Volume 36, Issue 6, June 2009
Index of content:
- Therapy Continuing Education Course: Ballroom C
- CE ‐ Therapy: RPC Programs: Credentialing — QA of Clinical Trials
36(2009); http://dx.doi.org/10.1118/1.3182193View Description Hide Description
Purpose: To describe the role of the Radiological Physics Center (RPC) in credentialing institutions for clinical trials involving advanced technology radiation therapy.Method and Materials: The RPC was founded in 1968 under an agreement between the AAPM and the Committee for Radiation Therapy Studies (CRTS). We have functioned continuously for 40 years to support medical physicists and radiation therapy departments that participate in NCI‐sponsored clinical trials. The focus of this presentation is on the RPC's evaluation of advanced technology radiation therapy. The use of the RPC phantoms has revealed a number of interesting conclusions about the delivery of IMRT and SBRT that should be understood by the community. Other credentialing programs address the use of brachytherapy, IGRT, and proton beam therapy. Results: At all participating institutions, the RPC monitors the basic machine output and brachytherapy source strengths, the dosimetry data utilized by the institutions, the calculation algorithms used for treatment planning, and the institutions' quality control procedures. The methods of monitoring include on‐site dosimetry review by an RPC physicist, and a variety of remote audit tools. During the on‐site evaluation, the institution's physicists and radiationoncologists are interviewed, physical measurements are made on the therapy machines, dosimetry and quality assurance data are reviewed, and patient dose calculations are evaluated. The remote audit tools include 1) mailed dosimeters evaluated on a periodic basis to verify output calibration and simple questionnaires to document changes in personnel, equipment, and dosimetry practices, 2) comparison of dosimetry data with RPC “standard” data to verify the compatibility of dosimetry data, 3) evaluation of reference and actual patient calculations to verify the validity of treatment planning algorithms, and 4) review of the institution's written quality assurance procedures and records. Mailable anthropomorphic phantoms are also used to verify tumordose delivery for special treatment techniques. Any discrepancies identified by the RPC are pursued to help the institution find the origin of the discrepancies and identify and implement methods to resolve them. The RPC has recently extended all of the monitoring and credentialing programs to include proton beam facilities. Conclusion: While conducting these reviews, the RPC has amassed a large amount of data describing the dosimetry at participating institutions. Representative data from the monitoring programs will be discussed and examples will be presented of specific instances in which the RPC contributed to the discovery and resolution of dosimetry errors. The results of credentialing programs for IMRT,SBRT,brachytherapy, and proton beams will be described.
The RPC is supported by PHS grants CA 10953 and CA 81647 awarded by NCI, DHHS.
1. Become familiar with the activities of the Radiological Physics Center.
2. Know how to contact the RPC for assistance or collaboration.
3. Understand the role of the RPC in monitoring institutions that participate in clinical trials.
4. Become familiar with the results of measurements using the RPC's anthropomorphic phantoms.
5. Review common errors and misconceptions regarding dosimetry, credentialing requirements, and other issues.
- CE ‐ Therapy: Radiation Therapy Shielding
36(2009); http://dx.doi.org/10.1118/1.3182200View Description Hide Description
The application of the structural shielding design techniques and goals as outlined in NCRP Report 147: Structural Shielding Design for Medical X‐ray Imaging Facilities (November 2004) and NCRP Report 151: Structural Shielding Design and Evaluation for Megavoltage X‐ and Gamma‐Ray Radiotherapy Facilities will be the basis for this practical course. Actual facility designs will be used as the example calculations of required shielding for Linear Accelerators to be installed in modern radiation therapy facilities. Examples of methods to minimize the amount of additional shielding needed for new departments due to well planned designs will be given.
As the equipment in radiationoncology departments has evolved to linear accelerators with IMRT as state of the art modalities, the requirements for adequate radiation shielding for these modalities has become more rigorous. Architectural designs no longer depend on standard maze design rectangular rooms. Innovative layouts and utilization of multiple layers of shielding materials allow much greater flexibility in room designs. Shielding calculations for these challenging designs will be covered in this presentation.
1. Understand the workload and occupancy factors to be used for dual energy linear accelerators with IMRT capabilities to determine required structural shielding to meet exposure limits for occupational personnel and the public.
