ESTRO 2021 Abstract Book

S1266

ESTRO 2021

Conclusion There occurs significant tumour regression during the course of radiotherapy in locally advanced carcinoma cervix, resulting in reduction in CTV and PTV volumes. Although the impact of tumour regression is seen as increased target coverage it however does not translate into better avoidance of normal surrounding structures. PO-1542 Influence of abdominal air cavities on the dose distribution of daily-adapted plans F. Belosi 1 , M. Bogowicz 1 , M. Chamberlain 1 , S. Ehrbar 1 , H. Garcia Schüler 2 , J. Krayenbuehl 2 , M. Guckenberger 3 , N. Andratschke 3 , S. Tanadini-Lang 2 , L. Wilke 2 1 University Hospital Zürich, Radiation Oncology, Zürich, Switzerland; 2 University Hospital Zürich , Radiation Oncology, Zürich, Switzerland; 3 University Hospital Zürich and University of Zürich, Radiation Oncology, Zürich, Switzerland Purpose or Objective The aim of this study was to evaluate the impact of air cavities (AC) on the dose distribution for abdominal radiotherapy treatments depending on their size and distance to the tumor. Firstly, it was investigated whether a correlation exists between the volume of an AC and its impact on the target coverage. Then, as more assessable measure during on-line adaptions, a threshold for the largest size of an AC in the 2-D plane to cause a difference >1% in the PTV (or GTV) volume receiving the reference isodose (V ref ) was identified. Materials and Methods Patients with abdominal lesions treated with stereotactic body radiotherapy (SBRT) on a ViewRay MRIdian Linac were included if presenting with an AC in at least one of the beam’s path. Prescription doses ranged between 5-9 Gy per fraction at the 80% or 65% isodose. All 38 patients received daily MRI and all treatment plans were adapted online, manually delineating ACs on the MR if their largest planar dimension was: These structures were overwritten to air-density in the electron density map used for dose optimization and calculation. All 38 on-line adapted plans were re-calculated without the air-density override on the largest AC present and the new dose distributions were compared to the ones clinically applied to the patients. Results In 19 cases, the AC was located within 2cm radius from the PTV (group 1; G1); in the remaining 19 cases, it was at a larger distance (group 2; G2). Dose difference between recalculated and nominal plans for the PTV/GTV V ref (ΔV ref ) mainly resulted in under-dosage. Median PTV_ΔV ref (GTV_ ΔV ref ) was -0.4% (-0.8%) in G1 with a largest PTV_ΔV ref (GTV_ ΔV ref ) of -3.8% (-14.6%); and it was -0.3% for both PTV and GTV in G2, with a largest PTV_ΔV ref (GTV_ ΔV ref ) of -2.2% (-4.8%). No correlation was found between the ΔV ref and the ACs’ volume, being the R 2 <0.5 in G1 and <0.05 in G2. Therefore, the volume of an AC can’t be used as indicator to predict its impact on the targets’ coverage. The box-plot in Fig. 1 shows the distribution of the largest AC’ sizes in the 2-D plane distinguished for cases with a PTV (or GTV) ΔV ref >1% and <1%. For median AC size of 3.2cm (3.6cm) in G1 (G2), the ΔV ref are still <1%. The ΔV ref increases >1% for median AC size of 3.6cm in G1 (9/19 cases), and of 5.3cm in G2 (5/19 cases). The PTV_D 0.1cc increased above tolerance for only 2 cases with AC located <1cm to the target. As for OARs, in only 2 plans a dose constraint was exceeded, being very close to the tolerance already in the original plan and with an AC of >3.6cm in size inside the OAR itself. § >1cm and they were located within a 2cm radius around the PTV; § >2cm and they were located outside a 2cm radius around the PTV.

Conclusion Delineating abdominal ACs with largest size in the 2-D plane <3cm doesn’t have a significant impact on the daily-adapted dose distribution and could be avoided, reducing the time for delineation during the on-line adaptive workflow. PO-1543 Can deformable registration of CT and CBCT predict plan adaptation need in proton therapy (H&N)? J. Gora 1 , A. De Leon 2 , G. Kragl 3 , A. Carlino 2 , M. Stock 2 1 MedAustron Ion Therapy Center, Medical Department, Wiener Neustadt, Austria; 2 MedAustron Ion Therapy Center, Medical Department , Wiener Neustadt, Austria; 3 Ordensklinikum Linz GmbH Barmherzige Schwestern, Strahlentherapie, Linz, Austria

Purpose or Objective