-modulated treatments one of the important advances in photon radiotherapy. Intensity modulation provides a greatly increased control over dose distributions. Such control can be maximally exploited to achieve significantly higher levels of conformation to the desired clinical objectives using sophisticated optimization techniques. Safe, rapid and efficient delivery of intensity-modulated treatments has become feasible using a dynamic multi-leaf collimator under computer control. The need for all other field shaping devices such as blocks, wedges and compensators is eliminated. Planning and delivery of intensity-modulated treatments is amenable to automation and development of class solutions for each treatment site and stage which can be implemented not only at major academic centers but on a wide scale. A typical treatment involving as many as 10 fields can be delivered in times shorter than much simpler conventional treatments. The main objective of the course is to give an overview of the current state of the art of planning and delivery methods of intensity-modulated treatments. Specifically, the following topics will be covered using representative optimized plans and treatments: 1. A typical procedure for planning and delivering an intensity-modulated treatment. 2. Quantitative definition of criteria (i.e., the objective function) of optimization of intensity-modulated treatments. Clinical relevance of objectives and the dependence of the quality of optimized intensity-modulated plans upon whether the objectives are stated purely in terms of simple dose or dose-volume criteria or whether they incorporate biological indices. 3. Importance of the lateral transport of radiation in the design of intensity-modulated treatments. Impact on dose homogeneity and the optimum choice of margins. 4. Use of intensity-modulated treatments in escalation of tumor dose for the same or lower normal tissue dose. Fractionation of intensity-modulated treatments. The minimum number of beams in an intensity-modulated treatment. 5. The computer-controlled delivery of intensity-modulated treatments using a dynamic MLC. Methods to transform an intensity distribution into patterns of leaf motion. Consideration of leaf transmission, 'tongue-and-groove' effect, head scatter and rounded edges of leaves. Treatment times. 6. Safety and quality assurance issues. Dosimetric verification of treatments. Record and verify aspects (confirmation of leaf travel pattern). Optimized intensity-modulated 3DCRT is a rapidly evolving field. Its potential is being recognized and it is expected that its development and implementation will permit a significant escalation of tumor dose for the same or lower probability of normal tissue damage with a potential for improvement in local control and hence, survival

[en] Optimized intensity-modulated treatments one of the important advances in photon radiotherapy. Intensity modulation provides a greatly increased control over dose distributions. Such control can be maximally exploited to achieve significantly higher levels of conformation to the desired clinical objectives using sophisticated optimization techniques. Safe, rapid and efficient delivery of intensity-modulated treatments has become feasible using a dynamic multi-leaf collimator under computer control. The need for all other field shaping devices such as blocks, wedges and compensators is eliminated. Planning and delivery of intensity-modulated treatments is amenable to automation and development of class solutions for each treatment site and stage which can be implemented not only at major academic centers but on a wide scale. A typical treatment involving as many as 10 fields can be delivered in times shorter than much simpler conventional treatments. The main objective of the course is to give an overview of the current state of the art of planning and delivery methods of intensity-modulated treatments. Specifically, the following topics will be covered using representative optimized plans and treatments: 1. A typical procedure for planning and delivering an intensity-modulated treatment. 2. Quantitative definition of criteria (i.e., the objective function) of optimization of intensity-modulated treatments. Clinical relevance of objectives and the dependence of the quality of optimized intensity-modulated plans upon whether the objectives are stated purely in terms of simple dose or dose-volume criteria or whether they incorporate biological indices. 3. Importance of the lateral transport of radiation in the design of intensity-modulated treatments. Impact on dose homogeneity and the optimum choice of margins. 4. Use of intensity-modulated treatments in escalation of tumor dose for the same or lower normal tissue dose. Fractionation of intensity-modulated treatments. The minimum number of beams in an intensity-modulated treatment. 5. The computer-controlled delivery of intensity-modulated treatments using a dynamic MLC. Methods to transform an intensity distribution into patterns of leaf motion. Consideration of leaf transmission, 'tongue-and-groove' effect, head scatter and rounded edges of leaves. Treatment times. 6. Safety and quality assurance issues. Dosimetric verification of treatments. Record and verify aspects (confirmation of leaf travel pattern). Optimized intensity-modulated 3DCRT is a rapidly evolving field. Its potential is being recognized and it is expected that its development and implementation will permit a significant escalation of tumor dose for the same or lower probability of normal tissue damage with a potential for improvement in local control and hence, survival