2. Understand the effectiveness of existing and additional structural shielding materials to provide radiation protection and methods to calculate the required amounts of these materials.
3. Understand the calculation methods to be used in performing the shielding calculations for all aspects of linear accelerator installations to insure adequate shielding is provided to meet applicable state and ALARA requirements.
- CE ‐ Therapy: Monte Carlo: Implementation of Clinical Systems
36(2009); http://dx.doi.org/10.1118/1.3182308View Description Hide Description
Monte Carlo (MC) based treatment planning systems are already commercially available for both photon and electron beams and more users are implementing them in a clinical setting. Therefore it is important that strategies and paradigms for clinical commissioning and implementation of such systems be formulated and discussed. The purpose of commissioning tests for MC based treatment planning systems is not only to evaluate the accuracy of the system, but also to define the optimum calculation parameters, such as number of histories, calculation voxel size, etc., which will be applied in clinical use of the system. This in turn requires that medical physicists responsible for clinical implementation of such systems are well educated in principles of Monte Carlo algorithms.
We provide a brief 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 radiation transport to the application of this approach in routine clinical photon and electron beamtreatment planning. We also describe possible clinical implementation issues, including dose‐to‐medium vs. dose‐to‐water differences and give comparison of computation times for typical patient anatomies and calculation parameters.
1. To provide an educational review of the physics of the MC method.
2. To discuss the factors associated with MCdose calculation within the patient‐specific geometry, such as statistical uncertainties, approximations of the underlying physics model, CT‐number to material density assignments, and reporting of dose‐to‐medium versus dose‐to‐water.
3. To briefly review the vendor transport codes currently used for clinical treatment planning.
4. To discuss the issues associated with experimental verification of MC algorithms.
5. To briefly review the potential clinical implications of MC calculated dose distributions.
6. To provide example timing comparisons of the major vendor MC codes in the clinical setting.
- CE ‐ Therapy: PET ‐ Imaging, Planning and Assessment
36(2009); http://dx.doi.org/10.1118/1.3182314View Description Hide Description
Positron Emission Tomography(PET)images show physiological and biological information through the in vivo distribution of radioactive, positron‐emitting agents. PETimaging shows focal and distributed regions of cancer and its metastases. Initial PET uses in oncology include diagnosis and staging, which are important for determining treatment decisions. Current PET uses now include target definition for radiation planning followed by PET‐based assessment of treatment. Hybrid PET‐CT devices are becoming attractive radiation treatment simulators.
PETimaging has coarse spatial resolution compared to CT and MR. PET's clinical use is valued because of its great sensitivity for cancer detection. While F‐18‐labled Fluoro‐deoxyglucose (FDG) remains the most promising agent for tumor diagnosis and staging, other, biologically more specific agents (e.g., FMISO, CuATSM for hypoxia imaging or FLT for cell proliferation imaging) might be more appropriate for target definition and treatment assessment.
Although PET brings to radiation therapy of cancer the critical advantage of defining the tumor based on its molecular properties, delineating the gross tumor volume (GTV) with PET is problematic due to the uncertainties in the biological and physiological processes governing the tracer uptake and due to physical limitations for the accuracy of PETimages. The main PETimage degrading factors, including limited resolution, photon scatter and attenuation will be described and some of the current correction strategies will be introduced. A brief summary of the PETimage segmentation methods will be presented.
This course reviews PET‐CT hybrid scanning devices, uses of FDG and non‐FDG PET for oncologyimaging, and quantitative aspects for PET‐based radiation target definition and treatment assessment. Example images demonstrate the potential contributions and limitations of FDG and non‐FDG PEToncologyimaging. This review course is intended for both imaging and radiationoncology physicists.
