ESTRO 2021 Abstract Book

S133

ESTRO 2021

Results Due to anatomical changes occurring during the treatment course, the CTV coverage of the non-adapted doses reduces on average by 9.7% (V95) compared to an ideal DAPT doses (Fig. 2a). Adapting the plan on propagated uncorrected structures restores the CTV coverage (average CTV V95 differences <1%) and reduces hotspots (Fig 2b). However, for some patients (for example patient 1 and 4, Fig. 2c) optimizing the dose on the structures contoured by a radiation oncologist reduces the dose to individual organs at risk (the dose to 20% of both lungs could be reduced about 3%).

Conclusion Our data suggest that an online DAPT workflow could be implemented, neglecting the online correction of the propagated structure. It was shown that the dosimetric uncertainties of daily omptimization on the propagated structures is smaller than non-adapting the plan. However, a careful (offline) structure review is advisable, and, if necessary, corrections to the daily structures can be included in an offline adaption.

[1] Albertini et al. BJR 2020 [2] Nenoff et al. IJRBP 2020

OC-0203 Nominal, daily and accumulated target coverage for photon and proton treatment of sinonasal cancer R. Argota Perez 1 , M.B. Sharma 1 , U.V. Elstroem 2 , D.S. Moeller 1 , C. Grau 1,2,3 , K. Jensen 2 , S.S. Korreman 1,2,3 , A.I. Holm 1 1 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark; 2 Aarhus University Hospital, Danish Center for Particle Therapy, Aarhus, Denmark; 3 Aarhus University, Department of Clinical Medicine, Aarhus, Denmark Purpose or Objective In selection of sinonasal cancer patients for proton therapy, robustness of the treatment to anatomical variations is an important addition to nominal dose comparison. We perform a comprehensive evaluation of dosimetric differences between photon and proton treatment, comparing nominal plans, effects of day-to-day anatomical and setup variations on daily delivered doses, and doses accumulated over the entire treatment. Materials and Methods Photon (VMAT) and proton (IMPT) plans were made retrospectively in Eclipse v15.6 for 24 sinonasal cancer patients treated with curative intent. Dose was 66-68Gy/60-66Gy for primary/postoperative radiotherapy. VMAT plans consisted of two full planar arcs and two 60-degree sub-arcs. Sub-arcs were delivered from the front of the patient, couch turned 90 degrees. PTV and PRV margins were 3 mm. IMPT plans consisted of 3-5 beam angles, using pencil beam scanning, multi-field optimization, and a 5 cm range shifter. Robustness optimization parameters included ±3mm setup uncertainty in all cardinal directions and ±3.5% range uncertainty. Plans were approved using PTV/PRV for photons, and robust evaluation (±2mm, 2%) for protons. Synthetic CTs (SynCT) were generated by deforming the planning CT (pCT) to the daily CBCTs in MIM v.7.0.2. Nominal plans were recalculated on SynCTs and resulting dose distributions were accumulated on the pCT. Acceptable target coverage was V95%>99% for the high-dose CTV. Results For photon plans, CTV coverage was stable throughout treatment; 4/24 patients had one or more fractions below the clinical acceptable limit. For proton plans, 13/24 patients had one or more fractions with under- dosage of the target. For the accumulated dose, good coverage was achieved for 22/24 patients for both photon and proton plans (Figure 1). The target under-dosage for photon plans was generally located superficially in the outer 3-4 mm of the patient. For the proton plans, the under-dosed volumes were in non- overlapping locations from fraction to fraction resulting in an averaging effect. For some patients the same cold spot locations were however observed on consecutive days. In general, the nominal dose to organs at risk (OARs) farther away from the target was lower for protons than for photons – mean dose to the brain was 5.7Gy/0.9Gy for photons/protons (population median). However, OARs in the proximity of the target received generally lower doses for photons - mean dose to ipsilateral