Effect of air gap under bolus on superficial dose and depth of dose maximum during radiotherapy

Abstract

To explore the influence of the air gap under bolus on the superficial dose and the maximum dose depth during radiotherapy for superficial tumors. A 30 cm×30 cm×30 cm phantom and an adjacent 10 mm-thick bolus were created in the Eclipse treatment planning system (TPS). The bolus and phantom materials were set as water. The gaps between the bolus and phantom were set as 0 mm, 2 mm, 5 mm, 10 mm, 20 mm, and 30 mm-thick air. The source skin distance was set as 100 cm, while the field sizes were set as 5 cm×5 cm, 10 cm×10 cm, 15 cm×15 cm, 20 cm×20 cm, and 25 cm×25 cm. The dose (Ds) at a depth of 1 mm on the superficial surface and the depth value (dmax) of the maximum dose point in the phantom were obtained under different air gaps and field sizes. A PTW water tank was used to measure the Ds and dmax under the same conditions as the TPS on the Varian Trilogy linear accelerator, the superficial dose Ds was measured with the EBT3 film, and the ionization chamber was used to obtain dmax. The TPS and experimental measurement results were then compared. The results showed that the Ds from the TPS was 0 when there was no bolus, while the dose from the measurement was not 0. The TPS and measurement results showed that when the field size was 5 cm×5 cm, Ds decreased the fastest with the increase in air gap thickness. When the field size was 15 cm×15 cm or above, the change in Ds with the air gap thickness was relatively stable. The TPS and experimental measurement results showed that dmax was the largest without a bolus. The change in dmax in the TPS was relatively stable with an increase in air gap thickness. The experimental results showed that when the field size was 10 cm×10 cm or smaller, dmax changed more significantly as the air gap thickness increased. When the field size was 15 cm×15 cm or larger, dmax remained relatively stable as the air gap thickness increased. The TPS and experimental measurement results showed that the smaller the field size, the larger the dmax, except for large differences in Ds between them when there was no bolus. The differences in Ds of the other field sizes and air gap thicknesses were all close to 0. The overall differences between the TPS and the experimental measurement dmax values were large for different air gap thicknesses. The differences were larger for small field sizes than for large field sizes, and when the air gap thickness was 0 mm, the difference in dmax for all field sizes was the closest to 0. The TPS and experimental measurement results showed that a bolus with an underlying 0-mm air gap is needed during superficial tumor radiotherapy.

Keywords

• bolus
• air gap
• radiotherapy
• surface dose
• maximum dose depth