Intensity-modulated radiation therapy (IMRT): the radiation oncologist's perspective.

Intensity-modulated radiation therapy (IMRT) is a new and evolving technological advance in high-precision radiation therapy. It is an extension of 3-dimensional conformal radiotherapy (3D-CRT) that allows the delivery of highly complex isodose profiles to the target while minimizing radiation exposure to surrounding normal tissues. Clinical data on IMRT are emerging and being collected, as more institutions are implementing or expanding the use of IMRT. However, the currently available IMRT and its applications are far from being well understood and established. In some circumstances, it remains impractical and too costly. This article discusses some practical issues from the radiation oncologist's perspective.     

二维电离室矩阵在调强适形放疗剂量学验证中的应用

剂量学验证是放射治疗中质量保证的重要一环,也是调强适形放射治疗(IMRT)应用于临床时必不可少的一个步骤,随着放射物理技术的发展,新设备和新方法在不断应用,为临床放射治疗提供了更好、更便捷的质量保证与控制手段.二维电离室矩阵(MatriXX)是一种快速的剂量测量系统,是目前较先进的调强治疗实时二维验证系统之一,它可以测量照射野的剂量分布和强度分布,在剂量学验证中,可以极大地简化验证工作量,提高验证效率.     

Application of intensity-modulated radiation therapy for pediatric malignancies

Novel radiation therapy delivery techniques have moved very slowly in the field of pediatric oncology. Some collaborative groups allow new radiation therapy delivery techniques in their trials. In many instances, the option of using these techniques is not addressed. These newer techniques of radiation delivery have the potential to reduce the probability of the common late effects of radiation and at the same time, potentially improve upon control and survival. The purpose of this study is to show the feasibility of IMRT in pediatric patients. No treatment results or toxicities will be presented. Five patients with a variety of pediatric malignancies received intensity-modulated radiation therapy (IMRT) at our institution as part of their disease management. A rigid immobilization device was developed for each patient and a computed tomography (CT) simulation was performed in the treatment position. In 3 of the patients, magnetic resonance imaging (MRI) scans were coregistered with the planning CT to facilitate target and critical structure delineation. In all but 1 patient, coplanar beam arrangements were used in the IMRT planning process. All IMRT plans exhibited a high degree of conformality. Dose homogeneity inside the tumor and rapid dose falloff outside the target volume is characteristic of IMRT plans, which allows for improved normal tissue sparing. Dose distributions were obtained for all plans, as well as dose and volume relationship histograms, to evaluate the fitness of the plans. IMRT is a viable alternative to conventional treatment techniques for pediatric cancer patients. The improved dose distributions coupled with the ease of delivery of the IMRT fields make this technique very attractive, especially in view of the potential to increase local control and possibly improve on survival.     

Application of intensity-modulated radiation therapy for pediatric malignancies

Novel radiation therapy delivery techniques have moved very slowly in the field of pediatric oncology. Some collaborative groups allow new radiation therapy delivery techniques in their trials. In many instances, the option of using these techniques is not addressed. These newer techniques of radiation delivery have the potential to reduce the probability of the common late effects of radiation and at the same time, potentially improve upon control and survival. The purpose of this study is to show the feasibility of IMRT in pediatric patients. No treatment results or toxicities will be presented. Five patients with a variety of pediatric malignancies received intensity-modulated radiation therapy (IMRT) at our institution as part of their disease management. A rigid immobilization device was developed for each patient and a computed tomography (CT) simulation was performed in the treatment position. In 3 of the patients, magnetic resonance imaging (MRI) scans were coregistered with the planning CT to facilitate target and critical structure delineation. In all but 1 patient, coplanar beam arrangements were used in the IMRT planning process. All IMRT plans exhibited a high degree of conformality. Dose homogeneity inside the tumor and rapid dose falloff outside the target volume is characteristic of IMRT plans, which allows for improved normal tissue sparing. Dose distributions were obtained for all plans, as well as dose and volume relationship histograms, to evaluate the fitness of the plans. IMRT is a viable alternative to conventional treatment techniques for pediatric cancer patients. The improved dose distributions coupled with the ease of delivery of the IMRT fields make this technique very attractive, especially in view of the potential to increase local control and possibly improve on survival.     

