Evaluation of Radiation Therapy Treatment Plans in a Randomized Phase 2 Trial Comparing 2 Schedules of Twice-Daily...

There is limited clinical data for recommendations on how to deliver thoracic radiation therapy (TRT) concurrently with chemotherapy in limited-stage small cell lung cancer. We reviewed radiation therapy treatment plans in a randomized phase 2 trial comparing high-dose with standard-dose twice-daily TRT to assess treatment planning techniques, dose-volume data for target volumes and organs at risk (OARs), evaluate compliance with the protocol, associations with radiation-induced toxicity, and whether an imbalance in treatment planning parameters might be a reason for the large survival benefit of the higher dose (median overall survival 43.6 vs 22.6 months).

In the study, 170 patients were to receive 4 courses of platinum/etoposide and were randomized to receive twice-daily TRT of 60 Gy/40 fractions (fx) or 45 Gy/30 fx. TRT treatment plans for those who received 1 or more fx of TRT (n = 166) were analyzed.

The most common treatment planning technique was 3-dimensional conformal radiation therapy (67%). The 75th percentile of the reported dose-volume parameters for the OARs were within the protocol-recommended limits for both groups. Mean doses to the esophagus of 25.5 Gy (IQR, 20.2-31.3; 60 Gy/40 fx) and 24.3 Gy (IQR, 20.3-27.5; 45 Gy/30 fx) were associated with 21% and 18% ≥ grade 3 acute esophagitis, respectively. In the 60 Gy/40 fx group, a mean dose to the lungs of 16.5 Gy (IQR, 15.8-16.9), V20 Gy of 29.5% (IQR, 28.8-30.4), and V5 Gy of 65.6% (IQR, 61.5-68.7) led to ≥ grade 3 pneumonitis in 4% of the patients. There was no ≥ grade 3 pneumonitis in the 45 Gy/30 fx group. The treatment planning techniques, the percentage change in volumes between original and redelineated OARs, planning target volumes, relative doses, and laterality were well balanced between the randomly assigned groups.

Considering the incidences of severe radiation-induced toxicities were within the range of other recent trials, the reported doses to the OARs appear to be safe. Treatment planning parameters were well balanced between the randomly assigned groups, supporting that the survival benefit of the twice-daily 60 Gy/40 fx TRT schedule was due to the higher dose.