Geometric Uncertainties in High Precision Radiotherapy for Brain Tumors: A Prospective Study

Abstract

62 ( P + 0.7 s) Stroom’s (2 P + 0.7 s) and Van Herk’s (2.5 P + 0.7 s) formula, ( P = systematic error and s = random error). Results:With GTC frame, mean displacement in AP axis is 0.15 mm (± 0.38), CC axis -0.17 mm (± 0.42) and ML axis 0.38 mm (± 0.56). The mean vector displacement for the population is 1.03 mm (± 0.34). The errors were within ±2 mm in ML direction in 99.6% cases, in 99.2% cases in AP and 97.6% cases in CC direction. The random error was 0.73 mm in CC, 1.86 mm in AP and 1.41 in ML axes and it was normally distributed. The margins (in mm) in ML CC and AP directions were 1.55, 0.93, 1.68 (ICRU 62), 2.1, 1.35, 2.06 (Stroom’s) and 2.39, 1.56 and 2.25 (van Herk’s). In BrainLAB, mean errors in ML, CC and AP directions was 0.14 cm (SD 0.27), -0.044cm (SD 0.151) and 0.059 cm (SD 0.150), respectively. The systematic errors in ML, SI and AP directions were 0.03 cm, 0.052 cm and 0.124 cm, respectively. The random errors in ML, SI and AP directions were 0.24 cm, 0.143 cm and 0.106 cm, respectively. The margins (in cm) in ML CC and AP directions were 0.19, 0.15, 0.19 (ICRU 62), 0.23, 0.20, 0.32 (Stroom’s) and 0.24, 0.23, and 0.38 (van Herk’s). In ray cast, the mean error in ML, SI, and AP directions was 0.15 cm (SD 0.14), -0.015 cm (SD 0.326), and 0.15 cm (SD 0.1668). The systematic errors in ML, SI and AP directions were 0.133 cm, 0.2831 cm and 0.0887 cm while random errors in the same directions were 0.160 cm, 0.1784 cm and 0.1429 cm respectively. The margins (in cm) in ML CC and AP directions were 0.24, 0.40, 0.18 (ICRU 62), 0.37, 0.69, 0.27 (Stroom’s) and 0.44, 0.83 and 0.32 (van Herk’s). Conclusions: Immobilization with GTC is superior to BrainLab device for brain. In 97-99% cases the error lies between ± 2 mm. Therefore, a margin of 2 mm around the tumor seems to be adequate. BrainLAB immobilization was better than thermoplastic ray cast in reducing set up errors in radiotherapy for brain tumors. Author Disclosure: S. Das, None; R. Isiah, None; R. B, None; P. S, None; S. John, None.