IMRT (intensity modulated radiation therapy): progress in technology and reimbursement.

For a new treatment technology to become widely accepted in today's healthcare environment, the technology must not only be effective but also financially viable. Intensity modulated radiation therapy (IMRT), a technology that enables radiation oncologists to precisely target and attack cancerous tumors with higher doses of radiation using strategically positioned beams while minimizing collateral damage to healthy cells, now meets both criteria. With IMRT, radiation oncologists for the first time have obtained the ability to divide the treatment field covered by each beam angle into hundreds of segments as small as 2.5 mm by 5 mm. Using the adjustable leaves of an MLC to shape the beam and by controlling exposure times, physicians can deliver a different dose to each segment and therefore modulate dose intensity across the entire treatment field. Development of optimal IMRT plans using conventional manual treatment planning methods would take days. To be clinically practical, IMRT required the development of "inverse treatment planning" software. With this software, a radiation oncologist can prescribe the ideal radiation dose for a specific tumor as well as maximum dose limits for surrounding healthy tissue. These numbers are entered into the treatment planning program which then calculates the optimal delivery approach that will best fit the oncologist's requirements. The radiation oncologist then reviews and approves the proposed treatment plan before it is initiated. The most recent advance in IMRT technology offers a "dynamic" mode or "sliding window" technique. In this more rapid delivery method, the beam remains on while the leaves of the collimator continually re-shape and move the beam aperture over the planned treatment area. This creates a moving beam that saturates the tumor volume with the desired radiation dose while leaving the surrounding healthy tissue in a protective shadow created by the leaves of the collimator. In the dynamic mode, an IMRT treatment session generally can be initiated and completed within the traditional 15-minute appointment window for radiation oncology clinics. In addition to being comforting for the patient, this rapid treatment delivery mode satisfies a key financial issue for hospitals and clinics by giving them the ability to handle high patient loads and achieve a more rapid return on their investment in an IMRT system. New IMRT reimbursement codes have been issued under the pass-through provisions of Medicare's Outpatient Prospective Payment System (OPPS), which authorize special or increased reimbursement levels for promising new developments in healthcare technology that previous reimbursement procedures did not address. These pass-through payments are generally applicable for defined periods during a promising new technology's early stage of adoption. In the case of codes G0174 and G0178, the effective period has been left open-ended. While the CMS adoption of these new IMRT reimbursement codes certainly paves the economic road for the diffusion of this technology by flattening out some of the economic obstacles, there are still bumps to overcome. The most obvious one is the investment in hardware and software that may be required. However, the added demands on staff and the cost of training cannot be ignored. IMRT is a treatment process involving FDA-approved medical devices, offering the hope of improved treatment outcomes with fewer complications for patients and higher reimbursement rates for hospital providers. By the end of the year 2001, there will probably be more than 75 hospitals with IMRT capabilities in place.     

Inverse planning algorithms for external beam radiation therapy.

Intensity-modulated radiation therapy (IMRT) is a new treatment technique that has the potential to produce superior dose distributions to those of conventional techniques. An important step in IMRT is inverse planning, or optimization. This is a process by which the optimum intensity distribution is determined by minimizing (or maximizing) an objective function. For radiation therapy, the objective function is used to describe the clinical goals, which can be expressed in terms of dose and dose/volume requirements, or in terms of biological indices. There are 2 types of search algorithms, stochastic and deterministic. Typical algorithms that are currently in use are presented. For clinical implementations, other issues are also discussed, such as global minimum vs. local minima, dose uniformity in the target and sparing of normal tissues, smoothing of the intensity profile, and skin flash. To illustrate the advantages of IMRT, clinical examples for the treatment of the prostate, nasopharynx, and breast are presented. IMRT is an emerging technique that has shown encouraging results thus far. However, the technique is still in its infancy and more research and improvements are needed. For example, the effects of treatment uncertainties on the planning and delivery of IMRT requires further study. As with any new technology, IMRT should be used with great caution.     

