Index of content:
Volume 33, Issue 6, June 2006
- Therapy Continuing Education Course: Room 224 A
- CE: IMRT Modeling Influence on Planning
33(2006); http://dx.doi.org/10.1118/1.2241667View Description Hide Description
The quality of an intensity modulated radiation therapy(IMRT)treatment plan can be strongly influenced by the physical and mathematical models underlying a clinical planning system. This lecture provides an overview of the models that are commonly employed for purposes of treatment planning for megavoltage (MV) photon‐beam IMRT achieved through multi‐leaf collimator(MLC) delivery. IMRTtreatment planningmodels will be examined which include models of: ionizing‐radiation dose computation; fluence‐map optimization; MLC delivery sequence optimization; dosimetric plan evaluation; and “biologic” plan evaluation. Models will be studied with focus on the assumptions, merits, and limitations involved with different models. The differences between reality and the models will be explored. Emphasis will be placed on plan characteristics that exist in reality but are not reflected in the model of the IMRT delivery. The influence of the algorithms employed to implement the models will also be discussed including methods that involve discretization, rounding, and limited numerical precision. Practical examples that demonstrate the influence of modeling differences on IMRT plan quality will be presented.
1. Understand the influence of the choice of physical models employed in clinical IMRT.
2. Understand the assumptions, merits, and limitations involved with different IMRT planning models.
3. Review the conditions where IMRT modeling is suspect and requires careful scrutiny in clinical implementation.
- CE: PET for Planning
33(2006); http://dx.doi.org/10.1118/1.2241483View Description Hide Description
Purpose:Positron Emission Tomography(PET)images show physiological and biological information through the in‐vivo distribution of radioactive, short‐lived, positron‐emitting species. PETimaging shows focal and distributed regions of cancer and its metastases. PET uses in oncology include diagnosis, staging, and disease monitoring, important for prognosis and treatment decisions. Quantitative uses of PET include assessment of the degree of malignancy and target definition for radiation treatment. Hybrid PET‐CT devices are being used as radiation treatment simulators. Method and Materials: F‐18‐labled Fluoro‐deoxyglucose (F‐18 FDG) is the most commonly used PETimaging agent. FDG shows regions of active glucose metabolism, such as local cancer, metastases, non‐cancer inflammation, and normal glucose use in brain. Non‐FDG PET agents can be highly specific in tissue targeting (binding), such as F‐18 Misonidazole and C‐11‐labeled amino acids, to imagetumor biology like hypoxia and cell proliferation, respectively. Results:PETimaging has coarse spatial resolution compared to CT and MR. PET's clinical use is valued because of its great sensitivity for cancer detection. Voxel intensity and image fidelity depend on equipment design, patient size, anatomic site, and imaging study parameters. PET‐CT units enable CT‐based attenuation corrections and inclusion of CTinformation in a registered PET‐CT dataset. Interest is high for PET‐CT simulators. The Standardized Uptake Value (SUV) is a normalized intensity measure for quantitative indication of disease, and can be used for target delineation, with certain constraints. Conclusion: This course reviews the basic physics of PET, uses of FDG and non‐FDG PET for oncologyimaging, and quantitative aspects for PET‐based radiation target definition. Example images demonstrate the potential contributions and limitations of FDG and non‐FDG PEToncologyimaging. This review course is intended for both imaging and radiation oncology physicists.
1. Review the basics physics of PETimaging.
2. Describe FDG PETimaging and oncologic indications.
3. Review the uses and limitations of PETimages in radiation treatment.
4. Describe the SUV and other threshold parameters for target delineation.
5. Review non‐FDG PETimaging of tumor biology.
Conflict of Interest: Research sponsored in part by Varian Medical Systems and GE Healthcare.
- CE: QA for Imaging Systems Used for Planning (CT, PET, MR)
33(2006); http://dx.doi.org/10.1118/1.2241825View Description Hide Description
Imaging for radiation therapytreatment planning has different goals than diagnosticimaging.Quality assurance (QA) of imaging devices (CT,PET, MR) which are used for radiation therapyimaging will therefore have different or additional goals compared to QA for diagnosticimaging purposes. Use of PET and MR imaging in radiation therapy is constantly increasing and reliable performance of these imaging modalities is important to avoid potentially significant errors.
