Accurate dose planning and delivery are very important in the intensity modulated radiation therapy. For helical TomoTherapy dose validation, a TomoTherapy second check software, called MU-Tomo, has been developed using archived patient documents, initial coordinates and planned dose of the point of calculation, and common dosimetric functions. Based on this software, sensitivity studies on 50 patient cases have been evaluated to show the impact of off-axis ratio profile misalignment on point dose calculation. Off-axis ratio is defined as the dose profile normalized to its maximum dose value. Sensitivity studies were done for three scenarios: oscillating the fluctuation regions of two off-axis profiles, shifting the profiles, and rotating the profiles. The result of the oscillation trial is linear along the change of longitudinal off-axis ratio (OARy), while oscillating the lateral off-axis ratio (OARx) has little influence on the dose calculation. For shifting, the variation in the percentage difference from the non-shifting value is about 15 times larger in OARy modification than in OARx modification. Rotating OARx by ±6' gave less than 1.5% ± 0.20% difference compared to the non-rotating value. Rotating OARy by ±1' changes the result more than 5% ± 2.69%. Therefore, for helical TomoTherapy dose validation, commissioned OARy profiles are more sensitive than OARx to oscillation, shifting and rotating. As a result, different tolerances for OARx and OARy may be required for annual quality assurance.

Key words: TomoTherapy; Second check; Independent dose calculation; Geometric uncertainties; Sensitivity; Off-axis ratio profile.