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Organ motion: Impact on conformal therapy and methods of compensation
Proceedings of the 40th A n n u a l A S T R O Meeting
PANEL V CONFORMAL RADIATION APPROACHES TO THE TREATMENT OF EARLY PROSTATE CANCER: TECHNIQUES, OUTCOMES & PITFALLS
Steven A. Leibel, M.D., John Blasko, M.D., P. William McLaughlin, M.D., James Purdy, Ph.D. Memorial Sloan-Kettering Cancer Center, Seattle Prostate Institute, Providence Cancer Center, Mallinckrodt Institue of Radiology
Three-dimensional conformal photon beam radiation therapy (3D-CRT) and brachytherapy alone or combined with external beam irradiation represent two approaches to conformal therapy in the treatment of patients with localized prostate cancer. Each of these approaches has its proponents. Patients who are candidates for each modality are often in a quandary as to which treatment to choose. There have been no randomized comparisons between these two modalities and little reporting of comparative data by patient characteristic stratification and using a common definition of PSA response. In this panel, the technical details of 3D CRT and brachytherapy will be reviewed and patient selection issues discussed. The biochemical outcome, acute morbidity and late toxicity of each modality will also be presented. Each of these approaches presents critical problems which remain unaddressed or unresolved. 3D-CRT represents a radical change in radiation oncology practice. Defining target volumes and organs at risk in 3D by drawing contours on CT images on a slice-by-slice basis, as opposed to drawing beam portals on a simulator radiograph, can be challenging. Another drawback of the 3D-CRT approach is the significant amount of time and effort required for contouring the target volume and sensitive normal tissue structures. In addition, considerably more dosimetric data must be analyzed when a 3D-CRT plan is evaluated. How large of margin to allow for the PTV (region to account for positional uncertainties) has not been fully determined, and it is now well-documented that there can be considerable motion of the prostate and seminal vesicles. Clearly, incorrect margins for PTV will result in a geographical miss. A more advanced 3D-CRT approach, intensity modulated radiation therapy (IMRT), is now being implemented. However, IMRT quality assurance procedures are not well established, introducing a potential for error. For brachytherapy, a variety of source distributions (peripheral vs homogeneous) have been used without a consensus regarding total prescription dose, margin and urethral dose. Post-implant studies have consistently demonstrated the potential for dosimetrically significant seed placement error, despite ultrasound guidance. Difficulties with image registration (ultrasound vs CT vs MRI) and prostatic edema complicate post-implant dosimetry and evaluation. Even if accurate post-implant dosimetry were possible, no guidelines regarding acceptability of implants have emerged. It is unclear without guidelines if a patient should undergo a second implant procedure to increase minimum target dosage. Further, without post-implant dosimetry and guidelines, dose-response studies (dose escalation or de-escalation) will not be feasible. Potential solutions to these pitfalls of 3D-CRT and brachytherapy will be discussed.
PANEL VI Organ Motion: Impact on Conformal Therapy and Methods of Compensation. Joos Lebesque, M.D., Ph.D., Randall K. Ten Haken, Ph.D. and John W. Wong, Ph.D. Moderator:
John W. Wong, Ph.D.
A fundamental requirement for effective conformal therapy is the proper prescription of a planning target volume (PTV). The PTV represents a 3-dimensional geometric region encompassing the clinical target volume to ensure that the latter will be treated to an adequate dose level despite the presence of setup variation and organ motion during the course of treatment. There is great impetus to optimize the prescription of the PTV. A properly reduced PTV will enhance dose escalation; whereas an inadequate one will lead to increased risks of treatment failure and/or deleterious complication. With increasing availability of imaging information from electronic portal imaging devices, volumetric CT and MRI scanners, significant strides have been made in understanding and addressing important issues about PTV prescription. Through the development of decision ruledriven correction strategies and advanced immobilization and repositioning devices, it is now possible to reduce the PTV margin for setup variation to within 5 mm. The problem of organ motion however poses a more difficult challenge and is the subject of several major studies. Organ motion can occur between (i.e., inter-) fractions as with the prostate and seminal vesicles; and during a (i.e. intra-) fraction as with motion associated with respiration. In either case, the nature of organ motion is complex as it is often associated, not only with spatial displacement, but also with deformation in shape and size. Such non-linear temporal and spatial characteristics of the motion greatly complicates the conformal therapy process. In this panel, the effects of organ motion on plan optimization, dose calculation and treatment delivery will be presented. Methods to minimize these effects will also be illustrated which include the adaptation of specific planning protocols; the implementation of decision rules for inter-fraction organ motion based on repeated daily CT scans; and the application of respiratory gating or breath-hold techniques for intra-fraction breathing motion. Finally, the difficult problem of calculating and evaluating 3-dimensional dose/volume and dose response data for an organ that deforms with time will be discussed.
