CONVENTIONAL 2-DIMENSIONAL RADIOTHERAPY

High-energy megavoltage radiation is produced by a linear accelerator (Fig. 15-1). The generated beam of radiation diverges from the head of the linear accelerator gantry and may be collimated or shaped using simple “jaws” that create square or rectangular fields. A secondary level of collimation can be achieved by placing lead-based alloy blocks in front of the beam to create a unique shape. Conventional radiotherapy using 2D techniques relies on simple 2D x-ray imaging and bony anatomical landmarks to determine where the beams of radiation should be focused. Historically, radiation therapists relied on a detailed knowledge of the relative relationship of internal organs with respect to the bony anatomy to define the radiation fields. These techniques are most commonly used for urgent radiation delivery or when treating simple bony lesions of the axial skeleton.

Fig. 15-1 Linear accelerator.

Figure 15-2 depicts a simulated radiation field created to treat a patient who presented with painful bony spinal metastasis at L2. In this x-ray image, the center of the beam is shown by the intersection of the perpendicular crosshairs. The thick continuous lines represent the “jaws,” which create a rectangular central area around the center of the beam that defines the target radiation field. It is customary to include at least one vertebral body above and below the target level when treating spinal metastases. The lateral borders of the field are set to encompass the paraspinal extent of the tumor with additional margin to account for uncertainties of internal organ motion, patient motion, and day-to-day variation in radiation field set-up. Parallel-opposed fields from anterior and posterior projections are used to treat the target within the rectangular field. Opposed lateral fields may be used for cervical spinal lesions. Thus, all the intervening organs and tissues contained in this field also will be irradiated. In this case, the amount of radiation that can be safely delivered is limited by the radiation sensitivity of the normal tissues, such as the spinal cord and small bowel.

Fig. 15-2 Simulation fields and portal images of lumbar spine for metastases.

THREE-DIMENSIONAL CONFORMAL RADIOTHERAPY AND INTENSITY MODULATED RADIOTHERAPY

With the advent of computed tomography (CT), creation of complex radiation doses that conform to the shape of the clinical target is feasible. After contouring the target and normal structures on individual CT slices, a 3D representation of these structures is created by digital reconstruction. Unlike simple 2D treatment planning, 3D conformal radiotherapy (3D-CRT) and intensity modulated radiotherapy (IMRT) use an array of radiation beam angles. Custom field blocking of each field is based on the relative relationship between the target and normal structures from the beam’s eye view. The blocking is achieved by either lead-based alloy blocks or by a multi-leaf collimator (MLC). With the MLC, 3- to 10-mm rectangular tungsten leaves are independently positioned a certain distance from the center of the field to create a custom shape of the field (Fig. 15-3). In most cases of 3D-CRT, the radiation dose delivery through each field is both homogeneous (i.e., radiation dose does not vary in intensity) and static (i.e., there is no change in the beam angle or field blocking). The resultant radiation dose distribution will conform to the shape of the target relatively homogeneously. This technique is widely used.

Fig. 15-3 Multi-leaf collimator.

When the tumor and highly radiosensitive normal structures are intimately associated with each other, IMRT is preferred. Unlike 3D-CRT, the dose intensity of each beam is not homogeneous, nor is the field blocking necessarily static with IMRT. The inhomogeneous intensity is achieved by allowing the MLC to change shape dynamically during the radiation delivery of each field. As each leaf of the MLC moves, it absorbs radiation and casts a shadow that results in modulation of the dose under the leaf. The intensity of each field is modulated based on the relative relation of the target and normal structures.

Figure 15-4 shows the radiation dose distribution of a seven-field IMRT plan for a spinal tumor. By converging beams from multiple angles and modulating the dose for each field, the prescription radiation dose, depicted by the yellow curve, tightly conforms to the shape of the target, depicted by the light red shaded contour. At the same time, the radiation dose to the bowel is about 25% of the prescribed dose.

Fig. 15-4 Complex IMRT seven-field beam arrangement for spinal cord tumor.