Comparison of gamma analysis in multiple static segment and sliding window techniques in nasopharyngeal radiotherapy

Abstract

The purpose of this study was to compare gamma an alysis by using portal dosimetry (PD) between static (SMLC) and dynamic (DMLC) intensity modulated radiation therapy (IMRT) techniques. Simultane Integrated Boost (SIB) treatment plans were created using four treatment volumes (PTV_54, PTV_59.4, PTV_66, PTV_70) for 10 patients with nasopharyngeal car cinoma. The treatment plans were performed using Varian DHX eclipse treatment planning system (TPS) version 15.1. Six out of ten patients treatment plans were planned with 7 fields IMRT technique using the ganrty angels of 0°, 52°, 104°, 156°, 204°, 256° and 308°. The remaining four patients treatment plans were planned using 9 fields IMRT technique with the gantry angles 0°, 40°, 80°, 120°,160°, 200°, 240°, 280° and 320°. A total dose 6996 cGy in 33 fractions were given and the dose calculation were performed using SW and MMS technique. The plans were measured by aS500 EPID integrated into Varian DHX linear accelerator. Dose distributions images were obtained. Gamma analyses were evaluated using different γ criteria (2%/2 mm, 3%/3 mm and 4%/4 mm) for the 6 MV photon beams. Gamma analysis was performed by using Portal Dosimetry software in 2, 3, 4 mm distance to agreement and 2%, 3% and 4% dose difference criteria and their algorithms were compared. When compared the calculated and measured dose distributions images of the gamma analyses the average result for SW technique in 7 fields for gamma criteria 2/2%, 3/3% 4/4% were 98.64±0.34 99.36±0.22 99.74±0.11 and 9 fields for the gamma criteria 2/2%, 3/3% 4/4% were 96,90±0.70 98,98±0,33 99,60±0,31 respectively. The average result for MMS technique in 7 fields for the gamma criteria 2/2%, 3/3% 4/4 were 97.46±0.22 98.92±0.39 99.42±0.18 and 9 fields for the gamma criteria 2/2%, 3/3% 4/4% were 94.94±1.13, 97.74±0.33, 98.82±0.24 respectively. SW and MMS techniques showed comparable results with the respect to gamma analysis so but both techniques are suitable for the gamma analysis criteria.