Volume 35, Issue 6, June 2008
Index of content:
- Therapy: Continuing Education Course: Room 350
- CE‐Therapy: Out of Field Dosimetry Involving Physical and Computational Phantoms
35(2008); http://dx.doi.org/10.1118/1.2962318View Description Hide Description
Patients treated with ionizing radiation carry a risk of developing second cancers in their life times. Factors contributing to the recently renewed concern about this risk include improved cancer survival rates, younger patient pool, as well as emerging treatment modalities such as IMRT and proton therapy that can potentially prolong secondary exposures to healthy tissues or expose patients to neutrons. In the past 20 years, external beam treatment technologies have evolved significantly, and there is an overwhelming amount of data on both dosimetry and epidemiological studies. This lecture reviews historical studies related to the assessment of scatter and leakage radiation from classical radiation therapy, 3D‐Conformal x‐ray therapy, intensity‐modulated x‐ray therapy (IMRT and tomotherapy), and proton therapy. The concept of high‐, intermediate‐ and low‐dose regions outside the primary treatment field is introduced. Terms such as “secondary cancers,” “organ‐average equivalent dose,” “out‐of‐field,” and “peripheral dose” are reviewed. Comparison of secondary doses from different treatment modalities, make/energy of accelerator and for adult and pediatric treatments is also made. Concomitant dose from IGRT localization and verification procedures involving cone‐beam CT is briefly discussed. This is followed by a detailed review on methods of performing measurements in physical phantoms using different types of radiation detectors, as well as Monte Carlo simulations using gender‐ and age‐specific “virtual patient” computational models. Finally future needs on dosimetry and epidemiological studies are discussed.
1. Understand the importance of secondary cancer risk in radiation treatment.
2. Understand basic concepts in radiationdosimetry associated with secondary cancer studies.
3. Understand practical methods of measurement and Monte Carlo simulation for the assessment of organ‐average equivalent doses for various treatment modalities.
- CE‐Therapy: Functional/Molecular Imaging: PET for Planning/Assessment
35(2008); http://dx.doi.org/10.1118/1.2962329View Description Hide Description
Purpose: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 beginning to be 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. Non‐FDG PET agents can be more specific in cell targeting (binding), and can image different aspects of tumor biology, like hypoxia (FMISO, CuATSM) and cell proliferation (FLT). 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. The Standardized Uptake Value (SUV) is a normalized intensity measure for quantitative indication of disease, and can be used to identify disease or possibly for target delineation, with certain limitations. While FDG remains the most promising agent for tumor detection, other, biologically more specific agents (e.g., FMISO, CuATSM for hypoxia imaging or FLT for cell proliferationimaging) might be more appropriate for target delineation and treatment assessment. Conclusion: 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 radiation oncology physicists.
1. Describe FDG PETimaging and oncologic indications.
2. Review the uses and limitations of PETimages in radiation treatment.
3. Describe the SUV and other threshold parameters for target delineation.
4. Review non‐FDG PETimaging of tumor biology.
5. Discuss quantitative aspects of PETimaging for treatment assessment.
- CE‐Therapy: Monte Carlo — II: Clinical Impact
35(2008); http://dx.doi.org/10.1118/1.2962411View Description Hide Description
This presentation will facilitate participants with the clinical application of Monte Carlodose calculation algorithms in radiotherapytreatment planning and dosimetry verification. Following a brief introduction to the clinical implementation and commissioning of the Monte Carlodose calculation software detailed discussions will be given on the actual and potential impact of Monte Carlodose calculation algorithms on conventional electron beam therapy and advanced mixed beam treatments.
Comparisons of patient treatment plans generated using conventional dose calculation algorithms and Monte Carlo methods will be made with an emphasis on the causes of the dose discrepancies. Further discussions will be conducted on the use of Monte Carlodose calculation as a radiotherapy treatment QA tool to validate individual patient plans and as an investigation tool to improve target dose conformity and normal tissue sparing using novel treatment techniques.
1. To describe clinical implement and commissioning of Monte Carlodose calculation algorithms for radiotherapytreatment planning.
2. To discuss the impact of Monte Carlodose calculation in electron therapy and advanced mixed beam therapy treatment planning.
3. To describe the applications of Monte Carlodose calculation in treatment planning and beam delivery QA for advanced radiotherapy treatments.
