Interleaf leakage for 5 and 10 mm dynamic multileaf collimation systems incorporating patient motion

Use of dynamic multileaf collimation (DMLC) for intensity modulated radiati on therapy (IMRT) is accelerating. Delivery systems have the ailment of int erleaf leakage (IL). This is compounded by the inefficiency of IMRT deliver y, estimated to be a factor of 5 for DMLC. With IL on the order of 4%, it i s possible to deliver as much as 20% of the prescribed dose to nonprescribe d regions. However, IL is characterized by narrow Gaussian peaks of similar to0.5-1.0 mm full-width-half-maximum (FWHM). We performed a leakage study for 5 and 10 mm leaf systems, accounting for intratreatment and intertreatm ent motions. In solid phantoms, film was placed perpendicular to beams. DML C patterns delivered step-wedged distributions. The same field was duplicat ed using a collimating jaw in a segmented fashion to obtain baseline data o f primary and scatter contributions. Longitudinal shifts up to 4 mm and ang ulations up to 4 degrees were introduced during beam delivery by running mu ltiple patterns, to arrive at a composite delivery. The intent of these rig id body motion experiments was to replicate patient motion. Clinical IMRT f ields using segmented MLC were also tested. Films were scanned and converte d to dose. A microionization chamber confirmed film data at discrete points . In all cases shifts diminished IL peak values. In the step-wedge case, th e net 18 MV IL peaks diminished from 3.6% to 3.2% for the 10 mm system. The 5 mm. system IL values decreased from 4.0% to 3.2% with a 2 mm shift but i ncreased to 4.0% with 4 nun shifts. The clinical field data followed the sa me pattern with a washing out of peak values, but the overall transmission to shielded regions slightly increased. Therefore nonprescribed regions are influenced by an effective transmission value rather than discrete peak IL values. The 5 mm leaf system does not introduce increased IL and is an app ropriate system for IMRT. (C) 2001 American Association of Physicists in Me dicine.