Intensity modulated radiotherapy dose delivery error from radiation field offset inaccuracy.

In Intensity Modulated Radiotherapy (IMRT), irradiation is delivered in a number of small aperture subfields. The fluences shaped by these small apertures are highly sensitive to inaccuracies in multileaf collimator (MLC) calibration. The Radiation Field Offset (RFO) is the difference between a radiation and a light field at the Source to Axis Distance (SAD) for a MLC. An Intensity Modulated Radiotherapy (IMRT) system must incorporate a RFO by closing in all leaf openings. In IMRT, RFO inaccuracy will result in a dose error to the interior of a target volume. We analyze dosimetric consequences of incorporating a wrong RFO into the CORVUS, 1 cm x 1 cm, step and shoot, IMRT system. The following method was employed. First an IMRT plan is generated for a target volume in a phantom, which produces a set of dynamic MLC (DMLC) files with the correct RFO value. To simulate delivery with a wrong RFO value, we wrote a computer code that reads in the DMLC file with the correct RFO value and produces another DMLC with an incorrect RFO specified by a user. Finally the phantom was irradiated with the correct and the incorrect RFO valued DMLC files, and the doses were measured with an ionization chamber. The method was applied to 9 fields, 6 MV, IMRT plans. We measured Dose Error Sensitivity Factor (DESF) for each plan, which ranged from (0-8)% mm(-1). The DESF(x) is defined as a fractional dose error to a point (x) in a target volume per mm of the RFO error, i.e., DESF(x) is equivalent to ¿deltaD(x)/D(x)deltaRFO¿. Therefore, we concluded that for CORVUS, 6 MV, 1 cm x 1 cm, step and shoot IMRT, RFO must be determined within an accuracy of 0.5 mm if a fractional dose error to a target volume is to be less than 4%. We propose an analytic framework to understand the measured DESF's. From the analysis we conclude that a large DESF was associated with an DMLC file with small average leaf openings. For 1 cm x 1 cm, step and shoot IMRT, the largest possible DESF is predicted to be 20% mm(-1). In addition, we wrote computer code that can calculate a DESF of a DMLC file. The code was written in Mathematica 3.0. The code can be used to screen patient IMRT plans that are highly sensitive to a RFO error.