ESTRO 2020 Abstract book

S927 ESTRO 2020

radiotherapy protocol: 67.5 Gy in 25 fractions, 2.7 Gy per fraction, with a simultaneous boost irradiation of 69 Gy. Three fiducial gold markers were implanted into the gland at least two weeks before the CT planning acquisition. At each fraction a pre-treatment CBCT was acquired and a rigid 3D fiducial markers-registration to the planning CT was performed to an accurate positioning of the patient. A post-treatment CBCT was acquired with the patient still in the treatment position on alternate days. The shifts based on the post-CBCT fiducial registration were used for total intrafractional motion determination, as we described in [1]. Organs at risk were re-contoured in each pre and post- CBCT, and CTV, PTV1 and PTV2 were re-contoured only on the post-CBCT. The plan was recalculated on daily pre- CBCT and post-CBCT. The delivered dose based on post- CBCT was calculated without applying the shifts corrections of intrafractional prostate motion, so we could evaluate the dosimetric impact maximum threshold of these displacement. A correlation analysis between intrafractional prostate motion and daily delivered doses was made. [1] Impact of rectum and bladder anatomy in intrafractional prostate motion during hypofractionated radiation therapy. M. Roch. Clin Transl Oncol DOI 10.1007/s12094-018-1960-y Results There is a correlation between rectum delivered doses and vertical intrafraction prostate motion. A prostate displacement in the anterior direction results in higher rectum doses, largest statistically significant differences were observe in the high dose range. Also we found that a prostate displacement in posterior and inferior direction results in higher bladder delivered doses. This dose increase was compensated by bladder filling during the treatment session that results in a reduction in the delivered dose at the end of the session, especially in medium dose range, as we show in Fig 1.

On average, the reduction in PTV1 and PTV2 coverage is considerable (Fig 2). This reduction was correlated with sagittal prostate intrafractional motion. Statistically significant differences were found from sagittal displacements > 4mm (p <0,01). In this case, the concomitant boost (PTV2) margin (3 mm to the GTV in all directions except at prostate–rectum interface) would not be enough to compensate the dosimetric impact of prostate intrafractional motion, instead the margin applied to the PTV1 (10mm except 7 mm at the prostate– rectum interface) would be enough.

Conclusion Intrafractional prostate motion has a considerable impact on the delivered dose to rectum, bladder and on to the coverage of the volumes of interest. It would be necessary to correct the intrafractional movement, for example by tracking techniques, to increase the precision in the hypofractionated prostate treatment. PO-1610 Robustness strategies towards respiratory motion for proton PBS treatments of oesophageal cancer L. Hoffmann 1 , P.R. Poulsen 2,3 , A. Hagner 2 , M. Dufour 4 , M. Nordsmark 2 , T. Nyeng 1 , D.S. Møller 1 1 Aarhus University Hospital, Department of Oncology- Medical physics, Aarhus, Denmark ; 2 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark ; 3 Aarhus University Hospital, Danish Center for Particle Physics, Aarhus, Denmark ; 4 University of Turin, Department of Physics, Turin, Italy Purpose or Objective Respiratory motion during proton pencil beam scanning (PBS) treatment of oesophageal cancer may compromise target coverage due to geometric misses and interplay effects. We investigate the effect of respiration for two target coverage strategies. Material and Methods The study included 26 oesophageal cancer patients (pts). All pts had a ten-phase 4DCT for planning (pCT) and at fraction (F) ten (surveillance, sCT). Retrospectively, PBS plans with two oblique posterior fields separated by 30 degrees were created using the mv-phase. Two strategies for robust optimization (RO) were tested: RO (3mm isocentre shifts, 3% density uncertainty) of the CTV (IMPT RO ), and CTV RO combined with coverage of PTV margins 5mm AP/LR, 8mm CC (IMPT PTVRO ). All plans fulfilled V95% CTV >99.5%. Dose calculations with interplay effects were performed by simulating treatment delivery at one F and distributing the spots into the 4DCT phases where they were delivered. The phase-specific doses were accumulated at the mv-phase (4D interplay dose), mimicking worst case scenario of delivering only one F. Additionally, the full plan was recalculated in all 4DCT phases and accumulated in the mv-phase to emulated motion blurring without interplay effects (4D dose).