Volume 35, Issue 6, June 2008
Index of content:
- Therapy: Continuing Education Course: Auditorium A
- CE‐Therapy: Monte Carlo I: Review of the AAPM TG105
35(2008); http://dx.doi.org/10.1118/1.2962319View Description Hide Description
A special class of Monte Carlo (MC)‐based dose calculation codes optimized for photon and electron beams in patient‐specific geometries has invigorated interest in the use of MC‐based dose calculations for radiotherapytreatment planning. In general, this class of “second generation” codes, including VMC++, XVMC, and DPM, among others, employ electron‐step algorithms that converge faster, i.e. fewer condensed‐history steps are required for the same precision versus “first‐generation” codes (such as EGS and MCNP). These advances, coupled with the use of sophisticated variance reduction techniques (e.g. directional bremsstrahlung splitting), have made it possible to perform MC‐based photon beam dose calculations, in some instances, of the entire linear acceleratortreatment head and patient‐geometry, within minutes on a single processor. Consequently, several commercial vendors have released or are currently in the process of releasing MC algorithms for photon and/or electron beamtreatment planning. With the impending availability of MC‐based dose calculation algorithms for routine clinical treatment planning, it is important that strategies and paradigms for clinical commissioning and implementation of these systems be formulated and discussed. We provide a review of AAPM Task Group Report No. 105 (Med. Phys. 34 (2007) 4818–4853), a document which outlines the important aspects of a MC‐based dose calculation algorithm, from the basic aspects of the use of the MC method for radiationtransport to the application of this approach in routine clinical photon and electron beamtreatment planning.
1. To provide an educational review of the physics of the MC method including discussion of the approaches used for coupled photon and electron transport.
2. To review the methods used to improve the MC simulation efficiency.
3. To briefly review the vendor transport codes currently used for clinical treatment planning.
4. To describe the development of beam models for clinical treatment planning.
5. To discuss the factors associated with MCdose calculation within the patient‐specific geometry, such as statistical uncertainties, CT‐number to material density assignments, and reporting of dose‐to‐medium versus dose‐to‐water.
6. To discuss the issues associated with experimental verification of MC algorithms.
7. To briefly review the potential clinical implications of MC calculated dose distributions.
8. To provide example timing comparisons of the major vendor MC codes in the clinical setting.
- CE‐Therapy: Stereotactic Cranial RS/RT
MO‐B‐AUD A‐01: Quality Assurance in Stereotactic Radiosurgery and Fractionated Stereotactic Radiotherapy35(2008); http://dx.doi.org/10.1118/1.2962330View Description Hide Description
In the 50‐plus years since it was first introduced, stereotactic radiosurgery, high‐dose irradiation of cranial neoplasms delivered in a single fraction, has become a standard of care in the treatmentbraintumors, vascular malformations, functional disorders, and pain. Modern radiosurgery can be performed non‐invasively and on an outpatient basis, yet with an extremely high degree of accuracy. Within the past ten years, the field of radiosurgery has seen numerous technological enhancements, including: the development of dedicated devices for stereotactic delivery, the use of relocatable frames to facilitate fractionated delivery, the development of image guided and “frameless” approaches, and the application to extracranial tumor sites. Each of these developments is accompanied by its own challenges in assuring targeting and dosimetric accuracy. In this presentation we review the technologies for stereotactic localization and treatment of cranial targets with particular emphasis on the quality assurance aspects associated with establishing and maintaining a clinical radiosurgery program.
1. Differentiate how radiation is delivered for Gamma Knife and Linac‐based (conventional and robotic)stereotactic radiosurgery.
2. Define the treatment planning parameters for Gamma Knife and Linac‐based stereotactic radiosurgery.
3. Discuss measures for assuring accuracy in stereotactic localization and dose delivery for Gamma Knife and Linac‐based stereotactic radiosurgery.
