Estimation of surface doses in the presence of an air gap under a bolus for a 6 MV clinical photon beam - a phantom study.

Abstract

Goal of the present study was to develop and build a phantom that replicates the air gaps under a gel bolus and to estimate the surface dose (D_surf) under normal incidence with a 6 MV photon beam. For this, an acrylic phantom with 10 plates, each including five open slots (one in the centre and four off axis) with a size of 2 cm × 2 cm at depths of 0.54 cm, 0.72 cm, 0.90 cm, 1.26 cm, and 1.62 cm from the phantom's surface was used. Computed tomography image sets were obtained without and with a gel bolus (thickness: 2 mm, 4 mm, and 6 mm) placed on top of the phantom. Dose calculations were performed with the XiO treatment planning system (TPS) for a 6 MV photon beam at normal incidence and a field size of 15 cm × 15 cm that covered all the slots. A virtual bolus in TPS was employed in CT picture sets that did not include a bolus. Six points of interest at a depth of 1 mm from the surface contour of each slot were used to determine the mean surface dose (D_surf) estimated by the TPS with and without the presence of a bolus. It turned out that, as the depth of the air gap (between skin surface and bolus surface) increased from 0.54 cm to 1.62 cm, there was a 25.2% increase in D_surf without bolus, followed by an increase of 7.6%, 6.4%, and 7.7% for a virtual bolus with 2 mm, 4 mm, and 6 mm thickness, while corresponding increases were 14.8%, 14.3%, and 8.3% for an actual bolus, respectively. However, as the thickness of the air gap increased, D_surf under the bolus decreased (from - 17.5% to -18.8%, and from - 10.4% to -16.9%, for a virtual and a physical bolus, respectively). It is concluded that, to ensure a homogeneous D_surf across the treatment area, extra attention should be given while utilizing a bolus in clinical radiation applications, to avoid any air gaps under the bolus.