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23 Inverse dose planning
Symposia
21
Wednesday, 31 January 2001
$7
oral
Radiation oncology services in developing countries: the backlog and constraints,
23
V. Levirt, H. Tatsuzaki International Atomic Energy Agency, Applied Radiobiology and Radiotherapy, Vienna, Austria Purpose: To identify the progress made in initiating and expanding radiotherapy services in developing countries and to determine whether equipment or personnel determine the extent of insufficiency of such services. Method: The resources utilised for radiotherapy in Africa, South East Asia, Southern and Central Americas and Eastern Europe have been surveyed by the IAEA. These have been related to economic, level of health care and incidence of cancer indices on a country and regional basis. Correlations identified. Results: Teletherapy equipment has expanded three4old in the last decade in most regions. The availability correlates with the economic index, GNP/Capita, but with regional distinctions. Brachytherapy equipment appears to be unrelated to cervical cancer incidences - the most common use of this modality. Personnel availability and training are highly significant constraints on the utilisation of existing equipment. Conclusion: National objectives in cancer control and especially cancer management need to be clearly identified. A plan of action tailored for each country with a realistic perception of constraints needs to be developed.
L. Verhey Univ. of California, San Francisco, USA Intensity-modulated radiotherapy (IMRT) is being investigated at a number of institutions around the world. This type of radiotherapy uses simple or complex variations of intensity across defined fields to yield additional degrees of freedom and more conformal distributions that can simultaneously deliver tolerable low doses to defined sensitive normal tissues and lethal doses to defined tumors. Simple IMRT can be defined as that which can be planned with existing 3DCRT treatment planning systems using iterative optimization. These IMRT plans include fields made up of two or more subfields, normally shaped with a multileaf collimator (MLC), at least one of which is designed to reduce dose to overlying normal tissues. A conceptual example of such a simple IMRT beam is one made up of 2 segments, one of which treats the entire projection of the target with normal margins and a second which treats only that portion of the target that is not shadowed by a sensitive normal tissue. This type of simple IMRT method is described in the literature and is in routine use in some facilities. Since these types of treatments can be planned with conventional 3D treatment planning programs, they can be considered part of 3DCRT. At UCSF, boost treatments of prostate tumors have been planned and treated using these simple IMRT methods. This group has reported the use of magnetic resonance spectroscopy (MRS) to identify the dominant intraprostatic lesion (DIL) which corresponds to the area of gross tumor within the prostate, presumably requiring a high radiation dose to control. Simple IMRT methods have been used to design an 18-20 field treatment (7 beam directions, 2 or 3 subfields per beam) that spares the rectum, treats the body of the prostate to 75.6 Gy and simultaneously boosts the DIL to ~ 90 Gy, As shown by this example, the extension of 3DCRT methods to include simple IMRT can be a very powerful method of dealing with complex radiotherapy problems. In our experience, treatments of up to 20 field segments can be easily treated in less than 15 minutes using modern accelerator control systems. General IMRT includes those beam delivery methods that require the use of inverse treatment planning programs with computerized optimization. Although a number of such programs are currently under development, the majority of worldwide experience is with a single commercial versiona. Planning programs that use computerized optimization are referred to as "inverse" since the process of determining the optimum intensity pattern for each given beam direction begins with integrating the electron density of the tissues from the distal edge of the target to the surface of the patient, i.e., the inverse of CT scanning. The planner begins by defining the desired doses to target and specified normal tissues, the number of beams, their directions and the maximum allowed complexity of the intensity pattern. The program returns with intensity distributions for each beam that will result in the best approximation to the desired dose distribution within constraints. A delivery method is then selected that can reproduce that intensity pattern. Once the contouring and prescription information is entered, the time required to obtain a satisfactory treatment plan can be fairly short (less than 30 minutes) with an appropriate choice of computer hardware. These inverse planning programs define a cost function that includes the dose goals to target and specified normal tissues. Once the beam directions have been selected, this cost function is minimized by varying the intensities and beam segment weights. A number of delivery methods have been proposed, including the use of specially manufactured high-Z compensators for each field, "step and shoot" segmental IMRT using multiple MLC fields (SMLC-IMRT), dynamic IMRT with velocity modulated MLC from fixed beam directions (DMLCIMRT), "tomotherapy" which is dynamic arcing IMRT with either contiguous fan beams (Peacock MIMiCb) or continuous spiral arc and intensity modulated arc therapy using a full-field MLC (IMAT). Most patients have been treated with the compensator, MIMIC, SMLC-IMRT or DMLC-IMRT delivery methods. This presentation will compare conventional "forward" and inverse planning systems in terms of achievable dose distributions, required planning time, quality assurance and delivery constraints.
