Volume 35, Issue 6, June 2008
Index of content:
- SAMs Therapy Symposium: Auditorium B
Clinical and Radiobiological Considerations and Physics and Dosimetry Considerations
MO‐SAMS‐AUD B‐01: Stereotactic Body Radiation Therapy (SBRT) I: Clinical and Radiobiological Considerations35(2008); http://dx.doi.org/10.1118/1.2962320View Description Hide Description
Stereotactic Body Radiation Therapy(SBRT) has emerged as an important form of cancer therapy with broad application across a spectrum of tumor types in the primary and metastatic settings. The capability of safely administering a very high dose of therapeuticradiation to discrete extracranial tumor sites has raised new questions about the radiobiology of high dose per fraction treatment. Accumulating clinical experiences are yielding new insights into practical aspects of tumor and normal tissue responses to high dose per fraction treatment.
The current practice of SBRT has evolved to some extent from knowledge gained from principles learned from the practice of cranial stereotactic radiosurgery(SRS), and this presentation will begin with discussion of the use of SBRT for sites where the extracranial nervous system (spinal cord, cauda equina) is the major dose‐limiting structure. Some recent preclinical studies of spinal cord tolerance to high dose therapy will be presented, as well as the results of recent clinical trials involving spine SBRT from various centers. Next, the discussion will focus on other major extracranial sites where SBRT has been applied, with emphasis on SBRT for tumors in the liver and lung in particular, again with inclusion of theoretical and preclinical studies as well as reported clinical outcomes.
1. Review and understand the major issues related to the use SBRT for tumors in the spine or paraspinous region, including key clinical observations.
2. Review and understand the common clinically observed normal tissue responses to SBRT and the inferences to be drawn regarding the practical radiobiology of high dose per fraction therapy.
3. Explore and highlight the major issues related to clinical application of SBRT for tumors in the lung and liver, including key clinical observations.
MO‐SAMS‐AUD B‐02: Stereotactic Body Radiation Therapy (SBRT) II: Physics and Dosimetry Considerations35(2008); http://dx.doi.org/10.1118/1.2962321View Description Hide Description
SBRT of well defined extracranial targets in various anatomical locations demands special treatment planning considerations due to limitations of dose calculation algorithms in heterogeneous medium and small field dosimetry coupled with tumor/organ motion and multiple radiobiological constraints. Unlike conventional intracranial radiosurgery,SBRTtreatment planning and delivery are confounded by reduced 3D physical space for beam placement due to possibility of treatment machine and patient collision as well as a multiplicity of surrounding serial and parallel architecture organs. Currently advanced treatment planning techniques with radiobiological considerations and precise image‐guidedtreatment delivery methods are practiced for SBRT delivery to a wide variety of tumor types.
The physics and dosimetry presentation will include an overview of the available technology for SBRT with an emphasis on image guidance, Quality Assurance, and clinical implementation of SBRT. The presentation will also include site specific treatment planning techniques with considerations to accuracy of dose calculation algorithms, radiobiological issues, and physics reporting strategies.
1. Understand the issues related to the clinical implementation and technical aspects of SBRT techniques and technology and become familiar with the preliminary reporting of AAPM Task Group 101.
2. Understand the importance of QA procedures and guidelines and reporting requirements for SBRT.
3. Understand the practical aspects of SBRTtreatment planning for paraspinal, lung, liver, and abdominal tumors and recognize the critical issues related to each site.
4. Understand the importance and limitations of dose calculation algorithms and heterogeneity correction methods for SBRT in various anatomical sites.
kV vs. MV CBCT
TU‐SAMS‐AUD B‐01: KV and MV Cone Beam CT Imaging for Daily Localization: Commissioning, QA, Clinical Use, and Limitations35(2008); http://dx.doi.org/10.1118/1.2962415View Description Hide Description
Three‐dimensional conformal radiotherapy (3D‐CRT), intensity‐modulated radiotherapy(IMRT), and body stereotactic radiotherapy (BSRT) allow for the generation of highly conformal dose distributions for patients with tumor volumes wrapped around or adjacent to critical structures. As a consequence of steep dose gradients between the target and organs‐at‐risk, precise localization of the target volume and surrounding normal tissue is essential. However, variations in patient setup and organ motion are limiting factors in the accurate delivery of radiation treatment with a high degree of precision. Recent online image‐guided RT techniques using kilo‐Voltage cone‐beam CT (kV‐CBCT) and Mega‐Voltage CBCT (MV‐CBCT) are now offering the opportunity to overcome these limitations and greatly improve treatment localization accuracy. These IGRT techniques using the latest imaging technologies represent an improvement over traditional techniques such as 2D portal imaging. It allows for the use of volumetric online imaging to account for organ motion and setup variation by providing multiple anatomical views of the patient during the full course of RT treatment. The selection of imaging modality and system to use in support of a treatment protocol is often a compromise between image quality, efficiency, availability, and dose.