Quality assurance for CTscanners used for radiation therapy scanning is relatively well established and defined. This process remains to be adequately defined for PET and MR scanners that are used for treatment planning. Evaluation of image quality of CT,PET, and MR scanners is generally adequately addressed by procedures which were established for diagnosticimaging. Design of QA protocols for radiation therapy scanning should be founded on procedures which are used for diagnosticimaging. Evaluation of mechanical accuracy and image spatial integrity is unfortunately not a major concern in diagnosticimaging and these parameters are often not sufficiently addressed in diagnostic QA protocols. All three imaging modalities have multiple potential sources of spatial and geometric errors and understanding of these parameters is necessary for design of an effective quality assurance program. The design of QA programs for these devices will be affected by the location of individual scanner and distribution of its utilization for diagnostic and radiation therapyimaging.Scanners which have dual purpose (diagnostic and treatment planningimaging) should have a QA program designed jointly by diagnostic and radiation therapy physicists to ensure that the program meets the needs of both groups.
The quality assurance for these imaging modalities in the radiation therapy goes beyond the QA of the scanners and should include evaluation of implementation of images in the treatment planning process. This should include evaluation of data transfer, image registration, potential degradation in image quality, image distortions, and evaluation of process for delineation of tumor and normal structure volumes. MR and PETimages can contain biologically active regions which may not correlate with any readily visible anatomic features. Correct identification and use of these regions in the treatment planning process should be a major concern of the QA program.
This lecture will provide an outline and description of QA program for CT,PET, and MR scanners used in radiation therapy. The fundamental goals of such program will be described and information which can be used for establishment institution specific QA programs will be provided.
1. Describe goals of QA programs for CT,PET, and MR scanners used for radiation therapyimaging.
2. Describe QA process for individual imaging devices.
3. Describe a concerns for verification of correct implementation of these imaging modalities in the treatment planning process.
- CE: QA for Linacs and MLC used for IMRT
33(2006); http://dx.doi.org/10.1118/1.2241833View Description Hide Description
Intensity‐modulated radiotherapy(IMRT) has become a part of our routine treatment for external beam radiotherapy. Most quality assurance procedures set for linear accelerators and multi‐leaf collimators(MLC) have been designed for conventional external beam radiotherapy. With IMRT, radiation portals are often irregular, small, off‐center, and abutting in the middle of the target volumes, which require specific IMRT QA for the linear accelerators and MLCs. Some of the QA issues are related to the specific IMRT delivery method, and the specific treatment planning system. This review course will discuss (1) the characteristics of three major MLCcollimators and the specific QA related to the unique MLC design; (2) additional QA for linear accelerators pertinent to the small MU and small field sizes used in IMRT; (3) tools often used to perform these QA tasks; (4) specific QA issues for different IMRT delivery methods, step and shoot vs sliding window.
1. Understand the characteristics of three major MLC systems.
2. Understand different IMRT delivery methods and their specific QA issues.
3. Understand effect of QA on the IMRT delivery accuracy.
- CE: Quality Assurance for IMRT and IGRT
33(2006); http://dx.doi.org/10.1118/1.2241862View Description Hide Description
Intensity modulated radiation therapy (IMRT) has become the standard‐of‐care for most cancertreatment programs. According to the 2004 AAPM professional information survey, 87 percent of the respondents had IMRT on one or more therapy units at their institution. The clinical successful or failure of an IMRTtreatment program is dependant on the correct delivery of the 3D dose distributions calculated by the planning system (Planned Dose) to the correct location in the patient (Delivered Dose). There are many sources of error that can arise during IMRT planning and delivery. However, it is ultimately the responsibility of the Medical Physicist to ensure that the Planned Dose “agrees” with the Delivered Dose. The difference between Planned Dose and Delivered Dose (i.e. Error) in IMRT can originate from at least three different sources: 1.) The treatment planning model, 2.) Treatment delivery dosimetry and mechanics, and 3.) Time‐dependant target/tissue positioning.
The IMRTtreatment planning model must be evaluated as part of the initial IMRT commissioning, and again after major software upgrades. The IMRT model evaluation can be performed using geometric test plans, anthropomorphic phantom test plans, and patient test plans. Calculated and measured absolute and relative doses should be compared using multiple measurement techniques, such as film,ionization chambers,TLDs, diodes, electronic imaging devices, etc. The test plans and techniques should be similar to those that will be used in clinical practice.
The multileaf collimator and the linear accelerator must be evaluated for dosimetric and mechanical accuracy. IMRT test sequences can be used to evaluate leaf positional accuracy, time‐dependant leaf positioning, motor performance, and dosimetry. These tests can be performed using film,ionization chambers,TLDs, diodes, electronic imaging devices, etc. In addition, patient‐specific IMRTquality assurance should be performed for each patient to verify that the planned and delivered doses are within tolerance.
An additional source of error in IMRT is the uncertainty in the position and shape of the target volume. The use of image‐guided radiation therapy can help minimize, but will not eliminate these errors. The impact of intra‐fraction motion on IMRT delivery can be measured using a dynamic motion phantom to simulate clinical conditions. Film,ionization chambers,TLDs, or diode dosimeters can be placed in the dynamic phantom to directly measure the impact of motion on IMRTdosimetry.