IMRT + P O R T A L IMAGING 22
Target movement and its impact on the probability of correct target dosage M. van Herk, P. Remeijer, J. V. Lebesque The Netherlands Cancer Institute, Radiotherapy Department, Amsterdam, The Netherlands Introduction: The accuracy of radiotherapy is limited by the following error sources: delineation uncertainties, organ (or target) movement and setup error. What is pften forgotten is that organ motion and setup error influence both treatment preparation (planning) and treatment execution. The purpose of this study is to investigate the effects of organ motion in combination with other errors on the dose received by the CTV and to devise ways to reduce the effect of organ motion. Material and methods: We developed a tool to evaluate probability distributions of various CTV dose parameters in realistic treatment plans when considering all error sources. The tool reads dose matrices and CTV contours from the planning system. The dose matrix is first convolved with the random errors to estimate the cumulative dose distribution. Systematic errors cause a shift of the CTV relative to the dose distribution. Therefore, the CTV is next displaced with respect to its planned position in all possible directions while computing several parameters (e.g., minimum dose, TCP) of the CTV dose. i.e., the resulting dose parameters are a function of the systematic error in x, y, and z. The interaction of the CTV shape with the dose distribution gives the function an irregular shape. For instance, due to the intended rectum sparing, a systematic error in posterior direction leads to a fast reduction of the minimum dose parameter in prostate treatments. The probability that a given dose parameter value is reached is computed by integrating the estimated error distribution within the isocontours for this value. Similarly, TCP curves including geometrical errors are computed. The tool also handles rotations, in which case the dose parameter function and probability distribution have more than three dimensions. Results: With 2 mm dose grid size the algorithm takes a few minutes and computed dose parameters are within 1% of the analytical solution for idealized situations. Next, we simulated prostate treatments (planned with 10 mm margin) using published uncertainty data including rotations, while varying selected error sources. When varying the systematic translations (using a normal distribution), the TCP starts to reduce at 5 mm SD. Since the measured systematic error is about 4 mm SD for each axis, the used margin of 10 mm is safe. When replacing the Gaussian systematic error distribution by a periodic function modeled after breathing, we found much smaller effects. Conclusions: This probability based analysis provides insight into the effects of geometrical deviations on a population of patients. Errors introduced during the treatment preparation phase have a much larger impact on the CTV dose than treatment preparation errors. A feasible method to reduce the impact of organ motion is therefore to acquire multiple planning CT scans to improve the estimation of the average position of the moving organ.
Inverse dose planning
21
Wednesday, 31 January 2001
$7
oral
Radiation oncology services in developing countries: the backlog and constraints,
23
V. Levirt, H. Tatsuzaki International Atomic Energy Agency, Applied Radiobiology and Radiotherapy, Vienna, Austria Purpose: To identify the progress made in initiating and expanding radiotherapy services in developing countries and to determine whether equipment or personnel determine the extent of insufficiency of such services. Method: The resources utilised for radiotherapy in Africa, South East Asia, Southern and Central Americas and Eastern Europe have been surveyed by the IAEA. These have been related to economic, level of health care and incidence of cancer indices on a country and regional basis. Correlations identified. Results: Teletherapy equipment has expanded three4old in the last decade in most regions. The availability correlates with the economic index, GNP/Capita, but with regional distinctions. Brachytherapy equipment appears to be unrelated to cervical cancer incidences - the most common use of this modality. Personnel availability and training are highly significant constraints on the utilisation of existing equipment. Conclusion: National objectives in cancer control and especially cancer management need to be clearly identified. A plan of action tailored for each country with a realistic perception of constraints needs to be developed.