35(2008); http://dx.doi.org/10.1118/1.2962412View Description Hide Description
Monte Carlo(MC) dose calculation algorithms are now available in some commercial treatment planning systems for applications in photon beam IMRT planning. This course will examine the various stages of a MCIMRT dose calculation, including methods for particle sourcing, fluence modulation, and patient dose computation dose. The use MC as a QA tool to validate commercial IMRT dose calculation algorithms and its use as a QA tool to validate 3D dose distributions for individual IMRT plans will be reviewed. To this end, large‐scale comparison of clinical IMRT plans created with a commercial planning system with MC dose re‐calculation will be presented. Advantages of using MC algorithms directly in the IMRToptimization loop to circumvent optimization convergence errors and strategies to reduce MC dose calculation time when used within an optimization framework will be discussed.
1. To understand the stages of a Monte Carlo dose calculation when applied to clinical IMRTtreatment planning.
2. To illustrate the role of Monte Carlo for IMRTtreatment planningquality assurance.
3. To evaluate the dose accuracy of a commercial non‐Monte Carlo IMRT dose calculation algorithms with respect to Monte Carlo dose computations.
4. To demonstrate methods for Monte Carlo‐based clinical IMRT dose optimization.
The author is supported in part by NCI Grant P01 CA116602, by Philips Medical Systems and by Varian Medical Systems.
- CE‐Therapy: RPC Programs
35(2008); http://dx.doi.org/10.1118/1.2962422View Description Hide Description
Purpose: To describe the role of the Radiological Physics Center (RPC) in evaluating 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). The agreement called for the AAPM to solicit applications to form a QA center that would be a resource in radiationdosimetry and physics for cooperative clinical trial groups and all radiotherapy facilities that deliver radiation treatments to patients entered onto cooperative group protocols. The RPC has functioned continuously for 40 years to support medical physicists and radiation therapy departments. Results: The RPC's mission has changed only slightly over the years. The primary responsibility is to assure NCI and the cooperative groups that the participating institutions have adequate quality assurance procedures and no major systematic dosimetry discrepancies, so that they can be expected to deliver radiation treatments that are clinically comparable to those delivered by other institutions in the cooperative groups. To accomplish this, 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 (TLD) 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 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. 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 RPC is supported by PHS grants CA 10953 and CA 81647 awarded by NCI, DHHS, and by an educational grant from Varian Medical Systems.
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: IMRT Planning of the Pelvis/GU
35(2008); http://dx.doi.org/10.1118/1.2962662View Description Hide Description
IMRT presented a major advancement in the radiotherapeutic management of gynecologic cancer. It has been shown that IMRT has reduced the incidence and severity of radiation‐related toxicity in this disease site. However, the precise dose distribution produced by IMRT is less forgiving and great care is required in order to achieve the intended results. An adequate understanding of the entire process ‐from proper patient selection to positioning/immobilization to treatment planning and delivery‐ is essential. The primary goal of this presentation is to provide a practical overview of IMRT for gynecological malignancies. A discussion of the steps in the GYN IMRT process will include patient selection, immobilization, simulation, structure delineation, planning strategies and parameters, PTV and critical organdose objectives, plan evaluation, QA, and potential delivery issues. Guidelines and practical examples of the GYN IMRT process will be presented.
At our institution, gynecologic patients are treated in the supine position with customized immobilization devices (alpha cradles), which are indexed to the treatment table. Oral, intravenous and rectal contrasts are used to aid in the delineation of the CTV and surrounding normal tissues. The CTV consists of the contrast enhanced vessels to identify common, external, and internal nodal regions along with the upper half of the vagina, parametrial tissues, presacral region and uterus (if present). A PTV is added to the CTV based on measured set‐up uncertainties and organ motion data. Several recent studies have addressed these issues and will be reviewed here. For treatment planning, 9 equally spaced co‐planar beams are generally used. Input parameters derived for treatment planning were developed over time, and their evolution will be discussed. Treatment plans are evaluated primarily based on the PTV coverage and normal tissue DVHs. Evaluation of small bowel is based on a normal tissue complication probability (NTCP) curve for the incidence of acute gastrointestinal toxicity of IMRT patients treated in our clinic. From this analysis, acceptable plans are those in which <200 cc of the small bowel region receives 45 Gy (prescription dose). We have also recently defined bone marrow constraints for patients receiving concomitant chemotherapy, and these will be discussed.
