Which Dose Specification Should Be Used for NRG Radiation Therapy Trials: Dose-to-Medium or Dose-to-Water?

To compare the doses calculated by the Analytical Anisotropic Algorithm (AAA), Acuros dose-to-medium, and Acuros dose-to-water for the patients with lung cancer treated at our institution and show that further investigation and clarification are needed about what dose specifications should be used for NRG clinical trials.

Twenty-one patients with lung cancer who previously received intensity modulated radiation therapy or volumetric modulated arc therapy-based treatments at our institution were analyzed by recalculating their plans for each one with the AAA algorithm (reviewed and approved by our radiation oncologists) and with both reporting modes of the Acuros algorithm. All plans used the same monitor units as the original approved plan and a 2.5-mm grid size. For each patient, D₁₀₀ of clinical target volume (CTV) and CTV coverage ratios in each plan were compared, and dose distributions and dose-volume histograms calculated by AAA, Acuros dose-to-water (D_w,m), and Acuros dose-to-medium (D_m,m) were compared as well.

Differences between CTV D₁₀₀ calculated by AAA and Acuros D_m,m were larger than the differences between AAA and Acuros XB D_w,m for all patients. When D₁₀₀ of CTV was evaluated, the largest difference between AAA and Acuros D_m,m was 14.12% and between AAA and Acuros XB D_w,m was 3.68%. The average differences between the CTV D₁₀₀ calculated by AAA and Acuros D_m,m was 5.39%. Coverage ratio between Acuros D_m,m and AAA ranges from 51.08% to 100% with an average of 91.32%; coverage ratio between Acuros D_w,m and AAA ranges from 87.2% to 100.41% with average of 98.94%; coverage ratio between Acuros D_m,m and Acuros D_w,m ranges from 58.58% to 100% with an average of 92.03%.

The present study shows large and systematic differences in doses calculated by AAA and Acuros D_m,m. Therefore, further investigation and clarification are needed about which dose reporting mode should be used.