L. Verhey Univ. of California, San Francisco, USA Intensity-modulated radiotherapy (IMRT) is being investigated at a number of institutions around the world. This type of radiotherapy uses simple or complex variations of intensity across defined fields to yield additional degrees of freedom and more conformal distributions that can simultaneously deliver tolerable low doses to defined sensitive normal tissues and lethal doses to defined tumors. Simple IMRT can be defined as that which can be planned with existing 3DCRT treatment planning systems using iterative optimization. These IMRT plans include fields made up of two or more subfields, normally shaped with a multileaf collimator (MLC), at least one of which is designed to reduce dose to overlying normal tissues. A conceptual example of such a simple IMRT beam is one made up of 2 segments, one of which treats the entire projection of the target with normal margins and a second which treats only that portion of the target that is not shadowed by a sensitive normal tissue. This type of simple IMRT method is described in the literature and is in routine use in some facilities. Since these types of treatments can be planned with conventional 3D treatment planning programs, they can be considered part of 3DCRT. At UCSF, boost treatments of prostate tumors have been planned and treated using these simple IMRT methods. This group has reported the use of magnetic resonance spectroscopy (MRS) to identify the dominant intraprostatic lesion (DIL) which corresponds to the area of gross tumor within the prostate, presumably requiring a high radiation dose to control. Simple IMRT methods have been used to design an 18-20 field treatment (7 beam directions, 2 or 3 subfields per beam) that spares the rectum, treats the body of the prostate to 75.6 Gy and simultaneously boosts the DIL to ~ 90 Gy, As shown by this example, the extension of 3DCRT methods to include simple IMRT can be a very powerful method of dealing with complex radiotherapy problems. In our experience, treatments of up to 20 field segments can be easily treated in less than 15 minutes using modern accelerator control systems. General IMRT includes those beam delivery methods that require the use of inverse treatment planning programs with computerized optimization. Although a number of such programs are currently under development, the majority of worldwide experience is with a single commercial versiona. Planning programs that use computerized optimization are referred to as "inverse" since the process of determining the optimum intensity pattern for each given beam direction begins with integrating the electron density of the tissues from the distal edge of the target to the surface of the patient, i.e., the inverse of CT scanning. The planner begins by defining the desired doses to target and specified normal tissues, the number of beams, their directions and the maximum allowed complexity of the intensity pattern. The program returns with intensity distributions for each beam that will result in the best approximation to the desired dose distribution within constraints. A delivery method is then selected that can reproduce that intensity pattern. Once the contouring and prescription information is entered, the time required to obtain a satisfactory treatment plan can be fairly short (less than 30 minutes) with an appropriate choice of computer hardware. These inverse planning programs define a cost function that includes the dose goals to target and specified normal tissues. Once the beam directions have been selected, this cost function is minimized by varying the intensities and beam segment weights. A number of delivery methods have been proposed, including the use of specially manufactured high-Z compensators for each field, "step and shoot" segmental IMRT using multiple MLC fields (SMLC-IMRT), dynamic IMRT with velocity modulated MLC from fixed beam directions (DMLCIMRT), "tomotherapy" which is dynamic arcing IMRT with either contiguous fan beams (Peacock MIMiCb) or continuous spiral arc and intensity modulated arc therapy using a full-field MLC (IMAT). Most patients have been treated with the compensator, MIMIC, SMLC-IMRT or DMLC-IMRT delivery methods. This presentation will compare conventional "forward" and inverse planning systems in terms of achievable dose distributions, required planning time, quality assurance and delivery constraints.
IMRT + P O R T A L IMAGING 22
Target movement and its impact on the probability of correct target dosage M. van Herk, P. Remeijer, J. V. Lebesque The Netherlands Cancer Institute, Radiotherapy Department, Amsterdam, The Netherlands Introduction: The accuracy of radiotherapy is limited by the following error sources: delineation uncertainties, organ (or target) movement and setup error. What is pften forgotten is that organ motion and setup error influence both treatment preparation (planning) and treatment execution. The purpose of this study is to investigate the effects of organ motion in combination with other errors on the dose received by the CTV and to devise ways to reduce the effect of organ motion. Material and methods: We developed a tool to evaluate probability distributions of various CTV dose parameters in realistic treatment plans when considering all error sources. The tool reads dose matrices and CTV contours from the planning system. The dose matrix is first convolved with the random errors to estimate the cumulative dose distribution. Systematic errors cause a shift of the CTV relative to the dose distribution. Therefore, the CTV is next displaced with respect to its planned position in all possible directions while computing several parameters (e.g., minimum dose, TCP) of the CTV dose. i.e., the resulting dose parameters are a function of the systematic error in x, y, and z. The interaction of the CTV shape with the dose distribution gives the function an irregular shape. For instance, due to the intended rectum sparing, a systematic error in posterior direction leads to a fast reduction of the minimum dose parameter in prostate treatments. The probability that a given dose parameter value is reached is computed by integrating the estimated error distribution within the isocontours for this value. Similarly, TCP curves including geometrical errors are computed. The tool also handles rotations, in which case the dose parameter function and probability distribution have more than three dimensions. Results: With 2 mm dose grid size the algorithm takes a few minutes and computed dose parameters are within 1% of the analytical solution for idealized situations. Next, we simulated prostate treatments (planned with 10 mm margin) using published uncertainty data including rotations, while varying selected error sources. When varying the systematic translations (using a normal distribution), the TCP starts to reduce at 5 mm SD. Since the measured systematic error is about 4 mm SD for each axis, the used margin of 10 mm is safe. When replacing the Gaussian systematic error distribution by a periodic function modeled after breathing, we found much smaller effects. Conclusions: This probability based analysis provides insight into the effects of geometrical deviations on a population of patients. Errors introduced during the treatment preparation phase have a much larger impact on the CTV dose than treatment preparation errors. A feasible method to reduce the impact of organ motion is therefore to acquire multiple planning CT scans to improve the estimation of the average position of the moving organ.
Inverse dose planning