Volume 36, Issue 6, June 2009
Index of content:
- Therapy Continuing Education Course: Room 211A
- CE ‐ Therapy: Pediatric Radiation Planning and Delivery
36(2009); http://dx.doi.org/10.1118/1.3182191View Description Hide Description
Most Medical Physicists working in radiotherapy departments see few pediatric patients. This is because, fortunately, children get cancer at a rate about 30 times lower than adults. Children have not smoked, abused alcohol, or been exposed to environmental carcinogens for decades, and of course, have not fallen victim to the aging process. Children get very different cancers than adults. Breast or prostate cancers, typical in adults, are rarely seen in children but instead a variety of tumors occur in children that are rarely seen in adults; examples are germinomas, ependymomas and primitive neuroectodermal tumors, which require treatment of the child's brain or neuroblastoma, requiring treatment in the abdomen. The treatment of children with cancer using radiation therapy is one of the most challenging planning and delivery problems facing the physicist. This is because bones, brain, breast tissue, and other organs are more sensitive in children than in adults while the required tumordose is frequently above 50 Gy. Because most therapy departments treat mostly adults, when the rare 8 year‐old patient comes to the department for treatment, the physicist may not understand the clinical issues of his disease which drive the planning and delivery decisions. There is a new set of dose constraints different from the adult patient, which, depending on the site of treatment, may require changing the routine beam arrangement for that site. Additionally, children are more prone than adults to developing secondary cancers after radiation. This fact has important implications for the choice of delivery techniques, especially when considering IMRT. For bilateral retinoblastoma, an irradiated child has a 50% chance of developing a second cancer by age 50.
In this presentation, an overview of childhood cancers and their corresponding treatment techniques will be given. These can be some of the most complex treatments that are delivered in the radiation therapy department. These cancers include Leukemia treated with total body irradiation, medulloblastoma, treated with craniospinal irradiation plus a conformal boost to the posterior fossa, neuroblastoma, requiring focal abdominal irradiation to avoid kidney, liver, and vertebral body damage, retinoblastoma, requiring treatment to an eye while minimizing dose to surrounding tissues, and a variety of other tumors which occur anywhere in the body. Case studies will be presented showing the treatment technique and resulting dosimetry, highlighting the objectives for tumor coverage and organ‐at‐risk sparing. Practical issues that have to be faced when treating children will also be discussed such as daily sedation and immobilization.
Finally, most children with cancer are treating within a clinical trial administered by the Children's Oncology Group. Examples of the protocol physics requirements will be discussed as well as the physicist's responsibility for providing data to the Quality Assurance Review Center.
The presenter is Chief of Physics at one of only two radiotherapy departments in the country that treat exclusively children (Chidlrens Hospital Los Angeles).
1. Improve understanding about childhood cancer and treatment with radiation
2. Understand treatment planning and delivery issues specific to children
3. Understand physicist responsibility for clinical trial participation
- CE ‐ Therapy: Biological/Clinical Outcome Models in RT Planning
36(2009); http://dx.doi.org/10.1118/1.3182197View Description Hide Description
Radiation therapy must strike a balance between clinically acceptable tumorcontrol probability (TCP) and normal tissue complication probability (NTCP). Typical treatments expose several normal organs to incidental dose with an associated risk of a variety of radiation‐induced complications, each of which may depend differently on the spatial and temporal dose distribution. Dose/volume effects, wherein the ‘iso‐complication dose’ increases if the irradiated volume fraction decreases, are particularly important in designing and evaluating treatment plans. These effects are pronounced for some complications and weak for others. The classic “Emami” paper of 1991 summarized guidelines based on decades of clinical experience derived from the simple beam arrangements of the 2‐dimensional planning era when discrete parts of normal tissues typically received uniform doses at conventional fractionation schedules (partial organ irradiation). More recently, 3‐dimensional planning (including IMRT) has led to more conformal distributions with superior patient‐specific dosimetric information. Three‐dimensionl techniques have also encouraged treatment to higher doses and exposed normal tissues to a more variable and inhomogeneous range of dose distributions and fraction sizes. These complex relationships are often summarized in dose‐volume histograms (DVH).
