Volume 34, Issue 6, June 2007
Index of content:
- Therapy Continuing Education Course: Auditorium
- CE‐Therapy Series: Topics in Radiation Therapy — I
34(2007); http://dx.doi.org/10.1118/1.2761197View Description Hide Description
As high‐precision 3‐D conformal radiation therapy and intensity‐modulated radiation therapy have become standard practice, radiographicimaging using kilovoltage (kV) x‐ray sources has been rapidly implemented for in‐room target localization and patient positioning to ensure conformal dosedelivery. Various types of imaging devices are commercially available for clinical applications and their typical imaging functionalities include 2‐D radiographic and fluoroscopic imaging as well as 3‐D cone beam CT. There are substantial demands for fundamental understanding of what kind of systems can be used for clinical practice, when and what imaging systems should be used, how they can be properly used for daily target localization for different anatomical sites, and what kind of quality assurance programs are needed.
In this session, we will briefly introduce the latest commercially available imaging systems using kV imaging for in‐room target localization and their imaging principles. Clinical applications and imaging protocols using these systems for accurate target localization and patient positioning will then be discussed. Finally, systematic quality assurance procedures will be presented.
1. Understand the latest commercially available technologies for in‐room kV radiography, fluoroscopy, and cone‐beam CT and their basic imaging principles.
2. Understand the basic clinical imaging applications for daily localization.
3. Understand the basic system limitations and QA components of a comprehensive QA program.
34(2007); http://dx.doi.org/10.1118/1.2761198View 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‐Therapy Series: Topics in Radiation Therapy — II
34(2007); http://dx.doi.org/10.1118/1.2761310View 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 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.
34(2007); http://dx.doi.org/10.1118/1.2761311View Description Hide Description
Intensity modulated radiation therapy(IMRT) is a method of treatment planning and delivery that conforms the high dose region to the shape of the target volume, while reducing the volume of normal tissue irradiated. Over the last several years, IMRT has played an important role in management of patients with gynecologic malignancies particularly for whole pelvic radiotherapy.IMRT planning begins with a CT simulation. Gynecologic patients are most often treated in the supine position, although several studies have demonstrated the advantages of planning in the prone position. At our institution, customized immobilization devices (alpha cradles) are fabricated which are subsequently indexed to the treatment table. Oral, intravenous and rectal contrast are used to aid in the delineation of the CTV and surrounding normal tissues (bladder, rectum, small bowel and pelvic bone marrow). The CTV consists of the contrast enhanced vessels (plus a 2 cm margin) 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 these will be reviewed. For treatment planning, 7 (small patients) or 9 (larger patients) equally spaced, co‐planar beams are generally used. Input parameters derived for treatment planning were developed over time, and their evolution will be discussed. Values used for a number of commercially available planning systems will also be presented. Treatment plans are evaluated primarily based on the PTV coverage and normal tissue DVHs. For the PTV, acceptable plans are defined as those which cover >98% of the volume with the prescription dose while <2% of the PTV receives >110% of the prescription dose. 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. Lastly, image‐guidedradiotherapy(IGRT) has received increasing attention as a component of treatmentdelivery. In gynecologic IMRT, there are three areas where IGRT may offer substantial benefit. First, IGRT may reduce geometric misses by providing daily information on isocenter displacements and patient rotations. Additionally, IGRT has an important role in cervical cancer patients where the tumor is shrinking during the course of treatment. Using IGRT,tumor size and position can be monitored and the treatment plan can be modified appropriately. In addition, IGRT approaches are currently being considered in the development of IMRT approaches to replace brachytherapy. Clinical examples of each of these approaches will be presented.
1. To understand the practical aspects of IMRT planning for gynecologic malignancies.
2. To describe the criteria for IMRT plan evaluation in gynecologic patients.
3. To consider the role of image‐guided technologies in this disease site.
34(2007); http://dx.doi.org/10.1118/1.2761312View Description Hide Description
Treatment of prostate cancer with IMRT requires great care in order to achieve the intended results. 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 or implanted fiducials and 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. 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 followed immediately by MR simulations with the rectum empty. These data are fused and all soft tissue structures contoured based on MR. We believe the apex of the prostate is more accurately visualized with MR without the potential prostate distortion associated with a retrograde urethrogram. 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. Additionally, emphasis is given to treatment time not only for throughput but also for patient comfort. 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.
- CE Therapy Series: Topics in Radiation Therapy — III
34(2007); http://dx.doi.org/10.1118/1.2761475View 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.
34(2007); http://dx.doi.org/10.1118/1.2761476View Description Hide Description
Four‐dimensional computed tomography (4D CT), also called respiratory correlated CT, was first published on, and commercially available in 2003. Since then this technology has gained widespread acceptance and clinical use. The 4D CT acquisition concept is relatively simple: acquire CT scans synchronized with the respiratory cycle such that sufficient data exists to reconstruct a volumetric image at or near a number of respiratory phases. There are a variety of commercial implementations of the basic acquisition concept.
