Geometric and dosimetric verification of step‐and‐shoot modulated fields with a new fast and high resolution beam imaging system
Authors: Bindoni Luca 
Published: June 2005

A technique for geometric and dosimetric pretreatment verification of step‐and‐shoot intensity modulated radiotherapy treatments (IMRT) using a beam imaging system (BIS) made up of a charge‐coupled device (CCD) digital camera optically coupled with a metal‐plate/phosphor screen is described. Some physical properties of BIS were investigated in order to demonstrate its capability to perform measurements with a high spatial resolution and a high sampling rate. High‐speed imaging, with a minimum charge integration time on the CCD of 120 ms, can be performed. The study of the signal‐to‐noise ratio as a function of sampling time is presented. In‐plane and cross‐line pixel size was measured to be , which agrees within 0.5% of the manufacturer value of 0.366 mm. Spatial linearity results are very good and there are no detectable image distortions on whole detector area. A software routine was written to automatically extract positions of the collimator leaves from the images of the field shaped by the multileaf collimator (MLC) and also to compare them with the coordinates from the treatment planning system (TPS), thus directly testing both the MLC positioning and the treatment parameters transfer from TPS to the linear accelerator in a fast and precise way. The dosimetric capabilities (characteristics) of the imaging device for photon beams with energies of 6 and 15 MV were studied. Additional plexiglass buildup layers, depending on x‐ray energy, were needed to reach maximum efficiency. The energy dependence of the BIS response versus dose and dose rate was found to be linear over a wide range. Relative output factors of BIS as a function of field size, compared with values measured with an ionization chamber, were in good accord for smaller field sizes but showed differences up to 4% for all the energies at the respective buildup depth for bigger fields. Square field profiles at water‐equivalent buildup depths, extracted from BIS maps, are compared with the corresponding scans performed with a diode detector. Disagreement is always shown in the regions outside the field penumbra (tails) and near the field edges only for field sizes due to the metal/phosphor screen higher sensitivity to low energy scattering x‐rays. A straightforward correction method for the “tails effect” was developed and then generalized to MLC‐shaped fields. In order to demonstrate the validity of this procedure, the comparison between the two‐dimensional (2D) dose distributions of a triangle MLC‐shaped field and of two simple IMRT fields created by the superimposition of five segments resulting from BIS images and the dose distribution of the same fields achieved by film, was measured and reported. The gamma index method, introduced by Low et al. (1998), was used for 2D dose distributions analysis. Agreements are good for both 6 and 15 MV energies. The described technique provides a lower time‐expensive mean to verify geometric and dosimetric accuracy of the treatment delivered in IMRT with the use of a high resolution beam imaging system and homemade software tools.