Depending on the suspected volume effect, treatment planners use various DVH‐based metrics to estimate complication risks. These include single DVH points (e.g. maximum dose or percent or absolute volume above a dose cut‐point), combinations of such points, generalized equivalent uniform dose (including mean dose) and models (e.g. Lyman model). Co‐morbidities, chemotherapies and other medical factors may also need to be considered. We will describe commonly used evaluation metrics, some of which date back to the early 1990's and others from more recent publications or in‐house experience.
The current metrics used to evaluate normal tissue DVHs are sub‐optimal. Under the QUANTEC (Quantitative Analysis of Normal Tissue Effects in the Clinic) initiative, a project that is jointly funded by AAPM and ASTRO, approximately 60 physicists and physicians have critically surveyed the NTCP literature. Their updated consensus guidelines will be published in the near future. Some of their preliminary findings will be discussed here (check the proffered sessions for other presentations).
1. Understand the general features of the volume effect as applied to NTCP.
2. Be familiar with the dose/volume metrics commonly used for complications risk estimation in treatment planning.
3. Understand some of the difficulties in determining reliable dosimetric predictors of normal tissue complications.
MO‐B‐211A‐02: Biological/Clinical Outcomes Models in Radiation Therapy Planning — (Equivalent Uniform Dose (EUD) and Tumor Control Probability (TCP))36(2009); http://dx.doi.org/10.1118/1.3182198View Description Hide Description
Designing radiation treatments is a difficult and complex art. Many considerations must be balanced to arrive at a satisfactory plan of treatment. Most of the tools and criteria that have been applied to designing radiation treatment plans involve constraints on the dose delivered to selected regions within the patient. These criteria indeed parallel some of those used by clinicians in evaluating plans. However, on the one hand, dose criteria are at best surrogates for biological considerations and, on the other hand, the criteria used have ignored a number of important considerations employed by therapists — not least of which is the assessment of non‐uniform irradiation of organs and tissues. These considerations have led to an interest in developing quantitative models that attempt to predict the likely biological or clinical response of organs and tissues to any arbitrary pattern of irradiation. The need to assess inhomogeneous dose distributions comes from two sources. First, even if the goal is to achieve uniform irradiation of the target volume, any scheme which is used in an automated procedure must be able to evaluate a non‐uniform pattern of irradiation, if only to ensure that, by giving it a low score, a more uniform dose distribution will be preferred. It is also possible that a somewhat non‐uniform target volume irradiation may lead to an overall more satisfactory plan than one in which there is an entirely uniform target coverage but which is associated with a higher dose to an adjacent critical organ. The second reason to assess inhomogeneous dose distributions is that these are the norm when it comes to the normal tissues outside the target volume — and there is thought to be a sometimes quite strong volume dependence of normal tissue tolerance of which clinicians wish to take advantage.
It is intuitively logical that, for any inhomogeneous dose distribution delivered to a volume of interest according to a certain fractionation scheme, there exists a unique uniform dose distribution delivered in the same number of fractions, over the same total time, which causes the same radiobiological effect. The important feature of this equivalent dose distribution would be its uniformity, which allows one to use a single number to describe the entire VOI dose distribution. This observation led to developing a concept of Equivalent Uniform Dose (EUD).
Models of EUD and models of tissue response to radiation can be classified into two broad categories. One category includes mechanistic models developed based on our best understanding of the underlying biological processes. The second category includes phenomenological models based on the observed phenomena and general laws governing these phenomena. Although these two categories are based on quite different philosophical approaches, they offer complimentary views. Both categories of models and their applications will be presented during the course.
The objectives of this course are:
1. To discuss the importance of biological considerations in treatment planning.
2. To present mechanistic and phenomenological approaches to modeling tissue/organ response to radiotherapy.
- CE ‐ Therapy: Margins in Radiotherapy
36(2009); http://dx.doi.org/10.1118/1.3182306View Description Hide Description
Conformal radiation therapy has historically used margins to account for geometrical uncertainties in the position of a target volume. The margins, in their simplest form, are simply expansions to the diameter of a treatment beam, to ensure that dosimetric planning criteria are met in the presence of inter‐ and intra‐fraction setup variations. Historically, the size of the margin in a given treatment site was difficult to determine due to the available technology and the time and effort required to obtaining accurate data. Consequently, margins were estimated to accommodate a population of patients. As new technologies have emerged, target volume position errors have become easier to measure, their accuracy has increased, and the measurements can be made much more frequently. As the quantity and quality of data has increased for patient populations, and even for individual patients, the conceptual basis of employing margins has evolved. Strategies for ensuring dosimetric coverage may now be individualized to a specific patient's geometric uncertainty characteristics with customized margins, or they may incorporate geometric correction based on predefined action levels. Other strategies may incorporate continuous monitoring to gate or modify the beam. And finally, other strategies seek to eliminate margins by including the estimated geometrical uncertainty into the development of the dose distribution.