- CE‐Therapy: QA for Linacs and MLC used for IMRT
35(2008); http://dx.doi.org/10.1118/1.2962413View Description Hide Description
Intensity‐modulated radiotherapy(IMRT) is now common in many therapy centers. Quality assurance (QA) procedures for a linear accelerator(linac) and multi‐leaf collimator(MLC) designed for conventional static fields do not address the unique dosimetric issues pertinent to IMRT planning and delivery. IMRT portals are composed of many irregular, small, off‐center, and abutting field segments throughout the target volume, each delivering only a few MU. In these regards, emphasis should be placed on beam stability for small MU and leaf position accuracy with gantry rotation. At the same time, the target volume, shielded by the MLC for much of the treatment, sees a larger transmitted dose, especially for MLC with rounded leaf ends, The impact of delivery technique, segmented (step and shoot) or dynamic (sliding window), on the required leaf position tolerance is not fully appreciated and will be addressed. While the mechanics and software of the various linacs and MLC distinguish them, there are many aspects of QA in common. This course will discuss: (1) the specific QA related to the MLC design; (2) additional QA for linear accelerators pertinent to the small MU and small field sizes used in IMRT; (3) tools and procedures often used to perform these QA tasks; (4) specific QA issues for different IMRT delivery methods.
1. Understand different IMRT delivery methods and their specific QA issues.
2. Understand effect of QA on the IMRT delivery accuracy.
3. Understand the importance of proper commissioning of the planning system to avoid confusion with dose delivery issues.
- CE‐Therapy: Biological/Clinical Outcome Models in RT Planning
35(2008); http://dx.doi.org/10.1118/1.2962423View Description Hide Description
The use of mathematical models to summarized clinical or biological outcomes with respect to the distribution of dose throughout an organ or tissue has great appeal. At its best, the overall cumulative effects of all parts of a heterogeneously irradiated object could be summarized in terms of a clinically relevant outcome such as a tumor control probability (TCP) or a normal tissue complication probability (NTCP). Further, it could be anticipated that a treatment planner would want to incorporate such information directly into the optimization of treatment planning parameters.
Despite this appeal, integration of these concepts into mainstream treatment planning has been slow in coming. Parameterization of the models is conceptually simple, requiring input data consisting primarily of dose, volume and outcome information from groups of patients. However, uncertainties in accumulated total effective dose and differential irradiated volumes together with heterogeneities in biological responses can make these determinations difficult. At best, it has been possible to obtain broad “population” based model parameters for some organs and tumors for “groups of similarly‐treated patients”. In general, as used today, these models are not within themselves (i.e., without additional stratification factors) “predictive” of outcome for “individual” subjects, nor can their use be generally extrapolated to the treatment of populations with techniques that vary greatly from those originally used.
However, disclaimers aside, the appeal remains great, and real progress has been made. In this session we will present some of the basic concepts associated with the models. Examples will be given as to how the models accommodate the responses of different tumor types and so‐called serial and parallel normal tissues. We will show how (as currently used) implementation of the models in evaluation or planning generally first requires reduction of a complex dose distribution (or DVH) into an equivalent “uniform” total (or partial) object dose.
Examples where the mathematical models appear to work well to describe the response of groups of patients will be provided. Specifically, the use of NTCP models for planning the treatment of tumors surrounded by volume effect normal tissues will be demonstrated, as will the use of EUD to allow rational heterogeneous irradiation of target volumes.
Some technical details associated with the implementation of mathematical/biological models into IMRT planning systems will be discussed. Since mathematical structures of biological models are nonlinear, an optimization algorithm that can explicitly handle nonlinear objective and constraint functions is important. We will describe various ways of incorporating the models into optimization problems.
This session will be concluded with a short survey of existing software tools that utilize the biological models for dose‐response data analysis, plan evaluation, and IMRT plan optimization.
1. Understand the basis and statistical nature of standard NTCP and TCP models.
2. Recognized that dose distribution reduction remains a basic component of their implementation and understand how this is accomplished.
