Analysis of Online Cone-beam CT Images for Non-small Cell Lung Cancer to Define an Appropriate CTV-to-PTV Margins

Abstract

Kilovoltage cone-beam computed tomography (CBCT) has been developed to provide accurate soft tissue and bony setup information. We evaluated the magnitude of interfraction and intrafraction tumor displacement using online CBCT for non-small-cell lung cancer (NSCLC) patients and estimated the appropriate planning target volume (PTV) margins. Ten NSCLC patients treated with three-dimensional conformal radiotherapy (3D-CRT) were investigated. The study procedure involved (1) initial patient setup using laser alignment to skin marks, (2) initial CBCT acquisition and analysis of these images with planning CT images using automatic and manual registration to assess the interfraction tumor displacement (setup error), (3) couch translation based on the registration results accordingly and treatment delivery, (4) a second CBCT were acquired on couch immediately after treatment to assess intrafraction tumor displacement. Online-guided correction data were recorded in the left-right (LR), cranio-caudal (CC), and anterior-posterior (AP) directions respectively. Each patient underwent this procedure 2∼3 times per week over the course of their treatment. The precorrection and postcorrection PTV margins were calculated using the 2.0 Σ+0.7σ recipe. The criterion to derive the recipe was that on average more than 99% of the CTV should at least get 95% of the dose. Interfraction (Σ inter, σ inter)and intrafraction errors (Σ intra, σ intra) were all involved in the calculation of the precorrection PTV margins, while postcorrection PTV margins accounts for intrafraction errors only. The complete procedure took approximately 6∼8 min, including deployment of CBCT components, scanning, reconstruction, images registration, and couch correction. Total of 159 sets of CBCT images were analyzed. The precorrection PTV margins required to account for total errors are 7.2 mm, 8.8 mm and 10.1 mm in the LR, CC and AP directions, respectively. When the tumor was corrected every fraction, the postcorrection PTV margins required to account for intrafraction errors are 2 mm,1.6 mm,1.6 mm in the LR, CC and AP directions respectively. There exists some extent of errors from setup and organ motion in 3D CRT for NSCLC. The approach based on the CBCT measurements can be used to reduce the impact of setup errors obviously and to estimate the PTV margins. When no CBCT correction was applied, the PTV margins should be 7 mm, 9 mm and 10 mm in the LR, CC and AP directions respectively, and if there is correction before treatment, the PTV margins were 2 mm in every direction.