Finally, current research and future directions for GYN IMRT will be introduced. For instance, image‐guided radiotherapy (IGRT) has received increasing attention as a component of IMRT planning and delivery. We will discuss the potential role of novel image‐guidance techniques in GYN IMRT. In addition, we will present IGRT/IMRT techniques that are currently being considered to provide an alternative or supplement to brachytherapy. Clinical examples of each of these approaches will be presented.
1. To provide an educational review of the practical aspects of IMRT planning for gynecologic malignancies.
2. To understand and review the criteria for IMRT plan evaluation in gynecologic patients.
3. To review the planning methods used to achieve plan acceptance criteria.
4. To discuss the role of IGRT technologies in this disease site.
35(2008); http://dx.doi.org/10.1118/1.2962663View 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 results in additional dose to surrounding normal structures. At FCCC we attempt to reduce the uncertainty by employing daily localization using BAT 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 inroom CT scanner or CBCT. These methods allow for minimal PTV expansion by moving the prostate or prostate bed into the intended dose region.
All patients are simulated and treated supine without a thermoplastic immobilizer to minimize respiratory related prostatic motion and to facilitate the use of ultrasound. Patients undergo CT and MR simulations with the rectum empty. These data are fused with emphasis on soft tissue structures particularly for those with implanted fiducials or beacons. Dose limiting structures primarily include the rectum, bladder, and femoral heads, but may also include bowel and erectile tissues. The delivery of high doses (70–80+Gy) using 3D CRT invariably includes rectal shielding to some degree in order to avoid unwanted complications. Rectal shielding also creates a dose gradient across the posterior prostate. Our initial comparisons at 78Gy between 3D CRT and IMRT resulted in an increase in 95% PTV coverage from approximately 76Gy to 78Gy, respectively and a reduction of approximately 6Gy to the “hottest” 20% of the rectum. 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 50% and 90% lines do not traverse the full or half width of the rectum on any CT slice, respectively. Quality assurance includes verification of absolute dose as well as the resultant spatial distribution and our plan acceptance is based on ±3% and 3mm DTA, respectively. We have been able to meet the absolute dose criteria in approximately 94% of cases.
1. To understand the practical steps associated with IMRT of the prostate.
2. To understand the planning methods utilized to achieve the numerical values presented for plan acceptance.
3. To be aware of MR‐CT fusion issues for patients with implanted fiducials or beacons.
- CE‐Therapy: Patient Motion: Adaptive RT
35(2008); http://dx.doi.org/10.1118/1.2962675View Description Hide Description
Patient‐specific target margin and 4D inverse planning are two typical methods to include treatment relevant patient motion in the treatment planning. Internal target volume (ITV) is probably the earliest technique in constructing patient‐specific target volume to compensate for target motion. However, this method only considers motion geometric information in the construction without utilizing inter‐patient heterogeneity of dose distribution, therefore the target margin has not been minimized. In contrast, a recent development in patient‐specific target margin construction includes motion probability density function (pdf) directly in the 4D dose evaluation; therefore individual dose distribution is included in the target margin design. Utilizing 4D dose calculation, treatment plan can also be optimized by modulating beam intensity with respect to a pre‐measured patient motion pdf. Concept of utilizing non‐uniform beam intensity to compensate for temporal displacements of target position has been pointed out long time ago, however it has not been implemented until recent years. Process of designing beam intensity by including patient geometric variation or motion pdf in the dose calculation and inverse planning has been called 4D inverse planning. Since this planning process is based on organ displacement information in a frequency domain, instead of the spatial domain used in the gating and tracking techniques, it is insensitive to the uncertainties in the motion phase. However, uncertainties in pdf characteristics could significantly detriment the treatment qualities.
The most significant variation in motion pattern is the target position baseline variation. Study has shown that inter‐treatment target baseline variation can be minimized efficiently using daily free breathing cone beam CTimaging location and patient repositioning. However, the relative position variation between target and normal organs, the dose response related organ volume variation, as well as intra‐treatment motion pattern variation cannot be easily corrected by repositioning the patient. Therefore, treatment feedback and adaptive planning modification become necessary. In principle, 4D inverse planning is a central part of adaptive planning modification, however to fully accomplish an adaptive planning, the dose distribution in organs of interest which has been delivered previously and may be delivered in future is also important factor and needs to be included in the objective evaluation for 4D planning optimization.
1. Understand the options of 4D planning.
2. Understand the sensitivity of 4D planning to the motion uncertainties.
3. Understand the key components of adaptive treatment process and their functions.
Research on adaptive management of patient motion is partially supported by Elekta and Philips Research Funds.
