Study of defining an appropriate CTV-to-PTV margins for non-small cell lung cancer based on online registration of cone.beam CT images and planning CT images

Abstract

Objective To estimate the appropriate margins for the clinical target volume (CTV) with or without online correction using cone-beam CT(CBCT) during the processs of radiation for non-small cell lung cancer(NSCLC) patients. Methods Eight patients with NSCLC treated with three-dimensional conformal ra-diotherapy(3D-CRT) were investigated, kV CBCT scans were performed before and immediately after radio-therapy. Then analysis of these images was performed using automatic and manual registration of the CBCT and planning computed tomography images. The patient positioning and organ motion were corrected by moving the couch in the left- right (X), cranio- caudal (Y), and anterior- posterior (Z) directions accordingly, and CBCTonline-guided correction data were recorded. The clinical study performed 2-3 times per week. CBCT data ac-quired before treatment delivery were used to evaluate the positioning error and organ motion, and that acquired after treatment were used to assess intrafraction tumor displacement and organ motion. These data were used in a standard formula to calculate CTV-to-PTV(planning target volume) margin of online-guided correction and non-online-guided correction. Results Total of 143 sets of CBCT images were analyzed. On the condition of non- correction, the margins required to account for total errors were 8 mm, 9 nun, 11 mm in X, Y and Z direc-tions respectively. When the tumor was corrected every fraction, margins required to account for intrafraction errors were 2 mm in each directions. Conclusion There are some extent of errors from positioning and organ motion in 3D-CRT for NSCLC. Online correction approach based on CBCT images analysis can be used to re-duce the impact obviously and to estimate the appropriate margins for the CTV. Key words: Carcinoma, non-small-cell lung; Radiotherapy, conformal; Cone-beam CT