5-Fraction Re-radiosurgery for Progression Following 8-Fraction Radiosurgery of Brain Metastases From Lung Adenocarcinoma: Importance of Gross Tumor Coverage With Biologically Effective Dose ≥80 Gy and Internal Dose Increase

The criteria for indication of salvage stereotactic radiosurgery (SRS) for local progression following multi-fraction (mf) SRS of brain metastases (BMs) remain controversial, along with the optimal planning scheme. Herein, we described a case of BMs from pan-negative lung adenocarcinoma (LAC), in which the two lesions of local progression following initial eight-fraction (8-fr) SRS were re-treated with 5-fr SRS with the biologically effective dose (BED(10)) of ≥80 Gy, based on the linear-quadratic (LQ) formula with an alpha/beta ratio of 10. The re-SRS resulted in the alleviation of symptoms and favorable tumor responses with minimal adverse effects during the 7.3-month follow-up. In the lesions of local progression, the gross tumor volume (GTV) coverage with 49.6 Gy (BED(10) 80 Gy) was generally insufficient, and the GTV dose wes relatively homogeneous with ≥87% isodose covering. In contrast, the 5-fr re-SRS was performed with sufficient GTV coverage with ≤68% isodose of 43 Gy (BED(10) 80 Gy). Taken together, sufficient GTV coverage with a BED(10) of ≥80 Gy and steep dose increase inside the GTV boundary, that is, extremely inhomogeneous GTV dose, are important in 8-fr SRS for ensuring excellent local control of BMs from pan-negative LAC. For local progression following mfSRS that does not fulfill both criteria, re-SRS with the above planning scheme can be an efficacious and safe treatment option for at least six months, especially in cases in which the prior SRS was performed with a dose/fractionation under adequate consideration of brain tolerance. The BED(10) seems to be the most suitable for estimating the anti-tumor efficacies of SRS doses in 3-8 fr, similar to that of a single fraction of 24 Gy.