S Webb and D McQuaid 2009 Phys. Med. Biol. 54 4345 doi:10.1088/0031-9155/54/14/001
S Webb and D McQuaid
Show affiliationsIn this paper it is formally shown that the dynamic multileaf collimator (MLC) IMRT delivery technique remains valid if the MLC is supported on a 1D moving platform. It is also shown that, in such circumstances, it is always time preferable to deliver overlapping modulating fields as a single swept field rather than as separate fields. The most general formulism is presented and then related to simpler equations in limiting cases. The paper explains in detail how a 'small-arc approximation' can be invoked to relate the 1D linear theory to the MLC-on-moving-platform-(gantry) delivery technique involving rotation therapy and known as volume-modulated arc therapy (VMAT). It is explained how volume-modulated arc therapy delivered with open unmodulated fields and which can deliver conformal dose distributions can be interpreted as an IMRT delivery. The (Elekta adopted) term VMAT will be used in a generic sense to include a similar (Varian) method known as RapidArc. Approximate expressions are derived for the 'amount of modulation' possible in a VMAT delivery. This paper does not discuss the actual VMAT planning but gives an insight at a deep level into VMAT delivery. No universal theory of VMAT is known in the sense that there is no theory that can predict precisely the performance of a VMAT delivery in terms of the free parameters available (variable gantry speed, variable fluence-delivery rate, set of MLC shapes, MLC orientation, number of arcs, coplanarity versus non-coplanarity, etc). This is in stark contrast to the situation with several other IMRT delivery techniques where such theoretical analyses are known. In this paper we do not provide such a theory; the material presented is a stepping stone on the path towards this.
General scientific summary. No complete general theory of the intensity-modulated radiotherapy (IMRT) technique known as volumetric arc therapy (VMAT) (or RapidArc) yet exists. We believe obtaining this will be very difficult given the complex interactions between the variables of the problem. The purpose of this paper is to present some 'stepping stone' analyses that help an understanding of VMAT and also to highlight outstanding problems for ongoing study. It is shown that the theory of the dynamic multileaf collimator IMRT technique remains valid when the MLC is on a carriage that moves in the same 1D direction as the MLC leaves. A small-arc approximation is introduced, thoroughly analysed and used to explain how VMAT can be related to this. Following this, the theory allows an insight into 'how much fluence modulation' is possible with VMAT knowing the parameters of the delivery. Another analysis gives a hint to why VMAT is temporally efficient. That VMAT can be considered an IMRT technique is explained in terms of re-arranging the unmodulated VMAT fan beams into equivalent modulated parallel beams.
Issue 14 (21 July 2009)
Received 25 February 2009, in final form 19 May 2009
Published 23 June 2009
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