Where goest the Peacock?

Since the treatment of the first patient in 1994, the Peacock system has maintained its presence as the dominant method of intensity-modulated radiation therapy (IMRT) delivery. Currently in use at nearly 80 institutions, over 8000 patients have been treated using the system. Peacock treatments have been delivered to sites throughout the body, including CNS, head & neck, prostate, liver, kidney, lung, mediastinum, and extremities. IMRT, however, is a young and rapidly evolving treatment methodology. As institutions have explored new ways of improving radiation therapy with intensity-modulated techniques, the requirements for the Peacock system have also expanded. More sophisticated planning algorithms have been implemented to satisfy these new requirements, as well as better tools for treatment verification and quality assurance. In addition, new delivery techniques are being examined to improve the ability of IMRT to increase target volume doses while limiting organ-at-risk doses. One such technique, using helical tomotherapy (Peacock is an example of sequential tomotherapy), is currently being evaluated at one institution. Both techniques use narrow, modulated delivery beams. However, helical tomotherapy requires continuous movement of the couch during radiation, similar to helical CT. This work reviews the development of tomotherapy with the Peacock system. It then looks at current IMRT treatment techniques using tomotherapy, and how the field has broadened since the first treatments were delivered. Finally, it looks at the future of tomotherapy techniques, and how these techniques will adapt to the changing requirements for radiation therapy.     

IMRT: high-definition radiation therapy in a community hospital.

A multileaf collimator (MLC)-based intensity-modulated radiation therapy (IMRT) program was implemented successfully at Monmouth Medical Center, a community hospital at Long Branch, New Jersey. Our clinical experience gained in the treatment of over 80 patients using IMRT for prostate, head and neck, and brain is reviewed, and some of the clinical issues are also, discussed. Implementation of the IMRT requires a treatment planning system, computer-controlled beam-shaping aperture, electronic record and verify system, and a good physics quality assurance program. These components, by grouping them efficiently, have created a seamless workflow for our complete radiotherapy process of IMRT. Each of these radiotherapy processes are discussed for clarity and the clinical importance is also evaluated. Of particular interest is inverse treatment planning that will impact treatment delivery such as beam orientation, treatment ports, and organ motion of IMRT. A checklist for physics and departmental quality assurance is suggested, with the intention of providing systematic workflow, making IMRT feasible at a community medical center setting. This is especially important because most of our cancer patients received radiation therapy locally. Lastly, the reimbursement issue affecting the implementation of IMRT at our medical center is also discussed to justify this new treatment protocol for future clinical outcomes.     

Intensity-modulated radiation therapy in the treatment of children.

Intensity-modulated radiation therapy (IMRT) is a relatively new method of conformal radiotherapy delivery that is rapidly being incorporated in clinical practice. Of all patients treated with conformal techniques, children are the most likely to benefit as normal, developing structures can be minimized in the radiation field. The advantages of IMRT, including increased conformality and possible dose escalation, are discussed in this review. Possible disadvantages of IMRT in children are also discussed, such as lack of dose homogeneity in the target volume, increased dose to nontarget tissues, reliability of treatment setup, increased anesthesia time in younger children, and prolonged treatment planning. The issue of increased risk of second malignancy in this very young population is important, as many of these children will be long-term survivors with current multimodality therapy.     

Dosimetric effects of weight loss or gain during volumetric modulated arc therapy and intensity-modulated radiation therapy for prostate cancer

