Method of identifying dynamic multileaf collimator irradiation that is highly sensitive to a systematic MLC calibration error.
In intensity modulated radiotherapy (IMRT), radiation is delivered in a multiple of multileaf collimator (MLC) subfields. A subfield with a small leaf-to-leaf opening is highly sensitive to a leaf-positional error. We introduce a method of identifying and rejecting IMRT plans that are highly sensitive to a systematic MLC gap error (sensitivity to possible random leaf-positional errors is not addressed here). There are two sources of a systematic MLC gap error: centerline mechanical offset (CMO) and, in the case of a rounded end MLC, radiation field offset (RFO). In IMRT planning system, using an incorrect value of RFO introduces a systematic error ARFO that results in all leaf-to-leaf gaps that are either too large or too small by (2*DeltaRFO), whereas assuming that CMO is zero introduces systematic error DeltaCMO that results in all gaps that are too large by DeltaCMO=CMO. We introduce a concept of the average leaf pair Opening (ALPO) that can be calculated from a dynamic MLC delivery file. We derive an analytic formula for a fractional average fluence error resulting from a systematic gap error of Deltax and show that it is inversely proportional to ALPO; explicitly it is equal to Deltax/(ALPO+ 2 * RFO+ epsilon), in which epsilon is generally of the order of 1 mm and Deltax =2 * Delta RFO + CMO. This analytic relationship is verified with independent numerical calculations.
Zygmanski
P
P
Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114, USA.
Kung
J H
JH
D000465:Algorithms; D002138:Calibration; D008962:Models, Theoretical; D011874:Radiometry; D020266:Radiotherapy, Conformal / Q000295:instrumentation* / Q000379:methods*; D015203:Reproducibility of Results