Margriet Kwint

Intra anatomical changes during radiotherapy 71 4 Introduction In the Netherlands, about 12000 new lung cancer patients are diagnosed annually. 80% of these patients are medically or technically inoperable at diagnosis [1]. Patients with inoperable locally advanced lung cancer are often treated with radical radiotherapy and depending on physical condition and/or tumor stage with or without chemotherapy. The overall treatment time of the radiotherapy courses in our clinic is 5/6 weeks and it is generally assumed that the anatomy of the patient is stable during this treatment. However, during this course of radiotherapy several anatomical changes may occur, such as atelectasis, infiltrative changes, tumor progression or regression and pleural effusion [2-14]. With the introduction of advanced image-guided systems like kilovoltage (kV) cone- beam computed tomography (CBCT), megavoltage (MV) CBCT, and tomotherapy, we have the ability to visualize the tumor and organs at risk (OAR) in 3D [15,16]. These modalities primarily minimize target misalignment and setup-error [9]. Many studies investigate setup precision in lung radiotherapy [17,18]. However, only a few studies reported anatomical changes during the course of lung radiotherapy [9,11,19]. In clinical practice repetitive CBCT’s make us aware of intra thoracic anatomical changes (ITACs) during the course of a radical treatment. In the Netherlands, CBCT’s are typically analyzed by radiation therapy technologists (RTTs), the radiation oncologist is only informed when a change is observed. In our institute, we developed an action level protocol as a decision support system to guide the RTT in prioritizing these changes. In this study, we quantified ITACs during radiotherapy and present a decision support system (Traffic Light Protocol). Methods and materials Patient selection The Traffic Light Protocol was introduced clinically in our institute in 2011. All CBCT’s of lung cancer patients radically treated with radiotherapy or chemo-radiotherapy (≥44Gy) in our institute in 2010, were retrospectively analyzed based on this Traffic Light Protocol. Patients treated with stereotactic radiotherapy were excluded as they had a different decision protocol. Inclusion criteria for this study were radical treatment with