Influence of air gap under bolus in the dosimetry of a clinical 6 mv photon beam

Aim: In some situations of radiotherapy treatments requiring application of tissue-equivalent bolus material (e.g., gel bolus), due to material's rigid/semi-rigid nature, undesirable air gaps may occur beneath it because of irregularity of body surface. The purpose of this study was to evaluate the dosimetric parameters such as surface dose (D_s), depth of dose maximum (d_max), and depth dose along central axis derived from the percentage depth dose (PDD) curve of a 6 MV clinical photon beam in the presence of air gaps between the gel bolus and the treatment surface. Materials and Methods: A bolus holder was designed to hold the gel bolus sheet to create an air gap between the bolus and the radiation field analyzer's (RFA-300) water surface. PDD curves were taken for field sizes of 5 cm × 5 cm, 10 cm × 10 cm, 15 cm × 15 cm, 20 cm × 20 cm, and 25 cm × 25 cm, with different thicknesses of gel bolus (0.5, 1.0, and 1.5 cm) and air gap (from 0.0 to 3.0 cm), using a compact ionization chamber (CC13) with RFA-300 keeping 100 cm source-to-surface (water) distance. The dosimetric parameters, for example, “D_s,” “d_max” and difference of PDD (maximum air gap vs. nil air gap), were analyzed from the obtained PDD curves. Results: Compared to ideal conditions of full contact of bolus with water surface, it has been found that there is a reduction in “D_s” ranging from 14.8% to 3.2%, 14.9% to 1.1%, and 12.6% to 0.7% with the increase of field size for 0.5, 1.0, and 1.5 cm thickness of gel boluses, respectively, for maximum air gap. The “d_max” shows a trend of moving away from the treatment surface, and the maximum shift was observed for smaller field size with thicker bolus and greater air gap. The effect of air gap on PDD is minimal (≤1%) beyond 0.4 cm depth for all bolus thicknesses and field sizes except for 5 cm × 5 cm with 1.5 cm bolus thickness. Conclusions: The measured data can be used to predict the probable effect on therapeutic outcome due to the presence of inevitable air gaps between the bolus and the treatment surface.