Stereotactic radiotherapy boost after definite chemoradiation for non-responding locally advanced NSCLC based on early response monitoring 18F-FDG-PET/CT.

BACKGROUND AND PURPOSE: Prognosis of locally advanced non-small cell lung cancer remains poor despite chemoradiation. This planning study evaluated a stereotactic boost after concurrent chemoradiotherapy (30 × 2 Gy) to improve local control. The maximum achievable boost directed to radioresistant primary tumor subvolumes based on pre-treatment fluorine-18 fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG-PET/CT) (pre-treatment-PET) and on early response monitoring 18F-FDG-PET/CT (ERM-PET) was compared. MATERIALS AND METHODS: For ten patients, a stereotactic boost (VMAT) was planned on ERM-PET (PTVboost;ERM) and on pre-treatment-PET (PTVboost;pre-treatment), using a 70% SUVmax threshold with 7 mm margin to segmentate radioresistant subvolumes. Dose was escalated till organ at risk (OAR) constraints were met, aiming to plan at least 18 Gy in 3 fractions (EQD2 84 Gy/BED 100.8 Gy). RESULTS: In five patients, PTVboost;ERM was 9-40% smaller relative to PTVboost;pre-treatment. Overlap of PTVboost;ERM with OARs decreased also compared to overlap of PTVboost;pre-treatment with OARs. However, any overlap with OAR remained in 4/5 patients resulting in minimal differences between planned dose before and during treatment. Median dose (EQD2) covering 99% and 95% of PTVboost;ERM were 15 Gy and 18 Gy respectively. Median boost volume receiving a physical dose of  ≥ 18 Gy (V18) was 88%. V18 was ≥ 80% for PTVboost in six patients. CONCLUSIONS: A significant stereotactic boost to volumes with high initial or persistent 18F-FDG-uptake could be planned above 60 Gy chemoradiation. Differences between planned dose before and during treatment were minimal. However, as an ERM-PET also monitors changes in tumor position, we recommend to plan the boost on the ERM-PET.