121
PANEL V CONFORMAL RADIATION APPROACHES TO THE TREATMENT OF EARLY PROSTATE CANCER: TECHNIQUES, OUTCOMES & PITFALLS
Steven A. Leibel, M.D., John Blasko, M.D., P. William McLaughlin, M.D., James Purdy, Ph.D. Memorial Sloan-Kettering Cancer Center, Seattle Prostate Institute, Providence Cancer Center, Mallinckrodt Institue of Radiology
Three-dimensional conformal photon beam radiation therapy (3D-CRT) and brachytherapy alone or combined with external beam irradiation represent two approaches to conformal therapy in the treatment of patients with localized prostate cancer. Each of these approaches has its proponents. Patients who are candidates for each modality are often in a quandary as to which treatment to choose. There have been no randomized comparisons between these two modalities and little reporting of comparative data by patient characteristic stratification and using a common definition of PSA response. In this panel, the technical details of 3D CRT and brachytherapy will be reviewed and patient selection issues discussed. The biochemical outcome, acute morbidity and late toxicity of each modality will also be presented. Each of these approaches presents critical problems which remain unaddressed or unresolved. 3D-CRT represents a radical change in radiation oncology practice. Defining target volumes and organs at risk in 3D by drawing contours on CT images on a slice-by-slice basis, as opposed to drawing beam portals on a simulator radiograph, can be challenging. Another drawback of the 3D-CRT approach is the significant amount of time and effort required for contouring the target volume and sensitive normal tissue structures. In addition, considerably more dosimetric data must be analyzed when a 3D-CRT plan is evaluated. How large of margin to allow for the PTV (region to account for positional uncertainties) has not been fully determined, and it is now well-documented that there can be considerable motion of the prostate and seminal vesicles. Clearly, incorrect margins for PTV will result in a geographical miss. A more advanced 3D-CRT approach, intensity modulated radiation therapy (IMRT), is now being implemented. However, IMRT quality assurance procedures are not well established, introducing a potential for error. For brachytherapy, a variety of source distributions (peripheral vs homogeneous) have been used without a consensus regarding total prescription dose, margin and urethral dose. Post-implant studies have consistently demonstrated the potential for dosimetrically significant seed placement error, despite ultrasound guidance. Difficulties with image registration (ultrasound vs CT vs MRI) and prostatic edema complicate post-implant dosimetry and evaluation. Even if accurate post-implant dosimetry were possible, no guidelines regarding acceptability of implants have emerged. It is unclear without guidelines if a patient should undergo a second implant procedure to increase minimum target dosage. Further, without post-implant dosimetry and guidelines, dose-response studies (dose escalation or de-escalation) will not be feasible. Potential solutions to these pitfalls of 3D-CRT and brachytherapy will be discussed.
PANEL VI Organ Motion: Impact on Conformal Therapy and Methods of Compensation. Joos Lebesque, M.D., Ph.D., Randall K. Ten Haken, Ph.D. and John W. Wong, Ph.D. Moderator:
John W. Wong, Ph.D.
A fundamental requirement for effective conformal therapy is the proper prescription of a planning target volume (PTV). The PTV represents a 3-dimensional geometric region encompassing the clinical target volume to ensure that the latter will be treated to an adequate dose level despite the presence of setup variation and organ motion during the course of treatment. There is great impetus to optimize the prescription of the PTV. A properly reduced PTV will enhance dose escalation; whereas an inadequate one will lead to increased risks of treatment failure and/or deleterious complication. With increasing availability of imaging information from electronic portal imaging devices, volumetric CT and MRI scanners, significant strides have been made in understanding and addressing important issues about PTV prescription. Through the development of decision ruledriven correction strategies and advanced immobilization and repositioning devices, it is now possible to reduce the PTV margin for setup variation to within 5 mm. The problem of organ motion however poses a more difficult challenge and is the subject of several major studies. Organ motion can occur between (i.e., inter-) fractions as with the prostate and seminal vesicles; and during a (i.e. intra-) fraction as with motion associated with respiration. In either case, the nature of organ motion is complex as it is often associated, not only with spatial displacement, but also with deformation in shape and size. Such non-linear temporal and spatial characteristics of the motion greatly complicates the conformal therapy process. In this panel, the effects of organ motion on plan optimization, dose calculation and treatment delivery will be presented. Methods to minimize these effects will also be illustrated which include the adaptation of specific planning protocols; the implementation of decision rules for inter-fraction organ motion based on repeated daily CT scans; and the application of respiratory gating or breath-hold techniques for intra-fraction breathing motion. Finally, the difficult problem of calculating and evaluating 3-dimensional dose/volume and dose response data for an organ that deforms with time will be discussed.
121