Chapter 19: Three-Dimensional Conformal Radiation Therapy

By three-dimensional conformal radiotherapy (3D-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.

• TCP = Tumor Control Probability
• NTCP = Normal Tissue Complication Probability

The 3D-CRT technique encompasses both the physical and biologic rationales in achieving the desired clinical results.

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 3D-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 3D-CRT than in techniques that use generously wide fields and simpler beam arrangements to compensate for the uncertainty in tumor localization.

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 3D-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 irradiate normal tissues.

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 3D-CRT concept that must be avoided at all cost.

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 3D-CRT and that of conventional radiation therapy is that the former requires the availability of 3D anatomic information and a treatment planning system that allows optimization of dose distribution in accordance with the clinical objectives.

Segmentation is the process of delineating targets and relevant anatomic structures using images acquired from imaging modalities that provide thin, transverse slices of the patient's ROI where the tumor is located.

One of the most useful features of these systems is the computer graphics, which allow beam's eye view visualization of the delineated targets and other structures. The term BEV denotes display of the segmented target and normal structures in a plan 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., > 94% of the prescribed dose). Ordinarily a field margin of approximately 2cm 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 uncertaintities in PTV.

Optimization of a treatment plan requires not only the design of optimal field apertures, but also...

• 1.) beam directions
• 2.) number of fields
• 3.) beam weights
• 4.) intensity modifiers

The time required to plan a 3D-CRT treatment depends on the...

• 1.) complexity of a given case
• 2.) experience of the treatment planning team
• 3.) speed of the treatment-planning system

• 1.) quality of input patient data
• 2.) image segmentation
• 3.) image registration
• 4.) field apertures
• 5.) dose computation
• 6.) plan evaluation
• 7.) plan optimization

Each pixel within a CT image is a measure of relative linear attenuation coefficient of the tissue for the scanning beam used in the CT scanner. By appropriate calibration of the CT scanner using phantoms containing tissue substitutes (CT phantoms), a relationship between pixel value (CT numbers) and tissue density can be established. This allows pixel-by-pixel correction for tisue inhomogeneities in computing dose distributions.

One of the important features of 3D treatment planning is the ability to reconstruct images in planes other than that of the original transverse image. These are called the digitally reconstructed radiographs (DRRs).

The spiral or helical CT scanner allows continuous rotation of the x-ray tube as the patient is translated through the scanner aperture. This substantially reduces the overall scanning time and therefore allows acquisition of a large number of thin slices required for high-quality CT images and DRRs.

Because CT images can be processed to generate DRRs in any plane, conventional simulation may be replaced by CT simulation. A CT simulator is a CT scanner equipped with some additional hardware such as laser localizers to set up the treatment isocenter, a flat table or couch insert, and image registration devices. A computer workstation with special software to process CT data, plan beam directions, and generate BEV DRRs allows CT simulation films with the same geometry as the treatment beams.

In general, MRI is considered superior to CT in soft-tissue discrimination such as central nervous system tumors and abnormalities in the brain. Also, MRI is well suited to imaging head and neck cancers, sarcomas, the prostate gland, and lymph nodes. 

On the other hand, it is insensitive to calcification and bony structures, which are best imaged with CT.

The term "registration" as applied to images connotes a process of correlating different image data sets to identify corresponding structures or regions. Image registration facilitates comparison of images from one study to another and fuses them into one data set that could be used for treatment planning.

The term "image segmentation" in treatment planning refers to slice-by-slice delineation of anatomic regions of interest, for example, external contours, targets, critical normal structures, anatomic landmarks, etc. The segmented regions can be rendered in different colors and can be viewed in BEV configuration or in other plans using DRRs. Segmentation is also essential for calculating dose volume histograms (DVHs) for the selected region of interest.

Image segmentation is one of the most laborious but important processes in treatment planning. Although the process can be aided for automatic delineation based on image contrast near the boundaries of structures, target delineation requires clinical judgement, which cannot be automated or completely image based.

Target delineation should not be delegated to personnel other than the physician in charge of the case, the radiation oncologist.

After image segmentation has been completed, the treatment planner gets to the task of selecting beam direction and designing beam apertures.

Beam directions that create greater separation between targets and critical structures are generally preferred unless other constraints such as obstructions in the path of the beam, gantry collision with the couch or patient, etc., preclude those choices.

Beam apertures can be designed automatically or manually depending on the proximity of the critical structures and the uncertainty involved in the allowed margins between the CTV and PTV. In the automatic option, the user sets a uniform margin around the PTV. A nonuniform margin requires manual drawing of the field outline. A considerable give and take occurs between target coverage and sparing of critical structures in cases where the spaces between the target and critical structures are tight, thus requiring manual design of the beam apertures.

A multiplicity of fields removes the need for using ultra-high-energy beams (>10 MV), which are required when treating thoracic or pelvic tumors with only two parallel opposed fields. In general, the greater the number of fields, the less stringent is the requirement on beam energy because the dose outside the PTV is distributed over a larger volume.

Noncoplanar beam directions can be useful in certain cases, for example, brain tumors, head and neck, and other regions where a critical structure can be avoided by choosing a noncoplanar beam direction.

To use a noncoplanar beam, the couch is rotated ("kicked") through a specified angle, making sure that it will not collide with the gantry.

Using a large number of fields (greater than four) creates the problem of designing an excessive number of beam-shaping blocks and requiring longer setup times as each block is individually inserted into the accessory mount and verified for correct placement of the field on the patient. Carrying so many heavy blocks, patient after patient, creates a nuisance for therapists who have to guard against dropping a block accidentally or using a wrong block.

A good alternative to multiple field blocking is the use of a multileaf collimator (MLC). MLCs can be used with great ease and conveniences to shape fields electronically. A field drawn on a simulator film or a BEV printout can be digitized to set the MLC setting. BEV field outlines can also be transmitted electronically to the accelerator to program the MLC. Because MLC fields can be set at the control console as programmed, a large number of fields can be treated efficiently and reproducibly.

Yes.

The dose distribution is usually normalized to be 100% at the point of dose prescription so that the isodose curves represent lines of equal dose as a percentage of the prescribed dose.

Display of dose distribution in the form of isodose curves or surfaces is useful because it shows not only regions of uniform dose, high dose, or low dose, but also their anatomic location and extent. In 3D treatment planning, this information is essential but should be supplemented by DVHs for the segmented structures, for example, targets, critical structures, etc. A DVH not only provides quantitative information with regard to how much dose is absorbed in how much volume, but also summarizes the entire dose distribution into a single curve for each anatomic structure of interest. It is therefore a great tool for evaluating a given plan or comparing competing plans.