Internal target volume uncertainty in adrenal stereotactic body radiotherapy: implications of respiratory motion and the value of combined 3DCT–4DCT delineation

Background Respiratory-induced motion introduces geometric uncertainty in adrenal stereotactic body radiotherapy (SBRT), but its magnitude and impact on internal target volume (ITV) delineation remain poorly characterized. This study quantified adrenal tumor motion, evaluated ITV delineation strategies, and assessed combined approaches to minimize uncertainty. Methods A retrospective review identified 19 patients with adrenal metastases who underwent SBRT from May 2022 to December 2024. All underwent free-breathing three-dimensional CT (FB-3DCT) and four-dimensional computed tomography (4DCT) simulation. Gross tumor volume on FB-3DCT (GTV 3D ) and five ITVs were generated using published lung SBRT methods. Amplitudes of tumor motion along the superior–inferior (SI), anterior–posterior (AP), and medial–lateral (ML) axes, as well as in three-dimensional (3D) directions were quantified; percentage volume differences (PVDs) between ITV pairs were calculated. Results Mean SI motion was 5.2 mm, AP 1.7 mm, ML 0.8 mm, and 3D 5.5 mm, and mean breathing variability was 1.6. The mean target volumes stepped up from GTV 3D (17.4 ± 15.1 cc) to ITV5 (25.0 ± 19.8 cc). Pairwise ITV comparisons were all significant (P < 0.05). The PVDs between ITV-based volumes and GTV 3D correlated negatively with effective tumor diameter, whereas the PVDs between ITV3–ITV5 and GTV 3D correlated positively with 3D motion amplitude; respiratory variability showed no consistent association. Conclusions Respiratory motion introduces clinically meaningful uncertainty in adrenal SBRT. A combined FB-3DCT and 4DCT approach was preferred because it provided better spatial coverage and closer agreement with the predefined reference composite ITV than either dataset alone.