A case report on the effect of fan beam thickness in helical tomotherapy of nasopharyngeal carcinoma.

The fan beam thickness (FBT) in helical tomotherapy is defined by a pair of collimators parallel to the rotational orbit of the radiation beam and is fixed for a specific patient treatment. The aim of this case study is to evaluate the dosimetric influence of changing the FBT in the treatment of a nasopharyngeal carcinoma (NPC) patient. The subject was a T2N1M0 stage NPC patient. The planning target volumes (PTVs) of the primary nasopharyngeal tumor and the left and right cervical lymphatics were delineated along with the organs at risk (OARs) in the corresponding computed tomography slices. Three treatment plans with FBT of 1.0 cm, 2.5 cm, and 5.0 cm (FBT-10, FBT-25, and FBT-50) were generated separately based on similar dose constraints and planning parameters. The dosimetric results of the PTV and OARs were collected and compared among the 3 treatment plans. The differences in the dose parameters of the PTVs were small among the 3 plans. The FBT-10 plan demonstrated the most homogeneous PTV doses with the smallest homogeneity indices (HIs). The FBT-50 plan delivered the highest dose to the OARs and the FBT-10 plan delivered the lowest. The differences between the 2 plans were more significant in the spinal cord, optic chiasm, optic nerves, and lens. This case study demonstrated that the variation of FBT in tomotherapy affected the quality of the treatment plan mainly in the OAR doses, but not so much in the PTV. Increasing the FBT reduced the effectiveness in the sparing of OARs.