The impact factors of longitudinal dose fall-off outside the target with helical tomotherapy / 中华放射医学与防护杂志

ABSTRACT

Objective:To study the changing characteristics and impact factors of helical tomotherapy (HT)for longitudinal dose fall-off outside the target, in order to guide the plan junction or pretreatment target and implementation efficiency in clinical.Methods:Eight patients with head and neck tumors admitted to the Department of Oncology Radiotherapy of the First Affiliated Hospital of Zhengzhou University in December 2019 were retrospectively selected as the research objects. The planning target area and dose drop structure were outlined in the head and neck images with a thickness of 1 mm obtained by Siemens SOMATOM Definition AS positioning computerized tomography (CT). Different field widths (FW, 5.0 cm/2.5 cm/1.0 cm) and pitches (0.430/0.287/0.215) were assembled for planning with the same modulation factor (1.8), finest does calculation grid (0.195 cm ×0.195 cm) and other planning parameters were consistent. The plans were designed by different parameters, and the result was analyzed by univariate analysis.Results:The that different pitch curves coincided under the same field width by comparative analyzing, so pitchs had no effect on dose drop. The different field width curves were independent of each other, indicating that the field width had an effect on dose drop in the head and foot direction. The relationship between the longitudinal dose drop speed outside the target and the change of the field width was inversely correlated the larger field widths meant the slower dose fall-off and the larger penumbra, while the smaller field widths meant the faster fall-off and the smaller penumbra. When the dose fall-off to 50% of the prescribed dose, the distance from the target was approximately equal to half the field widths, and the pitchs had not affect the rate of dose-drop, while the dose at different distances from the target boundary could be calculated by the fitting formulas. The field widths and pitchs had little effect on the CI and HI index of the target, relatively, the target area was best when the field width was 2.5 cm. The total beam-on time gradually decreased with the increase of the field widths and pitches.Conclusions:When segment target therapy needs to consider planning junction, execution efficiency, and controlling longitudinal dose fall-off and considered the execution, the optimal planned parameters such as field widths and pitches could be selected or the target at the junction regions could be adducted according to the longitudinal dose drop formula, so as to achieve the ideal dose distribution.