RESUMO
Objective To investigate the optimal distance between upper and lower target volumes and their correlated planning parameters by analyzing the dose distribution in the abutment regions during total body irradiation ( TBI) using helical tomotherapy. Methods A total of 10 patients with acute leukemia and with a height around 120 cm were enrolled. All patients were scanned by a Siemens simulation computerized tomography (CT) at a slice thickness of 5 mm. A lead wire was placed 10. 0 cm above the patella as a marker of the separation boundary for the upper and lower target volumes. The delineations of target volumes and organs at risk ( OARs ) were performed in the Varian Eclipse 13. 5 workstation with targets shrunk beyond the separation boundary at different distances. After contours and CT images were transferred to HT workstation, treatment plans were designed with different field width (FW, 5. 0 cm/2. 5 cm/1. 0 cm) and pitch values (0. 430/0. 287) at a modulation factor of 1. 8. All the plans were optimized with a dose calculation grid of 0. 195 cm × 0. 195 cm and identical planning parameters. The correlation between treatment planning parameters and targets shrunk distances were investigated by analyzing the dose distributions in the abutment area. Results The study demonstrated that the dose distributions in the abutment area were influenced only by the field width parameters: when the gap distance between the upper and lower targets was 5. 0 cm, the optimal FW is 5. 0 cm;Similarly when the gap distances were 2. 0 cm and 1. 0 cm, and the optimal FW 2. 5 cm and 1. 0 cm, respectively. In another words, the dose distribution of the abutment region was optimal when the target gap distance was equal to FW. Pitch values did not affect the quality of dose distribution in the abutment region and the overall treatment time ratio. Overall treatment time was inversely related to the FW. Conclusions Consistent target distance and FW is helpful to improve the dose homogeneity in the abutment area during TBI with HT. Appropriate planning parameters is critical to balance the treatment efficacy and efficiency.
RESUMO
Objective To investigate the optimal distance between the upper and lower targets in the subsection total body irradiation(TBI)using helical tomotherapy,and to analyze the dose distribution of abutment regions. Methods A total of 8 patients with acute leukemia with a height of about 120 cm were involved in the study. All patients were scanned from the calvarium to the toe by computerized tomography (CT,Siemens)with a thickness of 5 mm,and a lead wire was placed at a point 10 cm above the patella as a marker of the boundary between the upper and lower targets. The delineation of target volumes and organs at risk(OAR)was performed in the Varian Eclipse 10.0 doctor workstation. The different distances between the lead wires and the boundary of the two targets were delineated, and images were transferred to the HT workstation to design the radiotherapy planning,including Jaw width(5 cm),modulation factor(1.8),and pitch(0.43). The plans were superimposed together, and then the dose distribution in abutment regions with different target gaps was analyzed to find the optimal distance. Results When the target gap was 5 cm, the dose distribution in abutment regions was satisfactory. However,the dose was obviously insufficient when the gap was more than 5 cm;the doses in abutment regions significantly exceeded the prescribed doses when the gap was less than 5 cm. Conclusions In the subsection TBI using HT, different parameters were designed,including Jaw width(5 cm), modulation factor(1.8), pitch(0.43), and slice thickness(5 mm). The upper and lower borders of the targets should be 2.5 cm away from the lead wire,that is,a gap of 5 cm,thus avoiding the dose-related hot or cold spots in the target convergence and ensuring a safer and more accurate radiotherapy.