Purpose: Investigate the effect of increasing the number of beam angles per rotation to treat small off‐axis targets with helical tomotherapy. Methods: TomoTherapy® gantry rotates continuously although radiation beam angles are discretized in the optimization. At each projection angle, the leaf open/close pattern is assumed unchanged. However, for small off‐axis targets, the beam motion within a projection (7 degree) may require substantial portion of leaves to open/close for better target conformity. An off‐axis (17 cm) target of 1 cm diameter in a cheese phantom was used in the planning study. The plans were optimized with 180 and 51 beam angles with source super sampling and the results were compared. The optimization constraints are the same for the two plans. Measurements were taken for a plan with four cylindrical targets of 1 cm diameter located at different radii from the central axis along a line in a cheese phantom. The plan was optimized and delivered with 180 beam angles on a Hi‐Art® system. An EDR2 film was placed in the plane lined up with the four targets to measure the transverse dose distribution. Ion chamber point measurements were taken near three targets of the four. Results: Compared to the plan with 51 beam angles, the plan with 180 angles showed a 70% decrease of (D2‐D98) for target dose uniformity, a 27% decrease of D100 for OAR avoidance and a 12% decrease of beam‐on time. The film measurement showed good agreement between calculation and measurement. The point dose measurements showed agreement within a 1.6% difference. Conclusions: Delivery with 180 beam angles can improve the plan quality for small off‐ axis targets over 51 beam angles with source super sampling. Both target dose uniformity and OAR avoidance are improved. The plan with 180 beam angles was delivered and the measurements agreed with the calculation.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging