Beware: steep gradient ahead.

One criticism of stereotactic body radiation (SBRT), and sublobar resections, for NSCLC is the potential for occult disease in the involved lobe. Not only could this be a source of marginal failures, it could even lead to subsequent regional or distant metastases. One of the priorities of lung SBRT is a rapid dose fall-off outside the target to spare healthy heart and lung function. This study takes a novel look at whether rapid dose fall-off also spares extra-target tumor control. Using a large cohort (n=217) of patients with Stage I NSCLC treated with SBRT from two institutions, the mean dose within a 3 cm shell surrounding the target volume was calculated. In patients who received a mean dose of at least 20.8 Gy to this region, the rate of later distant disease was 5%. In patients with a mean periphery dose <20.8 Gy, that rate was...60%. The authors make two interesting hypotheses here. Moderate dose outside the target may 1) kill potentially metastatic tumor clones and/or 2) contribute to an immunogenic abscopal effect. TBL: Controlling moderate SBRT dose outside the primary target volume for NSCLC may adversely impact risk of distant failure. | Diamant, Radiother Oncol 2018