ESTRO 2023 - Abstract Book

S1599

Digital Posters

ESTRO 2023

Our study is one of the first to examine the dosimetric and volumetric effects of the EEBH technique in patients with oesophageal cancer. EEBH presents a promising method for reducing PTV volumes, as well as OAR doses. This, combined with the increased stability in motion that EEBH assures, compared to that of FB, may show EEBH as an effective breathing technique for optimising radiotherapy for oesophageal cancer treatment. Further work is ongoing to investigate the significance of current results in a larger patient cohort.

PO-1859 Real-time prostate and lymph node dose reconstruction accounting for independent 6DoF target motions

K. Klucznik 1 , T. Ravkilde 2 , S. Simon Skouboe 1 , D. Møller 2 , S. Hokland 2 , P. Keall 3 , S. Buus 2 , L. Bentzen 4 , P. Poulsen 5

1 Aarhus University Hospital, Danish Center for Particle Therapy, Aarhus, Denmark; 2 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark; 3 University of Sydney, ACRF Image X Institute,, Sydney, Australia; 4 Department of Oncology, Aarhus University Hospital, Aarhus, Denmark; 5 Aarhus University Hospital, Danish Center for Particle , Aarhus, Denmark Purpose or Objective High-risk prostate cancer patients require irradiation of both prostate and pelvic lymph nodes (LN) which undergo independent inter- and intrafraction motion composed of translations and rotations (six degrees of freedom, 6DoF). The dosimetric consequence of these motions have not been fully investigated. This study develops a method for real-time 6DoF motion-including dose reconstruction for two independently moving targets and applies it to investigate the actually delivered target doses for prostate patients. Materials and Methods The study includes 5 patients treated in 39 fractions with 78 Gy to prostate CTV and 56 Gy to LN CTV using 3-arc VMAT. The CTV-to-PTV margins were 7-9 mm (prostate) and 5-8 mm (LN). A CBCT matched on three implanted prostate markers was used for setup. Triggered kV images acquired every 3 s during treatment delivery at ten fractions per patient were used for post-treatment estimation of the time-resolved 6DoF prostate motion during treatment using the formalism of kilovoltage intrafraction monitoring (KIM) to estimate the 3D motion trajectory of each marker. A 6DoF match of a post treatment CBCT to the planning CBCT was used to estimate the 6DoF position of the LN assuming no intrafraction motion. Real-time motion-including dose reconstruction was performed post-treatment by sending data streams with machine parameters and the prostate and LN 6DoF motion to in-house developed software (DoseTracker) that calculated the 6DoF motion-including target doses by a simplified pencil beam algorithm. The motion-induced change in minimum dose to 95% of the prostate and LN CTVs ( Δ D95%) was calculated for each fraction and for all investigated fractions averaged (planned and reconstructed dose were calculated by DoseTracker). Results The largest observed 6DoF target position error (averaged over one treatment field) was 4.3 mm (LR), -6.7 mm (CC), -18.2 mm (AP) and -20.6° (LR), -2.7° (CC), -3.8° (AP) for prostate and 4.8 mm (LR), 10.9 mm (CC), 11.2 mm (AP) and 2.9° (LR), 2.1° (CC), 0.7° (AP) for LNs (see details in Fig 1). The largest observed intrafraction motion range occurring during one fraction was 5.1 mm (LR), 8.6 mm (CC), 18.5 mm (AP), 16.1° (LR), 13.9° (CC), and 5.2° (AP). For a single fraction the CTV Δ D95% range was [+0.3;-6.5]% for prostate and [+0.3;-12.4]% for LN, illustrating considerable motion-induced reduction in CTV dose coverage at certain fractions (Fig 2). For the averaged dose over all the fractions the CTV Δ D95% range decreased to [+0.1;2.1]% for prostate and [+0.3;-1.8]% for LN (Fig 2d).