ESTRO 2020 Abstract Book

S881 ESTRO 2020

PO-1617 Feasibility of spirometer-guided single breath-hold kV-CBCTs on Halcyon in lung cancer patients L. Delombaerde 1,2 , T. Depuydt 1,2 , P. Berkovic 1,2 , M. Lambrecht 1,2 1 University Hospital Gasthuisberg, Department of Radiation Oncology, Leuven, Belgium ; 2 KU Leuven, Department of Oncology, Leuven, Belgium Purpose or Objective The fast imaging (and radiation delivery) capabilities of the Halcyon linear accelerator (linac) (Varian Medical Systems) allow for single breath-hold kV-CBCT acquisitions (< 20 sec). The improved image quality, compared to free breathing CBCTs, will potentially allow improved automated contouring in adaptive radiotherapy and radiomics analysis. However, prior to performing breath- hold treatments, the intra fraction tumor position reproducibility has to be quantified. In this study we determined the intra-fraction tumor position reproducibility by acquiring single breath-hold kV-CBCTs in the treatment position at the Halcyon linac using the SDX spirometer (Dyn’R). Material and Methods Five locally advanced lung cancer patients participated in this imaging study (in progress). Prior to the simulation appointment, patients were coached in a training session in which patients were familiarized with the SDX spirometer system and an individualized deep inspiration breath-hold level was determined. During the simulation appointment one additional CT scan was acquired in breath-hold (DIBH-CT), after the free breathing scan with iodine contrast and the 4DCT. Patients were treated in free breathing - our standard of care – on the Halcyon linac. During four fractions (fx 2, 6, 11 and 16) patients received one additional kV-CBCT in breath-hold prior to treatment (PRE-CBCT), and one kV- CBCT in breath-hold after treatment (POST-CBCT). The POST-CBCT was matched to the PRE-CBCT using a 6 degree-of-freedom (DoF) automated rigid registration on the vertebrae. Subsequently, a 3 DoF automated rigid registration was performed on the primary tumor. The difference between the registrations quantified the intra- fraction tumor motion. The same procedure was applied to determine the inter-fraction errors by registering the PRE-CBCT to the DIBH-CT. Results One patient (out of five) was unable to maintain a breath- hold during the coaching session and was excluded from the study. All four remaining patients were able to perform the breath-hold CT and all kV-CBCTs. The CBCT acquisition time was 16 sec and all patients performed every CBCT in a single breath-hold. The improved image quality can be qualitatively assessed in figure 1. Motion blurring is limited resulting in a more clearly defined tumor boundary. The intra-fraction uncertainly was small: median 0.1 mm (range -2.5, 3 mm) in the craniocaudal, median 0.5 mm (range -2.3, 1 mm) in the mediolateral and median 0.7 mm (range -2.1, 2.7 mm) in the anteriorposterior direction. The intra-fraction systematic (Σ) and random (σ) errors are shown in the table.

comparison

with

TOPAS

simulations.

Conclusion The profile of scattered protons back-projected onto the isocenter plane parallel to the particle tracker of a pCT system provides high-resolution tumor position and beam fluence monitoring during high-dose-rate delivery of protons. This technique appears suitable for intra- treatment monitoring of FLASH therapy with shoot-through beams. PO-1616 Quantitative evaluation of prostate SABR verification workflow using triggered kV-imaging and CBCT G. Antal 1 , Á. Gulybán 2 , K. Kisiván 1 , A. Farkas 1 , F. Lakosi 1 1 Somogy County Kaposi Mor Hospital, Dr. József Baka Diagnostic- Oncoradiology and Research Center, Kaposvár, Hungary ; 2 Europa Hospitals, Department of Radiation Oncology, Brussels, Belgium Purpose or Objective During stereotactic ablative radiotherapy (SABR) rigorous verification strategy is required to ensure safe treatment delivery. We aimed to evaluate our clinical workflow for prostate SABR using triggered kV imaging (TkVI) in combination with repetitive CBCTs by focusing on intra- and inter-fractional changes on target coverage. Material and Methods Ten patients were treated with VMAT based SABR after gold marker implantation for a total dose of 36.25 Gy in 5 fractions. Following verification workflow was applied: 1) pre-radiotherapy CBCT (pre-CBCT) with gold marker-based correction; 2) treatment delivery with TkVI (≥3mm threshold for interruption/correction) 3) post-CBCT. Prostate, rectum, bladder were delineated on each CBCT. Difference in target coverage (D98) compared to planning CT were analyzed in three different clinical scenarios: 1) pre-CBCT 2) post-CBCT volumes with applied correction and 3) post-CBCT without TkVI. Intrafractional OAR dose differences using D10cc were also calculated. Results On average two beam interruptions (range: 0-10) were required during the 16±12 min treatment sessions. The average [min,max] D98 deviation from planned dose were modest in scenarios 1-2 (-0.14% [-0.51,0.32] and -0.32% [- 0.97,-0.1]) compared to -1.89% [-11.0,-0.1] without TkVI. In two cases the absence of TkVI would have led to a total of -11% and -5.6% target coverage loss caused by a single large and two intermediate shifts. The mean intrafractional changes of bladder D10cc were small, however variation were reduced by using TkVI (0.01±0.1Gy vs. -0.01±0.03Gy). For rectum D10cc no relevant difference were observed (-0.11±0.23Gy vs. - 0.14±0.33Gy). Conclusion Gold marker-based prostate SABR with triggered imaging and pre/post-treatment CBCT was successfully implemented. Dosimetric evaluation showed maintained target coverage through the whole clinical workflow underlying the importance of intrafractional imaging.

INTER FRACTIO N

ML (mm )

AP (mm )

CC (mm )

INTRA FRACTIO N

ML (mm )

AP (mm )

CC (mm )

Σ σ

0.7 0.8 1.6 Σ 1.0 2.1 3.2 σ

0.7 0.7 1.0 0.7 1.1 1.6