SU‐FF‐J‐09: An Analysis of Cone‐Beam CT for Determining Setup Errors and Designing Treatment Margins for Non‐Small Cell Lung Cancer Patients

Purpose: KV cone‐beam CT imaging (CBCT) was used to analyze setup errors when treating patients with non‐small cell lung cancer (NSCLC). For patients where an off‐line correction strategy was adopted, CBCT was also used to determine the patient specific planning target volume (PTV). Method and Materials: Eighteen patients with NSCLC were treated with CBCT image‐guided radiotherapy using an Elekta Synergy linac. 4D‐CT imaging was used for treatment planning. Patients were scanned in the supine position during normal relaxed free‐breathing and were immobilized with a wing‐board. The internal target volume (ITV) was determined using the maximum‐intensity‐projection (MiP) CT data to account for organ motion. A 5‐mm PTV margin was used to account for setup error. On‐line setup correction was based on the daily CBCT imaging prior to treatment. The systematic and random errors were analyzed retrospectively. The results were then fit to our CBCT off‐line correction strategy where a setup correction was made at the fifth treatment fraction to correct for the systematic error. The patient specific PTV margin was also analyzed. Results: The systematic and random errors were 5.7 ± 3.2 mm, 4.2 ± 2.6 mm and 4.8 ± 3.1 mm for anterior‐posterior (AP), medial‐lateral (ML) and superior‐inferior (SI) directions respectively. The 3D displacement vectors were 4.1 mm ± 3.1 mm (systematic ± random). If the patients were treated with the off‐line correction strategy, a patient specific PTV margin ranging from 7.0 to 17.4 mm would be needed. Conclusions: The use of a 5 mm PTV margin on the ITV determined by MiP dataset combined with daily CBCT on‐line correction is adequate to avoid geometric miss of the tumor for NSCLC patients. A larger patient specific PTV margin, ranging from 7.0 to 17.4 mm, would be needed if an off‐line correction is used. © 2007, American Association of Physicists in Medicine. All rights reserved.

Duke Scholars

Author Tien Yin Wong Ophthalmology