Top Line: How do you estimate the risk of symptomatic radiation pneumonitis (RP) from lung SBRT?
The Study: Here’s a large retrospective analysis of predictors of post-SBRT RP from Indiana University/VA. Over 300 cases had sufficient clinical and dosimetric data, and a single physician went back to re-contour lung volumes according to the RTOG 1106 atlas. Most treatment plans used 15 fixed, non-coplanar beams and 3D planning. The most common regimen was 48 Gy in 4 fractions (46%), and treatments were typically given on non-consecutive days. Overall, 9.7% of patients developed symptomatic RP (4.4% G2, 5% G3 (oxygen), and 0.3% G4). Non-dosimetric factors associated with an increased risk of RP were comorbid lung condition, prior lung RT, and right side tumor location. For dosimetric analysis, they converted to EQD2 (alpha/beta 3). For all-comers, there was a < 10% risk of grade 2+ RP (RP2) if the total mean lung dose (MLD-TL) was 6 Gy or less, ipsilateral MLD (MLD-IL) 20 Gy or less, and V20 10%. Converting 6 Gy and 20 Gy EQD2 back to 5 fractions is just under 7 Gy and just over 16 Gy, respectively. Slightly higher thresholds could be used for patients with no lung conditions and no prior RT. But for patients with a lung comorbidity, more strict thresholds of MLD-TL 4.2 Gy, MLD-IL 6.5 Gy, and V20 6.8% were required for a 10% risk of RP.
TBL: Lung comorbidities, prior lung RT, mean lung dose, and V20 are important predictors of symptomatic pneumonitis after lung SBRT. | Liu, Radiother Oncol 2020