ESTRO 2022 - Abstract Book

S701

Abstract book

ESTRO 2022

Head and Neck RT treatment information registered in our in-house software between February 2018 and September 2020 by means of our IG-TLP was analysed. Review orders from the IG-TLP resulting in a new CT as well as the patients’ characteristics were carefully evaluated to allow the identification of possible predictors. The patient related information were weight evolution and tumor site while the treatment parameters were the treatment regimen, the reason for new CT, the week of new CT and major/minor violation of re-planning criteria assessed by the physicians (Table 1).

Results Among the 266 patients analyzed, our protocol resulted in 807 review orders: 87% of them did not require further action after investigation; in 77 cases (10%), a new CT scan was made; and in 24 cases (3%), plan adaptations were performed. For these 24 re-planned patients, 12 resulted from criteria in the yellow action level, 6 from the orange and 6 from the red level. When analyzing the new CT scan orders, we realized that certain criteria or a combination of two criteria is more prone to result in plan adaptation (Figure 2). No strong statistical associations were found between the different criteria evaluated and the need for plan adaptation ( p> 0.05). However, 42% of re-planning was found in oropharyngeal cancer and 41% in primary RT. In 15/24 cases, re-planning was performed to ensure an adequate target coverage and in four cases because of a potential increased dose to the OARs. Those re-planning occurred mostly during the first (n=9) and the third week (n=6) of the treatment. Contrary to other studies, most re-planning was observed in patients with weight gain or weight loss up to 5%, rather than in patients with more substantial weight loss. Conclusion Only few RTT-reported IG-TLP changes led to re-planning. Moreover, no strong statistical associations were found between the TLP levels and plan adaptation. Therefore, ideally, TLP should serve as screening, followed by more objective dosimetric criteria (e.g delta dose and remaining sessions), preferably using actually delivered doses, in order to define a predictive model for adaptive planning.

OC-0782 Intrafraction motion of pulmonary tumors during sbrt and clinical consequences

S. van der Windt 1 , C. van Gijlswijk 1 , B. Gobets 1 , E. van Reij 1 , A. Langeveld 1 , M. Hol 1

1 Leiden University Medical Centre, Radiotherapy, Leiden, The Netherlands

Purpose or Objective To evaluate the intrafraction (IF) motion of pulmonary tumors during stereotactic body radiation therapy (SBRT) using online Cone Beam Computed Tomography (CBCT) data and to determine possible clinical consequences, such as margin adjustment.

Materials and Methods The Planning Target Volume (PTV) is defined as the Internal Tumor Volume with a 5 mm expansion.

For each fraction, CBCTs were acquired for set-up correction (scan A), to confirm correct positioning of the patient (scan B) and to monitor the IF motion (scan C, after completion of the fraction). Both a bone match (translations + rotations) and a soft tissue (tumor)match (translations only, rotations used from bone match) were performed for all CBCTs. In total 6930 CBCTs were reviewed (340 patients, 404 tumors) of SBRT treatments performed in the period 2018-2020. The IF motion of both tumor and patient were assessed (difference between match results of scan B and C for respectively soft