1. To review kV‐CBCT and MV‐CBCT imaging systems for daily localization including:
a. Commissioning, image quality, dose, registration process, and acquisition modes.
b. Clinical integration.
c. QA, stability over time, and downtime.
d. Standard clinical applications.
e. Novel clinical applications.
f. Technology evolution and future directions.
The Management of Motion: Technologies and Practical Limitations
35(2008); http://dx.doi.org/10.1118/1.2962666View Description Hide Description
The current climate of rapid technological evolution is reflected in newer and better methods to modulate and direct radiation beams for cancer therapy. This Continuing Education lecture focuses on one aspect of this evolution, locating and targeting moving tumors. The two processes — locating and targeting tumors— are somewhat independent and in principle different implementations of these processes can be interchanged. Advanced localization and targeting methods have an impact on treatment planning, and also present new challenges for quality assurance (QA), that of verifying real‐time delivery. Some methods to locate and target moving tumors with radiation beams are currently FDA approved for clinical use — and this availability and implementation will increase with time. Extensions of current capabilities will be the integration of higher order dimensionality into the estimate of the patient pose and real‐time reoptimization and adaption of delivery to the dynamically changing anatomy of cancer patients.
1. To describe the technology available to determine real‐time target position.
2. To review the systems for real‐time target‐beam alignment.
3. To discuss the practical considerations of real‐time target tracking systems.
Research sponsored by Accuray, Calypso and Varian.
35(2008); http://dx.doi.org/10.1118/1.2962667View Description Hide Description
Traditional fan‐beam scans provide an anatomical model of the patient which often does not correspond well with any of the patient's anatomy during respiration. As a consequence, dose distributions computed using such image data could deviate, sometimes significantly, from the doses received by patients during the course of their treatment. Moreover, the increased uncertainty in target definition due to imaging artifacts, together with the need to account for motion excursion, may lead to large planning margins, thus increasing unnecessarily normal tissue doses, while preventing potential dose escalations.
By contrast, time‐resolved computed tomography (4D CT) generates multiple volumetric imagesets that are more accurate descriptors of the various patient geometries during breathing; by using this information at the time of planning one can evaluate more accurately physical dose distributions in the presence of motion.
The question is how to use the available time‐dependent anatomical information when designing treatment plans. The answer will depend on whether the same treatment plan or different treatment plans will be delivered at various phases of the breathing cycle. Regardless, dose computations may be required on multiple image datasets, followed by an accumulation of doses from all datasets onto a single dataset (“planning dataset”), via time‐weighting coefficients representing the relative amount of time spent at a particular breathing phase. The accumulation of doses involves tracking anatomical voxels between various datasets, and is accomplished by using image registration techniques that provide non‐rigid (if organ deformations are involved) voxel mapping between datasets.
This lecture will provide an overview of dose accumulation techniques, planning aspects for various delivery approaches, sources of error specific to 4D planning. Also, several clinical lung and liver studies involving dose computations in the presence of motion will be reviewed.
1. Understand the rationale for accounting for respiratory motion during treatment planning.
2. Understand the principles of cumulative dose computation.
3. Learn about various sources of error specific to 4D‐based treatment planning.
4. Learn about the clinical importance of the changes induced by the respiratory motion.
35(2008); http://dx.doi.org/10.1118/1.2962668View Description Hide Description
Respiratory gated radiotherapy holds promise to reduce the incidence and severity of normal tissue complications and to increase local control through dose escalation for lungcancer patients. In this lecture, we discuss the current status, existing problems, and potential solutions for applying gating techniques to lungcancertreatment. First, the motion artifacts in CT simulation are discussed and the 4D CT scan technique is recommended for treatment simulation of lungcancer patients under gated radiotherapy. Second, we discuss two currently available forms of gated radiotherapy: internal (fluoroscopic) gating and external (optical) gating. Internal gating utilizes internal tumor motion surrogates such as implanted fiducial markers while external gating uses external respiratory surrogates such as markers placed on the surface of the patient's abdomen. The major strengths of external gating are that it is non‐invasive, is relatively easy and does not require any radiationdose for imaging. However, the relationship between the tumor motion and the surrogate signal may change over time, inter‐ and intra‐fractionally. The major strength of the internal gating systems is the precise and real‐time localization of the tumor position during the treatment. The two major weaknesses of internal gating are the risk of pneumothorax for implantation of markers in lung and the high imagingdose required for fluoroscopic tracking. Third, we discuss the potential solutions and future development of gated radiotherapy for lungtreatment. We propose the combination of external and internal surrogates (hybrid gating) to solve the imaging dos problem and the direct fluoroscopic tracking of lung mass to avoid seed implantation. Other related issues are also discussed.