Intensity-modulated radiation therapy using only jaws and a mask: II. A simplified concept of relocatable single-bixel attenuators.

Intensity-modulated radiation therapy (IMRT) can be delivered inefficiently using the movable jaws of a linac. The efficiency can be much improved by delivering IMRT with the movable jaws together with a relocatable mask below the jaws (Webb 2002 Phys. Med. Biol. 47 257-75). This paper extends the modelling work by showing that a much simpler relocatable mask than previously conceptualized can lead to very similar improvements in monitor-unit efficiency and a decrease in the number of field components compared with the use of jaws only (JO). The new concept comprises a set of relocatable single-bixel attenuators (SBAs) which can be moved into the field components otherwise collimated by jaws only. Typically for a 15 x 15 bixel2 2D matrix of fluence with a peak value of Ip = 10 MUs (or equivalently ten stratified fluence levels) and using just four SBAs the MU-efficiency is nearly three times that of the JO technique and the number of field components is reduced to about 0.6 the number required by the JO technique. These gains become greater by using more SBAs or for larger Ip values. Component reordering was achieved to minimize the total delivery time including intersegment deadtimes. Practicalities are discussed.