1. Understand the principle and the current physical limitations of PET
2. Become familiar with the types of PET‐based tumor segmentation methods
3. Discuss quantitative aspects of PETimaging for treatment assessment
- CE ‐ Therapy: IMRT Planning of the Pelvis/GU
36(2009); http://dx.doi.org/10.1118/1.3182446View Description Hide Description
Intensity modulated radiation therapy(IMRT) is increasingly used for the treatment of gynecological tumors. Successful implementation requires careful attention to every detail from patient selection to simulation, to planning, to setup, to localization, and to delivery. At the University of Chicago, all patients are simulated and treated in supine position using customized alpha cradles indexed to the treatment table. Oral, IV and rectal contrast are used to help in the delineation of the CTV and surrounding normal tissues. The CTV consists of the upper half of the vagina, uterus (if present), parametrial tissues, parasacral region, and the contrast enhanced vessels plus a 2cm margin to identify the common, external and internal nodal regions. PTV is constructed by adding a margin based on the setup uncertainty and organ motion data. Normal tissues contoured include bladder, rectum, small bowel and pelvic bone marrow. For treatment planning, 9 equally spaced co‐planar beams are used. Input parameters derived for the PTV and surrounding normal tissues were developed over the time, and their evolution will be discussed. Treatment plans are evaluated primarily based on the PTV coverage and normal tissue DVHs. 3‐D dose distribution plays an important role in the IMRT delivery and is rigorously evaluated by both physicists and physicians at our institutions. Acceptable plans are the ones that have a minimum coverage of 98% of the PTV volume with the prescription dose while no more than 2% of the PTV receives more than 110% of the prescription dose. Evaluation of small bowel dose is based on a NTCP analysis for the incidence of acute gastrointestinal toxicity of IMRT patients treated in our center. From this analysis, small bowel volume receiving prescription dose (45Gy) should be less than 200cc. Plan acceptance criteria for rectum and bladder will also be discussed during this course. Recent studies conducted in our center studies have associated the volume of pelvic and lumbosacral bone marrow (PBM and LSBM) receiving 10 and 20 Gy with acute hematologic toxicity (AHT) in patients receiving concurrent chemotherapy and IMRT. We will discuss and present plan consideration for bone marrow sparing IMRT to reduce AHT in these patients. QA is one of the most important and integral part of any radiation therapy procedure. IMRT QA, in our institution, is performed using independent monitor unit verification (MUV) and patient specific measurements and will be discussed in details. In addition, recent studies reporting adaptive treatment and image guided approaches in the treatment of gynecological tumors will be briefly reviewed.
1. To understand the practical and fundamental aspects of IMRT planning for gynecological malignancies
2. To discuss the GYN‐IMRT plan evaluation and acceptance criteria
3. To review and discuss the IMRT quality assurance issues for this disease site.
4. To briefly review and critique adaptive and image guided GYN‐IMRT approaches.
36(2009); http://dx.doi.org/10.1118/1.3182447View Description Hide Description
The prostate is a mobile structure compared to the surrounding bony anatomy. Daily setup, immobilization and localization uncertainties can be addressed by increasing the PTV but may result in additional dose to surrounding normal structures. At FCCC we attempt to reduce the uncertainty by employing daily localization using ultrasound, implanted fiducials or through target tracking using implanted Calypso beacons. We currently use an 8mm growth in all directions except posteriorly where 5mm is typical. Patients with fiducials and those being irradiated in the post‐prostatectomy setting undergo localization via an in‐room CT scanner or CBCT. We have recently begun target tracking on post‐prostatectomy patients as well. All patients are simulated and treated supine without a thermoplastic immobilizer to minimize respiratory related prostatic motion. Patients undergo MR and CT simulations with the rectum empty. These data are fused for target and normal structure delineation for use in treatment planning. Emphasis on soft tissue structures is important to avoid potential systematic localization errors due to the time difference between scans for patients with implanted fiducials or beacons. Dose limiting structures primarily include the rectum, bladder, and femoral heads, but may also include bowel and erectile tissues. We have developed “plan acceptance criteria” based on published data with respect to rectal complications. DVH analysis is used to ensure that the rectal volumes receiving 65Gy and 40Gy are less than 17% and 35%, respectively. Additionally, the bladder volumes receiving 65Gy and 40Gy are less than 25% and 50%, respectively. The volume of either femoral head receiving 50Gy should be less than 10%. PTV coverage should result in at least 95% of the volume receiving the prescription dose. It should be noted that the 3D dose distribution itself plays an important role in IMRT delivery and DVH analysis alone may not be sufficient. The isodose distribution should be such that the 90% and 50% lines do not traverse the half or full width of the rectum on any CT slice, respectively. Dose escalation and hypofractionation regimes continue to be of interest in the treatment of this disease site. However, most delivery techniques require PTV reductions to avoid increased normal tissue toxicity. A quantitative analysis of uncertainties contributing to PTV margins is addressed. Additionally, treatment methods that may not require margin consideration, are discussed. These include fiducial tracking using the Cyberknife and the use of high intensity focused ultrasound (HiFU) ablation.
