3D Treatment Planning and Intensity-Modulated Radiation Therapy

Introduction

Three-dimensional radiation therapy treatment planning (3DRTTP) systems have spurred the implementation of external beam radiation therapy techniques, in which the high-dose region can be conformed much more closely to the cancer patient’s target volume than was previously possible, thus reducing the volume of normal tissues receiving a high dose. This form of external beam irradiation is referred to as three-dimensional conformal radiation therapy (3DCRT), and its development and clinical use are discussed in considerable detail elsewhere.[1,2] Several groups have already reported on their early clinical experience with 3DCRT for the treatment of prostate cancer, and the results are encouraging.[3-5]

Three-dimensional planning is not just an addition to the current radiation oncology planning process, but rather represents a radical change in practice, particularly for the radiation oncologist. The two-dimensional (2D) treatment planning approach emphasizes the use of a conventional simulator for designing beam portals, based on standardized beam arrangement techniques applied to whole classes of comparable patients. Three-dimensional treatment planning emphasizes a virtual simulation, image-based approach for objectively defining tumor and critical structure volumes for the individual patient.

The use of the terms 2D and 3D as descriptors for the planning process has caused some confusion in the radiation oncology community. One should recognize that planning the cancer patient’s treatment is, and always has been, a 3D problem, and 2D planning refers to the process and tools used. Three-dimensional treatment planning does not require the use of “noncoplanar” beams, a common misconception. Noncoplanar beams have been used for years in selected sites such as breast cancer, even though 2D treatment planning systems could not accurately account for the geometry. Clearly, noncoplanar breast tangential fields represented on a 2DRTTP system by a single, or at most a few, slices cannot be considered a 3D treatment plan. Radiation oncologists will be able to transition to 3D planning much more easily if they approach 3DRTTP as a new treatment planning process, emphasizing image-based target volume design rather than as a reflection of a particular beam configuration.

One of the important factors contributing to the current 3D process is the standardization of nomenclature, published in the International Commission on Radiation Units and Measurements (ICRU) Report 50.[6] This report has provided a language and a way of thinking about the problem for defining the volume of known tumor, suspected microscopic spread, and marginal volumes necessary to account for set-up variations and organ and patient motion. Figure 1 illustrates the ICRU 50 formal definitions for the gross tumor volume, clinical target volume, planning target volume, and organs at risk. Some brief discussion on the use of the ICRU 50 methodology is presented in a later section. Also, the reader is referred to the literature for more details regarding the use of ICRU 50 nomenclature.[7,8]

Table 1 lists the various tasks that make up the 3D planning dose and delivery process in its current technology state. 3DCRT treatment plans generally use an increased number of radiation beams that are angled and shaped to conform to the planning target volume using the 3DRTTP system’s beam’s-eye view and room’s-eye view displays (Figures 2A and 2B). To improve the conformity of the dose distribution, conventional beam modifiers (eg, wedges or compensating filters) are sometimes used. This form of 3DCRT must now be referred to as “traditional” or “conventional” 3DCRT, because a more advanced form of 3DCRT, called intensity-modulated radiation therapy, is already emerging. Intensity-modulated radiation therapy can achieve even greater conformity by optimally modulating the radiation beam intensity (fluence) throughout each treatment field.[9]

These advances in radiation oncology technology are truly exciting and are occurring at a very rapid pace. However, as the implementation of 3D planning and dose delivery systems becomes more widespread, the radiation oncology team must understand that ensuring the safety and accuracy of this new modality is more difficult (in its current development state) than with the standard 2D process. Therefore, it is essential that the radiation oncology team stay well informed on the new technologies and maintain a strong commitment to a rigorous quality assurance program when this new treatment modality is implemented in the clinic. This article will update a previous review of advances in 3DRTTP and dose delivery.[10]

Advances In 3D Treatment Planning

Three-dimensional RTTP systems are currently going through a rapid transition period, having emerged from university-developed systems to Food and Drug Administration-approved commercially available systems in the early 1990s. These first-generation commercial systems provide specific functionality, such as virtual simulation or 3D external beam dose planning. Such limited functionality has resulted in clinics needing multiple treatment planning systems to provide for various high-tech treatment modalities, like virtual simulation, 3D external beam dose planning, high-dose-rate brachytherapy, prostate seed implant, and stereotactic radiosurgery. This is clearly not very satisfactory, resulting in increased costs and inefficiencies.

