Session Item

To evaluate the influence of a 6D couch correction on the stability of breast cancer patients and whether there is a relationship between the magnitude of pitch and roll movements and patient stability. In addition, to assess whether there are subgroups for which the effect of a 6D couch correction is greater and whether the stability of breast cancer patients is comparable to lung cancer patients.

The research population concerns the first 84 breast cancer patients treated between December 2020 and March 2021 on the Radiotherapy department in VUmc. All patients are treated on a Truebeam linear accelerator (Varian Medical Systems) and were positioned arms up in supine position on a thoraxsupport (MacroMedics). Online registration data were evaluated. In total this concerns 767 radiotherapy fractions. A CBCT was made for each fraction and a verification CBCT was made when pitch and/or roll was ˃1° (199 fractions). The CBCT’s were registered online using an automatic 6D bone match, the ROI was placed around the sternum and thoracic wall. Thereafter, in PBI or SIB treatments an automatic  or manual 3D registration was done on the surgery clips.

There is no correlation between the magnitude of pitch on the setup CBCT and the magnitude of vertical (r=-0.07) and lateral (r=0.05) corrections on the verification CBCT, but there is a moderate correlation between pitch and the longitudinal (r=0.57) correction. Longitudinal corrections up to 0.63 cm are required on the verification CBCT after pitch (figure 1). Every dot shows the executed correction on the verification CBCT. Pitch and roll data ≤1˚ also appear in the scatter plots. In the scatter plot of pitch a verification CBCT was made because of roll >1˚ and vice versa.

Secondly, there is no correlation between roll and the magnitude of vertical (r=-0.02) and longitudinal (r=-0.09) corrections on the verification CBCT. However, there is a strong correlation between roll and the lateral (r=-0.83) correction on the verification CBCT. Lateral corrections up to 0.74 cm are required (figure 1).

Except for some minor differences it appears that correlations of the subgroups breast cancer patients correspond to those for the entire group. Subgroups were PBI, breast FAST (+SIB), breast (+SIB), breast loco-regional, chest wall loco-regional and axilla.

There are no significant differences in correlation coefficients between breast and lung cancer patients (figure 2).

After performing pitch and roll on the setup CBCT the required corrections on the verification CBCT vary widely. There is a moderate correlation between the magnitude of pitch and the longitudinal correction and a strong correlation between the magnitude of roll and the lateral correction. Very similar statistical data and correlation coefficients show that the subgroups of breast cancer patients correspond to those for the entire group and that the stability of breast cancer patients is comparable to lung cancer patients.