Characterization of Delivery Systems for Intensity Modulated Radiation Therapy Using a Step-and-Shoot Approach

Abstract

Objective: In the step-and-shoot intensity modulated radiation therapy (IMRT), the superposition of many small area of distribution of short duration could potentially have a significant impact on the accuracy and precision of dose delivery. The purposes of our study were to evaluate the stability of dose delivery per monitor unit (MU) and the flatness and symmetry of the beams and relative output factor when applied to IMRT, and to realize the geometric and dosimetric characteristics of the multileaf collimator (MLC) system. Materials and Methods: The ion chambers and electrometer were used to measure the dose delivered per MU for 1 to 100 MU exposures. Relative output factors were measured using a 0.015 cm^3 pinpoint ion chamber for a series of square field with side lengths of 1, 2, 5, and 10 cm. The radiographic films were used for measuring the beam flatness, the beam symmetry, and the study of the MLC characteristics. Measurements included a determination of the penumbra width, leaf transmission, and tongue-and- groove effect. Films were analyzed using a scanner laser densitometer. Results: For 1 x 1 cm^2 field, a 2 mm deviation can lead to measured output that vary by as much as 10%. The dose delivered per MU was within ±2% for exposures more than 5 MU relative to a 100 MU exposure. The dose reduction of the tongue-and-groove effect at the depth of dmax (31.6%) was more serious than at the depth of 10 cm (29.3%). Measurements of 80%-20% penumbra widths at the depth of drnax (5.1-7.8 mm) were smaller than those at the 10 cm depth (7.8-9.6 mm). Conclusions: The exposure for each segment is recommended to be more than 5 MU to reach stability, and the characterization of the MLC system is necessary for clinical practice. We found that the performance of our accelerator is well suited to the delivery of step-and-shoot IMRT.