Verification of IMRT fields by film dosimetry.

In intensity modulated radiation therapy (IMRT) the aim of an accurate conformal dose distribution is obtained through a complex process. This ranges from the calculation of the optimal distribution of fluence by the treatment planning system (TPS), to the dose delivery through a multilamellar collimator (MLC), with several segments per beam in the step and shoot approach. The above-mentioned consideration makes mandatory an accurate dosimetric verification of the IM beams. A high resolution and integrating dosimeter, like the radiographic film, permits one to simultaneously measure the dose in a matrix of points, providing a good means of obtaining dose distributions. The intrinsic limitation of film dosimetry is the sensitivity dependence on the field size and on the measurement depth. However, the introduction of a scattered radiation filter permits the use of a single calibration curve for all field sizes and measurement depths. In this paper the quality control procedure developed for dosimetric verification of IMRT technique is reported. In particular a system of film dosimetry for the verification of a 6 MV photon beam has been implemented, with the introduction of the scattered radiation filter in the clinical practice that permits one to achieve an absolute dose determination with a global uncertainty within 3.4% (1 s.d.). The film has been calibrated to be used both in perpendicular and parallel configurations. The work also includes the characterization of the Elekta MLC. Ionimetric independent detectors have been used to check single point doses. The film dosimetry procedure has been applied to compare the measured absolute dose distributions with the ones calculated by the TPS, both for test and clinical plans. The agreement, quantified by the gamma index that seldom reaches the 1.5 value, is satisfying considering that the comparison is performed between absolute doses.