TU-E-BRB-10: Dosimetric Consequences of Setup Errors Using CBCT for SBRT Localization

Area of Science:

• Radiation Oncology
• Medical Imaging
• Cancer Treatment

Background:

• Stereotactic ablative radiation therapy (SABR) requires precise tumor targeting due to steep dose gradients and high fractional doses.
• Accurate patient setup and tumor localization are critical to minimize radiation dose to healthy tissues and maximize tumor coverage.
• Inter-fraction motion and setup errors can compromise treatment accuracy in SABR for lung cancer.

Purpose of the Study:

• To evaluate inter-fraction shifts in non-small cell lung cancer (NSCLC) patients undergoing SABR.
• To investigate the influence of tumor location and internal target volume (ITV) on inter-fraction shifts.
• To quantify residual errors after cone-beam CT (CBCT) correction and assess their dosimetric impact.

Main Methods:

• Retrospective analysis of daily free-breathing (FB) CBCT images from 78 NSCLC patients.
• ITV inter-fraction displacement assessed by matching CBCT and FB helical CT images.
• Setup errors quantified using orthogonal kV images; tumor locations categorized (e.g., central, peripheral).

Main Results:

• Mean systematic error (Σinter) was (2.1, 4.2, 2.9) mm, and random error (sinter) was (2.2, 3.2, 3.6) mm.
• No significant association found between inter-fraction shifts and tumor location or volume.
• CBCT image guidance reduced setup errors to Σsetup=(0.6, 0.6, 0.4) mm and ssetup=(1.2, 0.7, 0.7) mm.

Conclusions:

• CBCT image guidance significantly reduced setup errors in SABR for NSCLC.
• Population-based setup margins of 2-3 mm are sufficient for >95% ITV dose coverage.
• Further research on inter- and intra-fraction error classification by tumor location is recommended.