This prospective clinical study evaluates the accuracy of the currently used planning target volume (PTV) safety margin of 0.5 cm for stereotactic body radiotherapy (SBRT) of lung tumors at the Institute of Oncology Ljubljana. The study will assess setup errors and intrafractional motion using cone beam CT (CBCT) imaging in patients treated on three linear accelerators and with two different immobilization systems (vacuum cushion and long thermoplastic mask). Based on the collected data, the study aims to determine whether the current PTV margin is adequate and whether the SBRT setup protocol could be optimized.

This is a prospective cohort study designed to evaluate setup errors and intrafractional motion in patients undergoing stereotactic body radiotherapy (SBRT) for lung tumors. SBRT is a highly precise, image-guided form of hypofractionated radiotherapy in which high radiation doses are delivered in 1-8 fractions. Due to the high daily dose per fraction, accurate patient positioning and motion management are essential.

At the Institute of Oncology Ljubljana, lung SBRT has been performed since 2016 using a standard PTV margin of 0.5 cm, based on published literature. However, setup accuracy and residual positioning errors may depend on local clinical workflow, imaging verification protocols, and characteristics of the simulation and treatment equipment. Therefore, it is necessary to evaluate institution-specific setup uncertainties and verify whether the current margin remains appropriate.

The study will include consecutive patients treated with lung SBRT on three treatment machines (Ap4, Ap6, and Ap7). Two immobilization methods will be evaluated: a vacuum cushion and a long thermoplastic mask. For each treatment fraction, CBCT imaging will be performed before treatment, after online correction (residual setup error), and after treatment delivery (intrafractional motion). Setup errors and motion data will be recorded for analysis.

The primary outcomes will be systematic and random setup errors, residual intrafractional error, and calculation of an appropriate PTV margin for each machine and immobilization method using a modified van Herk formula. The study will also evaluate whether the current clinical workflow could be safely shortened by reducing the frequency of CBCT imaging.

The results of this study may support optimization of local SBRT protocols and improve efficiency while maintaining patient safety and treatment accuracy.

Inclusion Criteria:

• Female patients aged ≥18 years
• Diagnosed with gynecological malignancy (endometrial cancer, cervical cancer, ovarian cancer, or fallopian tube cancer)
• Undergoing radical surgical treatment
• Ability to complete quality-of-life questionnaires
• Signed informed consent

Exclusion Criteria:

• Age <18 years
• Patients who do not undergo surgical treatment
• Inability to complete questionnaires due to cognitive impairment or language barriers
• Refusal or inability to provide informed consent