3. Appreciate that as currently used, most results apply to “populations” of “similarly treated” patients.
4. Understand numerical issues of implementing and using the biological models within optimization systems.
- CE‐Therapy: Imaging for Planning/Verification: In‐room Imaging
35(2008); http://dx.doi.org/10.1118/1.2962664View Description Hide Description
Incorporating additional information into the initial model of the patient can improve the accuracy and precision of treatment planning. Multi‐modality and temporal imaging can add this additional information. The traditional goals of imaging for treatment planning, identifying the anatomical bounds of the tumor and the normal tissues, is being expanded to include functional and temporal information. Obtaining the highest quality images requires improved contrast, limiting artifacts, improving temporal and spatial resolution, and reducing or eliminating the interference of motion.
Combining the soft tissueimaging of MR, functional imaging of MR, CT, PET and SPECT with geometrically robust CTimaging improves the definition of the tumor and critical normal structures. In addition, dynamic information can be accurately quantified and incorporated into the treatment planning process through 4D imaging capabilities in CT and repeat or cine MR imaging. Reducing motion artifacts allows improvement in tumor definition. Methods of reducing the interference of motion on image acquisition include suspending the motion, through voluntary or assisted methods, and reducing the imaging session length, through multi‐slice acquisition and parallel imaging. Optimizing imaging sequences and contrast enhancement and timing improves the ability to define the tumor. The integration of these multi‐modality images into one more complete model of the patient is evolving through the use of automatic registration methods. The presentation will highlight the benefits of multi‐modality imaging in the treatment planning of tumors in the thorax, abdomen, and pelvis. Image optimization strategies will be discussed for each modality and developments to improve image acquisition and integration into treatment planning will be described.
1. Appreciate the benefits of including multi‐modality imaging in treatment planning.
2. Understand methods to optimize the acquisition of multi‐modality images for accurate treatment planning, including sequences, post‐processing, and timing.
3. Identify technical developments to improve image acquisition and integration into treatment planning.
35(2008); http://dx.doi.org/10.1118/1.2962665View Description Hide Description
It was not too long ago when CT based treatment planning was reserved for the few patients that was critical to have volumetric imaging, and treatment verification was limited to planar films of mediocre quality from a megavoltage imaging source. Today, we use multi‐modality based planning (CT,MRI,PET etc) and we employ sophisticated imaging tools and techniques to verify the correct delivery of the treatment. Such imaging capabilities include electronic portal imaging devices coupled with megavoltage (MV) or kilovoltage (kV) x‐ray sources, MV and kV cone beam CT,ultrasound guidance, optical systems, MV computed tomography,CT on rails and others. The goal of any in‐room imaging device is to verify that the patient setup is accurate and in accordance to the treatment plan. While some modalities allow for pre‐treatment verification, others can also provide on‐treatment verification feedback. The choice of the modality to use depends largely on user preference but also on the investment the clinic is willing to make to purchase the necessary equipment. Nonetheless, image based treatment verification has become a necessary and valuable aspect of our clinical practice and is enabling us to become a lot more aggressive with the planning and delivery of radiation treatments.Image Guided Radiation Therapy(IGRT) is undoubtedly the future of radiation therapy.
In this presentation we will discuss some of the most popular methods of imaging for treatment verification techniques. Clinical examples will also be presented to demonstrate the implementation, use, and clinical experience with these modalities.
1. Review of existing image based treatment verification modalities.
2. Discuss pros and cons of each modality and appropriateness of use.
- CE‐Therapy: Brachytherapy: General Clinical Applications
35(2008); http://dx.doi.org/10.1118/1.2962677View Description Hide Description
Brachytherapy has had a long history and formed the original intensity‐modulated, three‐dimensionally conformal radiotherapy. This presentation discusses some of the clinical applications of brachytherapy with an emphasis on the physical aspects of treatments. The presentation will focus particular attention on treatments with rapidly evolving procedures, particularly brachytherapy applications in breast, prostate, uterine cervix and liver. The discussion will consider how the physical nature of brachytherapy enhances and limits the treatments.