1. To provide an educational review of the technologies and methods of measuring target volume positioning errors.
2. To review methods of determining population margins from measured data.
3. To review corrective and intervention strategies for patients with individualized margins.
4. To review planning strategies which seek to eliminate margins.
- CE ‐ Therapy: Treatment Planning of Complex Cases: Strategies and Tradeoffs
36(2009); http://dx.doi.org/10.1118/1.3182313View Description Hide Description
The rapid increase in the complexity of treatment planning is a reflection of recent advances in the clinical technology. These include the use of multi‐modality and 4D imaging for planning, advanced treatment delivery and imaging capabilities available now on many linear accelerators.Intensity modulated radiotherapy(IMRT), already the most common form of treatment at some institutions, is increasingly being combined with other complex treatment capabilities such as respiratory monitoring or control, in‐room 2D or 3D imaging, or intra‐fractional patient motion monitoring.
Successful integration of any of these methods into the clinic relies on the increasing sophistication and ability of treatment planning staff to design and prepare not just the dose distribution but all data and images required for treatment. Although IMRT optimization and leaf sequencing algorithms are fairly mature, confidently identifying an “optimum” plan and appropriately evaluating the normal tissue and target trade‐offs can still be difficult and time consuming particularly for complex cases. Complex treatment procedures such as image‐guided treatment or respiratory monitoring can present a substantial challenge in terms of the effort required to manage, evaluate and appropriately use the many image sets acquired during planning and treatment. Dose escalation protocols and stereotactic body radiosurgery programs are being initiated at more institutions. Each of these demands a careful evaluation of initial and daily patient preparation and setup and a thorough understanding of the uncertainties associated with planning and treatment.
This course will focus on strategies and techniques for coping with some of the more complex planning issues that arise in today's technology‐intensive department on all stages, from planning image acquisition to dose delivery.
1. Understand the trade‐offs in IMRT planning and the role of target/normal tissue geometry and beam geometry in creating IMRT plans
2. Understand the increasing role of treatment planning staff in the analysis, use and preparation of images for planning and treatment
3. Identify sources of inter‐ and intra‐fractional uncertainty and their impact on dose escalation and stereotactic body radiosurgery programs
- CE ‐ Therapy: New Measurement Dosimetry — New Developments
36(2009); http://dx.doi.org/10.1118/1.3182442View Description Hide Description
Image‐guidedintensity modulated radiation therapydeliveries are becoming increasingly complex and their verification involves dosimetry measurements in situations that are no longer covered by reference dosimetry protocols or relative dosimetry procedures with simple corrections. These beams are generated as part of intensity modulation (IMRT),stereotactic radiosurgery(SRS), cyber‐knife and gamma‐knife deliveries and involve the use of extremely small fields in order of few millimeters. In this era of joint imaging‐therapy developments, accurate dosimetry techniques are sometimes trivialized and important effects not understood or ignored.
This presentation consists of three parts: the first part revisits the guiding principles of measurement dosimetry in standard and nonstandard conditions, definitions of detectors and phantoms, reference dosimetry of conventional beams and also discusses the new developments in reference dosimetry of nonstandard beams. The second part of the presentation concentrates on practical aspects of relative dose measurements with the goal to generate 3D distributions as well as integrated measurements and derived quantities. The third part concentrates on a discussion of relative dosimetry in narrow fields and deals with the choices of possible radiation detectors, the difficulties and possible solutions to measurements in disequilibrium situations created by this era's narrow photon fields.
1. To understand the principles of clinical measurement dosimetry and applicability to standard and nonstandard beams;
2. To understand standard clinical reference dosimetry techniques and be exposed to some of the new developments in reference dosimetry in nonstandard beam configurations;
3. To get an overview of practical aspects of relative dosimetry techniques for the purpose of 3D dose distributions in standard fields, small photon fields and buil‐up regions;
4. To be aware of the complications of measurements and detector choices in electronic disequilibrium situations created by small photon fields.