This continuing education course will discuss commercially available dose measurement tools, phantoms, and techniques for performing acceptance testing, commissioning, and routine quality assurance for the IMRT process. The three classes of error will be discussed, with a special emphasis on the lessons learned and manpower requirements.
1. To understand the issues surrounding IMRTquality assurance.
2. To identify potential sources of errors in IMRTquality assurance.
3. To review the tools used in IMRTquality assurance.
4. To understand the impact of target localization and patient positioning on IMRT.
33(2006); http://dx.doi.org/10.1118/1.2241863View Description Hide Description
The recent commercialization of linac‐mounted kilovoltage imaging systems (i.e., kV systems) has allowed high‐precision verification and correction of patient position immediately prior to the delivery of radiation therapy using fluoroscopic, radiographic, or cone‐beam tomographic modes. This novel technology challenges the radiotherapy community to redefine their patient positioning practice. One can now correct patient position using image information displaying not only bony anatomy and airways, like portal imaging, but also markers and soft tissue structures within the patient, including the target to be irradiated. On‐line kilovoltage imaging further allows, using daily imaging sessions, on‐line correction of patient translations and rotations, and the comparison for successive daily volumetric images permits to track changes of anatomy through the course of therapy. Introducing kV systems within busy radiation therapy clinics requires thoughtful testing and quality assurance protocols (QA) of the device, and judicious modification of existing radiation therapy processes and protocols.
The individual components making up kV systems are well‐established technologies, and the quality assurance for each component follows accepted standards. However, the specific features of the integrated systems require particular attention. First, the kilovoltage beam may not share a common central axis with the megavoltage treatment beam; therefore, the relation of the kilovoltage imaging matrix to the megavoltage treatment beam must be monitored to ensure adequate localization, scaling, and geometric accuracy. Second, cone‐beam tomography differs from conventional or helical CTimaging, and thus encounters specific image quality issues. A well‐planned QA program integrates closely the kV system procedures with linac procedures described in accepted QA standards such as the AAPM task group 40 report.
This lecture will present a brief review of commercial systems, but will focus on a suitable QA program, with special emphasis on issues germane to the system and the clinical processes relying on its use. Examples of clinical use in a number of sites will also be presented.
1. Understand the technical issues related to kV systems.
2. Understand the impact of kV systems on clinical processes.
3. Present a comprehensive commissioning protocol and QA program for kV systems.
This work is supported, in part, by a research grant from Elekta Oncology Systems.
33(2006); http://dx.doi.org/10.1118/1.2241911View Description Hide Description
Purpose: To describe the commissioning and quality assurance for helical tomotherapy machines. Method and Materials: Helical tomotherapy is a volumetric image guided, fully dynamic, IMRT delivery system. It was been developed at the University of Wisconsin and is now commercially manufactured as the ‘Hi‐Art’. At the core of this fully dynamic IMRT Delivery system lies a short gantry‐mounted linac that is used both for treatment and per‐treatment MVCT imaging. Aside from the primary collimator and the jaws, which set the width of the beam, it is also collimated by a binary multileaf collimator generating a fan beam of intensity‐modulated radiation. Modulation varies with gantry angle. Hence, the Hi‐Art allows for the acquisition of a helical pre‐treatment MVCT scan that is used for online image guidance and allows for precise interfraction positioning of the patient while in the actual treatment position just prior to the start of treatment. Due to its unique design the Hi‐Art system, allows highly conformal dose‐distributions to be delivered to patients in a helical fashion. Patients are treated lying on a couch that is translated through the bore of the machine as the gantry rotates, therefore the Hi‐Art is the therapy equivalent of helical CT. Since this approach to therapy is fully dynamic it requires synchrony of gantry rotation, couch translation, linac pulsing and the opening and closing of the leaves of the binary multileaf collimator used for beam modulation. Results: Over the course of the clinical implementation of the HiArt we have developed a quality assurance (QA) system that covers machine specific QA. The machine specific QA system is similar to that recommended for conventional linear accelerator QA by AAPM Task Group 40; however since the Hi‐Art design and operation differs from that of conventional medical linear accelerators, the tomotherapy QA system contains also novel components, such as QA checks for synchrony of gantry rotation, couch translation, linac pulsing and the opening and closing of the leaves of the binary multileaf collimator.Conclusion: In the first part of the presentation the design and dosimetric characteristics of Hi‐Art machines are summarized and the QA system is described along with experimental details of its implementation, while in the second part the pre‐treatment patient‐specific delivery QA for helical tomotherapy is discussed.
