Technical note: factors affecting dose distribution in the overlap region of two-segment total body irradiation by helical tomotherapy

OBJECTIVE: To assess the effects of various treatment planning parameters to identify the optimal gap distance for precise two-segment total body irradiation (TBI) using helical tomotherapy (HT) with fixed jaw mode. METHODS AND MATERIALS: Data of a treatment plan for 8 acute leukemia patients (height range: 109–130 cm) were analyzed. All patients underwent total-body computed tomography (CT) with 5-mm slice thickness. A lead wire, placed at 10 cm above the patella, was used to mark the boundary between the two segments. Target volumes and organs at risk were delineated using a Varian Eclipse 10.0 physician’s workstation. Different distances between the lead wire and the boundary of the two targets were used. CT images were transferred to the HT workstation to design the treatment plans, by adjusting parameters, including the field width (FW; 2.5 cm, and 5 cm), pitch (0.287 and 0.430), modulation factor (1.8). The plans were superimposed to analyze the dose distributions in the overlap region when varying target gap distances, FWs, pitches to determine the optimal combinations. RESULTS: The pitch did not affect the dose distribution in the overlap region. The dose distribution in the overlap region was mostly homogeneous when the target gap distance was equal to the FW. Increased FW diminished the effect of the target gap distance on the heterogeneous index of the overlap region. CONCLUSIONS: In two-segment TBI treatments by HT with Helix mode, a gap distance equal to the FW may achieve optimal dose distribution in the overlap region.