- CE ‐ Therapy: MRI — Imaging, Planning and Assessment
WE‐B‐211A‐01: Strengths and Limitations of Anatomical and Spectroscopic MRI in Radiation Oncology Treatment Planning36(2009); http://dx.doi.org/10.1118/1.3182451View Description Hide Description
Magnetic resonance imaging(MRI) is playing an increasingly important role in radiation oncologytreatment planning. As compared to computed tomography(CT),MRI provides excellent soft tissuecontrast making it ideal for imagingbraintumors. Other anatomical sites (e.g., prostate, pelvis, head and neck, extremities, abdomen, etc.) also benefit from the soft tissuecontrast provided by MRI, but are limited by difficulties in reproducing CT patient positioning in the MRI as well as geometric distortions introduced within the MRI datasets. These issues will be described, along with techniques for their minimization and quantification. MRI is also a powerful and non‐invasive imaging modality to assess tissue motion, caused by a host of physiological processes (respiration, peristalsis, bladder and/or rectal filling, etc.). Clinical approaches to using MRI for motion management will be described.
In addition to morphological imaging,magnetic resonance spectroscopic imaging(MRSI) provides a non‐invasive spatial mapping of abnormal tumor metabolism. This information may provide critically relevant assessment of the presence and extent of tumor activity, which may be substantially different from those considered to contain tumor based on T1 and T2 imaging. Many uncertainties remain: geometric and quantification accuracy of the acquired MRSI data due to magnetic field imperfections, lack of adequate anatomical details with MRSIimages for registration with planning CT, and lacking essential DICOM objects hamper the incorporation of MRSI information into the treatment planning process. Clinical approaches to using MRSI for tumor target delineation will also be described.
This lecture will outline the indications for MRI in radiation oncologytreatment planning for both external beam and brachytherapy. Examples are provided for many anatomical sites, along registration of MRI datasets with PET/CT. Approaches that can be used to validate MRI applications will be provided, along with a description of the current limitations.
1. Widespread applications of MRI in radiation oncologytreatment planning
2. Assessment of geometric distortion when MRI is utilized as one of several imaging modalities: reproducibility of patient setup and image distortion
3. Application of MRI to quantify and validate motion management models
4. Review obstacles to clinical utilization of magnetic resonance spectroscopic imaging(MRSI)
Research sponsored by Siemens Medical Solutions
36(2009); http://dx.doi.org/10.1118/1.3182452View Description Hide Description
The potential clinical value of in vivo functional and physiological MR imaging for assessment of therapeutic response has been explored in recent years. Such imaging may provide early prediction for treatment outcomes including local tumor control as well as normal tissue complications with greater predictive power compared to conventional clinical assessment methods and related anatomic imaging. The advantage of early assessment is to allow for re‐optimizing individual patients' treatment plans, including decisions of total dose, modifying risks of critical normal tissue, and optimizing use of concomitant therapies. A response model, e.g., predicting tumor or normal tissue response to doses based upon functional imaging biomarkers, can aid in clinical decision‐making. The volumetric data presented in both image‐derived metrics as well as local dose appear to provide a significant wealth of information. Care must be taken, however, to understand the limits of input data from which the model is developed.
This lecture will provide an overview on how to use functional imaging to develop a response model for individualized therapy. The limitations of these techniques will also be discussed.
1. Understand functional imaging as a biomarker for therapy response;
2. Understand influence of imaging parameters and image quality on quantitative metrics and uncertainty in the response model.
3. Understand clinical applications and limitations.
- CE ‐ Therapy: Digital Tomosynthesis
36(2009); http://dx.doi.org/10.1118/1.3182588View Description Hide Description
Any deviation between treated and planned volumes for 3‐D conformal therapy, such as IMRT, may cause an adverse clinical outcome. It is therefore critical to minimize all potential deviations using an on‐board (or real‐time) imaging procedure immediately prior to radiation delivery. At present, conventional 2‐D radiographicimaging and state‐of‐the‐art 3‐D cone‐beam CT(CBCT) are typically employed for treatment verification. However, 2‐D radiographic verification is mainly based on bony structures and/or implanted fiducials, and is sub‐optimal for soft‐tissue targets. While on‐board CBCT can provide 3‐D soft tissue information, it has three potential limitations: 1) The acquisition time is limited to a minimum of 60 seconds (≈15 breathing cycles) for on‐board CBCT, making single breath‐hold imaging impractical for organs which exhibit respiratory motion; 2) 360° mechanical clearance for CBCT acquisition may limit the use of CBCT for large patients, those with tumors at peripheral locations (e.g. breast), or those with substantial immobilization or support devices; 3) A high radiationdose (2–9 cGy) is delivered to the imaged volume with current imaging techniques, which is undesirable for daily imaging and may be a particular problem for those who are at high risk of developing second malignancies. To overcome these limitations, innovative digital tomosynthesis (DTS) imaging technologies are being developed for 3‐D and 4‐D target localization.
