Session Item

Patients with breast cancer treated in deep inspiration breath hold with external beam radiotherapy require a motion management (MM) system. The gating system for TrueBeam from Varian provides MM with an external surrogate. When an external surrogate is used for MM, placing it closer to the breast increases the precision of the tracked target motion (Skyttä et al. Acta Oncol. 2016;55:970-75). However, placing the surrogate closer to the target may result in the treatment beams going through the surrogate thus causing an unintended bolus effect. It is therefore crucial that the design of a surrogate minimizes the bolus effect to allow for placement near the breast. In this study, we have tested a new surrogate: Gating Device L (GDL) and compared it to Varian’s Truebeam reflector block (RB), as well as a modified version of the reflector block (MRB) (see figure 1), with respect to surface dose, bolus effect and tracking capabilities.

Figure 1: The three surrogates investigated: The Gating Device L (GDL, the Reflector Block (RB) and the Modified Reflector Block (MRB).

Percentage depth dose curves and profiles with and without surrogates were measured with Gafchromic film and performed in-silico in the treatment planning system (TPS) to assess the surface dose and bolus effect in a 6 MV beam. RB and GDL were compared in the TPS to evaluate the dosimetric foot print on dose distributions on a static field directly through the surrogates and by the Volumetric Modulated Arc Therapy (VMAT) technique and compared by calculating the Dice Similarity Coefficient (DSC). The tracking capabilities of RB and GDL by the gating system were investigated with induced pitch, roll and yaw of the surrogates.

 Surface dose was increased up to 295%, 277% and 190% for RB, MRB and GDL respectively compared to no surrogate with GDL having a smaller physical surface area of 11 cm² compared to 60 cm² of RB and MRB (see figure 2). In a static field directly through the surrogate, RB and GDL were found to be equivalent up to 4.5mm and 2.2mm water, respectively. By comparing dose distributions in the TPS, DSC was found to be 0.989 and 0.997 for GDL and 0.897 and 0.954 for RB for a static field and a VMAT plan, respectively. The RB and the GDL could be tracked in ranges [330˚, 60˚] and [330˚, 15˚], [-18˚, 18˚] and [-18°, +3˚], [-18˚, 18˚] and [-18˚, 11˚] in yaw, pitch and roll, respectively.

Figure 2: Film measurements. (A) Profiles for GDL, RB and MRB at the entrance dose of the beam compared to no surrogate (NS). (B) The position of the chosen profiles below the three surrogates.

Gating Device L was found to have a smaller dosimetric footprint than the Reflector Block. GDL had a lesser impact on dose distributions compared to RB. While GDL was trackable in a smaller volume of space, tracking was identical for GDL and RB in a clinically relevant range. Gating Device L is therefore a promising alternative to Varian’s Reflector Block.