ESTRO 37 Abstract book

S1127

ESTRO 37

Material and Methods The data of 15 left-sided breast cancer patients was retrospectively reviewed for this study. A moderate DIBH (approximately 80% of maximum inspiration) was visually instructed with the aid of an optical respiratory monitoring system (C-RAD AB, Uppsala, Sweden). The skin mark –based patient setup was verified for each treatment fraction with pre-treatment CBCT. The patients were treated with Infinity linear accelerators equipped with XVI imaging system (Elekta AB, Stockholm, Sweden). The CBCT images were registered with the planning CT in the Mosaiq system (v2.62) to record the final patient setup position in relation to skin mark – based setup. The registration was performed in 6D recording cranio-caudal (CC), anterior-posterior (AP) and left-right (LR) transitions as well as rotations in coronal, sagittal and transversal plane. The patients were treated according to a hypofractionated scheme (15x2.67Gy) resulting in 225 analyzed fractions. Results The mean shifts from the skin mark –based PTV setup were 1±5 mm anteriorly, 0±5 mm in left-right direction and 3±4 mm in caudally (Fig. 1). Mean absolute shifts from the skin mark based PTV setup were 3±3 mm, 3±3 mm and 4±3 mm in AP, LR and CC directions, respectively. When considering the typical 5mm tolerance margin used to create PTV from CTV, the shifts were smaller than 5mm in 82%, 80% and 69% of the fractions when considering AP, LR and CC directions independently. The difference between the planned position and the skin mark –based setup in AP, LR and CC directions was equal to or larger than 10 mm in 7%, 5% and 4% of fractions. The mean rotations were 0.6±1.5°, 0.3±1.7° and -0.2±1.9° in coronal, sagittal and transversal planes, respectively (Fig. 2). The corresponding absolute rotations were 1.1±1.2°, 1.4±1.1° and 1.3±1.3°. The rotational error in patient setup may add up a significant component to the total setup error. To illustrate, a rotation of 1° corresponds to a 1.7 mm shift at a distance of 10 cm.

Conclusion Even if the lung tumour motion during planning simulation is under 5 mm, variations in motion amplitude and larger motions can still occur during the course of treatment. This indicates that the use of daily tumour motion evaluation is required for PBS treatments, especially for hypo-fractionated treatments. Future work will include a risk-stratification for motion variations in lung patients based on initial tumour motion, tumour position and tumour volume for different PBS spot sizes, time structure and prescribed treatment courses. EP-2054 CBCT verified variation of PTV position in DIBH treatment of left breast T. Koivumäki 1 1 Central Finland Central Hospital, Medical Physics, Jyväskylä, Finland Purpose or Objective The aim of this study was to evaluate the variation in the position of the planning target volume (PTV) during the course of left breast irradiation in DIBH.

Figure 1. Histogram of the shifts in three orthogonal directions.

Figure 2. Histogram of the rotations in three orthogonal planes. For readability, the data is presented in 1° bins. In addition, 2 fractions with coronal rotation of more than 5.5° and 2 fractions with transversal rotation of less than -5.5° were omitted from the figure.