Tuesday, May 24, 2011

Direct optimization of treatment machine parameters in RT - Been there, done that!

In the early 90s I did my Ph.D. on radiotherapy optimization at Karolinska Insitute, supervised by professors Anders Brahme and Bengt Lind. The core part of this work concerned the development of a dose calculation formulation that facilitated the direct optimization of practically any beam delivery parameters that are relevant in external beam radiotherapy.

In the article Simultaneous optimization of dynamic multileaf collimation and scanning patterns or compensation filters using a generalized pencil beam algorithm, published in the AIP journal Medical Physics in July, 1995, I describe this algorithm in detail. In particular, I describe how the algorithm can be applied to optimize beam weights, effective wedge angles and jaw and MLC leaf positions.

To illustrate the power of the algorithm, it is applied to a number of different optimization scenarios. Two of these scenarios concern the optimization of leaf positions in multiply segmented MLC beams in combination with one or multiple beam scanning patterns (once relevant for the Scanditronix MM50 Racetrack Microtron, nowadays primarily applicable to proton therapy).

As far as I have been able to find out, this article is the first where MLC leaf positions of multiply segmented beams are optimized directly. Prior to my work, MLC segmented beams had been optimized in a two-step process, where initially an optimal fluence modulation pattern was obtained, and this pattern was then approximated by a finite number of MLC segments.

In retrospect, the direct MLC segment optimization could perhaps have merited an article of its own. Anyway, the new approach was recognized for example by professor Steve Webb who in his review books The physics of Conformal Radiotherapy: Advances in Technology and Intensity-modulated Radiation Therapy devotes one section per book on my algorithm. In Intensity-modulated Radiation Therapy, professor Webb also identifies subsequent work by other authors along the same lines of direct leaf position optimization of multiply segmented MLC beams.

In the IMRT optimization that I implemented in Helax-TMS in the late 90s, I applied some of the developments from my 1995 article. I deliberately excluded direct optimization of the leaf positions though, due to development time constraints. In the Helax-TMS successor, Oncentra External Beam, the direct optimization of leaf positions has eventually been incorporated through the optimization component that Raysearch has provided Nucletron with.

In 2002, Dr. David Shephard published his article on Direct Aperture Optimization, DAO, in Medical Physics. DAO is yet another implementation of direct optimization of jaw and MLC positions in single and multiply segmented beams. The main novelty is that Dr. Shephard uses a simulated annealing optimization engine, whereas I in my work from 1995 used a gradient-based optimization engine, since my dose calculation formulation allowed me to. Dr. Shephard references some of the direct optimization predecessors that were also identified by professor Webb in his Intensity-modulated Radiation Therapy book. Intriguingly enough, my article from 1995 is not referenced.

The DAO algorithm has been implemented in the commercially available Prowess Panther treatment planning system.

It is fascinating to see that, despite the preceding work that I and other authors have published on the direct optimization of jaw and leaf positions of multiply segmented radiation beams, Dr. Shephard and his co-workers have still managed to claim a US patent for the DAO technique.

Just recently, Prowess filed a complaint against Raysearch, claiming that Raysearch infringes on the DAO patent with their implementation of the direct optimization technique. It will be interesting to follow the legal process of this case. Who knows, I might even get a recognition in the court ruling? :-)

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