Volume 33, Issue 6, June 2006
Index of content:
- Therapy Continuing Education Course: Valencia A
CE: IMRT Site Specific ‐ I: Prostate, GYN, Pelvis
33(2006); http://dx.doi.org/10.1118/1.2241465View Description Hide Description
Intensity modulated radiation therapy(IMRT) is increasingly used for the treatment of gynecologic malignancies. A survey conducted in 2004 found that over 35% of the radiationoncology clinics with IMRT were using this modality in gynecologic patients. While treatment planning is an important aspect of gynecologic IMRT, successful implementation requires careful attention to detail throughout the entire planning process. At the University of Chicago, IMRT planning for whole‐pelvic gynecologic patients begins with a CT simulation. Patients are treated in the supine position, and 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. 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. Normal tissues that are contoured include the bladder, rectum, small bowel and pelvic bone marrow. For treatment planning, 7 (small patients) or 9 (larger patients) equally spaced, co‐planar beams are 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. Image‐guided radiotherapy (IGRT) has received increasing attention as a component of treatment delivery. 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. Lastly, IGRT approaches are currently being considered in the development of IMRT approaches to replace intracavitary 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.
33(2006); http://dx.doi.org/10.1118/1.2241466View 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: IMRT Site Specific ‐ II: H&N, CNS
33(2006); http://dx.doi.org/10.1118/1.2241620View Description Hide Description
Intensity modulated radiation therapy(IMRT) for the central nervous system(CNS) is being used more frequently in radiationoncology clinics. There are two general situations that the treatment of CNS disease may benefit from the use of IMRT compared to conventional, three‐dimensional conformal radiation therapy: 1) when multiple critical structures confined within the intracranial vault are to be avoided, one may desire an optimal dose distribution that allows for the dose to these structures to be minimized, and 2) since high‐grade gliomas tend to recur locally, IMRT should allow for dose escalation proportional to the corresponding heterogeneous cell populations. Based on the anatomic location of the treatment volumes, one can visualize examples where IMRT could be of benefit. Patients with a concave or irregularly‐shaped target in a frontal lobe may require IMRT in order to spare the adjacent globe and any uninvolved optic apparatus. In patients with well‐lateralized tumors involving the brain parenchyma, complete sparing of the contralateral hemisphere is desirable. Patients with infiltrative gliomas traditionally have large margins placed around the treatment volumes, and these may often encompass uninvolved critical normal structures. For such cases, IMRT may allow for non‐uniform reduction of the treatment volume around these normal structures. The primary goal of this presentation is to provide a practical overview of IMRT for the CNS.
Though much attention has been given to the inverse planning and quality assurance aspects of IMRT, one should have an adequate understanding of the entire process; from proper patient selection to positioning/immobilization and continuing through treatment. A discussion of the steps of the CNSIMRT process will include: patient selection, immobilization, recommended imaging acquisitions, structure delineation, planning strategies/parameters, dose objectives, plan evaluation, QA, and potential delivery issues. Guidelines and practical examples for each component of this process will be presented.
To gain further familiarization of CNSIMRT, one should review the corresponding technological and clinical outcome literature. Comparisons to conventional radiotherapy methods will be examined in terms of technique, dosimetry and clinical outcome. Finally, current research and future directions of CNSIMRT will be introduced such as the novel use of sophisticated imaging techniques for improved structure definition, patient positioning and dose modulation.
1. To understand the general practice of CNSIMRT from patient selection through actual treatment.
2. To become familiar with specific details pertaining to the CNSIMRT process through several illustrative examples.
3. To be introduced to some of the research and future directions of CNSIMRT.
Research supported, in part, by Varian Medical Systems.
33(2006); http://dx.doi.org/10.1118/1.2241621View Description Hide Description
The purpose of this presentation is to discuss key issues in IMRT treatments for head and neck cancers. Experience has matured over the past several years in this regard. The main focus will be on the details related to the planning process such as, immbolization, imaging and setup management, treatment planning, and plan evaluation. An emphasis will be given to describing one institution's implementation of a head and neck IMRT program with a lesser emphasis on a review of the relevant literature. The conformal nature of IMRTdose distributions requires additional consideration on the degree of immbolization and expected reproducibility of setup. Custom neck molds, masking systems and additional shoulder constraints are required to maximize reproducibility of the head, chin, and clavicals (supraclavicular nodes). Even with these constraints, daily variability can be expected and the treatment plan should account for those effects. New localization tools such as on‐board kV planar imaging and cone‐beam CT are becoming available to aid in localization and patient setup. The use of these new techniques will be described. Target and normal tissue segmentation are very important in the planning process and must be considered in detail by the physicist. Various imaging modalities are frequently used. Contrast enhanced CT and MRI‐CT fusion is useful for primary tumor segmentation. Fused 18FDG PET‐CT images can be used to identify positive neck nodes but lack anatomic definition and are not always useful for defining the primary tumor. Before treatment planning begins, a quick but pertinent conversation with the treating physician is necessary to clearly understand the dose/volume tolerances of normal tissues and other patient‐specific issues (e.g., previous treatment, chemotherapy, or already compromised tissues). This dialogue with the physician helps to limit the iterations of optimization and dose evaluation to efficiently arrive at the best IMRT plan. Once treatment planning begins there are many more techniques at the physicist's disposal to develop the best treatment plan compared to conventional planning. Each of these will be discussed. Evaluating IMRT plans is a determination of tradeoffs. An important principle regarding target coverage is the trade‐off between dose conformity and dose heterogeneity across the target. If the plan emphasis is conformity to the target, then one should accept increased dose heterogeneity and vice versa. It is absolutely essential to be realistic in the expectations of IMRT and be prepared to accept some dose to critical structures (but keeping them below tolerance) in order to get better target coverage than a conventional plan would provide. Detailed slice‐by‐slice evaluation of isodose coverage for the location and magnitude of hot and cold spots is essential during plan evaluation. Before starting treatment, a set‐up verification step is typically helpful, during which the immobilization system and isocenter location are checked. Orthogonal DRR images of the isocenter(s) location can be reproduced by simulator images for better visualization of bony landmarks. Kilovoltage and conebeam CTimaging techniques are more prevalent in the treatment room for this purpose. For on treatment set‐up verification, utilizing digital images and associated software tools can help to accurately identify isocenter translations and patient rotations (head or shoulder tilt).
1. Understand issues related to patient immobilization.
2. Identify normal tissues and know their dose/volume constraints.
3. Describe several planning techniques to achieve the best dose distribution.