Weight loss or gain during the course of radiation therapy for prostate cancer can alter the planned dose to the target volumes and critical organs. Typically, source-to-surface distance (SSD) measurements are documented by therapists on a weekly basis to ensure that patients' exterior surface and isocenter-to-skin surface distances remain stable. The radiation oncology team then determines whether the patient has undergone a physical change sufficient to require a new treatment plan. The effect of weight change (SSD increase or decrease) on intensity-modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) dosimetry is not well known, and it is unclear when rescanning or replanning is needed. The purpose of this study was to determine the effects of weight change (SSD increase or decrease) on IMRT or VMAT dose delivery in patients with prostate cancer and to determine the SSD change threshold for replanning. Whether IMRT or VMAT provides better dose stability under weight change conditions was also determined. We generated clinical IMRT and VMAT prostate and seminal vesicle treatment plans for varying SSDs for 10 randomly selected patients with prostate cancer. The differences due to SSD change were quantified by a specific dose change for a specified volume of interest. The target mean dose, decreased or increased by 2.9% per 1-cm SSD decrease or increase in IMRT and by 3.6% in VMAT. If the SSD deviation is more than 1 cm, the radiation oncology team should determine whether to continue treatment without modifications, to adjust monitor units, or to resimulate and replan.     

Clinical experience with intensity-modulated radiation therapy (IMRT) for prostate cancer with the use of rectal balloon for prostate immobilization.

The implementation of intensity-modulated radiation therapy (IMRT) is the result of advances in imaging, radiotherapy planning technologies, and computer-controlled linear accelerators. IMRT allows both conformal treatment of tumors and conformal avoidance of the surrounding normal structures. The first patient treated with Peacock IMRT at Baylor College of Medicine took place in March 1994. To date, more than 1500 patients have been treated with IMRT; more than 700 patients were treated for prostate cancer. Our experience in treating prostate cancer with IMRT was reviewed. Patient and prostate motions are important issues to address in delivering IMRT. The Vac-Lok bag-and-box system, as well as rectal balloon for immobilization of patient and prostate gland, respectively, are employed. Treatment planning also plays a very important role. IMRT as a boost after conventional external beam radiotherapy is not our treatment strategy. To derive maximal benefits with this new technology, all patients received full course IMRT. Three separate groups of patients receiving (1) primary IMRT, (2) combined radioactive seed implant and IMRT, and (3) post-prostatectomy IMRT were addressed. Overall, toxicity profiles in these patients were very favorable. IMRT has the potential to improve treatment outcome with dose escalation while minimizing treatment-related toxicity.     

Dose verification for patients undergoing IMRT.

At Emory Clinic intensity-modulated radiation therapy (IMRT) was started by using dynamic multileaf collimators (dMLC) as electronic tissue compensators in August 1998. Our IMRT program evolved with the inclusion of a commercially available inverse treatment planning system in September 1999. While the introduction of electronic tissue compensators into clinical use did not affect the customary radiation oncology practice, inverse treatment planning does alter our basic routines. Basic concepts of radiation therapy port designs for inverse treatment planning are different from conventional or 3D conformal treatments. With inverse treatment planning, clinicians are required to outline a gross tumor volume (GTV), a clinical target volume (CTV), critical normal structures, and to design a planning target volume (PTV). Clinicians do not designate the volume to be shielded. Because each IMRT radiation portal is composed of many beamlets with varying intensities, methods and practice used to verify delivered dose from IMRT portals are also different from conventional treatment portals. Often, the validity of measured data is in doubt. Therefore, checking treatment planning computer output with measurements are confusing and fruitless, at times. Commissioning an IMRT program and routine patient dose verification of IMRT require films and ionization chamber measurements in phantom. Additional specialized physics instrumentation is not required other than those available in a typical radiation oncology facility. At this time, we consider that routine quality assurance prior to patient treatments is necessary.     

Clinical experience of head-and-neck cancer IMRT with serial tomotherapy.

New radiotherapy planning and delivery techniques are undergoing rapid progress and change due to computer hardware and software technologies that have led to the development of sophisticated 3-dimensional (3D) radiation treatment planning and computer-controlled radiation therapy delivery systems. Intensity-modulated radiation therapy (IMRT) is the most recent and advanced form of external beam radiation therapy often used to perform 3D conformal radiotherapy. It represents one of the most important technical advances in radiotherapy. IMRT has the potential to achieve a much higher degree of target conformity and normal tissue sparing than most other treatment techniques, especially for target volumes at risk with complex shapes and concave regions such as head-and-neck cancer. In this review, we summarize our own IMRT treatment techniques with serial tomotherapy and our clinical experience with 126 patients with head-and-neck IMRT.     