1. The audience will understand the rationale for the proposed machine specific QA for Tomotherapy machines and how to carry out the various tests in a systematic and controlled fashion.
2. The rationale for the proposed patient specific pre‐treatment QA and how to carry it out in a systematic and controlled fashion will be understood.
33(2006); http://dx.doi.org/10.1118/1.2241912View Description Hide Description
Medical physicists play an important role in the success of clinical trials involving radiation therapy. An understanding of how these clinical trials work is necessary and expected of the physicist involved with these treatments. AAPM Report #86, Quality Assurance for Clinical Trials: A Primer for Physicists, provides background information to physicists participating in clinical trials and will be reviewed.
The second part of the presentation introduces a new initiative within the AAPM that will affect how clinical trials are conducted. The goal of AAPM Task Group 113 is to provide guidance to physicists and others involved in clinical trials on methods to improve the consistency and quality of data generated for trials involving external beam radiotherapy. This presentation will discuss factors that impact data quality for the entire treatment planning and delivery process with an emphasis on advanced technologies such as IMRT and IGRT. The scope of Task Group 113 includes image acquisition for volume definition, treatment planning systems, patient localization, treatment guidance and delivery, and credentialing for clinical trials.
1. To review the primer on QA for Clinical Trials.
2. To describe the goals of TG#113 Physics Practice Standards for Clinical Trials.
3. To highlight factors that directly impact clinical trials that involve IMRT and IGRT.
- CE: Shielding II: New NCRP Report: Linac
33(2006); http://dx.doi.org/10.1118/1.2241490View Description Hide Description
Regulatory agencies typically require shielding integrity radiation surveys during commissioning of radiation therapy linear accelerators(linacs). While concrete barriers that provide adequate shielding for photons also provide adequate shielding for neutrons, facilities operating at energies above 10 MV shall be checked for neutrons at the door, maze entrance, and any other openings through the shielding. Laminated barriers shall be monitored for neutrons beyond the shielding.
For the primary barrier measurements, the maximum field size is utilized without a phantom in the beam. Gantry angles of 0, 90, 180, 270 degrees as well oblique angles depending upon the shielding configuration are commonly used. Secondary barriers are surveyed with the maximum field size and a phantom in place.
Photon surveys outside the barriers are performed typically with a calibrated ionization chamber which has both rate and integrate modes, at 30 cm from the barrier. Head leakage in the linac room can be established with the use of film wrapped around the linac head and integrating dosimeters.
In this lecture neutron monitoring will be emphasized. Neutron measurements inside the treatment room are fraught with difficulties because of photon interference from the primary and leakage photon beam and the fact that neutrondetection is spread over many decades of energy. Thus no single neutrondetector can accurately measure neutron fluence or dose equivalent over the entire energy ranges. Additionally neutrondetectors can have photon‐induced reactions when used in the primary photon beam. Further because therapy linacs are operated in a pulsed mode, the intense photon pulse overwhelms any active detector that detects particles electronically. Thus active detectors such as such as neutron rem‐meters, fluence meters and spectrometers neutron cannot be used inside the treatment rooms except at or near the maze entrance. They can be used outside the shielded treatment room.
Passive monitors with high neutron sensitivity such as moderated activation foils (gold and indium) and threshold activation detectors (phosphorous) can be typically used inside the treatment room and inside the primary beam. Moderated activation foils can also be used inside the treatment room and outside the primary beam. Solid state neutrondetectors (SSNTDs) such as CR‐39 ® and bubble detectors can be used inside the treatment room, but outside the primary beam. Bubble detectors can also be used for radiation surveys outside the shielded treatment room.
1. Understand how to perform shielding integrity radiation surveys.
2. Understand the various neutron monitoring methods and instruments.
3. Understand under which conditions these monitors can be used.
- CE: Shielding III: Practical Examples, Including IMRT, TBI, SRS
33(2006); http://dx.doi.org/10.1118/1.2241674View Description Hide Description
The recently published NCRP Report No. 151, “Structural Shielding Design and Evaluation for Megavoltage X‐ and Gamma‐Ray Radiotherapy Facilities”, presents updated methods and data for radiation therapy room shielding design. These calculational methods are applied in examples representing the more common shielding design situations. A radiation therapy vault with a maze barrier and intended for use with IMRT and TBI procedures is the principal example. Detailed calculations of the barriers and door shielding structures are presented. Procedures for evaluating compliance with the NRC licensing constraint on dose equivalent in‐anyone‐hour are presented. An example of a high‐energy room where photoneutron production is of concern is presented with focus on assessing the dose equivalent at the entrance and the maze door design.