Although DTS technology has been used for digital chest and mammography, its use in target localization is new. DTS only requires limited gantry rotation (e.g., a scan angle of 40° or less) to reconstruct 3‐D anatomic information. Thus, imaging time and dose are substantially reduced compared to CBCT, making breath‐hold DTS a simple solution for daily imaging of moving organs. 4‐D DTS can also be generated much faster than 4‐D CBCT, which is desirable for on‐board 4‐D tomographic imaging. Further, the reduced mechanical clearance needed for DTS makes it more widely applicable than CBCT. At present, limited information has been published regarding the fundamentals of DTS for target localization, and/or preliminary clinical DTS localization results. The purpose of this presentation is to provide updated information about these topics, including the following objectives:
1. To understand the technical challenges and clinical potentials of using DTS technologies for target localization in radiation therapy
2. To understand the latest developments in DTS reconstruction and registration methods
3. To understand the clinical feasibility and efficacy of kV DTS compared to kV CBCT
4. To learn about the latest developments in MV DTS and brachytherapy DTS applications
Work is partially supported by grants from NIH, Varian Medical Systems, and GE Health Care.
- CE ‐ Therapy:Clinical Implementation of Respiration Motion Correlated Imaging, Treatment Planning and Delivery
TH‐B‐211A‐01: Practical Considerations for Respiratory Colarated CT Imaging and Target Volume Delineation36(2009); http://dx.doi.org/10.1118/1.3182595View Description Hide Description
Accurate and conformal treatment of tumors in abdomen and thorax is challenging due to respiratory related motion. Respiratory corralled CTimaging combined with gated treatment delivery can improve both accuracy and conformality of delivered dose distributions. Implementation of respiratory correlated imaging and treatment delivery can be performed in multiple ways and understanding of different techniques and technical challenges is critical for efficient and safe clinical implementation. Formal recommendations for commissioning and clinical use of this technology is scarce. Inadequate evaluation and understanding of these systems can lead to significant treatment errors. This presentation deals with practical issues of respiratory correlated CTimaging and commissioning. Examples of some commonly seen image artifacts are also discussed. Furthermore, contouring based on respiratory correlated images is discussed, again with examples of some of potential pitfalls and artifacts. Education Objectives:1. Commissioning process for respiratory correlated CTimaging and delivery techniques.2. Use of respiratory correlated images for target delineation.3. Common image artifacts and contouring errors seen with respiratory correlated imaging.
TH‐B‐211A‐02: Clinical Implementation of Respiration Motion Correlated Imaging, Treatment Planning and Delivery36(2009); http://dx.doi.org/10.1118/1.3182596View Description Hide Description
Respiratory motion is a significant factor in determining a suitable planning target volume (PTV) that surrounds the demonstrable gross tumor volume (GTV) in the radiotherapy of mobile lesions. Historically, a “one‐size‐fits‐all” margin derived from population statistics has been employed to account for such motion. The advent of respiratory phase correlation (4D) and multi‐slice CT technology has enabled patient‐specific determination of both demonstrable tumor volume and its motion from a single imaging study. Management of such additional information during treatment planning and delivery however presents several challenges. Furthermore, guidelines for establishing clinical treatment planning and delivery protocols based on respiratory correlated imaging data differ from one institution to the other. The purpose of this presentation is to educate the community on the different respiratory correlated radiotherapy approaches currently available and compare the benefits and pitfalls in each case. Special focus will be on treatment planning,dose calculation and treatment delivery related issues.
After the course, the attendees should be able to:
1. Understand the different respiratory correlated treatment planning and delivery approaches
2. Compare the benefits and pitfalls of each of the above approaches
3. Develop and establish respiratory correlated imaging,treatment planning and
4. delivery appropriate to the available equipment