In RTOG 0236 (Timmerman et al. JAMA 2010), a phase II trial treating patients with inoperable T1-2 non-small cell lung cancer with stereotactic body radiotherapy, what was the prescribed dose when the trial initiated?
Non-Small Cell Lung Cancer
RTOG 0236 (Timmerman et al. JAMA 2010) was a phase II trial of 55 patients with medically-inoperable peripheral NSCLC of up to 5 cm in size who underwent 18 Gy x 3 fractions using stereotactic body radiotherapy. The 3-year primary tumor and involved lobe control rate was 90.6% while the 3-year regional control rate was also high at 87.2%. Patients were at highest risk of distant failure with a 3-year rate of disseminated failure of 22.1%. Overall survival was 55.8% at 3-years.
The trial initially had a prescription dose of 20 Gy x 3 fractions. However, on review, it was determined that the dose delivered was overestimated due to lack of heterogeneity corrections (Xiao et al. IJROBP 2009). As photons pass through a low density material (e.g. lung) and incident on a higher density material (e.g. tumor), there is a required build-up region for electron production at the target periphery; this is similar to what happens when photons pass through air to first interact with a patient's body leading to skin sparing and a Dmax. Not accounting for this would lead to an overestimation of the dose to the target periphery. Thus in the final publication, the treatment dose was reported as 18 Gy x 3 fractions.