A novel surrogate for motion management in external beam radiotherapy of breast cancer patients

When treating breast cancer patients with external beam radiotherapy, deep inspiration breath hold (DIBH) has proven to be a valuable method to decrease dose to the heart and lungs [1–4]. Several commercial methods to monitor and regulate respiration during DIBH exist [5], and the most common methods are: external surrogate with an optical or infrared camera [1,3,4,6–10], spirometry [11], or surface monitoring with optical cameras and structured light [12–15]. When using a reflective surrogate, the closer the surrogate is placed to the treated breast in the cranio-caudal and lateral direction the better it represents the motion of the breast [8,16,17]. However, when positioning the surrogate close to the target, the surrogate will often be positioned in the path of the treatment beam. This may lead to an unwanted bolus effect, increasing the skin dose, thereby increasing the risk of erythema and skin irritation [18,19]. The commercially available two-marker real-time position management (RPM) box (TMB) was not suitable for placement within the treatment beams due the amount of material it consists of, as this may result in an unwanted skin reaction. Furthermore, due to the relatively wide lateral extent of the TMB, it may be positioned with a tilt toward the treated breast on some patients [16]. Therefore, a novel respiratory surrogate (NRS) was designed consisting of less material and with a smaller footprint to (1) allow placement within the treatment beam and (2) enable a stable placement of the reflective surrogate on the sternum approximately at the level of the papilla. In this study, we compared surface dose and water-equivalent thickness between the NRS and the TMB by radiosensitive film measurements. Tracking performance of the two types of surrogates was investigated in both a phantom and in clinical setting.