[en] Purpose: To develop computer software that assists the planner avoid potential gantry collisions with the patient or patient support assembly during the treatment planning process. Methods and Materials: The approach uses a simulation of the therapy room with a scale model of the treatment machine. Because the dimensions of the machine and patient are known, one can calculate a priori whether any desired therapy field is possible or will result in a collision. To assist the planner, we have developed a graphical interface enabling the accurate visualization of each treatment field configuration with a 'room's eye view' treatment planning window. This enables the planner to be aware of, and alleviate any potential collision hazards. To circumvent blind spots in the graphic representation, an analytical software module precomputes whether each update of the gantry or turntable position is safe. Results: If a collision is detected, the module alerts the planner and suggests collision evasive actions such as either an extended distance treatment or the gantry angle of closest approach. Conclusions: The model enables the planner to experiment with unconventional noncoplanar treatment fields, and immediately test their feasibility

[en] The high density and atomic number of hip prostheses for patients undergoing pelvic radiotherapy challenge our ability to accurately calculate dose. A new clinical dose calculation algorithm, Monte Carlo, will allow accurate calculation of the radiation transport both within and beyond hip prostheses. The aim of this research was to investigate, for both phantom and patient geometries, the capability of various dose calculation algorithms to yield accurate treatment plans. Dose distributions in phantom and patient geometries with high atomic number prostheses were calculated using Monte Carlo, superposition, pencil beam, and no-heterogeneity correction algorithms. The phantom dose distributions were analyzed by depth dose and dose profile curves. The patient dose distributions were analyzed by isodose curves, dose-volume histograms (DVHs) and tumor control probability/normal tissue complication probability (TCP/NTCP) calculations. Monte Carlo calculations predicted the dose enhancement and reduction at the proximal and distal prosthesis interfaces respectively, whereas superposition and pencil beam calculations did not. However, further from the prosthesis, the differences between the dose calculation algorithms diminished. Treatment plans calculated with superposition showed similar isodose curves, DVHs, and TCP/NTCP as the Monte Carlo plans, except in the bladder, where Monte Carlo predicted a slightly lower dose. Treatment plans calculated with either the pencil beam method or with no heterogeneity correction differed significantly from the Monte Carlo plans

[en] To overcome the limitations of the intensity modulation optimization techniques based on dose criteria, we introduce a method for optimizing intensity distributions in which we employ an objective function based on biological indices. The objective function also includes constraints on dose and dose-volume combinations to ensure that the results are consistent with the physician's judgment. We apply a variant of the steepest-descent method to optimize the objective function. The method is three-dimensional and incorporates scattered radiation in the optimization process using an iterative scheme employing the pencil beam convolution method. Previously we had shown that the inverse technique of obtaining optimum intensity distributions, for which the objectives are defined in terms of a desired uniform dose to the target volume and desired upper limits of dose to normal organs, produces satisfactory approximations of the desired dose distributions for prostate plans. However, for lung, the performance of this technique was considerably inferior. Our conclusion was that, in general, it is not sufficient to specify the objectives of optimization purely in terms of a desired pattern of dose and that the objectives should also incorporate biology, perhaps in the form of biological indices. We demonstrate that the biology-based approach produces lung plans that are superior to those produced when only dose-based objectives are used. For the treatment of prostate, the two methods produce comparable dose distributions