1. Consider the physical aspects of brachytherapy.
2. Understand the physical limitations in clinical applications of brachytherapy.
3. Know some of the emerging developments in the physical aspects of clinical applications in brachytherapy.
- CE‐Therapy: Quality Assurance for Image‐Guided Radiation Therapy
35(2008); http://dx.doi.org/10.1118/1.2962804View Description Hide Description
Image‐guidedradiation treatment(IGRT) is now widely available and implemented for routine clinical use in many clinics, allowing unprecedented level of precision and accuracy in treatmentdelivery. These systems bring a substantial change in clinical practice for all the disciplines involved in radiation medicine. One can now correct patient position immediately prior to administration of radiotherapy by registering IGRTimages to the reference planning CT scan.IGRT information displays not only two‐dimensional bony anatomy and airways, like portal imaging, but also three‐dimensional datasets representing the internal anatomy of the patient, implanted markers, soft tissue, and sometimes the target to be irradiated. Growing experience with IGRT has demonstrated the ability to verify, on a daily basis, the position of internal anatomy structures, such as the tumour or its surrogates, the spinal cord, and other organs at risk with respect to the treatment beam geometry for several anatomical sites. It is therefore logical to use IGRT daily for on‐line correction of patient translations and rotations, and to compare successive IGRTimages to monitor changes of internal anatomy through the course of therapy. Nevertheless, introducing IGRT within busy radiation therapy clinics requires thoughtful commissioning and quality assurance (QA) protocols, and judicious modification of existing radiation therapy processes and protocols. While the modes of use of these novel systems will continue to evolve into the distant future, their performance needs to be of the highest level, as they will be depended upon in the treatment process; indeed, increasing the precision of radiotherapydelivery may lead to reduced margins and therefore allow further dose escalation.
There are two key features of IGRT systems that require particular attention: geometric accuracy and image quality. First, the chosen IGRT system often does not share a common central axis with the megavoltage treatment beam; therefore, the geometric relation of the IGRT datasets to the megavoltage treatment beam must be assessed and monitored to ensure adequate localization, scaling, and geometric accuracy. Second, image quality metrics define the ability of the IGRT system to consistently produce an image of sufficient quality to localize the structures of interest, through either manual au automatic registrat5ion of IGRTimages to the reference planning CT scan. A well‐planned QA program integrates closely the IGRT system procedures with linac procedures described in accepted QA standards such as the report of the AAPM task group 40.
This lecture will present a brief review of IGRT systems, including kilovoltage and megavoltage cone‐beam CT, ultrasound, CT on rails, Tomotherapy, and projection radiography. The emphasis will be on QA issues germane to these systems and the clinical processes relying on their use. QA procedures and phantoms will be suggested, and some QA metrics, with their associated tolerance levels based on clinical experience will be presented. Successful strategies for clinical implementation of IGRT will also be discussed.
1. Understand the technical issues related to IGRT systems.
2. Understand the impact of IGRT on clinical processes.
3. Present a QA program for IGRT systems focusing on geometric accuracy and image quality.
- CE‐Therapy: Accurate Clinical Measurements
35(2008); http://dx.doi.org/10.1118/1.2962820View Description Hide Description
Image‐guidedradiation therapydeliveries are becoming increasingly complex and their verification involves clinical measurements in situations that are no longer covered by reference dosimetry protocols or relative dosimetry procedures with simple corrections. In this era of joint imaging‐therapy developments, accurate dosimetry techniques are sometimes trivialized and important effects not understood or ignored. However, clinical measurements increasingly involve reference and relative dosimetry of complex charged particle disequilibrium configurations and their interpretation.
This presentation consists of two parts: the first part revisits principles of measurement dosimetry, definitions of detectors and phantoms, reference dosimetry for conventional beams and also discusses some new developments in reference dosimetry of non‐conventional beams. The second part of the presentation concentrates on relative dose measurements with the goal to generate 3D distributions as well as integrated measurements and derived quantities. We conclude with a discussion of relative dosimetry in special cases including the photon build‐up region as well as narrow fields.
1. To understand the principles of clinical measurement dosimetry.
2. To get an overview of detectors and phantoms for reference and relative dosimetry.
3. To understand standard clinical reference dosimetry techniques and be exposed to some of the new developments in reference dosimetry in non‐standard beam configurations.
4. To get an overview of relative dosimetry techniques for the purpose of 3D dose distributions
5. To be aware of the complications of measurements in electronic disequilibrium such as in the photon build‐up and for small photon fields.