ESTRO 2020 Abstract Book

S892 ESTRO 2020

vagina respectively, and > 94.2% for lymph nodes. ReCTs target doses showed for one patient a reduction in vagina coverage if the planCT rectum volume was large (Figure 1A). If rectum volume at planning was <200 cc, pelvic target coverage was maintained for all individual reCTs. The mean DVHs for bowel bag and bone marrow for the planCT and reCTs showed that both PBS-PT techniques provided better OAR sparing compared to VMAT. PBS-PT 2F showed slightly better bowel bag sparing than PBS-PT 4F, while PBS-PT 4F resulted in better reduction of higher bone marrow doses compared to PBS-PT 2F (Figure 1B and 1C). Patients with larger bowel bag volume had the most benefit of PBS-PT considering low bowel bag dose reduction (Figure 1D). The dose in the majority of OARs was significantly lower using PBS-PT compared to VMAT, mean V15Gy bowel bag decreased 30.3% and 28.5% and V20Gy bone marrow decreased 10% and 28.4% for PBS-PT 2F and 4F, respectively (Table 1).

Fig. 2 Conclusion

A straightforward offline rotation correction protocol can effectively reduce the systematic rotations to less than 0.5 o in fractionated SRT without the need of a 6D couch. ORC can also decrease the systematic rotations in treatments delivered with a lower number of fractions. PO-1631 Potential benefit of robust ITV-based proton therapy in cervical cancer patients E.M. Gort 1 , J.C. Beukema 1 , M.J. Spijkerman-Bergsma 1 , M.L. De Vries-de Groot 1 , S. Both 1 , J.A. Langendijk 1 , W.P. Matysiak 1 , C.L. Brouwer 1 1 University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands Purpose or Objective Current standard treatment of cervical cancer is radiotherapy (IMRT or VMAT) with concurrent chemotherapy. This treatment strategy is associated with chronic bowel toxicity, affecting quality of life, and hematologic toxicity, which can lead to chemotherapy discontinuation. Pencil Beam Scanning Proton Therapy (PBS-PT) can reduce low and medium dose areas in organs at risk (OARs), but inter-fraction motion and increased sensitivity to range uncertainties may affect target dose coverage and OAR dose. New treatment tools like robust treatment planning combined with image-guided and adaptive strategies could solve this problem. The aim of this study is: 1. to evaluate target dose coverage robustness of robustly optimized PBS-PT compared to VMAT against inter-fraction motion and 2. to investigate the potential of PBS-PT to reduce OAR doses. Material and Methods Twelve cervical cancer patients were included in a prospective study undergoing 5 weekly repeated CT scans (reCTs). The primary and para-aortic lymph node target volumes were delineated on both the planCT and reCTs. Two-arcs VMAT and robustly optimized two- and four-field (2F and 4F) PBS-PT plans for 25 fractions of 1.8 Gy RBE using RayStation 6.99 were made on the planCT and recalculated on the reCTs. Robustness evaluation using a 5-mm setup error on planCT, 1.5-mm (x, y) and 1.8-mm (z) setup error on reCTs and 3% range uncertainty margin was performed, based on literature [1] and department guidelines. The three planning techniques were evaluated for reCT target coverage (voxel-wise minimum (vox min) D98 > 95%) and nominal OAR dose [2-8]. Also, accumulated dose distributions were evaluated. Results For all techniques, mean accumulated vox min D98 coverage relative to prescribed dose was > 97.4%, > 96.6% and > 96.8% for target substructures GTV, uterus and