[en] Purpose: The dose distributions of intensity-modulated radiotherapy (IMRT) treatment plans can be shown to be significantly superior in terms of higher conformality if designed to simultaneously deliver high dose to the primary disease and lower dose to the subclinical disease or electively treated regions. We use the term 'simultaneous integrated boost' (SIB) to define such a treatment. The purpose of this paper is to develop suitable fractionation strategies based on radiobiological principles for clinical trials and routine use of IMRT of head and neck (HN) cancers. The fractionation strategies are intended to allow escalation of tumor dose while adequately sparing normal tissues outside the target volume and considering the tolerances of normal tissues embedded within the primary target volume. Methods and Materials: IMRT fractionation regimens are specified in terms of 'normalized total dose' (NTD), i.e., the biologically equivalent dose given in 2 Gy/fx. A linear-quadratic isoeffect formula is applied to convert NTDs into 'nominal' prescription doses. Nominal prescription doses for a high dose to the primary disease, an intermediate dose to regional microscopic disease, and lower dose to electively treated nodes are used for optimizing IMRT plans. The resulting nominal dose distributions are converted back into NTD distributions for the evaluation of treatment plans. Similar calculations for critical normal tissues are also performed. Methods developed were applied for the intercomparison of several HN treatment regimens, including conventional regimens used currently and in the past, as well as SIB strategies. This was accomplished by comparing the biologically equivalent NTD values for the gross tumor and regional disease, and bone, muscle, and mucosa embedded in the gross tumor volume. Results: (1) A schematic HN example was used to demonstrate that dose distributions for SIB IMRT are more conformal compared to dose distributions when IMRT is divided into a large-field phase and a boost phase. Both were shown to be significantly superior compared to dose distributions obtained using conventional beams for the large-field phase followed by IMRT for the boost phase. (2) The relationship between NTD and nominal dose for HN tumors was found to be quite sensitive to the choice of tumor clonogen doubling time but relatively insensitive to other parameters. (3) For late effect normal tissues embedded in the tumor volume and assumed to receive the same dose as the tumor, the biologically equivalent NTD for the SIB IMRT may be significantly higher. (4) Normal tissues outside the target volume receive lower dose due to the higher conformality of the IMRT plans. The biologically equivalent NTDs are even lower due to the lower dose per fraction in the SIB strategy. Conclusions: IMRT dose distributions are most conformal when designed to be delivered as SIB. Using isoeffect radiobiological relationships and published HN data, fractionation strategies can be designed in which the nominal dose levels to the primary, regional disease and electively treated volumes are appropriately adjusted, each receiving different dose/fx. Normal tissues outside the treated volumes are at reduced risk in such strategies since they receive lower total dose as well as lower dose/fx. However, the late effect toxicities of tissues embedded within the primary target volume and assumed to receive the same dose as the primary may pose a problem. The efficacy and safety of the proposed fractionation strategies will need to be evaluated with careful clinical trials

[en] High atomic number inserts, such as hip prostheses and dental fillings, cause streak artifacts on computed tomography (CT) images when filtered back-projection (FBP) methods are used. These streak artifacts severely degrade our ability to differentiate the tumor volume. Also, incorrect Hounsfield numbers yield incorrect electron density information that may lead to erroneous dose calculations, and, as a result, compromise clinical outcomes. The aim of this research was to evaluate the dosimetric consequences of artifacts during radiotherapy planning of a prostate patient containing a hip prosthesis. The CT numbers corresponding to an iron prosthesis were inserted into the right femoral head of an existing CT image set. This artifact-free image was used as the standard image set. CT projections through the image set formed the sinogram, from which filtered back projection and iterative deblurring methods were used to create reconstructed image sets. These reconstructed image sets contained artifacts. Prostate treatment plans were then calculated using a Monte Carlo system for the standard and reconstructed CT image sets. Close to the prosthesis, the CT numbers between the reconstructed and standard image sets differed substantially. However, because the CT number differences covered only a small area, the dose distributions on the reconstructed and standard image sets were not significantly different. The dose-volume histograms for the prostate, rectum, and bladder were virtually identical. Our results indicate that even though CT image artifacts restrict our ability to differentiate tumors and critical structures, the dose distributions for a prostate plan containing a hip prosthesis, calculated on both artifact-free image sets and image sets containing artifacts, are not significantly different

[en] Purpose: Three dimensional conformal radiation treatments are complex, often involving large numbers of blocked or multileaf collimated fields that shape regions of high dose to conform to the treatment volume. As manual definition and digitization of aperture shapes and their corresponding multileaf configurations can be impractically time consuming, it was necessary to integrate the planning of multileaf fields into an existing three dimensional treatment planning system and improve the efficiency of treatment delivery to make these treatments feasible on a routine basis. Methods and Materials: A subfunction of the Beam's Eye View (BEV) component can be used to automatically generate a continuous aperture shape with a margin around the tumor to account for beam penumbra, and excluding any normal structures to be spared (each with its own margin). To convert a continuous aperture shape into one defined by the multileaf collimator (MLC), a leaf coverage mode is chosen to determine how leaves are fitted to aperture shapes. The conversion process also considers parameters of the specific MLC system, e.g., leaf thickness and the number of leaves. If normal structures to be shielded split the target into multiple regions, more than one multileaf aperture can result. An interactive leaf adjustment routine is also provided to allow for modification of individual leaf positions. Dose calculation programs then take into account multileaf apertures for computation of dose distributions using a pencil beam convolution model. Finally, prescription files specifying leaf and jaw configurations are prepared in treatment machine specific formats and downloaded to the computers driving the multileaf collimators and other components of the treatment machines. Results and Conclusion: An example is presented of a prostate treatment plan, with MLC configurations, dose distributions, and treatment delivery description, along with discussion of clinical implementation at Memorial Hospital

