Volume 33, Issue 6, June 2006
Index of content:
- Therapy Continuing Education Course: Room 224 C
CE: Action Levels for IMRT QA
33(2006); http://dx.doi.org/10.1118/1.2241399View Description Hide Description
Each intensity‐modulated radiation therapy(IMRT) field includes many small, irregular, and asymmetric fields that completely obscure the relationship between monitor unit (MU) setting and radiationdose. Uncertainty and inaccuracy of dose delivery with IMRT is primarily attributed to the leaf positioning accuracy, modeling of radiation output for small field sizes, modeling of beam penumbra, and the dose outside the IMRT field. Dose‐difference distribution, distance‐to‐agreement (DTA), and a numerical gamma index are often used to evaluate the quality of agreement between measured and calculated dose distributions for the IMRT fields. The tolerance limits based on these indices for IMRT QA are often not adequate because all these methodologies do not account for space‐specific dose uncertainty information. In other words, single tolerance criterion is applied to all test points even when dose uncertainty is significantly different from point to point. At any given point, the dose uncertainty depends on different levels of dose and gradients from multiple small beams rather than that of the overall dose profile. Therefore, new methodologies are needed that determine dose uncertainties based on the dose level and gradient information of each small field. In IMRT, it is sometimes difficult to have agreement between calculation and measurement of dose at all points in a 3‐D dose distribution. A disagreement at a few points does not necessarily lead to negative overall result if other comparable points are well within the established tolerance limits. We will describe a new approach in establishing tolerance limits and action levels for IMRT QA that will ensure delivery of prescribed radiationdose within an acceptable limit of 5%.
1. To describe the uncertainties in IMRT planning and delivery.
2. To describe the impact of spatial and dosimetric uncertainties on the IMRTdose distribution.
3. To describe the limitations of current methodologies of establishing tolerance limits for IMRT QA.
4. To describe new methodologies for establishing tolerance limits for IMRT QA.
CE: Dailiy Localization ‐ II: EPID, MVCT
33(2006); http://dx.doi.org/10.1118/1.2241484View Description Hide Description
The Mega‐Voltage Cone‐Beam CT (MVCBCT) system provides a 3D image of the patient anatomy in the actual treatment position that can be tightly aligned to the planning CT, allowing daily verification and correction of the patient position moments before the dose delivery. Integrated onto a linear accelerator, the system consists of a new a‐Si flat panel adapted for MV imaging and a workflow application allowing the automatic acquisition of projection images at low dose rate, CBCTimage reconstruction,CT to CBCTimage registration and couch position adjustment. Moreover, MVCBCT provides accurate electron density and allows studies of dosimetrical impact of setup error, anatomical changes or presence of implanted metallic objects.
This lecture will provide an overview of the physics characteristics of MV and MV CBCTimaging, acceptance testing and commissioning, acquisition and reconstruction,image registration, alignment precision and quality assurance procedures. An overview of the clinical applications will be provided. Finally, current challenges and future developments will be addressed.
1. Understand the basics concepts of MV Cone‐Beam CTimaging.
2. Understand the workflow and issues related to clinical applications of MV CBCT, including acquisition, reconstruction, registration and patient alignment.
3. Understand the possibilities of 3D Imaging for patient alignment.
This work was supported by Siemens Oncology Care Systems.
CE: 4D Scanning
33(2006); http://dx.doi.org/10.1118/1.2241491View Description Hide Description
The purpose of CT simulation in radiotherapy is to acquire patient geometrical information and to build a patient geometrical model for treatment planning. Errors in patient model caused by motion artifacts will influence all treatment fractions and therefore should be handled carefully. Due to the tumor respiratory motion, the captured tumor position and shape can be heavily distorted. The distortions along the axis of motion could result in either a lengthening or shortening of the target. The center of the imaged target can be displaced by as much as the amplitude of the motion.
A newly developed technique that can reduce motion artifacts and provide patient geometry throughout the whole breathing cycle is called respiration‐correlated or 4D CT scan. The basic idea for 4D CT scan is that, at every position of interest along patient's long axis, images are over‐sampled and each image is tagged with breathing phase information. After the scan is done, images are sorted based on the corresponding breathing phase signals. Thus, many 3D CT sets are obtained, each corresponding to a particular breathing phase, and together constitutes a 4D CT set that covers that the whole breathing cycle. 4D CT scan has been developed at various institutions with slightly different flavors. In this lecture, we will provide an overview of various implementations of 4D CT scan.