Output factor calculations for intensity modulation radiation therapy as dosimetry quality assurance

Because intensity-modulated radiation therapy (IMRT) is complicated by many small, irregular, and off-center fields, dosimetry quality assurance (QA) is extremely important. QA is performed with verifications of both dose distributions and some arbitrary point doses. In most institutes, verifications are carried out in comparison with dose values generated from radiation treatment planning systems (RTPs) and actually measured doses. However, the estimation of arbitrary point doses without RTPs should be feasible in order to perform IMRT delivery more safely and accurately in terms of the clinical aspect. In this paper, we propose a new algorithm for calculating output factors at the center point of the collimations in an IMRT field with step and shoot delivery machines in which the lower jaws were replaced with multileaf collimators (MLC). We assumed that output is independently affected by collimator scatter and total scatter according to the position of the upper jaws and each of the MLC leaves (lower jaws). Then, the two scatter factors are accurately measured when changing their position. Thus, the output factor for an irregular field could be calculated with the new algorithm. We adopted this technique for some irregular fields and actual IMRT fields for head-and-neck cancer and found that the differences between calculated and measured output values were both small and acceptable. This study suggests that our methods and this algorithm are useful for dosimetry quality assurance.     

Patient setup and verification for intensity-modulated radiation therapy (IMRT).

Intensity Modulated Radiation Therapy (IMRT) is now widely used in the radiation therapy community. The ability of IMRT to deliver complex dose distributions has allowed dose escalation to targets while sparing normal tissues. In IMRT the roles of the physicist, dosimetrist, and physician are changed. Inverse planning, which is inherent to IMRT, requires that the final dose solution be defined at the beginning of the planning process. The physician must define specific dose volume constraints for the target as well as normal tissues. The physicist and dosimetrist must evaluate the final plan and determine if it meets the goals of the treatment, even if it does not completely satisfy the initial constraints. Once a plan is decided upon, the ability of the clinic to safely and accurately deliver that plan to the patient must be confirmed. As with any new technology, IMRT has created a need for new quality assurance procedures. Here we describe our IMRT process from simulation through planning and treatment. By standardizing our simulations we have decreased setup times and decreased the threat of collisions. Comparison of pseudo-DRR's and multiple-exposure port films allows confirmation of patient positioning on the linac. Our treatment delivery quality assurance program using film and MOSFET detectors in a polystyrene phantom is also described. We provide insight on how to overcome some of the common problems encountered in treatment planning and delivery such as isocenter location, collision avoidance, table indexing, dose confirmation, and plan analysis.     

Clinical use of intensity-modulated radiotherapy: part II

Intensity-modulated radiotherapy (IMRT) is a novel conformal radiotherapy technique which is gaining increasingly widespread use. This second clinical article aims to summarize the published data pertaining to prostate cancer, pelvic irradiation, gynaecological and breast cancer. Prostate cancer patients represent the largest group treated to date. The main indication has been radiation dose escalation within acceptable normal tissue late toxicity. Phase II data are promising, but no randomized clinical trial data are available to support its use. Pelvic IMRT aims to deliver radical radiation doses to pelvic lymph nodes while sparing the bowel and bladder. Indications for breast IMRT data are reviewed, and current data presented. Further data from randomized trials are required to confirm the anticipated benefits of IMRT in patients.     

Treatment of pancreatic cancer tumors with intensity-modulated radiation therapy (IMRT) using the volume at risk approach (VARA): employing dose-volume histogram (DVH) and normal tissue complication probability (NTCP) to evaluate small bowel toxicity.