[en] Purpose: The equivalent uniform dose (EUD) for tumors is defined as the biologically equivalent dose that, if given uniformly, will lead to the same cell kill in the tumor volume as the actual nonuniform dose distribution. Recently, a new formulation of EUD was introduced that applies to normal tissues as well. EUD can be a useful end point in evaluating treatment plans with nonuniform dose distributions for three-dimensional conformal radiotherapy and intensity-modulated radiotherapy. In this study, we introduce an objective function based on the EUD and investigate the feasibility and usefulness of using it for intensity-modulated radiotherapy optimization. Methods and Materials: We applied the EUD-based optimization to obtain intensity-modulated radiotherapy plans for prostate and head-and-neck cancer patients and compared them with the corresponding plans optimized with dose-volume-based criteria. Results: We found that, for the same or better target coverage, EUD-based optimization is capable of improving the sparing of critical structures beyond the specified requirements. We also found that, in the absence of constraints on the maximal target dose, the target dose distributions are more inhomogeneous, with significant hot spots within the target volume. This is an obvious consequence of unrestricted maximization target cell kill and, although this may be considered beneficial for some cases, it is generally not desirable. To minimize the magnitude of hot spots, we applied dose inhomogeneity constraints to the target by treating it as a 'virtual' normal structure as well. This led to much-improved target dose homogeneity, with a small, but expected, degradation in normal structure sparing. We also found that, in principle, the dose-volume objective function may be able to arrive at similar optimum dose distributions by using multiple dose-volume constraints for each anatomic structure and with considerably greater trial-and-error to adjust a large number of objective function parameters. Conclusion: The general inference drawn from our investigation is that the EUD-based objective function has the advantages that it needs only a small number of parameters and allows exploration of a much larger universe of solutions, making it easier for the optimization system to balance competing requirements in search of a better solution

[en] Purpose: Fixed field fractionated stereotactic conformal radiotherapy (FFSRT) combines the field shaping properties of multiple custom blocked fields with precise stereotactic immobilization for use as an alternative to traditional radiosurgery in the treatment of brain tumors and AVMs. The use of NTCP (normal tissue complication probability) may be of value in selecting block margin and tumor coverage criteria for individual plans or to compare different FFSRT methods (standard 3D vs. intensity modulation). In particular, NTCP may augment traditional parameters such as PITV (prescription isodose vol./target vol.), MDPD (maximum dose/prescription dose), DVH (dose volume histogram), integral dose and 3D surface isodose display in this regard. Materials and Methods: Selected intracranial targets (2-6 cm) were planned using 6 noncoplanar fixed fields. The planning target volume (PTV) was defined as the gross tumor volume plus 1.5 mm to account for patient set-up uncertainty with a relocatable stereotactic frame. First; FFSRT treatment plans for each target were generated; each with varying block margins (0-10 mm). Each plan required 100% target (PTV) coverage by the prescription isodose. Treatment plans were then generated using a constant block margin of 5 mm for each target and PTV coverage was varied from 90 - 100%. Complication probabilities of the whole brain and non-target whole brain were generated for each plan using the Lyman model with several different sets of parameters (TD50, slope and volume factors). A dose of 100 Gy was prescribed. DVH, PITV, MDPD and integral dose were calculated. Also, a DVH analysis of the component of PTV outside the prescription isodose was performed for each case. For some targets intensity modulated treatment plans and corresponding NTCP and volume parameters were computed and compared to standard 3D FFSRT plans. Results: The average NTCPs (target coverage 100%) for margins of 0, 2.5, 3.75, 5, 6.25, 7.5, and 10 mm were 92, 44, 30, 26, 34, 41, and 63 respectively. The average margin of minimum NTCP was 4 mm and this varied with target shape, volume and location in the brain. The same general trends for NTCP were seen for inclusion and exclusion of the target volume and for various sets of NTCP parameters. These minimum NTCPs resulted in a MDPD of 1.35 on average. Further increasing MDPD by decreasing block margin did not result in improved NTCP (even for non-target whole brain). When the margin was kept constant at the minimum NTCP value and the tumor coverage re-normalized to 100, 99, 97.5, 95, 92.5 and 90%, the resulting NTCPs varied significantly (range; 26 for 100% coverage and 10 for 90% coverage). DVHs performed on the portion of PTV outside the prescription isodose showed an average minimum PTV dose of 90 - 100 Gy. Intensity modulated treatment plans had lower NTCP values for irregularly shaped targets. Conclusion: Complication probabilities varied significantly with small changes in block margin and tumor coverage despite minimal changes in brain DVH, and other dose based criteria. Changing NTCP parameters changed the magnitude and not relative differences in NTCP values. We have demonstrated a utility for using complication probabilities to compare plans and to guide us in selecting margin and tumor coverage criteria for employing FFSRT