35(2008); http://dx.doi.org/10.1118/1.2962676View Description Hide Description
Motion of the patient and inner organs does not substantially change the spatial dose distribution in photon therapy in the room coordinate system, aside from the build‐up region. In other words, organs move within a static “dose cloud”. Using this approximation it is easy to understand the consequences of motion: systematic errors (lack of accuracy) lead to an offset of the dose distribution in the frame of the organs, and random errors (lack of precision) lead to dose blurring.
Dose blurring can be described in a statistical way by use of a motion probability (density) function (PDF). The motion‐blurred dose distribution is obtained by a convolution of the “sharp” (static case) dose distribution with the motion PDF. This holds true for both inter‐ and intra‐fraction motions. If intra‐fraction motion is present during an IMRT treatment, the dose distribution will also be affected by an “interplay” effect, in addition to the blurring. It has been shown that the interplay effect averages out during the course of a fractionated treatment, and that it is usually negligible after a typical number of fractions. The convolution model relies on the linear superimposition principle, which holds true for dose values but not for the biological effect. This issue has recently been addressed and will be discussed.
Several investigations have now looked at the feasibility of un‐doing the motion blur through the use if intensity‐modulation. In principle it should indeed be possible to de‐convolve the motion PDF from the intensity maps, to compensate for motion effects. This approach has been called 4D optimization or 4D inverse planning. Motion de‐convolution cannot, however, compensate motion effects exactly and it cannot be applied in a naïve straight‐forward way, because that would lead to undeliverable intensity maps with sharp spikes and negative values. The method of choice is rather to include the motion PDF in the IMRToptimization process. It has been shown that this can indeed yield a surprisingly high degree of motion compensation and it can even compete with other motion compensation methods such as gated delivery. However, this is only true if the motion characteristics (the PDF) are known with great precision. If the actually realized motion PDF deviates substantially from the planned PDF, the method becomes less useful and can, in principle, make things worse.
More recently, uncertainties in the knowledge of the motion characteristics have been taken into account by use of robust optimization techniques. With these one can now compensate for motion effects in an approximate way for a large class of motion characteristics. In terms of the sparing of normal structures, the results are in between the use of conventional margins and the idealistic case of perfect motion compensation. The resulting intensity maps exhibit “horns”, which can shave off a few mm from the margins.
1. Understand the concepts of motion blur and PDF.
2. Understand the idea of de‐blurring a dose distribution through “4D” motion optimization.
3. Be able to discuss the relative potential and limitations of 4D motion optimization in comparison with margins and gating.
- CE‐Therapy: Scintillator Dosimetry
35(2008); http://dx.doi.org/10.1118/1.2962819View Description Hide Description
Interest in the development of plastic scintillation detector systems for dosimetry has been evolving for more than a decade. Plastic scintillation materials have many properties that make them ideal for dosimetry including water equivalence and energy independence for MV photons, linearity with dose,dose rate independence, and high spatial resolution. Therefore, these detectors do not require the usual conversion and/or correction factors used for other commonly used detectors to convert the dosimeter reading to absorbed dose. The only disadvantage of plastic scintillation detectors is the spurious effect arising from Cerenkov radiation produced in the optical fiber that guides the scintillation light, which has been solved. This evolution started with point detectors and is leading to matrix arrays to respond to the ever‐increasing complexity of radiotherapy treatment fields such as IMRT. Small fields, high dose gradients and other challenging conditions could soon require the development of commercial scintillation detectors. Moreover by using a photodetector with a large sensitive area (e.g. a CCD camera), arrays of several hundreds of scintillation detectors could be made to simplify long and arduous quality assurance tests.
This lecture will provide an overview of the dosimetric characteristics and properties of plastic scintillation detectors when exposed to high‐energy photon, electron as well as proton beams. We will discuss both forms of plastic‐based scintillation detectors: plastic scintillators and plastic scintillating fibers. Finally, we will present few applications for plastic scintillation detectors in clinical radiotherapy:stereotactic radiosurgery,quality assurance and in vivo dosimetry applications.
1. Review the underlying physics of scintillation materials.
2. Review the properties of plastic scintillation materials used in radiationdosimetry.
3. Understand the principles of this method and recent innovations in scintillation dosimetry.
4. Identify applications that could benefit from scintillation detectors.