Optimal clinical implementation of the Siemens virtual wedge.

Installation of a modern high-energy Siemens Primus linear accelerator at the Northern Centre for Cancer Treatment (NCCT) provided the opportunity to investigate the optimal clinical implementation of the Siemens virtual wedge filter. Previously published work has concentrated on the production of virtual wedge angles at 15 degrees, 30 degrees, 45 degrees, and 60 degrees as replacements for the Siemens hard wedges of the same nominal angles. However, treatment plan optimization of the dose distribution can be achieved with the Primus, as its control software permits the selection of any virtual wedge angle from 15 degrees to 60 degrees in increments of 1 degrees. The same result can also be produced from a combination of open and 60 degrees wedged fields. Helax-TMS models both of these modes of virtual wedge delivery by the wedge angle and the wedge fraction methods respectively. This paper describes results of timing studies in the planning of optimized patient dose distributions by both methods and in the subsequent treatment delivery procedures. Employment of the wedge fraction method results in the delivery of small numbers of monitor units to the beam's central axis; therefore, wedge profile stability and delivered dose with low numbers of monitor units were also investigated. The wedge fraction was proven to be the most efficient method when the time taken for both planning and treatment delivery were taken into consideration, and is now used exclusively for virtual wedge treatment delivery in Newcastle. It has also been shown that there are no unfavorable dosimetric consequences from its practical implementation.