# Safety and Efficacy of Stereotactic Magnetic Resonance-Guided Adaptive Radiation Therapy (SMART) for Ultracentral Metastases in Non-Small Cell Lung Cancer

## TL;DR

SMART radiation therapy safely and effectively treats hard-to-reach lung cancer metastases near critical organs.

## Contribution

Demonstrates the safety and efficacy of SMART for ultracentral NSCLC lesions with daily plan adaptation.

## Key findings

- No grade ≥ 3 toxicities were observed in 11 patients treated with SMART.
- One-year freedom from local progression was 93% with a median overall survival of 20 months.
- Daily online plan adaptation improved safety and local control compared to conventional SABR.

## Abstract

Stereotactic ablative radiation therapy (SABR) is a standard of care for early-stage lung cancer and thoracic oligometastatic or oligoprogressive disease. However, ultracentral lesions remain challenging because of their proximity to critical mediastinal structures and the associated risk of severe toxicity. Stereotactic magnetic resonance-guided adaptive radiation therapy (SMART) allows for daily plan adaptation and real-time tracking in breath-hold, enhancing target coverage while improving sparing of adjacent organs compared to conventional SABR.

This retrospective study analyzed outcomes of SMART-based SABR for ultracentral metastatic lesions in patients with histologically confirmed non-small cell lung cancer (NSCLC). Ultracentral lesions were defined by planning target volume overlapping with the proximal bronchial tree, esophagus, or pulmonary vessels. Endpoints included grade ≥ 3 SMART-related toxicity, freedom from local progression, progression-free survival, and overall survival.

Between 2020 and 2023, 11 patients with 18 ultracentral NSCLC lesions underwent SMART. All treatments were delivered in breath-hold. The median dose was 40 Gy (range, 30-60 Gy) in 5 to 8 fractions. Online plan adaptation was performed for 100% of the 78 delivered fractions. No grade ≥ 3 toxicities were observed. Rates of grade 1 to 2 acute and late toxicities were 54% and 18%, respectively. At a median follow-up of 28 months (range, 5-41 months), 66.7% of patients were alive. One-year freedom from local progression was 93%. Median progression-free survival was 5.8 months (range, 1-39 months), and median overall survival was 20 months (range, 5-41 months).

SMART with daily online adaptation achieved excellent local control and a favorable safety profile in ultracentral NSCLC, comparable to conventional non-adaptive SABR, but without severe toxicity.