The effect on tumour control probability of varying the setting of a multileaf collimator with respect to the planning target volume.
The multileaf collimator (MLC) is being used increasingly to replace Cerrobend blocks when shaping radiation fields. When the MLC has a stepped edge in relation to the planning target volume (PTV) there might be concern that the MLC is not equivalent to the use of a block. This concern is countered by studying the effect on the tumour control probability of varying the distance between the stepped edge of the MLC and the contour of the PTV. It is observed that provided an adequate margin is added to the PTV when using the MLC the TCP is not reduced. The meaning of adequate has been accurately quantitated as being > or = 7 mm when the projected leaf width is 10 mm at the isocentre. This is the margin which when added to the outline of the planning target volume will ensure no reduction in TCP compared with the use of a block.
Webb
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Joint Department of Physics, Institute of Cancer Research, Sutton, Surrey, UK.
D006801:Humans; D009369:Neoplasms / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods*; D011882:Radiotherapy, High-Energy / Q000295:instrumentation / Q000379:methods*