ESTRO 2020 Abstract book

S943 ESTRO 2020

rotational errors were derived from a 6D grey-value registration of the daily acquired CBCT scan with the planning CT scan. Only translational errors were corrected prior to treatment. For each patient the average rotational error during the first 3 fractions (R 1–3 ), the remaining fractions (R 4–N ), and all fractions (R all ) was calculated around the LR, CC, and AP axis. We proposed the following offline rotation correction (ORC) protocol to correct for systematic rotations: The dose distribution will be rotated (not the patient/couch) and R 1–3 is used to predict the systematic rotations of the remaining fractions R 4–N . If R 1–3 is higher than 0.5 o or 1.0 o , the planning CT with delineated structures is rotated in opposite direction of R 1–3 and used to adapt the treatment plan for fractions 4 to N. The ORC protocol can be simulated for each patient by subtracting R 1–3 from the daily rotations in fractions 4 to N. R all,ORC (around 3 axes) is the residual rotational error after the applied ORC. ORC was also simulated for N=3. Results The systematic rotational errors R all were 2.2 o or less for all patients. The population standard deviations Σ were 0.5 o , 0.5 o , 1.0 o for the LR, CC, and AP rotation axes, respectively. Figure 1 shows that R 1–3 was highly correlated with R 4–N : the Pearson correlation coefficient was 0.88, 0.90, and 0.97 for the LR, CC, and AP rotation axes, respectively (error bars denote the standard deviation). For ORC with an action level of 0.5 o , Σ reduced to 0.3 o , 0.2 o , 0.3 o (for LR, CC, AP) and R all,ORC was 0.5 o or less for all patients (Fig. 2; plan adaptation was required for 13/18 patients). For an action level of 1.0 o , these rotations were slightly higher: Σ = 0.4 o , 0.2 o , 0.4 o (for LR, CC, AP; plan adaptation required for 8/19 patients) and R all,ORC ≤ 0.9 o . When using the rotation of the first fraction only (R 1 ) to predict the average rotation of fractions 2, 3, and 4 (R 2–4 ), ORC with an action level of 1.0 o reduced Σ from 0.6 o , 0.6 o , 1.0 o (LR, CC, AP; R 2–4 ≤ 2.3 o ; without ORC) to 0.5 o , 0.3 o , 0.5 o (R 2–4,ORC ≤ 1.3 o ; with ORC).

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

Fig. 1