Utilizing deformable image registration in evaluation of anatomic changes over the course of lung cancer stereotactic body radiation therapy and the accumulated dosimetric deviation

Abstract

Background. The overall treatment time for a course of lung cancer stereotactic body radiation therapy (SBRT) is much shorter than a conventional fractionated radiation therapy. However, there has been increasing number of recent studies showing remarkable inter-fraction anatomic changes of both lung tumors and surrounding organs during SBRT. The purpose of this study is to investigate the inter-fraction variations of tumor during a course of SBRT and their relationship to the deviation of actual dose delivered to the targets and organs at risk (OARs). Materials and Methods. Twenty patients with non-small cell lung carcinoma (NSCLC) or pulmonary oligometastases treated by SBRT were retrospectively recruited. The anatomic variation as revealed in daily cone-beam computed tomography (CBCT) images was evaluated in terms of internal target volume (ITV), center of mass (COM) displacement, ITV Dice similarity coefficient (DSC) and planning target volume (PTV) DSC. Deformable registration was used to deform the planning computed tomography (PCT) images to the CBCT images for more accurate dose calculation. Dosimetric comparison was carried out between the planned dose and actual delivered dose. The relationship between the anatomic variation and dosimetric deviation was then analyzed by correlation test. Results. No statistically significant time trend was found in the ITV volume, COM displacement, ITV DSC and PTV DSC. However, twelve patients (60%) demonstrated similar tendency of ITV volume changes, with tumor expansion from the first fraction and tumor shrinkage by the final fraction. The largest tumor expansion found in this study was 20% of the initial volume. In the dosimetric comparison, the PTV D95%, conformation number (CN) and maximum dose at 2cm from the PTV surface (D2cm) significantly deteriorated from the planned dose to the actual delivered dose, but ITV coverage was not significantly compromised. Only one patient (5%) showed ITV D99% < 95% of the planned dose and also had a suboptimal V100% value of 96.9%. The dosimetric deviation was found to be significantly correlated with ITV volume, ITV DSC and PTV DSC. Conclusion. Significant anatomic and dosimetric variations were observed in some patients during the course of SBRT. With image guidance in SBRT, re-planning or adaptive radiation therapy should be considered for patients demonstrating remarkable anatomic changes to avoid compromised target coverage. ITV volume, ITV DSC and PTV DSC might be potential indicators for severe dosimetric deviation to aid the decision making of re-planning.