Chapter 14: IMRT and VMAT

Perhaps the most concrete way to understand the rationale for intensity-modulated radiation therapy (IMRT) is to consider a case study. Figure 14.1.1 shows a patient in which the goal is to treat the target volume (blue) to a high dose while sparing the nearby organ at risk (green). One approach is to use open fields from various angles (Figure 14.1.1a). This is called 3D conformal radiation therapy (3D-CRT) because the shape of the beam conforms to the shape of the target as viewed from the particular angle being treated. Each of these beams provides a uniform dose fluence in the region being treated. However, a disadvantage is that the high dose may not conform well to the target, and the organ at risk may not be spared well. This case is particularly challenging since the target presents a concave shape with the organ located inside the concave region. In this case, IMRT can provide a benefit by allowing for more complex dose distributions. In IMRT, the fluence of each beam is no longer uniform but is varied or “modulated” across the face of the beam to produce higher or lower intensities where needed. In the example, the posterior beam requires high intensities at the peripheral regions while lower intensities are needed for the region that passes through the organ at risk. IMRT allows for complex dose distributions since many parameters can be varied.