ESTRO 2024 - Abstract Book

S5543

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2024

Purpose/Objective:

Respiratory motion is commonly determined in 4DCT scans using external surrogates to correlate the patient-specific breathing cycle. With a different approach, the internal anatomy and diaphragm motion can be determined at the treatment couch with a 4D-CBCT, without surrogates. This study aimed to evaluate the differences between the two imaging modalities in ITV delineation and motion amplitude for lung SBRT.

Material/Methods:

A mono-institutional cohort of 6 lung SBRT patients underwent 4DCT simulation using a tension belt to generate the breathing trace. A 4DCT scan without contrast media was acquired with 2 mm-thick slices and a selected pitch value according to patients’ breathing rate. A retrospective phase-binning reconstruction in 10 phases was performed. The ITV was defined as the envelope of GTVs delineated on each breathing phase and the PTV was defined as the ITV 5 mm isotropic expansion. VMAT 6 MV-FFF single-isocentre SBRT plans were optimized for the Versa HD linac. A 4D CBCT scan with Elekta Symmetry protocol was acquired during the first treatment session to perform a dual registration image matching. At first, an experienced Radiation Oncologist (RO) evaluated the positioning deviation in the spinal region (clipbox registration). A following mask registration was used to correct motion deviation in the surrounding of the ITV. Clipbox and mask data were analysed to evaluate the baseline shift. The quantitative evaluation of the differences in the target definition and volume has been performed using a MIM automated workflow allowing the contour propagation from a single manually contoured breathing phase and its clinical check. To avoid any delineation bias, MIM workflow was used on both 4DCT and 4D CBTC images by the same RO. ITV volume differences and relative centroid position variations (ΔCP) between the two methods were recorded and analysed.

Results:

Collected data included 6 lung SBRT patients treated between September 2023 and October 2023 (50 Gy in 5 fractions). Five patients had an upper lobe lesion, one patient in the lower lobe near the diaphragm.

The baseline shift of upper lobe lesions was almost negligible in all the directions with a maximum of 1.9 mm, while a cranio-caudal shift of 11.2 mm was observed for the diaphragmatic tumour.

Mean ITV volume delineated in 4DCT scans was 9.8 [1.2 – 34.7] cm3. Mean ITV volume on 4D CBCT was 11.9 [1.5 – 38.6] cm3 with a mean increase of 20.6% [-2.2 – 56.0]. Only one patient had a limited reduction in ITV delineation, while the diaphragmatic lesion’s ITV resulted 56.0% larger at the CBCT contouring. The mean lateral ΔCP was less than 1 [0.1 – 1.1] mm. In the antero-posterior direction, the mean ΔCP was 1.6 mm, with a maximum of 5.1 mm, while a mean cranio-caudal ΔCP of 2.1 mm was recorded with a maximum of 9.5 mm for the diaphragmatic lesion.

Conclusion: