The effect of the number of segments in IMRT plans

In this study, the effect of segment numbers on dosimetric verification of IMRT fields obtained by the static IMRT (step and shoot) method is investigated by considering 15 patient with different kind of cancer. IMRT plans of each patient were created in CMS XIO 4.8 treatment planning system (TPS), for both high and low segment number by using "step and shoot" technique and conserving the number of fields. In the Treatment Planning system (TPS), a 30x30x30 cm virtual phantom created in the Quality Assurance (OA) Plan, where all the fields were normalized by the value at a gantry angle of 0o and coronal dose mapping has been calculated by simulation made at source-surface distance (SSD) of 95 cm and ısocenter at 5 cm. Afterwards, dose distributions for these 30 IMRT plans have been measured by using 2D Array phantom which has a set-up under the identical conditions with the virtual simulations. For both calculated and measured flow charts, the percentage of point numbers passing the gamma (g) index for 3mm Distance to Agreement (DTA) and 3% Dose Difference (DD) criteria, have been found. The percentages of the points passing the gamma index in the plans with high and low segment numbers have been analyzed.