ESTRO 2021 Abstract Book

S1433

ESTRO 2021

Purpose or Objective To investigate setup uncertainties in stereotactic body radiotherapy (SBRT) for bone metastases with respect to different target localizations, patient immobilization, time from setup imaging to treatment and total treatment time as part of implementation of a phase II SBRT trial for oligometastatic bone metastases. Materials and Methods Eight patients with a total of nine targets were treated with either 30 Gy or 37.5 Gy in three fractions, prescribed to the GTV. Target locations were spine, ribs, sternum, pelvis and acetabulum. Long vacuum cushions were used for immobilization. The patient with the sternum metastasis was treated in deep inspiration breath hold (DIBH). CBCT was acquired for position verification and correction before, midway and after treatment in all treatment fractions. Intra fractional patient positioning deviations were calculated for all fractions and patients as the difference between the patient position before and after treatment. Setup time, i.e. time from start of first CBCT acquisition to first beam on, and total treatment time, i.e. time from start of first CBCT acquisition until end of treatment, were registered for all fractions. The CTV-PTV margin was 2 mm for all targets except from the sternum target, where the margin was 5 mm. Results Median total treatment time (inter quartile range (IQR)) was 14 min 57 s (13 min 21 s - 17 min 49 s). Setup time ranged between 3 min 28 s and 11 min, with a median of 5 min 2 s. For all patients the setup time was longer for the first fraction than for the second- and third fraction. Median (IQR) of the absolute values of the intra fractional patient positioning in mm were 0.9 (0.4 - 1.6), 0.6 (0.2 - 1.2), and 0.4 (0.2 - 0.7) in the vertical, longitudinal and lateral directions, respectively (Figure 1). The outlier (patient 4) is the patient with the sternum target. Median (IQR) of the absolute value of the rotational intra fractional patient positioning deviations were 0.2° (0.1° - 0.4°), 0.4° (0.2° - 0.7°), and 0.2° (0.1° - 0.3°) for pitch, roll and rotation, respectively (Figure 2). Rotational deviations were minimal. Conclusion For most patients and fractions, the translational setup deviations were <2 mm. The small rotational deviations indicate that the long vacuum cushions were suitable for SBRT immobilization. We will further investigate how to streamline workflows to shorten time from CBCT to treatment, especially for the first fraction. With the inclusion of more patients in the protocol, we will explore how different target locations impact set-up uncertainty.