ESTRO 37 Abstract book

S1124

ESTRO 37

EP-2050 Geometrical uncertainties of humerus and breast in breast cancer radiotherapy. J.T.J. Honkanen 1 , M. Loukkola 1 , J. Seppälä 1 1 Kuopio University Hospital, Cancer Center, Kuopio, Finland Purpose or Objective This study aimed to determine the geometrical uncertainties of humerus and breast during radiotherapy treatment course of breast cancer (BCa) patients and compare two patient fixation devices. Material and Methods Changes in the daily positioning of BCa patients were investigated by comparing low-dose CBCT images with planning CT images with two patient fixation devices. Patients ( n =40) treated with whole breast irradiation including axillary lymph nodes were aligned using either C-Qual breastboard with arm and wrist support ( n =21) or C-Qual M breastboard with Monarch overhead arm positioner ( n =19) (CIVCO). The patients were treated with conventional fractionation (50 Gy/25 fr) using a VMAT technique and daily CBCT image guidance (Elekta Infinity/XVI). CBCT images were co-registered with the respective planning CT in six degrees of freedom with a box shaped region-of-interest (ROI) using maximization mutual information (MMI) or Chamfer registration algorithm (Mosaiq v2.62, Elekta AB). For each patient, the co- registrations were computed for 4-9 treatment fractions (182 and 160 fractions in total for C-Qual and C-Qual M patients, respectively) using two different ROIs covering 1) the PTV and 2) proximal humerus (Fig. 1). The orthogonal shifts and rotations (direction of the rotation was omitted) were recorded and compared between the two fixation devices.

The treatment plan was obtained by using a 4D robust optimizer developed in-house (MIROpt), without considering breathing amplitude variations. Its robustness was verified using the presented method.

Fig 1. Planning CT (magenta) and CBCT (green) images were co-registered based on the mutual information using two different ROIs (delineated with yellow line) covering ( A ) the target volume and ( B ) the proximal humerus. Results The average orthogonal shifts were -3.0±4.0 mm, - 1.2±3.9 mm and -1.6±4.5 mm for PTV and -1.3±5.1 mm, - 1.6±6.3 mm and -2.1±6.3 mm for humerus in longitudinal, lateral and vertical directions, respectively. Significant differences in orthogonal shifts between the two breastboards were found in lateral and vertical directions with both ROIs (Table 1). Moreover, the lateral shifts were larger with C-Qual M breastboard but the vertical shifts were larger with C-Qual breastboard. The average rotations were 0.9±0.7°, 1.0±1.0° and 1.0±0.9° for PTV and 2.0±1.7°, 2.1±1.7° and 2.0±1.8° for humerus in coronal, sagittal and transversal planes, respectively. Transversal rotations were significantly larger with C-Qual M when the co-registration was conducted using ROI covering the target volume. However, when the co-registration was conducted based on the proximal humerus, transversal rotations were significantly larger with C-Qual (2.2±2.0°) than with C- Qual M (1.7±1.5°) breastboard.

The robustness test results, reported as DVH-bands, revealed a sufficient plan robustness against treatment uncertainties typically reported in the literature. However, without repainting, the interplay effect significantly degraded the dose distributions. Conclusion The developed model enables realistic robustness evaluation of proton therapy treatments, including for mobile targets. Most uncertainty models apply to both photon and particle treatments. Moreover, the generated 4DCTs can be directly used as uncertainty scenarios to feed robust optimizers in commercial TPS.