Aug 31, 2007
Choicest cuts for IMRT
Intensity-modulated radiation therapy (IMRT) is increasingly being used to treat large tumours, owing to its ability to deploy the available dose more efficiently. Target regions of cancer can be blasted with a high dose of radiation while surrounding healthy tissue is spared. Some tumours, however, are just too large to be covered by a single IMRT treatment field and the field has to be split. The question is, how?
Researchers at the University of Florida (Gainesville, FL) have now devised an algorithm that shows exactly how to divide IMRT fields most efficiently. The field-splitting solutions generated by their mathematical equations are designed to make the best use of dose delivered by the treatment beam (Phys. Med. Biol. 52 5483).
"We wanted to minimize the total time that the radiation beam was on during a treatment session, that is, the total number of monitor units delivered during therapy," said lead author Srijit Kamath. "With this algorithm, we have optimized that to the maximum extent possible."
When faced with an area that cannot be covered by a single IMRT treatment field, practitioners have a wide array of options. Commercially-available solutions tend to simply bisect the treatment field to produce two identical-width subfields. The positioning of this divide does not necessarily need to be central, though. It may be more efficient to split the field into three subfields, or to plan for a region of overlap.
"Cases where the IMRT field does not need to be split have been studied extensively, and many people have tried to optimize the time that the treatment beam needs to be on," Kamath told medicalphysicsweb. "But for the case where the beam does need to be divided, there was no analysis of what type of split would lead to a lower beam time. The problem just hadn't been studied until we took it up."
The team tested the potential clinical impact of their work using 30 sets of IMRT field data that had been used to treat patients with head-and-neck tumours at the University of Florida. Head-and-neck cancers can spread considerably and tumours often require IMRT treatment widths that are not deliverable with a single field.
They found that the number of monitor units required for each case was an average 18.8% lower using the field split computed by their algorithm, rather than the central split suggested by a commercial treatment-planning system. The highest decrease in monitor units that would have been achieved was 45%. All solutions proposed by the mathematical model included a degree of overlap between the subfields.
The commercial future of the new algorithm for actual treatment planning will depend on the results of clinical validation. The change in 3D dose distribution delivered to patients will also need to be quantified. This angle has not been explored by the University of Florida team. However, Kamath is hopeful that researchers with a clinical interest in IMRT will pursue this line of enquiry.
In terms of beam-time optimization, the proposed solution is as good as it gets. "Mathematically, there is nothing better that we can do," Kamath said. This assumes, of course, that the equipment used to deliver the conformal treatment is not overhauled. Changes to the structure of the dose-delivery system, or the development of technology that permits large tumours to be treated with a single field could help to minimize beam times further. At which point, the mathematical solution may have to be revisited…
About the author
Paula Gould is a contributing editor on medicalphysicsweb.