4D CT scan can be used to account for respiratory motion to generate images with less distortion than 3D CT scan. 4D images also contain respiratory motion information of tumor and organs that is not available in a 3D CTimage. This technology can be used for respiratory‐gated treatment to identify the patient‐specific phase of minimum tumor motion, determine residual tumor motion within the gate interval, and compare treatment plans at different phases. It can also be used for non‐gated treatment planning to define ITV by combining gross tumor volume at all breathing phases or using a method called maximum intensity projection. Of course 4D CT will also play a vital role in the futuristic 4D radiotherapy where the tumor is tracked dynamically during the treatment using multi‐leaf collimator.
Existing problems for 4D CT scan include the increased imagingdose,CT tube heating, and data management. More importantly, one has to keep in mind that 4D CT scan is not really 4D. Temporal information is mapped into one breathing cycle. Irregular respiration will cause artifacts in 4D CTimages. Patient coaching can improve the regularity of breathing pattern and thus reduce the residual artifacts. However this issue still deserves further studies.
1. Understand the origin and magnitude of motion artifacts in free breathing helical CT scan.
2. Understand how 4D CT scan works.
3. Understand how 4D CT can be used in radiotherapy.
4. Understand the remaining artifacts in 4D CT scan and possible future improvements.
CE: Daily Localization III: Tomotheraphy
33(2006); http://dx.doi.org/10.1118/1.2241668View Description Hide Description
Helical tomotherapy using the Hi‐ART II is analogous to helical CTimaging where the gantry and the couch are in simultaneous motion. Hence, beam delivery is continuous over all 360 in transverse planes about the patient. Temporal beam modulation is achieved by using a binary multi‐leaf collimator. In addition to its ability to deliver IMRT, the Hi‐ART II has the ability to obtain helical megavoltage CT (MVCT) images. These MVCT images have adequate spatial and contrast resolution for image guidance, and also for identification of many soft‐tissue structures. The incorporation of daily three dimensional soft‐tissue imaging into the radiotherapy process also enables dose recalculation and periodic evaluation of the treatment delivery during a course of radiotherapy. Hence, the subsequent treatment delivery can be modified using a systematic feedback of the geometric and dosimetric information in the previous fractions. This requires many components as feedback, including CT guidance to achieve soft tissue localization, dose recalculation, dose accumulation, treatment evaluation, re‐contouring, and re‐optimization.
This lecture will provide an overview of the physical characteristics of the Hi‐ART II, acceptance testing and commissioning, image quality tests, image registration, basic quality assurance, and an overview of clinical applications. Finally, system limitations and future developments will be addressed.
1. Understand the basics concepts of helical tomotherapy.
2. Understand the basic QA requirements and system limitations association with helical tomotherapy.
3. Understand the workflow and issues related to clinical applications of helical tomotherapy, including acquisition, reconstruction, registration and patient alignment.
4. Understand the possibilities of daily soft‐tissue imaging for patient alignment and evaluation of treatment accuracy.
CE: 4D / Gated Treatment
33(2006); http://dx.doi.org/10.1118/1.2241675View Description Hide Description
Human anatomy and physiology change with time. Solid tumors also exhibit temporal behavior, particularly when assaulted with radiotherapy. In the era of image‐guided therapy, technology is being developed to explicitly account for these changes with time, both in cancerous and healthy tissue. One source of temporal anatomic changes is respiratory motion, which affects organs (and tumors) in the thorax, abdomen and pelvis. This motion causes deleterious effects during the imaging, planning and delivery of radiotherapy:
• Imaging: Motion causes a misrepresentation of the positions, shapes and volumes of both the tumor and normal anatomy during CT scanning and other imaging modalities. This phenomenon potentially leads to geometric misses of the tumor during treatment delivery.
• Planning: If tumor motion is present, and not explicitly being accounted for, larger safety margins are needed. These larger safety margins increase normal tissuedose, increase treatment‐related toxicity and limit dose escalation.
• Treatment delivery: The motion of the tumor during treatment can cause unplanned under‐ and over‐dosage regions, particularly for IMRT.
Clinical studies have demonstrated evidence of a dose response for both tumors and healthy lung tissue. Thus it is hypothesized that increased targeting accuracy will allow for dose escalation, facilitating improved local control, and/or a reduction in treatment‐related toxicities, predominantly pneumonitis. Two methods that can account for respiratory motion and hence increase targeting accuracy are respiratory gated radiotherapy and four‐dimensional (4D) tumor tracking radiotherapy.
An implicit assumption common to all techniques that base delivery decisions on the respiratory signal is that the tumor motion is correlated with this signal. The strength of this correlation is dependent on the patient, tumor type and location and the source of the respiratory signal.