There are several limitations of 4D CT. One problem is artifacts. Though 4D CT was developed to account for the deleterious effects of respiratory motion on 3D image acquisition techniques, irregular respiratory motion causes artifacts in 4D scans. Free breathing, unlike the cardiac cycle on which the technology is based, is typically irregular and artifacts can be found in nearly all 4D CT scans with current technology. There are several strategies to deal with this irregular signal: (1) improve the regularity of the signal itself, using audio‐visual biofeedback tools, (2) during imaging only acquire data during regular cycles and (3) use post‐processing methods to reduce artifacts. Another limitation of 4D CT, at least in its application to radiotherapy, is that the time interval during which images are acquired over, ∼5 seconds per anatomic location for a ∼1 minute total scan time, is a small sample of the respiratory induced anatomic changes occurring over a course of radiotherapy, which can be between a single fraction to several weeks.
Despite these limitations 4D CT has been found to be very useful for a number of applications in radiotherapy planning. 4D CT can be used for measuring target motion, and motion inclusive, respiratory gated and target tracking treatment scenarios. Fully utilizing 4D CTimages for treatment planning requires deformable image registration algorithms for automatic contour propagation and dose summation. For this application, several studies have shown that current algorithms have acceptable geometric performance with respect to expert observers. The dosimetric impact of the geometric uncertainty of deformable registration algorithms appears low.
A more recent development in 4D CT is the extension to 4D cone beam CT (4D CBCT) which offers the ability for pre‐treatment anatomic position and motion verification. This application is a major innovation and will increase treatment accuracy. Residual uncertainties from anatomic changes between the time of imaging and time of treatment have been observed, and intra‐fraction position monitoring is desired to complement 4D CBCT.
1. Understand the principles of 4D CTimage acquisition and reconstruction.
2. Understand the limitations of current 4D CT technology.
3. Understand the ongoing developments in 4D CT and 4D CBCTimaging.
4. Understand the application of 4D CT to treatment planning.
34(2007); http://dx.doi.org/10.1118/1.2761477View Description Hide Description
The report of Task Group 51 on a “Protocol for Clinical Reference Dosimetry of High‐Energy Photon and Electron Beams” was published nearly 8 years ago (Medical Physics26 (1999) 1847 – 1870) and according to the RPC has been adopted by about 80% of all clinics.
This talk will review how the various factors in TG‐51 were calculated, including a derivation of the relevant equations and a discussion of the sources of physical data used. The rationale for the choice of beam quality specifiers will be given and the role of the lead foil explained.
The talk will conclude with a review of some of the published experimental data which confirms the calculated kQ values used in TG‐51 and review some of the post‐TG51‐publication research which has an impact on the physics in the protocol. The protocol should continue to be used as written.
1. To review the basic physics and sources of data underlying TG‐51.
2. To derive the equations used to calculate kQ and k′R50.
3. To review some experimental verifications of TG‐51 quantities, especially kQ.
4. To discuss the implications of more recent dosimetry research and its effects on future dosimetry protocols.
- CE‐Therapy Series: Topics in Radiation Theraphy — IV
34(2007); http://dx.doi.org/10.1118/1.2761621View Description Hide Description
Brachytherapy has had a long history and formed the original intensity‐modulated, three‐dimensionally conformal radiotherapy. This presentation discusses some of the clinical applications of brachytherapy with an emphasis on the physical aspects of treatments. The presentation will focus particular attention on treatments with rapidly evolving procedures, particularly brachytherapy applications in breast, prostate, uterine cervix and liver. The discussion will consider how the physical nature of brachytherapy enhances and limits the treatments.
1. Consider the physical aspects of brachytherapy.
2. Understand the physical limitations in clinical applications of brachytherapy.
3. Know some of the emerging developments in the physical aspects of clinical applications in brachytherapy.
34(2007); http://dx.doi.org/10.1118/1.2761622View Description Hide Description
Intensity modulated radiation therapy(IMRT) has revolutionized the treatment planning process. We can now produce treatment plans for complex target shapes and obtain remarkable dose conformity while respecting the tolerance doses to critical structures. While the emphasis has been given in the implementation of faster, more efficient, and more comprehensive optimization algorithms to solve the inverse problem, some progress has also been made in the implementation of accurate dose calculation algorithms. The convolution/superposition algorithm is the most popular photondose engine used in treatment planning, while fast, pencil beam like implementations continue to be popular with IMRT. The Monte Carlo algorithm although available, is becoming an option as a secondary dose verification rather than as the principal optimization engine for IMRT planning, primarily due to calculation speed issues.
In this presentation we will discuss the algorithms that have historically been used for photon beam treatment planning with emphasis on the convolution and Monte Carlo based methodologies and their application in IMRT planning. Clinical examples will also be presented to demonstrate the use and outcome of dose calculations in homogeneous and heterogeneous media.
1. Review of dose calculation algorithms for photon beams.
2. Demonstrate the effect of dose algorithm selection in IMRT planning.