Over the next few years, the treatment planning system manufacturers will begin to combine these planning programs into a single integrated image-based 3DRTTP system that provides planning capability for multiple treatment modalities, resulting in considerable cost savings. In addition, we should soon see improved efficiencies based on the use of “intranets.” An intranet is a special type of internet-related technology with the potential to dramatically improve a radiation oncology clinic’s ability to use 3DRTTP. A radiation oncology clinic’s intranet will make use of client server (and browser) technology that will provide easy-to-use Windows-like displays, permitting users to easily navigate through multiple treatment planning and other clinical applications, data, and graphical displays using point-and-click technology. However, prior to further discussion on information technology, it is important to describe some of the improved features now readily available on 3DRTTP systems.

Imaging for 3D Planning

The development of powerful but relatively inexpensive computer systems has allowed the integration of computed tomography (CT)–based target volume and normal tissue definition with the process of radiation therapy treatment planning, creating an entirely new device, the CT simulator. The CT simulator differs in both purpose and function from the conventional simulator.[11,12] A typical CT simulation uses a helical CT scanner with localization software, a patient registration and laser marking system, and a dedicated computer workstation for virtual simulation and generation of digitally reconstructed radiographs.

Although the current generation of CT simulators provides powerful new 3D capabilities, several improvements in the hardware are still needed for treatment planning purposes because the CT scanner currently used was designed for diagnostic radiology use. Specifically, a larger CT gantry aperture is needed. Current scanners have a 70-cm diameter, which limits some treatment set-ups. Also, the CT reconstruction size needs to be larger (currently only a 52-cm diameter is provided). CT simulator couch tops that mimic the geometry of a medical linear accelerator are also needed, as are improved registration systems for going from the real patient to the virtual patient and then back to the real patient for treatment. Other issues that continue to be of concern are data storage and data transfer. Also, a satisfactory process for quantitating the effects of respiratory organ motion or other organ movements has not yet been developed for CT simulation.

For effective use of CT simulation, the radiation oncologist must become familiar with CT cross-sectional anatomy and with the appropriate use of CT contrast materials that aid in the identification of the gross tumor volume and organs at risk. When CT simulation/3DRTTP is first introduced in the clinic, assistance from a diagnostic radiologist in contouring the organs at risk and gross tumor volume/clinical target volumes can be invaluable. However, identifying normal tissues and tumor on a treatment-planning CT can be difficult even for an experienced diagnostic radiologist, as the planning CT images are typically acquired in a nonstandard diagnostic patient position. Since 3D planning requires the definition of target volumes and normal tissue on a CT scan, strong consideration should be given to incorporating image-based, cross-sectional anatomy training into radiation oncology residency training programs. In the interim, short courses on image-based anatomy are needed for practicing radiation oncologists.

While CT is the principal source of image data for 3DRTTP, there is a growing demand to incorporate the complementary information available from magnetic resonance imaging (MRI). The sharply demonstrated tumor–soft-tissue interface often seen on an MRI scan in tumors such as in the brain can be used to better define the gross tumor volume.[13] Several groups have also demonstrated the value of MRI in distinguishing the prostate gland from surrounding normal structures.[14,15] In addition, functional imaging modalities such as single photon emission computed tomography (SPECT) and positron emission tomography (PET) will likely prove to be important in both the target definition phase of treatment planning and also in the follow-up studies needed to assess efficacy. For example, Marks et al have reported on the use of SPECT lung perfusion scans to determine functioning regions of the lung.[16,17] The functional lung volume data are used in calculating dose-volume histograms rather than the CT-defined anatomy and are referred to as functional dose-volume histograms. Radiation beams are planned that minimize irradiation of these functioning areas. Similarly, PET imaging is used to aid in defining the patient’s lung cancer gross tumor volume/clinical target volumes. However, use of multimodality imaging in the treatment planning process brings with it the need for better image registration tools.