1. To understand the practical steps associated with IMRT of the prostate
2. To understand the numerical values presented for plan acceptance
3. To be aware of MR‐CT fusion issues for patients with implanted fiducials or beacons
4. To be aware of issues related to PTV reduction
- CE ‐ Therapy: IMRT Metrology
36(2009); http://dx.doi.org/10.1118/1.3182453View Description Hide Description
This session will present dosimetry tools and techniques for intensity modulated radiation therapy based on the AAPM Task Group 120 report. First, advantages and disadvantages of different detectors including ion chambers and 2D dosimetry systems will be discussed in the context of IMRT distributions. Second, considerations with respect to phantoms will be discussed. Next, the application of different dosimetric analysis tools will be presented. The final topic is the next generation of detectors for QA such as polymergels.
The educational objectives of this session are to provide information to:
1. Determine the best detector for a given measurement scenario
2. Assess the impact of phantom selection on the measurement
3. Understand the benefits and limitations of different analysis tools for evaluating calculations and measurements of IMRT fields
4. Learn about new dosimetry systems for IMRT measurements
- CE ‐ Therapy: The Use of Ultrasound Imaging in RT Planning and Delivery
36(2009); http://dx.doi.org/10.1118/1.3182590View Description Hide Description
While numerous image guidance tools have evolved in recent years, in‐room ultrasound(US) guidance remains a viable, efficient and still widely utilized tool for localization of prostate and other abdominal sites. In the presence of proper training and implementation, the method has been found to be an acceptably accurate, relatively inexpensive, non‐invasive method of localizing prostate that requires no delivered radiation dose. In this refresher course we will review the fundamentals of the US guidance process and discuss methods for implementation and ongoing QA of an effective US Guidance program.
- CE ‐ Therapy: Tools for IMRT Commissioning: Static and Rotating Gantries
36(2009); http://dx.doi.org/10.1118/1.3182597View Description Hide Description
Commissioning an IMRT system refers to the initial verification by phantom studies that treatments can be planned, prepared, and delivered with sufficient accuracy. Our profession has struggled since the inception of IMRT to define what measurements to make and what degree of accuracy is sufficient. Two AAPM task groups have been dealing with complementary issues. AAPM TG 120 is finalizing a report on dose measurement techniques for IMRT validation. AAPM TG 119 is finalizing a report that recommends a specific set of tests for IMRT commissioning and presents the results of those tests from several institutions in order to illustrate the accuracy that is achievable. The report presents a methodology for other facilities to perform the tests and determine if their achieved accuracy is comparable to this reference baseline.
The TG119 tests pose a range of optimization problems requiring simple to complex modulation patterns. The tests include mock prostate, head and neck, and para‐spinal geometries. Members of the group have planned and delivered the treatments using their local planning and delivery systems, and then assessed the resulting doses using broadly available dosimetry tools following a specified protocol. Measurements included ion chamber “point” doses and film dosimetry on selected planes for all fields irradiating the phantom. Institutions also evaluated dose distributions produced by individual fields using detector arrays, film, or EPID. Planar dosimetry results were analyzed with gamma criteria of 3% dose/ 3mm distance to agreement. The mean values and standard deviations of the results were used to develop confidence limits for the test results using the concept [Confidence Limit = |Mean| + 1.96 σ]. Other facilities can use the test protocol and results as a basis for comparison to this group.
Learning objectives for this presentation:
1. Describe the TG119 test suite, analysis protocol, and initial results.
2. Demonstrate how it can be applied by other facilities, including delivery with rotating gantries.
3. Provide examples of IMRT QA failures that can be identified and corrected using these processes.