Respiratory gating is a method of synchronizing radiation with respiration, during the imaging and treatment processes. Image acquisition occurs either by prospectively triggering acquisition during a certain part of the breathing cycle, or retrospectively sorting the sinogram/images based on the part of the breathing cycle in which they were acquired. Respiratory gating has been successfully clinically implemented in a number of academic and community settings for both conformal and IMRT treatments.
4D radiotherapy can be defined as the explicit inclusion of the temporal changes in anatomy during the imaging, planning and delivery of radiotherapy:
• 4D CTimaging: Acquisition of a sequence of CTimage sets over consecutive phases of a breathing cycle.
• 4D planning: Designing deliverable treatment plans on 4D CTimage sets.
• 4D treatment delivery: Continuous delivery of the 4D treatment plan throughout the breathing cycle.
• 4D delivery can be achieved by continuously aligning the beam and patient during treatment using a robotic linac, DMLC, block motion or couch motion.
1. Understand the rationale for accounting for respiratory motion during imaging, treatment planning and radiation delivery.
2. Learn about the clinical implementation of respiratory gated radiotherapy.
3. Learn about 4D tumor tracking radiotherapy.
4. Understand the advantages and disadvantages of respiratory gated and tumor tracking radiotherapy.
Conflict of Interest: PI's research supported by Varian Medical Systems.
CE: Daily Localization ‐ IV: Ultrasound and Implantable Devices
33(2006); http://dx.doi.org/10.1118/1.2241826View Description Hide Description
The use of ultrasound localization in radiotherapy has been in wide clinical use since the late 90's. It is the one of the first imaged guided radiotherapy(IGRT) systems to be widely used. The most common site of localization has been the prostate using a transabdominal approach. The technique makes use of treatment planning contour volumes with their associated isocenter overlaid on spatially localized ultrasoundimages. The ultrasoundimages are localized to the linac isocenter using various methods that include: a robotic arm, inferred stereotactic cameras, or various optical tracking methods. The difference in position between the overlaid contours and the observed ultrasound structures provides the three dimensional patient correction.
Ultrasound localization has shown to be an efficient method of alignment but it does have limitations. Ultrasoundimages are often difficult to interpret. For prostate, the transabdominal approach takes advantage of the increased resolution when imaging through the bladder, typically positioned anterior and superior to the prostate. When patient's bladders are empty, degradation in prostate imaging typically occurs. Often, large patients necessitate using lower frequency ultrasound, which also degrades the image quality. Large patients may also limit the ability to perform a transabdominal ultrasound with the presence of a pannis. Additionally, the operator's capacity to capture and interoperate ultrasoundimages can vary, limiting the quality of the alignment.
This lecture will touch on some of the techniques used to improve ultrasound alignment quality. These techniques were developed at an institution where over 100,000 ultrasound alignments have been performed since 1998. Also, the results from several clinical studies will be presented. These include abdominal pressure effects, procedural changes improving alignment quality, comparison with daily CT, and complication rates of a hypo‐fractionated prostate protocol using ultrasound alignment.
1. Techniques in improving ultrasoundimage quality during alignment.
2. Understanding the limitations in performing ultrasound alignment.
3. Procedural changes that can improve alignment quality.
33(2006); http://dx.doi.org/10.1118/1.2241827View Description Hide Description
Two technologies have been recently applied to commercial product development for localization and monitoring of treatment. Electromagnetic tracking, quite mature in aerospace and surgical guidance, has evolved to development of implantable markers. Video‐based surface mapping systems have been applied to a product that rapidly extracts the anterior surface of a patient as an aid to localization and motion measurement.
These new systems show dramatic promise as aids to initial setup. Precision on the order of 1–2 mm has been described for both technologies. These systems act as surrogates for tumor localization inferring that either a) the patient's skin or b) implanted fiducial locations can be correlated to tumor position. For the prostate, implanted fiducials have shown acceptable accuracy when properly placed. For surface imaging, targets near/at the surface (e.g. breast cancer) should work very well. For other body sites, investigations are ongoing.
In addition to rapid setup, the monitoring capability of these systems presents a paradigm that has been extremely limited to date in treatment rooms. The ability to more directly infer target position to aid in gating and tracking, especially without the use of additional ionizing radiation to the patient, may have dramatic impact on targeting accuracy.
This lecture will overview these technologies, and will discuss core measurements to establish accuracy and compatibility in the radiotherapy environment. Procedures for use will be outlined, as well as potential limitations of these systems.
1. Understand the operating principles of electromagnetic and surface localization.
2. Overview systems using these technologies.
3. Understand critical issues for commissioning of such systems.