By three-dimensional conformal radiotherapy (3-D CRT), we mean treatments that are based on 3-D anatomic information and use dose distributions that conform as closely as possible to the target volume in terms of adequate dose to the tumor and minimum possible dose to normal tissue. The concept of conformal dose distribution has also been extended to include clinical objectives such as maximizing tumor control probability (TCP) and minimizing normal tissue complication probability (NTCP). Thus, the 3-D CRT technique encompasses both the physical and biologic rationales in achieving the desired clinical results.

Although 3-D CRT calls for optimal dose distribution, there are many obstacles to achieving these objectives. The most major limitation is the knowledge of the tumor extent. Despite the modern advances in imaging, the clinical target volume (CTV) is often not fully discernible. Depending on the invasive capacity of the disease, what is imaged is usually not the CTV. It may be what is called the gross tumor volume (GTV). Thus, if the CTVs drawn on the cross-sectional images do not fully include the microscopic spread of the disease, the 3-D CRT loses its meaning of being conformal. If any part of the diseased tissue is missed or seriously underdosed, it will inevitably result in failure despite all the care and effort expended in treatment planning, treatment delivery, and quality assurance. From the TCP point of view, accuracy in localization of CTV is more critical in 3-D CRT than in techniques that use generously wide fields and simpler beam arrangements to compensate for the uncertainty in tumor localization.

In addition to the difficulties in the assessment and localization of CTV, there are other potential errors that must be considered before planning 3-D CRT. Patient motion, including that of tumor volume, critical organs, and external fiducial marks during imaging, simulation, and treatment, can give rise to systematic as well as random errors that must be accounted for when designing the planning target volume (PTV). If sufficient margins have been allowed for in the localization of PTV, the beam apertures are then shaped to conform and adequately cover the PTV (e.g., within 95%–105% isodose surface relative to prescribed dose). In the design of conformal fields to adequately treat the PTV, consideration must be given to the cross-beam profile, penumbra, and lateral radiation transport as a function of depth, radial distance, and tissue density. Therefore, sufficient margins must be given between the PTV outline and the field boundary to ensure adequate dose to the PTV at every treatment session.

Even if the fields have been optimally designed, biologic response of the tumor and the normal tissues needs to be considered in achieving the goals of 3-D CRT. In other words, the optimization of a treatment plan has to be evaluated not only in terms of dose distribution (e.g., dose volume histograms), but also in terms of dose-response characteristics of the given disease and the irradiated normal tissues. Various models involving TCP and NTCP have been proposed, but the clinical data to validate these models are scarce. Until more reliable data are available, caution is needed in using these concepts to evaluate treatment plans. This is especially important in considering dose-escalation schemes that invariably test the limits of normal tissue tolerance within or in proximity to the PTV.

Notwithstanding the formidable obstacles in defining and outlining the true extent of the disease, the clinician must follow an analytic plan recommended by the International Commission on Radiation Units and Measurements (ICRU) (1). Various target volumes (GTV, CTV, PTV, etc.) should be carefully designed considering the inherent limitations or uncertainties at each step of the process. The final PTV should be based not only on the given imaging data and other diagnostic studies, but also the clinical experience that has been obtained in the management of that disease. Tightening of field margins around image-based GTV, with little attention to occult disease, patient motion, or technical limitations of dose delivery, is a misuse of the 3-D CRT concept that must be avoided at all cost. It should be recognized that 3-D CRT is not a new modality of treatment, nor is it synonymous with better results than successful and well-tested conventional radiation therapy. Its superiority rests entirely on how accurate the PTV is and how much better the dose distribution is. So, instead of calling it a new modality, it should be considered as a superior tool for treatment planning with a potential of achieving better results.

The main distinction between treatment planning of 3-D CRT and that of conventional radiation therapy is that the former requires the availability of 3-D anatomic information and a treatment-planning system that allows optimization of dose distribution in accordance with the clinical objectives. The anatomic information is usually obtained in the form of closely spaced transverse images, which can be processed to reconstruct anatomy in any plane, or in three dimensions. Depending on the imaging modality, visible tumor, critical structures, and other relevant landmarks are outlined slice by slice by the planner. The radiation oncologist draws the target volumes in each slice with appropriate margins to include visible tumor, the suspected tumor spread, and patient motion uncertainties. This process of delineating targets and relevant anatomic structures is called segmentation.

The next step is to follow the 3-D treatment-planning software to design fields and beam arrangements. One of the most useful features of these systems is the computer graphics, which allow beam’s-eye-view (BEV) visualization of the delineated targets and other structures. The term BEV denotes display of the segmented target and normal structures in a plane perpendicular to the central axis of the beam, as if being viewed from the vantage point of the radiation source. Using the BEV option, field margins (distance between field edge and the PTV outline) are set to cover the PTV dosimetrically within a sufficiently high isodose level (e.g., ≥95% of the prescribed dose). Ordinarily a field margin of approximately 2 cm is considered sufficient to achieve this, but it may need further adjustments depending on the given beam profile and the presence of critical structures in the vicinity of the PTV.

Nonetheless, it is important to remember that each beam has a physical penumbra (e.g., region between 90% and 20% isodose level) where the dose varies rapidly and that the dose at the field edge is approximately 50% of the dose at the center of the field. For a uniform and adequate irradiation of the PTV, the field penumbra should lie sufficiently outside the PTV to offset any uncertainties in PTV.

Optimization of a treatment plan requires not only the design of optimal field apertures, but also appropriate beam directions, number of fields, beam weights, and intensity modifiers (e.g., wedges, compensators, dynamic multileaf collimators, etc.). In a forward-planning system, these parameters are selected iteratively or on a trial-and-error basis and therefore, for a complex case, the whole process can become very labor intensive if a high degree of optimization is desired. In practice, however, most planners start with a standard technique and optimize it for the given patient using 3-D treatment-planning tools such as BEV, 3-D dose displays, noncoplanar beam options, intensity modulation, and dose volume histograms. The time required to plan a 3-D CRT treatment depends on the complexity of a given case, experience of the treatment-planning team, and speed of the treatment-planning system. The final product, the treatment plan, is as good as its individual components, namely the quality of input patient data, image segmentation, image registration, field apertures, dose computation, plan evaluation, and plan optimization.