Microscopic disease extensions as a risk factor for loco-regional recurrence of NSCLC after SBRT

Stereotactic body radiotherapy (SBRT) is a highly conformal technique that allows a more accurate irradiation of lung tumors. However, a highly conformal dose distribution may underdose undetected microscopic disease extensions (MDE) near the tumor leading to loco-regional failure in tumor control. The purpose of the current work is to assess the risk of loco-regional failure in SBRT by analyzing pre-treatment scans.

A model to predict the risk of occurrence of MDE from pretreatment images was developed based on pathology samples of 47 lung cancer patients. This model was used to assess the outcome of 238 SBRT treatments.

Patients with high risk of MDE presence showed significantly lower 2-year loco-regional control (82.0% vs. 91.8%) and shorter time to loco-regional failure (8.4 months vs. 20.7 months) than low risk patients. The minimum dose delivered in the volume surrounding the GTV affected the model predictive power. The model remained predictive for patients who received less than 31 Gy in that volume. For patients who received larger doses, the MDE risk classification was not significant.

The results show that MDEs are, at least partially, responsible of loco-regional failure in highly conformal radiotherapy. This information could be used to optimize dose distributions.