The emergent use of a combined modality approach (chemotherapy and radiation) in pancreatic cancer is associated with increased gastrointestinal toxicity. Intensity-modulated radiation therapy (IMRT) has the potential to deliver adequate dose to the tumor volume while decreasing the dose to critical structures such as the small bowel. We evaluated the influence of IMRT with inverse treatment planning on the dose-volume histograms (DVHs) of normal tissue compared to standard 3-dimensional conformal radiation treatment (3D-CRT) in patients with pancreatic cancer. Between July 1999 and May 2001, 10 randomly selected patients with adenocarcinoma of the pancreatic head were planned simultaneously with 3D-CRT and inverse-planned IMRT using the volume at risk approach (VaRA) and compared for various dosimetric parameters. DVH and normal tissue complication probability (NTCP) were calculated using IMRT and 3D-CRT plans. The aim of the treatment plan was to deliver 61.2 Gy to the gross tumor volume (GTV) and 45 Gy to the clinical treatment volume (CTV) while maintaining critical normal tissues to below specified tolerances. IMRT plans were more conformal than 3D-CRT plans. The average dose delivered to one third of the small bowel was lower with the IMRT plan compared to 3D-CRT. The IMRT plan resulted in one third of the small bowel receiving 30.2+/-12.9 Gy vs. 38.5+/-14.2 Gy with 3D-CRT (p = 0.006). The median volume of small bowel that received greater than either 50 or 60 Gy was reduced with IMRT. The median volume of small bowel exceeding 50 Gy was 19.2+/-11.2% (range 3% to 45%) compared to 31.4+/-21.3 (range 7% to 70%) for 3D-CRT (p = 0.048). The median volume of small bowel that received greater than 60 Gy was 12.5+/-4.8% for IMRT compared to 19.8+/-18.6% for 3D-CRT (p = 0.034). The VaRA approach employing IMRT techniques resulted in a lower dose per volume of small bowel that exceeded 60 Gy. We used the Lyman-Kutcher models to compare the probability of small bowel injury employing IMRT compared to 3D-CRT. The BIOPLAN model predicted a small bowel complication probability of 9.3+/-6% with IMRT compared to 24.4+/-18.9% with 3D-CRT delivery of dose (p = 0.021). IMRT with an inverse treatment plan has the potential to significantly improve radiation therapy of pancreatic cancers by reducing normal tissue dose, and simultaneously allow escalation of dose to further enhance locoregional control.     

Intensity-modulated radiation therapy reduces the dose to normal tissue in T2N0M0 squamous cell carcinoma of the glottic larynx

The purpose of this paper was to compare intensity-modulated radiation therapy (IMRT) and conventional planning for T2N0M0 squamous cell carcinoma (SQCC) of the glottic larynx. Three patients with T2N0M0 SQCC are presented who were treated with IMRT. Conventional plans were also generated for comparison purposes. Isodose distributions and dose-volume histograms (DVHs) were generated for all the plans to evaluate the fitness of the plan as well as the differential benefit of IMRT vs. conventional treatment. The isodose distributions that were obtained by the IMRT plan are much more conformal to the planning target volume (PTV) and clearly show that less healthy tissue is subjected to a high-dose level, thus reducing toxicity. IMRT offers better comformality without compromising the PTV coverage and delivers less dose to normal tissues as compared to conventional radiation therapy in T2N0M0 SQCC of the glottic larynx. With an increase in conformality, it is expected to have an increase in the therapeutic ratio.     

Intensity-modulated radiation therapy reduces the dose to normal tissue in T2N0M0 squamous cell carcinoma of the glottic larynx

The purpose of this paper was to compare intensity-modulated radiation therapy (IMRT) and conventional planning for T2N0M0 squamous cell carcinoma (SQCC) of the glottic larynx. Three patients with T2N0M0 SQCC are presented who were treated with IMRT. Conventional plans were also generated for comparison purposes. Isodose distributions and dose-volume histograms (DVHs) were generated for all the plans to evaluate the fitness of the plan as well as the differential benefit of IMRT vs. conventional treatment. The isodose distributions that were obtained by the IMRT plan are much more conformal to the planning target volume (PTV) and clearly show that less healthy tissue is subjected to a high-dose level, thus reducing toxicity. IMRT offers better comformality without compromising the PTV coverage and delivers less dose to normal tissues as compared to conventional radiation therapy in T2N0M0 SQCC of the glottic larynx. With an increase in conformality, it is expected to have an increase in the therapeutic ratio.