[Study of feasibility of verifying setup errors for patients with head tumor by observing patients' SSD]

To verify the feasibility of setup error verification by observing patents' source-skin distance (SSD) for patients of head tumor.

Films for 21 patients with head tumor were recorded using simulator (Varian Acuity 8.6), and comparison with reference digitally reconstructed radiograph (DRR) from Treatment plan system (TPS). The deviation of setup for 21 patients in the left-right, anterior-posterior and superior-inferior directions were measured by using 2D match, and SSD error was recorded when gantry angle was 0 degrees, 45 degrees, 315 degrees. Then setup error and corresponding SSD error were analyzed.

The systematic errors and random errors of 21 patients in the left-right, anterior-posterior and superior-inferior directions were (1.1 +/- 11.6) mm, (0.7 +/- 1.2) mm, (0.9 +/- 1.5) mm, and (1.51 +/- 3.1) mm, (1.05 +/- 3.3) mm, (1.60 +/- 2.3) mm. The systematic SSD errors and random SSD errors were (1.25 +/- 1.3) mm, (1.04 +/- 1.3) mm. (1.10 +/- 2.3) mm, and (2.03 +/- 1.7) mm, (2.81 +/- 2.3) mm, (2.33 +/- 3.0) mm for gantry angle was 0 degrees, 45 degrees, 315 degrees, respectively.

It is simple and feasible for setup error verification by observing patients' SSD and can be auxiliary to other verification means.