*4.3.1 Equally spaced beams (ESB)*

The delivery of IMRT requires several beams to achieve the assigned dose distribution [29]. It has been a common practice to use the 5–9 beams arrangement in IMRT for head and neck cancer [55, 56]. Theoretically, a greater number of beams can have a higher chance to achieve the planned dose distribution, which increases the time for delivery and quality assurance. Hence, effort should be put to minimize the number of beams to use. Another concern in the beam placement is that opposing beams should be avoided in IMRT because it reduces the effectiveness of the optimization [57]. Furthermore, it has been calculated that the optimal number of beams is 7–9 after striking a balance between the gain in dose distribution and the expenses of treatment time in further addition of beams [58].
