Effect of cavity under bolus on the dose of superficial tissue / 中华放射肿瘤学杂志

ABSTRACT

Objective To investigate the effect of cavity thickness, area and distance under the bolus upon the dose in the superficial tissues. Methods An accelerator model was constructed based on Geant4.The model accuracy was validated by the comparison of the calculated data with the measured data. A 30×30×30 cm3 water phantom with the upper surface located at the isocenter level and a 30×30×1 cm3 water film were constructed. Different models with the water film close to or different cavities with the water phantom were established. Under the 10×10 cm2 field with 6 MV X-ray beam,the central axis depth dose distribution and the lateral dose profiles at a depth of 0. 1 cm ( profile1) of the models with different cavities were calculated. The calculated data of different model with the water film close to or different cavities with the water phantom were statistically compared. Results When the cavity thickness was ≤ 0. 5 cm, the cavity exerted slight effect upon the depth of maximum dose ( Dmax ) and superficial dose. As the cavity thickness was increased,the Dmax was also increased,the PDD at 0. 1 cm ( PDD1) was decreased rapidly and the profile1 was increased from the cavity center to the edge. Along with the increase of cavity area,the Dmax was initially increased and then decreased,whereas the PDD1 was first decreased followed by an increase. When the cavity area was small,the profile1 was gradually increased from the cavity center to the edge. When the cavity area was large,the profile1 was initially decreased and subsequently increased. When the distance was ≥0. 2 cm,it was qualified for the clinical requirement and it exerted no effect when the distance was≥1. 0 cm. The profile1 distant from the cavity was not affected. Conclusion The cavity under the bolus should be minimized to reduce the cavity thickness,area and distance as possible.